First of all I would like to thank the institutions involved in my cotutelle PhD thesis: the Universitè Jean Monnet de Saint Etienne, which funded this PhD thesis; the Università degli Studi di Palermo, which welcomed me as a cotutelle PhD student; the Univeristé Franco-Italienne, which financially supported my mobility towards Italy.

I would like also to acknowledge the contribution of the CEA-DAM-DIF of Arpajon and iXFiber SAS to the realization of this PhD thesis. I am infinitely grateful to the professors I had the opportunity to work with: Simonpietro Agnello, Aziz Boukenter, Sylvain Girard and Youcef Ouerdane. They supported me throughout these years and working with them has been a truly enriching experience for me. They always granted me the freedom to use my own initiative to carry out other experiments alongside my core work, no matter how exotic they were... In some cases the final result was positive only because of their encouragement to try again, after a first disheartening attempt. Thanks to them, some of these results are shown in one of the chapters of this thesis.

I would like to thank all the members of the LAMP Group of Palermo, especially Prof. Marco Cannas, for the many enlightening discussions we have had during these years. Special thanks also go to Dr. Antonino Alessi, for such a formative collaboration and to Giampiero Buscarino for his precious advices. I am also grateful to Jean-Yves Michalon of the Laboratoire Hubert Curien, and to Claude Marcandella of the CEA-DAM-DIF of Arpajon, for their help in performing several experiments.

Working at the University of Saint Etienne has been my first opportunity to experience a multicultural environment. I have had the pleasure to meet and work with some very special people, with whom I shared many enjoyable moments. I cannot name them all but I must express my gratitude to Adriana Morana and Xavier Pheron. If this were a chronological list of acknowledgments, they would be at the very top: when I arrived in France three years ago they were the first people I could count on, who took it upon themselves to make sure I settled into my new life.

I also need to send my thanks overseas, to my hometown Cefalù, in Italy, where my family and my dearest friends live. Their moral support is what has kept me going, as well as becoming an uncle in the meantime, which has been the best gift of all and for which I thank my sister Manuela and my dear friend Nicola.

Last but not least, I am grateful to Antonella. After all these acknowledgements, what is left of the credits I could claim I must share with her.

Table of Contents i ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Table of Contents

Introduction

In our society the need of fast and reliable communication becomes every year more and more demanding. Fiber optics is the keystone technology developed in the last five decades to match such requirements. Any kind of information (text, music, images, etc.) can be encoded in light pulses that travel for thousands of kilometers along dielectric waveguides as thin as a human hair. Optical fibers (OF) offer several advantages compared to the other telecommunication technologies: higher speed of data transmission, lighter weight, immunity to external electromagnetic fields and lower production costs. Although many years have passed since the beginning of the intense study of OFs and related technologies, there are still strong efforts in developing new fibers capable of better performances. The scientific community has recently recognized the importance of fiber optics by assigning the Nobel Prize in Physics (2009) to C. K. Kao [START_REF] Kao | Proc. IEE[END_REF], who started working on optical fibers in the sixties and first understood, in 1966, the potentiality of this technology in telecommunications.

Since then, the progress in fiber optics has never stopped and in fact, more and more applications related to OF technology have risen. The research in the domain of OFs has greatly widened under the pressure of its new potentialities and, in many cases, the achieved technological successes departed from the original communications purposes. A remarkable application of the current crucial role of OFs is represented by their employment in radiative environments as devices, data transmission components and sensors. Following the interests related to such applications, this PhD thesis is devoted to the study of the behavior of silica-based OFs exposed to ionizing radiation.

In presence of ionizing radiation point defects are created into the silica-based material which the optical fibers are made of. These defects are at the origin of the degradation of the optical transmission of the dielectric material. The understanding of the microscopic structure of point defects and their mechanisms of creation is necessary to improve the radiation resistance of the OFs or, vice versa, to exploit their radiation sensitivity. In this context, the use of doping of fibers for changing the refractive index has posed the question regarding dopant related defects, both native and radiation induced, and the role they could have in the fiber resistance to harsh environments [START_REF] Origlio | Properties and Radiation Response of Optical Fibers: Role of Dopants[END_REF].

Of main concerns are the Ge, P, F et Al dopants due to their technological role, but until now the Introduction ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ research has been mostly focused on the presence of a dopant at a time. The current PhD thesis work focuses on the role of codoping silica and on the influence of small molecules on the radiation sensitivity of standard and codoped fibers. First we have investigated the radiation response of codoped optical fibers trying to highlight the interactions between the different co-dopants among the most widely used: Ge, P, Ce, F. Secondly, we have studied the effects due to the variation of the irradiation temperature on the radiation response of the fibers as well as the effects of different drawing conditions to highlight technological relevant features to be applied in production sites.

Finally, we started the study of the effects of an O 2 -loading treatment of some OFs with particular respect to the subsequent impact on the respective radiation responses in order to deepen the understanding of alternative passivation procedure with respect to the traditional H 2 loading.

The thesis can be divided in two main parts. The first part is composed by chapter 1 and chapter 2. The first chapter reports on the main features of an OF, the working principle, the most important types, the production processes. Moreover, in this chapter we provide a wide survey on the many point defects that are involved in the OFs typologies experimentally investigated in this PhD thesis. In the second chapter we report on the theoretical background concerning the spectroscopic techniques employed in our experimental research. In this chapter we also provide a description of the investigated samples and experimental setups employed for our studies.

The second part of the thesis is composed by chapter 3 and chapter 4. In the third chapter we report on all the experimental results involving:  P-doped and P-Ce-doped OFs;  Ge-doped, Ge-F-doped and Ge-Ce-doped OFs;  N-doped OF.

Moreover, in this chapter we also describe the variation of the radiation response of the P-doped OF as a function of the temperature of irradiation. The fourth and last chapter deals with the O 2 -loading treatment of the pure-silica-core, F-doped and P-doped OFs. We demonstrate the incorporation of molecular oxygen into the OF and, subsequently, we propose a comparative study of the radiation response of the O 2 loaded samples and the unloaded one.

A summary of the most significant results is reported in the conclusion paragraph, as well as a list of publications and communications related to this PhD thesis work.

Light propagation in an OF ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Chapter 1 1 Background on optical fibers and point defects

This chapter is devoted to the description of the basic features of silica-based Optical Fibers (OFs) and their response to ionizing radiation.

First we report on the structure of OFs, their fabrication processes and their working principles. In the second part we review the main literature results concerning the radiation response of OFs and the radiation induced point defects in several types of doped silica of interest for the present PhD thesis work. In this literature background, we preferentially focus on the bibliographic data which are of major importance for the analysis of our experimental results.

Silica-based optical fibers

An OF is a dielectric waveguide of electromagnetic radiation which allows reliable and fast information transmission over long distances [START_REF] Keiser | Optical Fiber Communications[END_REF][START_REF]Fiber-Optic Communication Systems[END_REF]. The transmitted signals are typically in the near infrared (NIR) spectral domain for telecommunication applications, whereas UV-Vis spectral domains are also exploited for technology and scientific applications. The basic structure of an OF is reported in Figure 1.1. The central part of the OF is called core and is an optical medium characterized by a refractive index n 1 . The intermediate part is called cladding and it is characterized by a refractive index n 2 . The outer part is the so called coating (usually a polymer) which is a protective layer applied to the OF to improve its mechanical properties.

Chap. 1: Background on optical fibers and point defects ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ In order to be able to guide the light the refractive indexes of core and cladding are such that n 1 >n 2 .

In silica (SiO 2 ) based OFs, the refractive index change between the core and cladding zone is obtained by changing the chemical composition of the material. For instance, as shown in Figure 1.2, doping silica with germanium or phosphorous increases the refractive index of the glass, whereas doping it with fluorine or boron decreases the refractive index.

The geometry of the core of an OF and more precisely its refractive index profile determines the light propagation properties of that OF. We discuss these aspects in the following paragraphs.

Light propagation in an OF

The propagation properties of light inside an OF, with a known refractive index profile, are obtained by solving the wave equation for the electromagnetic field [START_REF] Jackson | Classical Electrodynamics[END_REF]. The solutions are expressed in terms of waveguide modes, which are the electromagnetic field spatial configurations that maintain their intensity profile during the propagation. The fiber design (radius, refractive index profile) and the 1.1.1 Light propagation in an OF ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ signal wavelength determine the number and characteristics of propagating modes inside an OF. On the basis of this, OFs are then divided into two large families:

 single-mode (SM) OFs: only one mode, called fundamental mode, is guided by the OF at the signal wavelength. Typical SM OFs operating in the NIR domain have core diameters of 8 µm and a small refractive index difference between the core and the cladding (~0.3 %).  multi-mode (MM) OFs: several modes are supported in the OF. MM OFs operating in the NIR have large core diameters (>40 µm) and larger refractive index differences between the core and the cladding.

In order to simply explain the guiding effect of an OF it is common to refer to the phenomenon of total internal reflection. When a light ray impinges on an interface separating two homogeneous optical media characterized by refractive indexes n 1 and n 2 , with n 2 <n 1 , it is partially reflected and partially transmitted as shown in Figure 1.3. If the incident angle is i, the angle r of the emerging refracted ray satisfies the Snell's law [START_REF] Jackson | Classical Electrodynamics[END_REF]:

𝑛 1 sin(𝑖) = 𝑛 2 sin(𝑟). ) are the ones that are guided by the OF. It is worth to mention that this simple ray optics approximation cannot be used in describing the propagation of light in SM OFs. However, it can be useful in treating the cases of MM OFs when large numbers of modes are involved.

Chap. 1: Background on optical fibers and point defects ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ Up to now we have treated the case of an OF with a so called step-index profile, which means that the refractive index profile (perpendicularly to the fiber axis) is described by only two numbers, the refractive index of the core and the one of the cladding. This is often the case for SM OFs. However, for MM OFs several other designs are today employed in the production of commercial OFs such as the graded-index profile and the multistep-index one. The first one consists typically of a smooth parabolic increase of the refractive index in the core of the OF which reaches its maximum in center of the OF. In the second case the refractive index increases from the cladding to the center of the OF in a succession of steps as shown in Figure 1.4. In this PhD Thesis we employed only MM OFs. As described in details in § 2.5, all the studied samples have step-index profiles.

Losses in silica-based OFs

The success of fiber optics technology relies mainly in its capability of transmitting digitized information over several tens of kilometers without amplification and at high rates. Similar performances are not achievable using either electrical signal in coaxial cables. Therefore, one of the most important parameters that characterize an OF is the attenuation (see § 2.1.1), which is the reduction of the intensity of propagating light signal along the OF. In Figure 1.5 we report a typical attenuation spectrum of silica-based OF. Several phenomena contribute to the reduction of the intensity of light propagating along the fiber:  Rayleigh scattering, which arises from microscopic variations of the material density and depends on the wavelength as 1/λ 4 .  The infrared absorption tail, which arises from the absorption of light due to the vibrations of atoms.  The ultraviolet (UV) tail, which arises from absorption of light associated with the electronic transition in the UV spectral domain.  The absorption of light by point defects, such as the OH groups that absorb light at 2.73, 1.38 and 0.95 µm.  Scattering due to the waveguide imperfections and bending losses.

For the study of the radiation induced losses we will be concerned by the creation of point defects (see further) and the appearance of the corresponding absorption bands.

Fabrication of OFs

The fabrication of an OF takes place in two steps: preform fabrication and drawing process. The preform is a cylinder of properly doped silica glass with a radial refractive index profile which reproduces, on much larger scale, the refractive index profile of the final OF. Typical sizes of preforms range from few millimeters in thickness to some centimeters, whereas they can be as long as one meter. The preform can be produced by several techniques: Outside Vapor-Deposition (OVD), Modified Chap. 1: Background on optical fibers and point defects ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ Chemical Vapor Deposition (MCVD), Axial Vapor Phase Deposition (AVD), Plasma Chemical Vapor Deposition (PCVD) and Surface Plasma Chemical Vapor Deposition (SPCVD) [START_REF] Ungar | Fibre optics: theory and applications[END_REF][START_REF] Golant | Proceedings of SPIE "Advances in Fiber Optics[END_REF][START_REF] Pacchioni | Defects in SiO2 and Related Dielectrics[END_REF].

These production processes rely on the thermal chemical reaction in which gaseous SiCl 4 reacts with O 2 according to the following reaction:

𝑆𝑖𝐶𝑙 4 + 𝑂 2 → 𝑆𝑖𝑂 2 + 2𝐶𝑙 2 .
1.2

In order to change the refractive index profile of the core of the preform, some other chemical compounds are added to the reaction. These compounds may contain Ge, P, F and the other dopants responsible of the refractive index change. With the exception of one sample produced by SPCVD method, all the OFs studied in this thesis were produced by MCVD method, schematically reported in Figure 1.6.

Figure 1.6: Representation of the MCVD process for the fabrication of OF preforms. Adapted from ref. [START_REF]Fiber-Optic Communication Systems[END_REF].

After the preform has been produced it is mounted vertically at the top of the drawing tower where it is heated up to 2000 °C. A drop of molten preform starts to fall dawn starting the drawing process which goes on until the whole preform has been drawn. The drawing parameters, such as speed, tension and temperature are constantly controlled in such a way that the OF maintains the desired parameters such as the size of the outer diameter of the cladding. All the samples studied in this thesis have a total diameter of 125 µm. Their preparation procedure explored different drawing conditions as well as different doping, as detailed in the following. During the drawing process a dual acrylate protective coating is applied to the OF. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ Figure 1.7: Representation of the drawing process of an OF. Adapted from ref. [START_REF]Fiber-Optic Communication Systems[END_REF].

Background on pure and doped silica

In the first part of this section we report on the main features of silica glass. First of all we describe the structure of the ideal glass and we report on the intrinsic defects of the pure material.

Subsequently, we describe the defects associated with the primary dopants of the OFs (Ge, P, F, N)

and the Ce co-dopant. In the last part we describe the main processes induced in a glass by several types of irradiation.

Pure silica 1.2.1.1 Ideal silica glass

Silicon dioxide is a compound of Si that exists in nature in both crystalline and amorphous states [START_REF] Pacchioni | Defects in SiO2 and Related Dielectrics[END_REF].

The structures of these solids have been investigated by means of X-rays and neutron scattering and

Chap. 1: Background on optical fibers and point defects ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ these techniques have shown that (with the exception of stishovite) the structural unit of silicon dioxide is tetrahedral in both the crystalline and amorphous forms: a silicon atom forming four covalent bonds with four oxygen atoms (see Figure 1.8) [START_REF] Pacchioni | Defects in SiO2 and Related Dielectrics[END_REF]. In α-quartz each tetrahedron is characterized by two long bonds, 1.62 Å, and two short bonds, 1.60 Å, whereas the angle Si-O-Si is 144° [START_REF] Pacchioni | Defects in SiO2 and Related Dielectrics[END_REF]. The amorphous silicon dioxide, which is referred to as silica glass or simply silica, is different from the corresponding crystalline form due to the absence of the translational symmetry and long range order.

The first theoretical model that describes the structural properties of silica is the so called continuous random network (CRN) developed by Zachariasen [START_REF] Zachariesen | [END_REF]. This model is based on the observation that the nature of the bonds of the crystal and the amorphous material is the same. Moreover, in order for the internal energies of the two structures to be comparable it is necessary that the basic unit structure is the same. The main difference between crystal and glass is that in the latter two tetrahedra sharing an oxygen atom are randomly oriented to one another. The angle between Si-O-Si is not 144° as in the crystal but it has a certain distribution within 120 and 180 °C, as shown in Figure 1.9. An ideal amorphous solid of the type AX 2 is composed by a chemically invariable and continuous structure in which two tetrahedra are connected by only one vertex. There are no broken bonds or homopolar bonds.

1.2.1 Pure silica ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ Figure 1.9: Interconnection between two structural units in silica and distribution of the Si-O-Si angle. Adapted from ref. [START_REF] Pacchioni | Defects in SiO2 and Related Dielectrics[END_REF].

It is convenient to describe silica in terms of four ranges associated to internal structure of increasing size [START_REF] Pacchioni | Defects in SiO2 and Related Dielectrics[END_REF]:

I Range: it is the basic structural unit which is tetrahedral with a silicon atom at the center and four oxygen atoms at the vertices. To better define the characteristics of this range the lengths of the Si-O bonds and the amplitude of the angles O-Si-O must be provided. II Range: it consists of the interconnections between the fundamental units, i.e. the way in which two tetrahedra are connected through a vertex. The parameters relative to this range are the distribution of the amplitudes of the Si-O-Si β angles and two other torsional angles (α 1 and α 2 in Figure 1.9) necessary to define the relative orientation of two tetrahedra. These three variables are not uncorrelated with each other due to the possible steric effects. The origin of the disorder of the structure of amorphous silica must be looked for in this range.

III Range: within this range the topology of the network should be defined. However, because of the disordered nature of the matrix this operation is not possible. It is common to characterize this range by studying the statistics of particular structures present in the silica, the so-called rings. A ring is defined as the smallest closed path that contains a pair of Si-O bonds relative to a same silicon atom. The order of a ring, n, is the number of tetrahedra that participate in the ring formation (or equivalently the number of O atoms which are encountered along the ring). Silica, unlike the crystalline forms of silicon dioxide, has rings of different orders and with different statistics. Most of the rings have order n=6, but there are rings of order 3 and 4 whose vibrations can be easily observed by Raman measurements [10,11].

Chap. 1: Background on optical fibers and point defects ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ IV Range: the density fluctuations in the long range (tens of Å) and the possible microheterogeneity present on this scale of sizes belong to this range.

Actually, the glass structure is different from the ideal model for the presence of defects, which are called point defects in all the cases in which their spatial extension is comparable with the interatomic distance [START_REF] Pacchioni | Defects in SiO2 and Related Dielectrics[END_REF]. The study of point defects has a remarkable interest for practical reasons: point defects are associated to the properties of optical absorption and luminescence of the material. Point defects are commonly divided in two categories, intrinsic defects and extrinsic defects. The first concerns only atomic species that belong to the material (Si and O), the second, on the other hand, involves atomic species which are not supposed to be present in the glass. Furthermore, the defects could be paramagnetic or diamagnetic if they possess or not unpaired electrons. In general, defects lead to erroneous atomic arrangements with incomplete bonds, under-coordination or over-coordination.

The presence of defects in an amorphous glass depends strongly on the process of production of the glass. Moreover, it is possible to induce more defects by subjecting the glass to different types of irradiation: UV, X, γ-rays, β-rays, neutrons, etc.

Silica-related defects

As mentioned in the previous paragraph, the intrinsic point defects involve only the Si and O atoms of the silica matrix. In Figure 1.10 we show their most accepted microscopic structures [12].

Figure 1.10: Microscopic structures of the basic silica point defects. Adapted from ref. [12]. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ The shown defects have been investigated for several years and the optical properties of some of them are still not completely understood. In the following we summarize the most accepted principal features of each defect.

The oxygen deficient center I (ODC(I), ≡Si-Si≡), is a diamagnetic defect consisting in the bonding between two Si atoms. An optical absorption band peaking at 7.6 eV is assigned to this defect. The same absorption band was observed in the crystalline silicon dioxide [13].

The oxygen deficient center II (ODC(II), =Si: ) is a diamagnetic center characterized by a twofold coordinated Si with two electrons not involved in any bond. An optical absorption band peaking at 5 eV is assigned to this defect together with two emission bands peaked at 4.4 and 2.7 eV [13].

The peroxy linkage (POL, ≡Si-O-O-Si≡) is constituted by silicon atoms connected via two oxygen atoms. It is a diamagnetic center usually associated with oxygen rich silica samples. A weak optical absorption band at 7.1 eV is tentatively associated with this defect [14]. It is worth to mention that another absorption band associated with POL is located at 3.77 eV, although this assignment is quite controversial [15].

The E'(Si) center (≡Si • ) is the most studied paramagnetic defect in silica and it consists of a Si dangling bond with an unpaired electron in the sp 3 orbital of Si [START_REF] Pacchioni | Defects in SiO2 and Related Dielectrics[END_REF]. Several varieties of E'-like defects have been investigated by EPR (Electron Paramagnetic Resonance) and reported [16,17,18]. The most abundant variety, which is called E' γ [19], i absorbs light at 5.8 eV [13,20].

The non-bridging oxygen hole center (NBOHC, ≡Si-O • ) is a paramagnetic center that consists of a O dangling bond [21,22], the O atom being bonded to a silicon. NBOHC absorbs light at 2 eV and in the UV spectral domain from 4 to 8 eV [21,23], as shown in Figure 1.11. NBOHC has a luminescence band at 1.9 eV [24]. In order to study the EPR signal of NBOHC the measurements are usually performed at low temperature (77 K) which allows to distinguish its signal from the overlapping ones [25,26].

Chap. 1: Background on optical fibers and point defects ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ The peroxy radical (POR, ≡Si-O-O • ) is a paramagnetic center which can be clearly isolated by EPR measurements at low temperature (77 K) as the NBOHC [25,26]. The optical absorption band of POR in bulk silica has not been clearly assigned yet because of the usual overlap with optical absorption bands of other defects. However, silica surface PORs show a large absorption band peaked at 5.4 eV and the best estimate for the absorption of the corresponding bulk defect is 5.3 eV [27,12].

Another typology of defects is represented by the interstitial species. In several works it has been pointed out that in silica atomic and molecular species can diffuse [START_REF] Doremus | Diffusion of reactive molecules in solids and melts[END_REF]. Regarding this PhD thesis it is worth to introduce two of them: ozone molecule, O 3 , and molecular oxygen, O 2 .

The ozone molecule is usually created under irradiation in oxygen rich silica samples. Similarly to free ozone molecules, the O 3 molecules embedded in the silica matrix absorb light at 4.8 eV [START_REF] Kajihara | [END_REF]30,31].

Molecular oxygen can be present in oxygen rich as-grown samples, can be created by irradiation and at sufficiently high temperature can diffuse in silica from the atmosphere [32,33,34]. The diffusion of oxygen was also proved to be possible at room temperature on nanometric lengthscale in silica using nanoparticles [START_REF] Francesca | Studio del processo di diffusione di O2 a temperatura ambiente in nanoparticelle di silice[END_REF]. O 2 embedded in SiO 2 can be detected by a typical Raman band at 1549 cm -1 (see Figure 1.12) or more efficiently by its NIR photoluminescence at 1272 nm that can be excited at 765 or 1069 nm [START_REF] Carvalho | [END_REF]37]. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ Figure 1.12: Raman spectra of O2 loaded silica nanoparticles. In the inset the Raman bands of O2 in air at about 1556 cm -1 and in silica under N2 flux or not at about 1549 cm -1 . Adapted from ref. [38].

Concerning silica extrinsic defects not related to doping, we introduce two common H-related defects which could be present in silica.

The H(I) center (=Si • -H) is analogous to an E'(Si) center in which one of the bonds to an oxygen atom has been substituted with an hydrogen atom. Because of the hyperfine interaction between the unpaired electron and the nuclear spin of the hydrogen atom, the EPR signature of this defect is easily recognized: a doublet of lines with a separation of 74 G [39,40].

The OH group is another extrinsic H-related defect often present in as-drawn OFs. The main spectroscopic features associated to this defect in OFs are the absorption bands in the NIR domain [41]. These bands (1380, 1250, 1140, 945, 880 nm) are overtones and combination modes of the fundamental absorption band at 2.7 µm. Although the presence of such absorptions can be undesirable for some application, H 2 loading is an efficient technique to passivate the UV-Visible absorption bands of some several defects. Moreover, H 2 loading is also exploited to enhance the photosensitivity of Ge-doped OFs [START_REF] Golant | Proceedings of SPIE "Advances in Fiber Optics[END_REF][START_REF] Pacchioni | Defects in SiO2 and Related Dielectrics[END_REF].

The main unwanted impurity due to the production process of silica-based OFs is chlorine. For instance, the OF samples investigated in this thesis have a concentration of Cl of about 0.1 wt% in their cores. Some absorption bands are related to the presence of such impurity. On the basis of low temperature time resolved studies it has been established that molecular chlorine embedded in silica is the interstitial defect absorbing light at 3.8 eV [START_REF] Pacchioni | Defects in SiO2 and Related Dielectrics[END_REF]42]. However, in the same spectral region other chlorine related defects could absorb light. In particular, Cl 0 and Cl 2 -can be responsible of light

Chap. 1: Background on optical fibers and point defects ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ attenuation as reported in some studies [43,44,45]. In particular, OFs exposed to ionizing radiation develop an absorption band centered at 3.5 eV which has been assigned to radiolytic Cl 0 [45,46].

In this context it is worth noticing that in OF technology very important improvements of the glass transparency and radiation resistance have been achieved also by fluorine doping [47,48,49]. The incorporation of F atoms helps to relax the silica network without introducing new absorption bands. This aspect will not be considered further, whereas other doping of interest in this PhD thesis will be illustrated in the following paragraphs.

Ge-doped silica

Germanium doping increases the refractive index of silica glass and is commonly employed to design the core of OFs. The main reason why Ge-doped OFs have been intensively studied is that they are widely used for telecommunications purposes. Germanium belongs to the same chemical group of silicon and when it is incorporated in silica it plays the same role than a silicon atom: it creates four bonds with four tetrahedrally disposed oxygen atoms [50,51]. A remarkable feature of Ge-doped silica (and Ge-doped OFs) which increased the interest for this material is its photosensitivity. The latter is the property of the material to change permanently its refractive index under UV light exposure and it was first observed by Hill in 1978 [52]. From its discovery the researchers have developed several techniques which allow to enhance photosensitivity and exploit it in interesting applications such as fiber Bragg grating (FBG) [53]. This latter fiber technology consists in inducing a periodic refractive index modulation inside the core of an OF by exposing it to a spatially modulated UV light irradiation (typically obtained with the use of optical interferometric methods). Another interesting property related to Ge-doped silica is the second harmonic generation observed for the first time in 1985 by Osterberg and Margulis [54]. In their experiment they injected pulsed light at 1064 nm from a Nd:YAG laser in a germanosilicate OF and they observed the generation of light at 532 nm with an intensity growing with the exposure time to the Nd:YAG laser. Finally, recently it has been shown that the thermo-luminescence properties of Ge-doped OF can be also exploited for dosimetry purposes in radiotherapy and in-vivo radiation dosimetry and in nuclear facilities [55].

The potentialities of Ge-doped silica led to an extensive study of its optical properties and obviously of Ge-related defects. In the following section we report on the spectroscopic characteristics of the known Ge-related defects in silica.

Ge-related defects in silica

As mentioned in the previous paragraph, Ge is incorporated in silica mostly as substitutional to silicon. However, Ge atoms can be arranged in silica in several configurations which represent different defects. In view of our study, it is worth to mention that in the case of the OFs, whenever the core of the OF has been doped with Ge, the radiation induced absorption of the fiber in the UV-Visible domain is usually controlled by the generation of Ge-related defects, and the silica related defects are essentially negligible [56].

Since Si and Ge are isovalent atoms, it is natural to expect that some of the intrinsic defects of silica would have their Ge-modified counterpart in which the silicon atom has been substituted by a germanium. This is indeed the case [57]. In the following, we briefly review the main Ge-related defects and the associated optical properties. Figure 1.13, shows a set of absorption bands which are employed for decomposition of radiation induced attenuation spectra in Ge-doped OFs. The germanium lone pair center (GLPC, =Ge:) consists of a germanium atom linked to two oxygen atoms by single bonds and with a pair of electrons not involved in any bond: a twofold coordinated Ge atom [57,58,59]. This diamagnetic defect, which is abundant in as-grown oxygen deficient Gedoped silica, is responsible for an absorption band at 5.2 eV, called B 2β [START_REF] Leone | Silicon-based Materilas and Devices[END_REF].

Chap. 1: Background on optical fibers and point defects ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ The same defect is characterized by two photoluminescence bands centered at 4.2 and 3.1 eV [58].

Under UV and X-ray irradiation the concentration of GLPC decreases, as shown in Figure 1.14, and other absorbing and paramagnetic defects are created [59,62].

Three main paramagnetic defects, usually induced by irradiation, are associated to Ge in silica: they are called Ge [START_REF] Kao | Proc. IEE[END_REF], Ge( 2) and E'(Ge) [START_REF] Pacchioni | Defects in SiO2 and Related Dielectrics[END_REF]57].

The microscopic structure of Ge(1) has been unambiguously identified by EPR studies and it consists of an electron trapped on a tetra-coordinated Ge atom [63,64]. It has been shown that the EPR signature of this center depends on the concentration of Ge in the silica sample (see Figure 1.15) [65]. An absorption band peaked at 4.4 eV is related to the Ge(1) center [57,64]. The E'(Ge) center is structurally identical to the E'(Si) center in which a silicon has been substituted by a germanium [64,66]. Its EPR signal, shown in Figure 1.16, has been unambiguously related to the latter structure. Moreover, an absorption band at 6.3 eV is associated to the E'(Ge) [57]. The Ge( 2) is a center whose microscopic structure is still debated. The defining EPR signal is shown in Figure 1.17.

Figure 1.17: EPR signals associated to the Ge(2) defect in Ge-doped silica samples. Adapted from ref. [65].

In literature three models are attributed to Ge [START_REF] Origlio | Properties and Radiation Response of Optical Fibers: Role of Dopants[END_REF]. In the first model the Ge(2) is like a Ge(1) in which one of the nearest Si atoms has been substituted by another Ge atom [57,63]. In the second model it is a positively ionized GLPC [59]. In the third one the Ge(2) and Ge(1) are considered to be two energetically inequivalent configurations of a single trapped-electron defect (like the above reported Ge(1)), in analogy to what is known to be the case for the Ge(II) and Ge(I) centers respectively in quartz [67]. An OA band has been associated to this defect at about 5.8 eV [48,67], but also this attribution is questioned and deserves further investigation [68,69,70].

The relevance of Ge doped silica in view of OF applications has pushed the interest in exploring the effects of co-doping with other elements to further exploit the optical properties of this material.

Chap. 1: Background on optical fibers and point defects ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ In the past years few authors have investigated the effects of F-codoping of germanosilicate OFs and the experimental results are still debated. As in the case of pure silica, incorporating fluorine in germanosilicate glass decreases its refractive index [START_REF] Bubnov | Proc. SPIE[END_REF]. Moreover, concerning the production processes of Ge-F-doped OFs, it has been reported that the F-codoping can reduce the optical losses of the as-drawn fibers and their dependence on the drawing parameters [START_REF] Bubnov | Proc. SPIE[END_REF][START_REF] Bubnov | [END_REF]73]. In ref. [START_REF] Bubnov | [END_REF] it has been proven that F-codoping has no impact on the Rayleigh scattering of Ge-doped fibers and the reduction of the optical losses are attributed to a reduction of the anomalous (small-angle) scattering at the core cladding interface. Regarding the OF radiation response of Ge-F-doped OFs, the main effect that has been highlighted is that F can reduce the radiation induced losses acting as terminator of some preexisting precursor defects (GLPC) [74,[START_REF] Wei | Proc. SPIE[END_REF].

Another interesting study that was performed involves the co-doping with cerium of Ge-doped silica. When silica is doped with cerium the Ce atoms are incorporated as Ce 3+ or as Ce 4+ in the silica network [START_REF] Stroud | [END_REF]77]. The relative concentration of the two typologies of ion depends critically on the production processes, in particular an oxygen rich atmosphere promotes the creation of Ce 4+ whereas an oxygen poor one the Ce 3+ [78]. The two ions have different optical properties: Ce 3+ absorbs light at 3.9 eV, whereas Ce 4+ absorbs light at 4.8 eV, as shown in Figure 1.18 [79]. Under UV irradiation as well as X-rays irradiation, the Ce 3+ ion is further ionized and converted to (Ce 3+ ) + [START_REF] Stroud | [END_REF]80]:

1.2.2 Ge-doped silica ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ 𝐶𝑒 3+ → (𝐶𝑒 3+ ) + + 𝑒 - 1.3
The optical properties of the ionized Ce 3+ differ from the ones of the Ce 4+ because of the different environments around the ion and therefore the ionized Ce 3+ is sometimes indicated as (Ce 3+ ) + .

Anoikin et al. have shown that in Ge-Ce-doped silica the electron released by the photoionization of Ce 3+ can be trapped by a tetra-coordinated Ge atom and creates a Ge(1) [57,80]. On the other hand, they did not observe the formation of Ge [START_REF] Origlio | Properties and Radiation Response of Optical Fibers: Role of Dopants[END_REF]. On the basis of these results the authors concluded that Ge(2) must be a hole center which supports the model in which the microscopic structure of this defect is a (GLPC) + . In general, the release of electrons by Ce ions during exposure to ionizing radiation could be of interest for the radiation sensitivity of Ge in co-doped fibers since it can strongly impact its radiation response. It is important to mention that the luminescence decreases above a certain concentration of Ce, as

shown in Figure 1.20 [82]. Such effect is attributed to the formation of CeO 2 clusters in the silica samples and the consequent introduction of non-radiative decay channels of the Ce 3+ radioluminescence.

P-doped silica

As shown in Figure 1.2, the incorporation of phosphorus in silica increases the refractive index of the material and therefore it can be employed to synthetize the core of optical fibers with pure silica cladding. At the same time P-doping reduces the melting temperature of silica [START_REF] Pacchioni | Defects in SiO2 and Related Dielectrics[END_REF] and in optical fibers it is often used as co-dopant of the core and cladding zones because it changes the viscosity of the material [83]. Phosphate glasses are also studied as transmitting material in the UV spectral domain [83]. One of the most remarkable applications of P-doped silica concerns OF amplifiers and laser. This special types of OF are also doped with rare-earth elements such as erbium, ytterbium and cerium [START_REF] Becker | rbium-Doped Fiber Amplifiers, Fundamental and Technology[END_REF]. Rare-earth dopants indeed are hardly dissolved in pure silica and in order to increase their concentration preventing clustering of the rare-earth (which is detrimental for the envisaged applications) aluminosilicate host-matrix is usually preferred [START_REF] Laegsgaard | [END_REF]86]. However, recent findings have shown that P-doped silica is able not only to increase the solubility of rare-earth ions compared to pure silica but it can also provide a higher clustering threshold than aluminosilicate glasses [87].

P-doped silica 27 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

For the present study it is important to remark that OFs doped with phosphorus show higher radiation sensitivity than the other basic dopants [48,88]. Therefore, several authors have proposed to exploit such a property for the fabrication of an OF radiation sensor or dosimeter [89,90,91].

Despite the technological interests related to P-doped silica and P-doped OFs there are not many literature studies which are dedicated to this dopant as compared to germanium. In fact, in most of the published works phosphorus plays the role of a co-dopant and is therefore difficult to separate its contribution to the optical properties of the material from the ones of the other elements [92,93,94]. In order to optimize the behavior of P-doped silica glasses under irradiation further investigations of the radiation induced defect generation mechanisms are needed.

P-related defects in silica

The basic structure of P-doped silica glasses is described by the CRN model in which the P atoms are mainly incorporated as in the stoichiometric phosphate glasses P 2 O 5 : the phosphorous atom is bonded to four oxygen atoms, three of them are bridging oxygens whereas the fourth one is a nonbridging oxygen linked by a double bond (≡P=O) [95,96]. The vibration related to the doubly bonded oxygen atom is easily detected by Raman measurements because of the associated band at 1320 cm -1 which in phosphate glasses is located at 1390 cm -1 [97]. The stoichiometric structure of phosphorous is the most frequent configuration of P atoms also in silica and all the other possible microscopic structures are considered defects of the P-doped silica glass. Origlio et al. have shown that pristine OFs and preforms produced by MCVD method do not show any significant P-related EPR signal demonstrating that the as-grown P-doped samples do not contain significant amounts of P-related paramagnetic defects [98]. However, irradiation causes the appearance of strong absorption bands in the UV-Visible spectral domain as well as in the NIR domain [88,98,99]. At the same time, in the irradiated samples several EPR doublets are detected. Since P has nuclear spin ½ (100% of the natural abundance) these EPR signals are naturally associated to P-related paramagnetic centers [98,99].

The main paper, upon which the interpretation of the studies concerning P-related defects relies, has been published in 1983 by D. Griscom et al. [99]. In their work the authors were able to provide an extensive study of the P-related defects which is based on EPR and optical absorption measurements.

Chap. 1: Background on optical fibers and point defects ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ to belong to the E'(Si) centers with a P-atom as next-nearest-neighbors. It is worth to mention that in the original paper the metastable variety of POHC was thought to be unstable at room temperature, whereas successive studies have shown that this is not the case [98]. Finally, the suggested structures of P1, P2 and P4 defects received further support by simulation studies [100]. Concerning the use of P-doped silica for the fabrication of OF, the P1 defect takes on a major importance. As a matter of fact, its absorption band, which is peaked at 1.6 µm (~0.78 ev), could strongly affect the radiation resistance of this type of OF in the NIR domain as compared to the other OFs (PSC, F-doped, Ge-doped) and could severely compromise the fiber performances when 1.2.3 P-doped silica ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ exposed to irradiation [88]. Moreover, it has been shown that one of the post irradiation recovery processes is able to cause the concurrent decrease of POHC centers and the increase of P1 defects [99,101].

Although a quite complete representation of the paramagnetic defects and their properties has been reported in literature, the same cannot be said for the diamagnetic defects. The precursors of the radiation induced defects shown in Figure 1.21 are basically postulated and further investigation should be done to support the actual picture.

Concerning the study of photoluminescence related to P centers, Origlio et al have reported a luminescence band at 3 eV that can be excited at 4.8 and 6.3 eV as shown in Figure 1.27 [102] This emission is tentatively attributed to the precursor of the P2 defect (see Figure 1.21) which is a positively charged tetra-coordinated P atom. Chap. 1: Background on optical fibers and point defects ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ Chan et al. have reported another photoluminescence, around 600 nm (2.1 eV), which they have attributed to the POHC centers [103]. As shown in Figure 1.28, such photoluminescence was excited and photobleached at 488 nm.

As above stated, the use of P is associated to the need of doping the OF with rare-earth ions. In this context the overall radiation sensitivity is of particular concern. In order to improve the radiation resistance of phosphosilicate OFs doped with rare-earth ions several approaches have been attempted [104,105,106]. It was shown that co-doping the core of the OFs with cerium improves its radiation resistance in the NIR spectral domain at low doses (<100 krad). In fact, as already explained in the paragraph 1.2.2.1, cerium is mainly incorporated as Ce 3+ which under irradiation is further ionized and converted to Ce 4+ . The electron which is released during the process can then be trapped by the P1 defect which is the hole center that absorbs light in the NIR spectral domain inhibiting its formation.

It is worth to mention that Ce-codoping of phosphosilicate glasses introduces new absorption bands in the UV-Visible domain as reported in ref. [107], that should influence the OF transmission. To clarify this aspect, in Figure 1.29, we report the normalized excitation and emission spectra of the Ce 3+ ions in P-Ce-doped silica. The maximum of the excitation curve is located at 275 nm, whereas the emission is peaked at 370 nm. The excitation and emission bands are blue-shifted as compared to the Ce 3+ ions in pure silica because of the different host matrix [107].

N-doped silica

One of the most interesting types of OFs developed in the recent years is the nitrogen-doped OF, also called silicon oxynitride (SiO x N y ) OF. Doping silica with nitrogen increases its refractive index and can be employed for the doping of OF cores [108]. In particular, one specific feature of Ndoping is that substituting 1% of the oxygen atoms by nitrogen atoms produces a refractive index increase of 0.015 [109], which allows obtaining usual refractive index profiles of the OFs with a rather low N-doping level in comparison with Ge-doping and P-doping. Moreover, nitrogen (as oxygen and silicon) is a very abundant element which is a desirable condition for the production of inexpensive OFs. One of the drawbacks of N-doped OFs is that the common MCVD, OVD and VAD production methods of the fiber preforms cannot incorporate nitrogen into the silica matrix [START_REF] Golant | Proceedings of SPIE "Advances in Fiber Optics[END_REF]. In order to overcome such difficulty, the employed production methods are based on plasmachemical glass deposition process (SPCVD) [START_REF] Golant | Proceedings of SPIE "Advances in Fiber Optics[END_REF].

As shown in Figure 1.30, a characteristic spectral feature of N-doped OFs is an intense absorption band centered at 1.5 µm which is assigned to the presence of N-H groups [108]. As a consequence, whenever the envisioned application of the OFs involves the transmission of light at 1.55 µm the production processes must avoid the incorporation of high concentrations of hydrogen. Chap. 1: Background on optical fibers and point defects ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ under conventional conditions has a stronger resistance to high temperature (900 °C) than FBG in Ge-doped OFs [109]. Bufetov et al. have demonstrated that in N-doped OFs intense (10 W) NIR light from a Nd:YAG laser can induce third harmonic generation domain, with higher efficiency than in Ge-doped OFs, as well as other nonlinear effects (stimulated Raman scattering, lasing of N-color centers) [111,[START_REF] Golant | Proceedings of SPIE "Advances in Fiber Optics[END_REF]. Moreover, after a few minutes long measurement the post-irradiation recovery shows that N-doped OFs have a faster recovery compared to the Ge-doped ones [113]. On the other hand, in pulsed irradiation regimes the N-doped OFs have the best radiation response among Ge-and F-doped OFs [114].

Although the N-doped OFs represent for several reasons a promising technology, not much is known about defects and radiation induced microscopic processes.

N-related defects in silica

In this section we review the main literature data in relation with our studies about N-related defects in silica.

N-doped silica 35 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

When nitrogen is incorporated in silica it tends to form bonds with Si atoms because of weak character of the N-O chemical bond in a =N-O-unit [115]. The most common microscopic structure is the threefold coordinated N atom linked to three Si atoms [116,117]. Therefore, depending on the doping level, silicon oxynitride glasses are often represented as SiO 2-x N x . A part from the stoichiometric structure N-(Si≡) 3 , the presence of two N-related defects is suggested [116,117]. The di-bridging nitrogen atoms ≡Si-N --Si≡ are equivalent of stoichiometric oxygen atoms in silica, whereas the non-bridging N atoms -N 2-are equivalent to the non-bridging oxygens. Furthermore, Tsai et al. in ref. [118] suggested the existence of positively charged tetra-coordinated N atoms. All the presented configurations of the N atoms in silica are diamagnetic and cannot be investigated by EPR. At the same time, not much is known about the optical properties of these diamagnetic defects.

In some studies the EPR signals of N-related paramagnetic centers have been detected and in the following we report the three of them. Dealing with N-related paramagnetic defects, the first thing to consider is that the natural abundance of the isotope 14 N, which has nuclear spin 1, is equal to 99.6%.

As a consequence, any N-related EPR signal is expected to show a hyperfine interaction which leads to a triplet-type signal.

In Figure 1.32 we report the EPR spectrum of a N-doped silica sample in which several signals are present. The strong line in the center of Figure 1.32 is due to the E'(Si) center and the first two peaks close to it (indicated by the a arrows) are due to E'(Si) centers with a 29 Si atom as next-nearest-neighbor Chap. 1: Background on optical fibers and point defects ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ (( 29 Si-O)-Si • =) [115,119,120]. These two types of defects do not involve nitrogen and are present or can be induced in all types of silica. Moving away from the center of the spectrum, two peaks separated by ~25 G are visible (indicated by the b arrows). This signal is part of a triplet that by simulation is assigned to an E'(Si)-like center in which one of the three bonded oxygen atoms has been replaced by a nitrogen atom (=Si • -(N<)) [115]. Finally, the two most external spectral features (indicated by the c arrows) belong to another center which is assigned to an electron localized on an N atom which is bonded to two Si atoms (≡Si-N • -Si≡) [115,121]. In Figure 1.33(b), the simulated EPR spectrum of this defect is shown. The last N-related paramagnetic center, that was observed in SiO 2 films, is attributed to an electron trapped on a tetra-coordinated N atom [121,122], also called N 4 (=N • =). In Figure 1.34, the EPR spectrum at room temperature of this defect is reported. It is constituted by three lines of equal area and the most external ones, which are supposed to remain isolated in the presence of strong overlapping central signals (E'(Si)), are separated by 38 G.

In order to explain the existence of the neutral tetra-coordinated atoms, Tsai et al. have proposed a model in which an electron is captured by a positively charged precursor center, which is a tetracoordinated N atom [118]. There are only few publications dealing with the optical properties of N-doped silica OFs [START_REF] Golant | Proceedings of SPIE "Advances in Fiber Optics[END_REF]123,124]. In Figure 1.35, we report the optical absorption spectrum of an N-doped OF preform. The spectrum shows three absorption bands at 4.54, 5.00 and 5.77 eV. The absorption band at 5 eV is related to the ODC(II) defects of silica [58]. The absorption band at 5.77 would be assigned to the E'(Si) [START_REF] Pacchioni | Defects in SiO2 and Related Dielectrics[END_REF]. However, the authors of ref. [123] ruled out this possibility since the corresponding (intense) EPR signal was not detected. Therefore, this absorption can also be considered unknown.

The assignment of the absorption band at 4.54 is also unknown. Chap. 1: Background on optical fibers and point defects ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ In Figure 1.36, the photoluminescence of the N-doped silica excited at 4.86 eV is shown. Four emission bands are visible: two fast emissions (<20 ns), 4.45 and 3.55 eV, and two slow emissions (~10 ms), 3.03 and 2.7 eV. The emission bands at 4.45 and 2.7 belong to the ODC(II) centers [58].

The other two are N-related emission bands [123].

Figure 1.36: Photoluminescence spectra excited at 4.83 eV in N-doped OF preform (silicon oxynitride). The modulation frequency of the exciting light is 10 Hz (solid lines) and 300 Hz (dashed line). Adapted from ref. [123].

On the basis of the similarities of the lifetime characteristics of the emission bands, the authors of ref. [123] tentatively assigned the absorption band at 4.54 eV and the two emission bands at 3.55 and 3.03 eV to an ODC(II)-like defect neighboring a nitrogen atom.

The summary of defects reported in the previous paragraphs relating to the different doping sketches the complexity of the scenario for the investigation of the irradiation effects in doped and co-doped OFs. The interpretation of the spectroscopic studies related to the radiation induced defects in such glasses should tend to find the possible interaction mechanisms between the dopants and their effects on the generation of related defects [114]. This task introduces the need to use different spectroscopic techniques to shed light on the irradiation response of the different types of OFs.

Radiative environments and basic processes

The effects of irradiation on a material depend on several parameters that characterize the irradiation itself. The OFs that are developed in order to be employed in radiative environments are subjected to different working conditions depending on the envisaged application and consequently they are 1.2.4 N-doped silica ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ exposed to different dose, ii dose-rate and type of irradiation [125]. In Figure 1.37, we report a graph in which the characteristic doses, dose-rates of several important radiative environments are shown [START_REF] Berghmans | Proc. of NATO Science for Peace and Security Series[END_REF].

Figure 1.37: Characteristic doses and dose-rates of several radiative environments of applicative and research interest. Adapted from ref. [START_REF] Berghmans | Proc. of NATO Science for Peace and Security Series[END_REF].

It is important to remark that, as shown in Figure 1.37, the radiation dose and dose-rate ranges span several orders of magnitudes. When an OF is tested to assess its radiation resistance and reliability, it should be exposed to conditions that are as close as possible to the ones of the future application. Sometimes this is not possible, for instance in the case of years long irradiations, and therefore a complementary simulation approach must be adopted.

As mentioned above, the OFs can be exposed to several types of radiation such as UV, X-rays, γrays, neutrons, ions. Depending on the nature of the radiation many processes can take place in the irradiated material. In Figure 1.38, we report a schematic synthesis of the cascade mechanisms that lead to the formation of defects in silica (and silica-based OFs) as a function of the nature of the irradiation.

ii The dose is defined as the quantity of energy deposited for mass unit on the target material and in our work is expressed as rad(SiO2) (100 rad = 1 Gray = 1 Joule/kg). Naturally, the rate of deposited dose is called dose-rate. Mechanisms that lead to the creation of defects for the different types of irradiation (UV, X, γ, neutrons, ions). Adapted from ref. [START_REF] Griscom | [END_REF].

Depending on the nature of the radiations and their energy, two main types of mechanisms of formation are activated: knock-on and radiolysis [START_REF] Devine | Structure and imperfections in amorphous and crystalline silicon dioxide[END_REF]. In the knock-on process the incident particle interacts directly with an atom of the material causing its displacement or distortion of the silica network. The knock-on process can be elastic or inelastic whether or not the total kinetic energy is conserved (in the inelastic case nuclear reactions can take place). In radiolytic processes the irradiation acts on the electrons of the material which are promoted to the conduction band with the contextual creation of an hole in the valence band (in the case of UV irradiation defects are created by sub-band gap photons). The creation of electron-hole pairs is the first step toward the formation of a stable defect: some of the electron-hole pairs recombine immediately whereas others can be trapped at impurities or other network sites.

It is worth to mention that in the case of X-rays and γ-rays irradiations, which are the radiation types involved in this thesis work, the radiolysis mechanisms is the dominant one and exceeds the knockon process in creation of defects by at least one order of magnitude [START_REF] Devine | Structure and imperfections in amorphous and crystalline silicon dioxide[END_REF][START_REF] Devine | [END_REF]. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Chapter 2 2 Experimental setups and samples

This chapter is devoted to the brief description of the experimental techniques employed in our study and the physical phenomena upon which they are based.

In order to characterize the effects of irradiation on our samples, it is necessary to adopt an approach which exploits several spectroscopic techniques. In our investigations we took advantage of three As schematically reported in Figure 2.1, these experimental techniques allow probing different defects on the basis of their physical properties. Loosely speaking, in favorable conditions the absorbing defects can be revealed by RIA experiments, the paramagnetic ones by EPR and the luminescent ones by CML. Obviously, some of the studied defects could be revealed by more than Chap. 2: Experimental setups and samples ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ one technique or all of them. For instance, the NBOHC defect, which was introduced in the § 1.2.1.2, can be revealed by RIA, EPR and CML.

In the following, we first present a simplified theoretical background of the physical phenomena investigated. Subsequently, we report on the experimental setups that we employed. In the last section we describe optical fiber samples that we have experimentally investigated.

Theoretical background of the employed spectroscopic techniques

In order to characterize the radiation response of the OFs, identify the radiation induced point defects and infer the respective formation mechanisms, we employed four spectroscopic techniques: Optical Absorption (OA); Photoluminescence (PL); Raman scattering; Electron Paramagnetic Resonance (EPR). In this section we present a brief theoretical background of the physical phenomena upon which the above experimental techniques are based.

Optical absorption

Light propagating inside a transparent medium in a fixed direction can undergo several processes (e. g. Rayleigh scattering, absorption, etc.) that result in an attenuation of its initial intensity. In particular, the presence of defects inside a doped silica matrix introduces new electronic levels inside the energy gap between the valence and conduction bands. Therefore, photons propagating in the medium can be absorbed by the point defects causing an electron promotion from a given initial state to a final excited state, and subsequently a recovery of the initial state occupation by radiative or non-radiative decay mechanisms. In this sequence, the number of photons initially impinging on the sample at a given wavelength could be reduced due to the energy absorption process.

According to the Lambert-Beer law, considering the medium to be homogeneous the intensity of the transmitted light I T is related to the intensity of the incident light I 0 by the following relation [START_REF]Fiber-Optic Communication Systems[END_REF]:

𝐼 𝑇 (𝜆) = 𝐼 0 • 𝑒 -𝛼(𝜆)•𝑑 2.1
in which λ is the wavelength of the incident light, d is the distance travelled by the light in the medium and α is the absorption coefficient. When an OF is exposed to ionizing radiation many point defects are created at microscopic level and the related absorption bands, which are mainly located in the UV-Visible range, are the main cause of the degradation of the transparency of the OF. In general, then, the absorption coefficient can be expressed as ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

𝛼(𝜆) = ∑ 𝜎 𝑖 (𝜆) • 𝑛 𝑖 𝑖 2.2
in which σ i is the cross-section of the i th absorbing species and n i the corresponding concentration [START_REF] Pacchioni | Defects in SiO2 and Related Dielectrics[END_REF].

The absorption coefficient is usually measured in cm -1 , but very often in works that concern OF characterization it is measured in dB/m or dB/km after the following definition:

𝛼 ′ = - 10 𝑑 • log ( 𝐼 𝑇 𝐼 0 ) 2.3
in which d can be expressed in meters or kilometers. iii

It is worth to mention that, in contrast to several other experimental techniques, an optical absorption spectrum is not affected by the instrumental response of the employed apparatus since the ratio of light impinging on the sample with that emerging from it is determined at each wavelength.

From an applicative point of view, optical absorption measurements are essential for the study and development of all types of optical fibers, especially the passive OFs, such as the ones investigated in this work and that have to be exposed to radiative environments. The principal limitation of this experimental technique is related to its frequently poor selectivity in the study of the absorbing defects. Indeed, very often several absorption bands overlap in the same spectral range without allowing simple decomposition (see previous chapter for some examples).

Photoluminescence

Some of the defects that are found in silica and doped silica not only absorb photons, they can also emit them. Such process is called photoluminescence (PL) and is a widely studied phenomenon.

iii The absorption α' in dB/m is equal to 434 times α expressed in cm -1 . In Figure 2.2, a summary of the basic excitation and relaxation mechanisms concerning a molecule or a point defect are reported, referring to 0 K. First, during the absorption process, the involved electrons complete a transition from the ground state to the excited one. Immediately after, the nuclei relax toward the minimum energy configurations in a time scale of the order of 10 -12 s.

Successively, since the electronic system is not in the minimum energy level it relaxes back towards the ground state. Although the relaxation from the excited state to the ground state can be completely non-radiative, it can happen that a photon is reemitted and finally the nuclei undergo a last relaxation toward the initial vibrational ground state. Since the excitation of phonons reduces the energy available for the emitted photon, the photoluminescence is shifted towards lower energies compared to the excitation light. The energy shift is known as Stokes shift. The time decay constant can vary from ~10 -9 s, in the case of allowed radiative transitions (fluorescence), to long transition for forbidden radiative transitions (phosphorescence).

Assuming that no non-radiative decay mechanisms are present, the PL intensity spectrum is given by [START_REF] Pacchioni | Defects in SiO2 and Related Dielectrics[END_REF][START_REF] Leone | Silicon-based Materilas and Devices[END_REF]:

𝐼 𝑃𝐿 (𝜆 𝑒𝑥𝑐 , 𝜆 𝑒𝑚 ) = 𝑘 𝑟 • 𝑁(𝜆 𝑒𝑥𝑐 ) • 𝑓 𝑃𝐿 (𝜆 𝑒𝑚 ) 2.4
where N is the number of excited centers, f PL is the emission lineshape of the PL and k r is the radiative decay rate. When the molecules (or defects) of a sample are excited by a continuous source ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ (such as a laser or a lamp) we observe a stationary PL in which the intensity I PL of the emitted light under excitation at 𝜆 𝑒𝑥𝑐 can be written as:

𝐼 𝑃𝐿 (𝜆 𝑒𝑚 ) = 𝜂 • 𝐼 0 (𝜆 𝑒𝑥𝑐 ) • (1 -𝑒 -𝛼(𝜆 𝑒𝑥𝑐 )𝑑 ) • 𝑓 𝑃𝐿 (𝜆 𝑒𝑚 ) 2.5
The η coefficient is called quantum yield and it represents the ratio of the emitted and absorbed photons. Therefore, its definition is:

𝜂 = 𝑘 𝑟 𝑘 𝑟 + 𝑘 𝑛𝑟 2.6
where k r and k nr represent the radiative and non-radiative decay rates, respectively. This expression takes into account the competition between the emission process and the overall processes of relaxation from the excited state. The k nr typically depends on the temperature of the sample and in this case it is not active at 0 K. From a dynamic point of view, the relaxation from the excited state of a population of excited molecules or point defects occurs in a time scale given by =(k r + k nr ) -1 .

This quantity is called emission lifetime (or decay time) and establishes the time needed to reduce the emission amplitude by a factor e after pulsed excitation. Eq. 2.6 enables also to highlight that when ( exc )d <<1 the emission of a sample of thickness d is proportional to the concentration of chromophores, an useful feature to investigate materials properties.

In order to measure a true PL spectrum care should be taken in the correction of the instrumental response [START_REF] Lakowicz | Priciples of Fluorescence Spectroscopy[END_REF]. In fact, in general

𝐼 𝑃𝐿 𝑒𝑥𝑝 (𝜆 𝑒𝑚 ) = 𝑅(𝜆 𝑒𝑚 ) • 𝐼 𝑃𝐿 (𝜆 𝑒𝑚 ) 2.7
where R represents the response function of the experimental apparatus.

PL is a very useful technique for the investigation of defects in silica and doped silica because it often allows selective study of the defects and high sensitivity. When two or more overlapping emission bands are found in a sample, it is also possible to try to distinguish them performing time resolved luminescence measurements for which usually a tunable pulsed laser source is employed together with a fast recording spectrometer. In fact, if the emissions are characterized by different decay times it is possible to observe the change of the relative intensity of one PL component with respect to the other and eventually isolate them.

Raman spectroscopy

The Raman effect is the inelastic scattering of a photon due to the excitation or relaxation of a phonon in the illuminated material (or molecule). When the electromagnetic waves hit a polarizable molecule its charges start oscillating at the same frequency of the incident wave. The electromagnetic wave generated by the charge (and polarization) oscillations will have the same frequency (same photon energy) of the incident wave. Such scattering process is called Rayleigh scattering and represents an elastic scattering of the incident light. However, the oscillating polarization of the molecule can couple to some vibrational motions of the molecule that cause the appearance of new frequencies that can be both higher and lower than the one of the exciting field. In Figure 2.3 a diagram summarizing the described processes employing the quantum mechanics is reported.

𝐸 = ℎ • 𝑐 • 𝑘 𝑅𝑎𝑚𝑎𝑛 2.8
Where h is the Planck constant and c the speed of light. Because the Raman spectrum is a measure of the vibrational modes inside a material, such as silica, from a practical point of view it provides global information about microscopic structures and chemical bonds. Raman spectroscopy is generally insensitive to the creation of point defects in a silica-based material because their concentrations rarely reach values higher than 10 -4 the concentration of the SiO 2 molecular units. In this thesis work, Raman spectroscopy was mainly employed in relation to the detection of O 2 molecules embedded in silica matrix (see Figure 1.12).

Electron Paramagnetic Resonance

The Electron Paramagnetic Resonance (EPR), or Electron Spin Resonance (ESR), is a very powerful experimental technique which permits the detection of the paramagnetic centers (usually unpaired electrons) in the material under test [START_REF] Weil | Electron Paramagnetic Resonance[END_REF].

An electron has a magnetic moment which is given by:

𝜇 = - 𝑔|𝑒|ħ 2𝑚 𝑒 𝑆 = -𝑔𝛽𝑆 2.9
where β≈9.3·10 -24 J/Tesla is the Bohr magnetic moment , g is the adimensional Landé factor , and S is the spin angular momentum (m e the electron mass; e the electron charge; and ħ the reduced Planck constant).

When a molecule (or a silica defect) containing an unpaired electron is placed in a magnetic field, the magnetic moment of the unpaired electron interacts with it according to the Zeeman hamiltonian,

𝐻 𝑍𝑒𝑒𝑚𝑎𝑛 = -𝜇 • 𝐵 = 𝑔𝛽𝑆 • 𝐵 2.10
which in the case of 2S+1 degenerate energy levels gives rise to the energy levels:

iv

𝑘 𝑅𝑎𝑚𝑎𝑛 = 𝑘 𝑑𝑖𝑓𝑓 -𝑘 𝑒𝑥𝑐 = 𝐸 𝑑𝑖𝑓𝑓 ℎ𝑐 - 𝐸 𝑒𝑥𝑐 ℎ𝑐 48
Chap. 2: Experimental setups and samples ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

𝐸 𝑚 𝑆 = 𝑔𝑚 𝑠 𝛽𝐵 2.11
corresponding to different projections, m s , of S along the magnetic field B direction. An oscillating magnetic field B 1 perpendicular to B can induce a transition between two Zeeman magnetic levels if its frequency satisfies the resonance condition:

ħ𝜔 = 𝑔𝛽𝐵 2.12
The selection rule m=1 generally applies.

In complex systems, as a crystal or a glass, g must be replaced by a tensor in order to take into account its interaction with the surrounding. In the case of an unpaired electron localized on a single atom, the spin-orbit coupling must be considered [START_REF] Leone | Silicon-based Materilas and Devices[END_REF][START_REF] Weil | Electron Paramagnetic Resonance[END_REF]. The Zeeman Hamiltonian then becomes:

𝐻 𝑍𝑒𝑒𝑚𝑎𝑛 = 𝛽𝑺 • 𝒈 ̂• 𝑩 2.13 with 𝒈 ̂= | 𝒈 𝒙𝒙 𝒈 𝒙𝒚 𝒈 𝒙𝒛 𝒈 𝒚𝒙 𝒈 𝒚𝒚 𝒈 𝒚𝒛 𝒈 𝒛𝒙 𝒈 𝒛𝒚 𝒈 𝒛𝒛 | 2.14
The ĝ tensor is characteristic of the paramagnetic center and can help determining its microscopic structure.

In some cases the unpaired electron is mainly localized around a nucleus whose nuclear spin is different from zero, or some of the closest nuclei have nuclear spin different from zero. In this case the magnetic dipolar interaction between the nuclear and electron spins (hyperfine interaction) must be taken into account. In addition, another contribution to the hyperfine interaction arises from the Fermi-contact term that is isotropic [START_REF] Weil | Electron Paramagnetic Resonance[END_REF]. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ If the overall hyperfine interaction is weaker than the Zeeman interaction term in the spin Hamiltonian, the following contribution must be added to evaluate the spin state energy:

𝐻 ℎ𝑓𝑠 = 𝑺 • 𝑨 ̂• 𝑰 2.15
where  is the hyperfine tensor and I the nuclear spin operator. The hyperfine interaction gives a further splitting of the energy eigenvalues. For instance, in the oversimplified case in which the ĝ and  tensors are isotropic and the hyperfine interaction is much weaker than the Zeeman interaction, the energy eigenvalues can be expressed as:

𝐸 𝑚 𝑆 𝑚 𝐼 = 𝛽𝑔𝐵𝑚 𝑠 + 𝐴𝑚 𝑠 𝑚 𝐼 2.16
in which m S and m I are the projections of the electron spin and nuclear spin on the z-axis, assumed to coincide with the external magnetic field direction. The allowed transitions are the ones which satisfy the selection rules Δm S =±1 and Δm I =0. Consequently, a microwave photon is absorbed if

ħ𝜔 = 𝑔𝛽𝐵 + 𝐴𝑚 𝐼 2.17 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
In this case the absorption spectrum in presence of nuclear spin will be split in 2I+1 resonance lines and the frequency distance between two adjacent resonance lines is A/h. An EPR spectrum of a paramagnetic center involving hyperfine interaction is reported in Figure 2.5 showing the multiplet of lines arising from the electron-nucleus spin interaction. The interest in EPR measurements is relevant not only for the determination of geometric and atomic properties of the centers, but also for the properties of its electronic orbital. It is also a very sensitive technique that often allows detecting and distinguishing among several paramagnetic defects and evaluating the corresponding concentrations if the measurement method has been calibrated.

Spectroscopic setups

In this section we describe the experimental setups employed for the investigation of the OF samples and the most relevant terms considered for carrying out the measurements.

Light attenuation in irradiated OFs: online RIA and cut-back technique

The online Radiation Induced Attenuation (RIA) technique consists in the evaluation of the losses of transmitted light in an OF during the irradiation. In this experiment an OF is connected to a light source and a spectrometer and, at the same time, a part of the OF is irradiated. The time evolution of the induced losses is determined relatively to the initial transmitted spectrum with a temporal 2.2.1 Light attenuation in irradiated OFs: online RIA and cut-back technique 51 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ resolution not higher than 1 second. Very often, in order to ensure the same irradiation conditions, two or more fibers are tested at the same time. The experimental setup of a typical RIA experiment in which the radiation responses of two OFs are tested, shown in Figure 2.6, consists of two light sources (Deuterium-Halogen DH2000 light source by Ocean Optics) and two UV-Visible spectrometers (HR4000 or Maya by Ocean Optics, spectral resolution higher than 1 nm). The available equipment (spectrometers and light source) allowed us to test up to four OFs at the same time. The irradiated part of the sample, whose maximum length is 2.5 cm because of the size of the radiation beam, is placed on a temperature controlled plate which can span temperatures from 25 to 280 °C with stabilization within ±4 °C at 25 °C and ±1 °C at higher temperatures. The irradiations were performed with the PROBIX machine of CEA (Commissariat à l'énergie atomique et aux énergies alternatives) which is a 10 keV X-rays source (which is described in the following paragraph 2.3.1). On the basis of the repeatability tests we performed the error associated to the RIA spectra is not greater than 10%. In Figure 2.7 we show a schematic diagram representing the two measurements that must be performed in order to derive the OA spectrum of the OF by applying the following equation:

𝛼 ′ (𝜆) = - 10 𝐿 1 -𝐿 2 • log ( 𝐼 𝐿 1 (𝜆) 𝐼 𝐿 2 (𝜆) ) 2.18
The method is applied for the characterization of irradiated OFs as well as the pristine ones. The difference between the post-irradiation absorption spectrum and the pre-irradiation one gives the post-mortem radiation induced attenuation spectrum. It is worth to mention that, depending on the tested OF and the irradiation conditions, the OA before the irradiation is negligible compared to the one after the irradiation, therefore the latter one coincides with the post-mortem radiation induced attenuation

The light source we used to perform OA measurements is a DH2000 by Ocean Optics and a NIRQuest Ocean Optics spectrometer (spectral resolution 5 nm) was employed for the detection of light in the near infrared spectral domain. The cut-back was employed for the study of the postmortem radiation induced attenuation of the γ-irradiated samples.

Online Radiation Induced Emission

The online Radiation Induced Emission (RIE) is a type of measurement similar to the online RIA except for the fact that no light source is employed and the ionizing irradiation induced light emission is detected. while the radio-luminescence spectrum is recorded by a NIR512 Ocean Optics spectrometer (spectral resolution 3 nm) connected to one of the OF extremities (depending on the envisaged experimental investigation other spectrometers, working for instance in the UV-Visible domains, could also be employed). The irradiated region of the OF is 2.5 cm long.

Confocal micro-Raman and PL: experimental setup

The Confocal Micro-Luminescence (CML) and Raman measurements were carried out with a LabRam Aramis (Jobin-Yvon) instrument, schematically shown in Figure 2.9, which is an integrated confocal micro-Raman and micro-photoluminescence system [START_REF] Aramis | Bruker EMX User Manual[END_REF].

Figure 2.9: Optical path in the LabRAM Aramis (Jobin-Yvon) integrated confocal system. Adapted from [START_REF] Aramis | Bruker EMX User Manual[END_REF]. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

The system is equipped with four excitation lasers: a HeNe laser working at 633 nm (1.96 eV); an Ar+ laser working at 488 nm (2.54 eV); a HeCd laser providing both a 442 nm (2.81 eV) and 325 nm (3.82 eV) excitation lines. The excitation laser is focused upon the sample by means of one of the available objectives (UV-40x, 10x, 50x and 100x) resulting in a beam of less than 10 µm in size at the waist point. The sample is located on a translational stage whose position can be adjusted in the X, Y and Z directions with micrometric precision. Additionally, it is possible to reduce the size of the pinhole of the confocal microscope in order to limit the volume of the sample from which the backscattered light is collected and get, at best, a probed spot size of about 1µm in the X-Y directions. As shown in Figure 2.10, the fibers in the measurements were opportunely fixed with their axis parallel to the impinging laser light beam direction, so allowing a mapping of the surface perpendicular to the axis. In this experiment the cleaved tip of a 4 mm long OF is probed in several points along its diameter. The recorded set of spectra can be then used to derive the cartography of a particular spectral feature as a function of the distance from the center of the OF.

In the present thesis work all the measurements were performed with a regular spacing between recorded points of 3 µm and with a probed spot diameter of 3 µm. The latter choice allowed obtaining a satisfying signal to noise ratio and, at the same time, completing the measurements quickly enough for any photo-bleaching effect to take place.

EPR setup 55 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

The collected light is finally diffracted by one of the four available gratings on a CCD camera. The experimental conditions employed for the PL measurements in this thesis assure a spectral resolution of 0.5 nm. The best spectral resolution achievable performing Raman measurement is 2 cm -1 .

EPR setup

The EPR apparatus we used is a Bruker EMX micro-Bay spectrometer working at 9.8 GHz (X-band) whose block scheme is reported in Figure 2.11. The sample is placed in the center of a rectangular metallic resonant cavity, to which the microwave radiation generated by a Gunn diode is coupled by means of metallic waveguides. The excited microwave mode of the cavity (TE 102 ) is such that in the center of the cavity the oscillating magnetic field amplitude is maximum and the electric field minimum. The cavity is placed between the magnetic expansions of an electromagnet which generates a uniform static magnetic field B, perpendicular to the microwave field, responsible of the Zeeman splitting. While the magnetic field ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ spans the spectral domain of interest (the microwave frequency is kept fixed), the microwave power reflected by the cavity (not absorbed by the sample) is measured by a Schottky diode, which generates a current that is proportional to the square root of the reflected microwave power. To enhance the signal to noise ratio, an oscillating (100 kHz) magnetic field, called modulation field, is added to B allowing the use of a lock-in detection system for the reflected microwave power. In addition, the reflected microwave radiation is passed through two more stages consisting of a filtering stage (RC filter) and an integrating stage (analog-to-digital converter). Furthermore, an automatic frequency control (AFC) device keeps the microwave radiation tuned to the resonant frequency of the cavity-sample system to avoid erroneous detection of reflected power arising from source-cavity frequency mismatch. Finally, the microwave radiation power can be opportunely varied, changing its attenuation, from 200 nW to 200 mW.

In order to avoid distortion or bad quality spectra, the experimental parameters linked to the filtering, integrating stages, microwave power and modulation field amplitude are always chosen accordingly to some technical constrains known from literature [135].

Two detection modes have been employed during the EPR experiments enabling to record the signal reflected by the microwave cavity at the modulation field frequency ( m , first-harmonic detection mode) or at the doubled frequency (2 m , second-harmonic detection mode) [START_REF] Buscarino | Experimental investigation on the microscopic structure of intrinsic paramagnetic point defects in amorphous silicon dioxide[END_REF].

When we performed EPR spectra in the first-harmonic detection mode, the resulting spectrum is proportional to the first derivative of the absorption spectrum. The corresponding double integral, opportunely scaled by instrumental parameters, is proportional to the concentration of defects in the sample [START_REF] Buscarino | Experimental investigation on the microscopic structure of intrinsic paramagnetic point defects in amorphous silicon dioxide[END_REF]. Finally, the concentration of paramagnetic centers is calculated by comparison with a reference irradiated silica sample containing a known spin concentration. This concentration reference was obtained by means of spin echo measurements [START_REF] Agnello | [END_REF]. However, first-harmonic detection mode can provide absolute spin concentrations only if the microwave power is sufficiently low to allow the system of spins to dissipate the absorbed power. Therefore, for each EPR signal a study of the corresponding saturation curve with the microwave power is mandatory. All the quantitative measurements reported in this thesis were recorded in non-saturated conditions. The absolute concentration error is not higher than 20%.

The EPR spectrometer also allows performing measurements in the second-harmonic detection mode controlling the modulation field phase shift around 90° with 1° resolution. For some experiments the 2.3.1 X-rays irradiations: Probix machine 57 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ use of the second harmonic detection mode was necessary in order to increase the detection sensitivity and reveal some interesting paramagnetic signals: E'(Si), P1, P2. In this thesis such measurements are referred to as SH-EPR. Even if the sensitivity of the SH-EPR is higher than that of the first-harmonic detection mode, working in second harmonic detection mode has a remarkable disadvantage: the dependence of the signal intensity on the microwave power cannot be characterized by a simple power law of general validity avoiding an estimation of defects concentration [START_REF] Buscarino | Experimental investigation on the microscopic structure of intrinsic paramagnetic point defects in amorphous silicon dioxide[END_REF]. As a consequence, for each new defect investigated by means of SH-EPR measurements, some evidences must be provided for the applicability of this estimate by this technique. Furthermore, the SH-EPR signal lineshape not always resembles the true absorption line of the paramagnetic center and some distortions could occur depending on the phase shift optimization. In the case of E'(Si), the SH-EPR signal optimization work was done by G. Buscarino in the context of his Ph.D. thesis and is here used [START_REF] Buscarino | Experimental investigation on the microscopic structure of intrinsic paramagnetic point defects in amorphous silicon dioxide[END_REF]. For the P2 defects the detection of their signal by SH-EPR was applied here for the first time with the used instrument with fixed experimental parameters and their concentration estimate was legitimated by the linear correlation between first and second harmonic measurements obtained in favorable conditions. In the case of the P1 defect, which was never detected in first harmonic mode, the detection parameters by SH-EPR were here optimized and fixed for all the investigated samples but their concentration has not been estimated and only a relative signal amplitude comparison has been carried out during the here reported work.

Irradiation setups

In this section we briefly present the irradiation setups we employed for the experiments with the OFs.

X-rays irradiations: Probix machine

The 10 keV X-rays irradiations were performed by using the Probix machine of the CEA-Bruyères le Châtel, Commissariat à l'énergie atomique et aux énergies alternatives (Arpajon, France). This machine is operated at 60 kV and provides a source with the spectral distribution reported in Figure 2.12. The calculated dose spectrum indicates that about 80 percent of the deposited dose results from the tungsten characteristic radiation (X-ray lines around 10 keV) and as such the dose deposition is independent of the operating voltage ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ Figure 2.12: The spectral characteristics of the X-ray source of the Probix machine. Adapted from [138].

The X-ray beam is homogeneous and it has a rectangular spatial shape whose sizes are 2.5 cm and 1 cm. During the irradiation, whether it is performed for a RIA experiment or for the preparation of EPR samples, the OF are transversally irradiated (perpendicularly to the OF axis). It is important to mention that the employed X-rays source assures a nearly uniform dose deposition on the cross section of the OFs. In fact, Figure 2.12 provides the depth-dose profile in SiO 2 , which shows a 20% dose variation in a 100 µm thick silica sample. The total size of the OFs we used is 125 µm and since the core of the OF is less than 45 µm thick we can consider it uniformly irradiated. More importantly, our confocal microluminescence studies of OF samples, show a radially symmetrical concentration profile of the luminescent radiation-induced defects (such as the NBOHC, see further), which provides further proof of the homogeneity of the irradiation.

As reported in Figure 2.6, the samples during the irradiation are placed on a temperature controlled plate which can range from 25 °C to 280 °C. The dose-rate is also adjustable up to a maximum value of 5 krad/s, which is the dose-rate employed for almost all the here reported experiments.

2.3.2 γ-rays irradiations: 60Co source (Mol) 59 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

γ-rays irradiations: 60 Co source (Mol)

The γ-rays irradiations were performed by a 60 Co source having 1.25 MeV photons in the underwater irradiation facility BRIGITTE in SCK-CEN (in Mol, Belgium) [START_REF] Fernandez | Radiation Effects Data Workshop[END_REF]. The irradiation has the same duration for all the samples (about 23 days) and in order to obtain different total doses samples are located at different distances from the 60 Co source, so that the dose-rate can vary between 50 and 500 rad/s. During the irradiation the temperature reaches ~60 °C. The total deposited radiation dose goes from 100 Mrad up to 1 Grad. The γ-rays irradiated OFs are employed for post-mortem EPR and OA measurements.

High pressure and high temperature O 2 loading treatments

On some of the studied OFs we performed an O 2 loading treatment [START_REF] Francesca | Studio del processo di diffusione di O2 a temperatura ambiente in nanoparticelle di silice[END_REF]38].

For each sample, some fiber lengths were uncoated, coiled in 5 cm diameter loops and treated in O 2 atmosphere. In order to load molecular oxygen in the OFs, they were thermally treated in a 250 mL high-pressure vessel, PARR 4651 (schematic representation in Figure 2.13). The temperature was increased in few hours from room temperature up to 390 °C. The vessel was filled up with high-purity technical O 2 gas, having overall impurities content lower than 5 ppm mol., up to a maximum pressure of 95 bar at the maximum temperature reached. The loading treatment lasted 21 days.

The treatment conditions were chosen on the basis of the cylindrical geometry of the samples and the diffusion coefficient of molecular oxygen in silica [START_REF] Norton | [END_REF]141,142]. The goal was to load as much oxygen as possible reaching an almost constant concentration profile of O 2 along the fiber diameter.

Materials: canonical samples

In this last section of this chapter we provide a description of the OF samples investigated in this thesis and introduce the scope of their investigation.

All the studied fibers were produced by iXFiber SAS [143]. The OF samples, which we refer to as canonical samples [45,144,145], were produced to be representative of the current commercial fibers and to allow developing the next OF generation that will be employed in future irradiation and nuclear facilities. Moreover, one of the actual research purposes of the investigated canonical samples is to implement a multi-scale simulative approach that can help overcoming the difficulties that arise from the large variety of radiative environments and working conditions that OF can be subjected to, and the impossibility to qualify the OF in the radiative conditions of the future facilities [145].

Concerning this thesis work, which is devoted to perform meaningful quantitative spectroscopic analysis of the effects of dopants, co-dopants and treatments on the radiation response of the OFs, it is important to minimize the interpretative errors that could arise from the non-negligible influence of inconstant production processes of the commercial OF samples so rigid industrial production protocols have been applied during the fiber preparation.

Germanium doped OFs: GeDi, GeFDi and GeCeDi

The first family of used fibers is of pure silica cladding and with Ge-doped sole or codoped by other elements core type. Three types of core doping have been specifically used: only Ge; Ge and F codoping; Ge and Ce codoping. These fibers are detailed in the following.

2.5.1 Germanium doped OFs: GeDi, GeFDi and GeCeDi

¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

The hereafter named GeD1, GeD2, GeD3 (collectively named GeDi), GeFD1, GeFD2, GeFD3 (collectively name GeFDi), GeCeD1, GeCeD2, GeCeD3 (collectively named GeCeDi) OFs are produced by the Modified Chemical Vapor Deposition process (MCVD) [START_REF] Ungar | Fibre optics: theory and applications[END_REF][START_REF] Pacchioni | Defects in SiO2 and Related Dielectrics[END_REF]. The main difference among the fibers of a given group is the drawing speed. The GeDi, GeFDi and GeCeDi OFs [146].

The fibers differing only for the drawing conditions were drawn from the same preform. The principal chemical characteristics and drawing features of the samples are summarized in the following Table 2 The declared concentration of OH groups in the core of each of the studied fibers is less than 0.5 ppm. Cl impurities left over by the production process (starting from SiCl 4 ) in the fiber core are usually less than 0.1-0.2 wt%.

2.5.2 Phosphorous doped OFs: PDi and PCe 63 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Phosphorous doped OFs: PDi and PCe

The second group of samples investigated is constituted by pure silica cladding and P doped core fibers or P and Ce codoped core fibers. The hereafter named PD1, PD2 and PD3 (collectively named PDi), and PCe OFs are produced by MCVD. The PDi samples are two steps index multimode OFs with a core size 62.5 µm and outer cladding diameter of 125 µm. They were drawn from the same preform and they differ from one another by the drawing conditions. The chemical compositions of the P-doped OFs and drawing features are summarized in Table 2 In Figure 2.16 we show the chemical concentration profiles of dopants obtained by Electron Micro-Probe Analysis (EMPA) on the PDi OFs.

Nitrogen doped OFs: NDi

The last family of studied fibers is constituted by pure silica cladding and N doped core samples.

They are hereafter named ND1, ND2 and ND3 (collectively named NDi). The preform of the NDi OFs was produced by Surface Plasma Chemical Vapor Deposition process (SPCVD) by FIBERUS.

The NDi fibers were successively drawn by iXFiber. These samples are two step-index multimode OFs with a core size 62.5 µm and outer cladding diameter of 125 µm. Once more the three NDi OFs were drawn from the same preform and they differ from one another by the drawing conditions. The chemical compositions and drawing features of the N-doped OFs are summarized in Table 2 In Figure 2.17 we show the chemical concentration profiles of dopants obtained by Electron Micro-Probe Analysis (EMPA) on the NDi OF. It is worth noticing that the Cl content of these samples is much larger than all the other here investigated.

The concentration of OH groups is ~0.015 ppm.

Fluorine doped and Pure-Silica-Core OF

The F-doped and Pure-Silica-Core (PSC) OFs were only employed in the study of the effects of the O 2 loading treatments. The radiation response of these types of OF had been already studied in refs [147,148]. These fibers were produced by iXfiber by MCVD method. The F2D3 OF has a single step core diameter of 43 µm, whereas the SiD3 has a core of 45 µm.

Chapter 3 3 Study of Radiation effects on P-, Ge-and N-doped OFs

In this chapter we report and discuss the experimental results obtained for the studied OFs. RIA, EPR and CML measurements were performed in order to characterize the responses of the OFs under irradiation, and to identify the presence of specific point defects and their formation mechanisms.

All the irradiations were carried out with X-rays, as explained in § 2.3.1, unless otherwise specified.

Radiation effects on P-doped OFs

In this section all the results related to the P and P/Ce doped fibers are collected.

First we discuss the effects of irradiation at room temperature on the OFs doped with P. In this first part the effect of drawing, is also extensively investigated. Successively, the effect of irradiation at different temperatures is studied. In the last section the results concerning cerium codoping are reported and discussed.

Study of Radiation and Drawing effects by X-ray irradiation

RIA study

In Figure 3.1 we report the RIA spectra of the PD2 OF irradiated at several doses (the RIA measurements were performed under 10 KeV X-rays as explained in § 2.3.1). During the irradiation the temperature inside the irradiation chamber varied between 26 and 30 °C. The dose-rate was 5

Chap. 3: Study of Radiation effects on P-, Ge-and N-doped OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ krad/s and the irradiation time was 20000 s long, i.e. until the maximum dose of 100 Mrad was reached. It is evident that the irradiation caused a very strong increase of the absorption of light in the UV-Visible domain. This is mainly due to the generation of the P-related defects presented in § 1.2.3. In particular, the absorption features around 400 and 600 nm could be reasonably assigned to the metastable POHC and stable POHC bands, respectively, whereas the one peaked at 270 nm could be associated to the P2 centers band [99]. Moreover, it is also evident the presence of an UV absorption tail at short wavelengths which is attributable to the sum of several not resolved absorption bands either related to P-defects or intrinsic defects of silica. In Figure 3.2 the RIA growth kinetics at 270, 400 and 600 nm are reported. The RIA increases linearly for doses below 100 krad. For higher values the growth of the RIA at 270 nm and 400 nm almost reaches a saturation level, whereas at 600 nm the RIA keeps growing with a sublinear law, which highlights a different behavior of the stable and metastable POHC centers. Close to the highest dose of 100 Mrad, the RIA at 270 nm and 400 nm seems to start growing again with a sublinear law probably because of the contribution of underlying unresolved absorption bands. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ The RIA experiment was performed in parallel to the PD2 OF on the PD1 and PD3 in order to assure the same radiation conditions. In The curves in Figure 3.3 clearly show that the radiation responses of PD1, PD2 and PD3 are the same. In accordance with this, similar growth kinetics (data not reported) are found at representative spectral positions in the three OFs.

These results evidence the absence of any significant effect on the RIA in the drawing condition range here investigated (see § 2.5.2), which is the one employed for the drawing of specialty fibers.

The experiment we carried out shows that it is not possible to improve the radiation resistance of a P-doped OF by changing the drawing conditions in the investigated range. This fact also implies that the fibers can be drawn at the highest speed without impacting their radiation response and that the radiation hardness of this type of OF is robust against the variation of drawing conditions. The latter property is essential for the dosimetry applications that were suggested for this type of OF by several authors [89,90,91]. In fact, such devices should rely on the (online) measurement of the radiation induced losses in the OF in the visible domain at one or more wavelengths, which involve the creation of both metastable and stable POHC centers.

EPR study

In this section we present the results of the EPR measurements on the irradiated OFs samples. For this study the measurements were carried out a few weeks after the irradiation. We performed few tests that evidenced that after the irradiation the OF experiences a partial recovery of the radiation induced defects, therefore the paramagnetic defects detected by EPR are considered to be the stable ones which are leftover by the recovery process which takes place after the irradiation at room temperature. In Figure 3 [98] about the fact that such defects are stable at room temperature although they were originally considered to be stable only at low temperatures (about 100 K) [99]. In Figure 3.5, the absolute concentrations of POHC, both stable and metastable, in the core of the OFs are shown. vi As in the case of the RIA experiment no differences are recorded among the three differently drawn OFs suggesting that during irradiation as well as at the equilibrium (after the postirradiation recovery processes) the defect population is unaffected by the different OF drawing conditions. At low doses (<100 krad) the growth kinetics is linear and changes to sublinear for higher doses, in qualitative agreement with the findings reported for RIA in Figure 3. In Figure 3.6, we report the EPR signals normalized by the samples masses of the E'(Si) for the three optical fibers. In this case the measurements were performed in the second harmonic detection mode in order to improve the sensitivity of the measurements [START_REF] Buscarino | Experimental investigation on the microscopic structure of intrinsic paramagnetic point defects in amorphous silicon dioxide[END_REF]. We notice that the pristine PD1 OF shows a slightly lower concentration of E'(Si) than the others. It is worth to mention that the PD1 OF is the one drawn at lowest speed and lowest tension and the slight difference is probably due to a reduced creation of E'(Si) induced by the drawing process. The concentration of E'(Si) is very low in the pristine samples and grows sublinearly with the irradiation dose as shown in Figure 3.7. vii vii Since the E'(Si) are Si related defects it is not possible to distinguish between the ones located in the core of the OF and those located in the cladding. The concentration of E'(Si) is an average of the number of defects contained in the whole OF mass. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ The low signal per unit of magnetic field, at all radiation doses, gives rise to a low signal to noise ratio also with optimized recording conditions. In order to determine the growth kinetics of the P2 center, only the high field line of the P2 doublet was recorded and a baseline was conveniently subtracted. In Figure 3.9 the growth kinetics of the P2 defects are reported. viii . In the case of P2 we observe that a maximum concentration is reached after a total irradiation dose in the range 1 to 10 Mrad. Beyond these doses the concentration of P2 shows a decreasing trend. All the reported EPR measurements confirmed no significant differences among the radiation response of PD1, PD2 and PD3 providing the additional evidence that the recovery processes that takes place after the irradiation are similar in all three OFs. Finally, in Figure 3.10 the defects growth kinetics of POHC (stable and metastable), E'(Si) and P2 are shown together. This comparative graph shows that no relevant similarities are present in the growth kinetics of these defects suggesting that their generation processes are not strictly related. We can observe that the POHC defects have a growth kinetics that resembles the one reported in Figure 3.2 concerning the RIA experiment. Although the data are not strictly correlated, we can intuitively say that the POHC do not reach a saturation level because of the contribution of the stable POHC which, at high doses, increases relatively to the metastable ones (e.g. compare Figure 3.4(a) and Figure 3.4(c)). Regarding the P2 defects, absorbing light around 270 nm, the different trend of RIA and EPR growth kinetics at doses higher than 10 Mrad is probably due to the presence of other absorption bands which prevent to isolate the contribution to the RIA of the P2. Moreover, the viii The concentrations of P2 here reported are employed for the calibration of the second-harmonic measurements reported in Appendix A and which allows higher sensitivity.

Chap. 3: Study of Radiation effects on P-, Ge-and N-doped OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ possibility that the post-irradiation recovery processes could impact differently highly irradiated samples than a slightly irradiated one should be taken into account and further investigated.

CML study

In this section we present some results concerning the investigation of luminescent centers in the irradiated PD2 OF. With the exception of a very weak luminescence, due to a modest presence of native NBOHC centers, the pristine P-doped fibers do not show any luminescence which can be investigated with the used equipment and excitation laser light. After the irradiation, however, the intensity of the NBOHC band increases and also a new band appears.

In Figure 3.11, the PL spectra, excited at 325 nm, in the PD2 OF irradiated at 300 Mrad are shown.

As explained in § 2.2.3, the CML apparatus allows to probe the OF in several points of its cross section, with micrometric spatial resolution, and this figure presents the spectra of the cladding and of different regions of the core (see § 2.5.2 for the PD2 doping profiles). By inspection of the spectra reported in Figure 3.11, it is possible to state that a contribution of the band at 520 nm is present especially in the cladding zone of the OF, which is the reason why at distances greater than 30 µm (outside the doped region of the core), this PL intensity is not zero.

Basing on this finding this band cannot be associated to P doping, and will not be further explored (a dedicated investigation is deserved). The presence of at least three overlapping bands makes it difficult to decompose the spectra. However, in order to study the spatial profile of the NBOHC and the nearby unknown (and maybe P-related) band, we plotted the cartography of the integral of two narrow spectral ranges: [645÷650] nm for the NBOHC (shown in Figure 3.12); [585÷595] nm for the other band (shown in Figure 3.13). In Figure 3.12, the cartography of the PL band amplitude of NBOHC is shown. The intensity of the PL increases inside the core of the OF. This fact is interpreted as resulting from a higher concentration of radiation induced NBOHC centers in the core than in the cladding. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ as can be guessed by the spectrum reported in Figure 3.11 for the fiber cladding. Figure 3.13 shows the cartography of this integral. First of all, comparing the cartographies of the two bands evidences that the defects responsible for the two luminescences are not correlated. Moreover, the cartography of Figure 3.13 reproduces the two step concentration profile of the dopant (see § 2.5.2), which leads us to attribute this emission to a P-related defect. In particular, this band is related to an irradiation induced defects since its presence has not been detected before the irradiation. Since the excitation wavelength of 325 nm (3.81 eV), falls within the absorption band of m-POHC centers and some previously reported data [103] suggest that POHCs have an emission band around 600 nm, we consider the observed emission related to the radiation induced m-POHC centers. To further support this assignment, it is worth to note that the intensity of this photoluminescence is roughly twice intense in the inner part of the core than in the outer and, since the concentrations of the dopant in this two zones differ by a factor two as well, it is natural to assume that the luminescent center must involve only one P atom in its microscopic structure.

Study of the -irradiated P-doped OFs

In this section we report the results obtained on the γ-irradiated PD1 OFs. The measurements performed on this series of samples allowed us to detect the P1 defect induced in the irradiated P- doped OFs by EPR measurements in second harmonic detection mode. The P1 defect could not be detected by RIA (in X-rays irradiated OFs) measurements because of the very short length of our Xray irradiated sample. At the same time, the P2 defect is detected in the γ-irradiated PD1 OF employing too the EPR second harmonic detection mode with higher sensitivity compared to the first harmonic one. Finally, the measures were exploited to compare the effects of γ-irradiation as opposed to X-irradiation and to extend investigated dose range up to 1 Grad.

Correlation of NIR absorption of the P-doped OFs and the EPR signal of the P1 defect

The PD1 OF was γ-irradiated at four different doses in order to perform OA measurements in the NIR domain, by means of the cut-back technique, (see § 2.2.1) and EPR measurements as well. On the basis of the X-irradiation study this fiber is considered representative of all the differently drawn OFs. Figure 3.14 shows the absorption spectra of the irradiated samples in the NIR spectral domain.

The radiation induced losses in the P-doped OFs are very high and for the following we can consider the losses of the pristine fibers to be negligible. As known from literature, this absorption band is associated to the P1 defect, which is peaked at 1.6 µm (0.78 eV with a FWHM of 0.35 eV) [88,99].

Chap. 3: Study of Radiation effects on P-, Ge-and N-doped OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ The P1 is also a paramagnetic defect (≡P • ) which has a known EPR signature as described in § 1.2.3.

After revealing its presence by optical measurements, it is expected to confirm this defect generation by irradiation also by the EPR technique. In details, as shown in Figure 3.16, the spectral region related to the P1 and P2 defects was decomposed using four gaussian bands two for each defect. A good fit is found for all the investigated doses. We verified that the two gaussian bands corresponding to the two lines of the same doublet have the same area within 5% error. In Figure 3.17, we report the total area of the P1 gaussian bands determined by the decomposition of the EPR spectra as a function of the optical attenuation at 1.6 m. A good correlation between the OA at 1.6 µm and the intensity of P1 signature in the corresponding EPR measurements is found.

The experimental points in the graph are fitted by a linear law, which, on one hand, confirms the result already reported in ref. [99] about the assignment of the NIR absorption band to the radiation induced P1 defects and, on the other hand, legitimates the use of the second harmonic detection mode for the investigation of the P1 centers, although their absolute concentration cannot be easily estimated (see § 2.2.4).

Comparison of γ and X irradiation on PD1 OF

As in the previous EPR study of the X-irradiated PDi samples, the γ-irradiated samples were investigated in order to evaluate the absolute concentration of POHC, P2 and E'(Si). In Figure 3.18, the EPR growth kinetics of the γ-irradiated PD1 samples are reported together with the measurements of the X-irradiated ones. First of all the γ-irradiations extend the radiation dose domain up to 1 Grad. However, there are two doses, 100 Mrad and 300 Mrad, for which both X and γ irradiations were done. In the case of POHC and P2 defects, the results at the two doses are compatible. At doses higher than 300 Mrad we can see that the POHC concentration keeps growing (as suggested by the RIA experiments and the 3.1.1 Study of Radiation and Drawing effects by X-ray irradiation 83 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ previous analysis on the X-irradiated samples), whereas the P2 has reached its saturation value of ~2.5•10 17 defects/cm 3 . In contrast to these results, the γ-irradiation induces a significantly higher concentration of E'(Si). This finding is highlighted in the following Figure 3.19 where we report the spectra of the samples irradiated at 100 and 300 Mrad by X and γ irradiation. The spectra shown in Figure 3.19 were normalized by the masses of the samples in order to compare the relative intensities of the EPR signals. Although the EPR lines of E'(Si) are here recorded in saturation conditions (other measurements are employed for the evaluation of the corresponding concentrations), these spectra clearly show that the EPR signal of the POHC defects is basically the same whether the samples were X-irradiated or γ-irradiated. On the contrary, the γ-irradiation induces unequivocally higher concentration of E'(Si). Such differences can be partially due to the different types of radiation. However, in the next § 3.1.2, we will show that the generation of E'(Si) is very sensitive to the irradiation temperature below 100 °C, and, in particular, that increasing the temperature promotes the creation of E'(Si). In this context it is worth to note that during the γirradiation the samples reach temperatures around 60 °C, whereas during the X-irradiation the maximum temperature is 30 °C. We assume that this is the main reason for the different contents of radiation induced E'(Si) in X-and γ-irradiated samples.

Chap. 3: Study of Radiation effects on P-, Ge-and N-doped OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Study of Radiation and Temperature effects

In recent papers it has been shown that the radiation response of OFs depends strongly on the temperature at which the irradiation is performed [149,150]. In particular, depending on the dopant of the OF, the increase of the temperature of irradiation can cause the decrease or the increase of the RIA. Since the scenario is very complex an extensive study of each fiber typology is required in order to assess its vulnerability or radiation hardness. In this section we report the results that concern the effects of irradiation at different temperatures on the P-doped OFs.

After it was established, in the previous paragraph, that PD1, PD2 and PD3 OFs have the same radiation response at room temperature, this series of experiments was performed with only one of the three OFS, the PD3, considered to be representative of all the other.

RIA study

During the RIA experiments the dose-rate was 5 krad/s and the X-ray irradiation duration 20000 seconds, i.e. until the maximum dose of 100 Mrad was reached. The results obtained at room temperature (30°C), which are presented in the previous sections, are here reported in order to compare them with the ones corresponding to higher temperatures. We performed a decomposition of the RIA spectra corresponding to the total dose of 2 Mrad for the three temperatures of 30, 150 and 280 °C with a best-fit procedure using gaussian bands. Figure 3.21

shows the result of such procedure. Each spectrum can be fitted univocally with four gaussian bands whose parameters are summarized in Table 3.1. However, the spectral parameters of the fitting bands show some minor variability with temperature. Moreover, it is worth to notice that the UV tail employed to fit the curves at high energies has a quite large FWHM which is probably due to the presence in this spectral region of several radiation induced absorption bands, both P-and SiO 2 - In order to describe the irradiation effects as a function of the total dose, in Figure 3.22 we report the RIA at three specific wavelengths: 270, 400 and 600 nm. Each graph of this figure contains five curves corresponding to the five investigated temperatures. For each temperature, the RIA increases almost linearly for doses lower than 100 krad. For higher values the increase with the dose slows Chap. 3: Study of Radiation effects on P-, Ge-and N-doped OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ down. At all doses the maximum RIA values are obtained for the PD3 OF irradiated at 100 °C. In the case of 190 °C and 280 °C a strong decrease of the RIA is detected, especially around 400 nm (3.1 eV). Basing on the spectral analysis summarized in Table 3.1, such effect is attributed to a lower creation rate of the m-POHC at high temperature or higher transformation rate. This effect is not surprising if we consider that originally the metastable-POHC centers were considered to be unstable at room temperature [99].

From a practical point of view it is interesting to observe that increasing the radiation temperature strongly impacts the RIA of the P-doped OFs in a non-trivial way: from 30 to 100 °C the radiation sensitivity of the OF increases and for higher temperatures it decreases. P-doped OFs, that are produced for future applications as sensor in radiative environments where the temperature can vary, should be fully characterized against combined radiation-temperature effects. Moreover, P-doped optical fibers are often employed as host-matrix for the incorporation of rare-earth elements and design of fiber amplifiers. Such devices could also be strongly affected by a change of the irradiation temperature and, depending on their envisioned working conditions, it would be difficult to characterize their performances without extensive tests campaigns. These aspects could be the subject of future experiments.

EPR study

In this section we present a study, complementary to the one by RIA reported in the previous paragraph, based on the investigation by EPR of the radiation induced paramagnetic defects in the Pdoped OFs. For this study the samples were first irradiated at four different temperatures (100,150,190 and 280 °C). After the end of the irradiation the samples were cooled down in air to room temperature in less than one hour and the EPR measurements were finally performed a few weeks after the irradiation. Therefore, the detected defects are considered to be the stable ones which are leftover in the OFs by the irradiations and the transient recovery processes. Although these measurements cannot be strictly correlated to online RIA, they could confirm the results shown in the previous paragraph, which concerns the UV-Visible absorbing defects. Furthermore, the EPR measurements by the detection of the P1 defect, that is known to feature a NIR absorption band, can provide further information regarding the expected response of P-doped fibers in the NIR domain. This series of measurements were performed on the PD2 OF. The EPR results relative to In Figure 3.23, we report the EPR spectra of the POHC centers detected in the PD2 OF irradiated at four different temperatures up to a total dose of 20 Mrad. As reported in § 1.2.3 the EPR signatures of the two varieties of POHC, stable and metastable, are overlapped and difficult to separate. However, Figure 3.23 shows that increasing the irradiation temperature impacts strongly the m-POHC to such an extent that it is not detectable in samples irradiated at 280 °C. In Figure 3.24, we report two graphs where the absolute concentrations of the POHC defects, both stable and metastable, are estimated. In Figure 3.24.a we report the growth kinetics of the POHC at the five investigated temperatures. A monotonic increase of the concentration with the dose for all the temperatures, with a lower efficiency on increasing the dose above 10 3 krad, is found. Figure 3.24.b, displays the dependence of the POHC concentration on the temperature for several radiation doses.

In this case it is observed that for low doses the POHC concentration decreases monotonically with the temperature, whereas at high doses such dependence gets weaker and weaker.

Chap. 3: Study of Radiation effects on P-, Ge-and N-doped OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ It is worth to mention that these results agree and support the ones obtained by the RIA experiments where a strong reduction of the absorption band of the m-POHC was detected on increasing the temperature above 100 °C, as well as a weaker reduction of the stable POHC. However, in the case of the EPR measurements we observe that the concentration of POHC induced at 100 °C is lower than the concentration of defects induced at 30 °C, not in agreement with the RIA spectra. In order to explain such discrepancy we must take into account the post-irradiation recovery process. In fact, after an irradiation at high temperature is completed it takes some minutes for the samples to get back to room temperature. In this time the thermal post-irradiation recovery processes can take place and reduce the concentration of POHCs that are later observed by EPR.

We also characterized the evolution of E'(Si) centers performing EPR measurements in second- The concentration of E'(Si) center increases monotonically with the dose. However, unlike the case of POHC, increasing the temperature facilitates the creation of E'(Si) especially at low doses. At high doses (>100 Mrad) and high temperature (280 °C) a tendency to a leveling value of 10 17 E'(Si)/cm 3 is reached.

These results are in agreement with the data reported by Girard et al. in [149]. In their article the authors have reported the RIA experiment performed on PSC OFs performed in the same temperature range. Working on PSC OFs allowed them to clearly highlight an increase of generation of E'(Si) at temperatures higher than room temperature. Although the OFs studied here are P-doped, we must consider that the EPR measurements provide an estimate of the average concentration of E'(Si) defect in the whole fiber sample, core and cladding. Since the 75% of the mass of our OFs is constituted by pure silica (cladding zone) the results of ref. [149] and ours can be qualitatively This finding evidences a qualitative agreement between RIA and EPR results and shows that the P2 defects can be monitored by both techniques. Furthermore, since the EPR measurements have been carried out a few weeks after irradiation it can be concluded that at least a part of these defects is stable at room temperature.

In Figure 3.28, we report the results concerning the dependence of the P1 defect on dose and temperature. The EPR signal of this defect is only detectable in second harmonic detection mode and, as a consequence, we are not able to provide the corresponding concentration but only proportional values ix . However, on the basis of this consideration, we can say that the concentration of these defects increases sublinearly with the dose and, as in the case of the E'(Si), increasing the temperature promotes the creation of P1 centers.

For some of the samples it was not possible to evaluate the intensity of the EPR band associated to the P1 defects. In particular, at low radiation doses and low temperature the intensity of P1 signal is very weak and overlapped with the much stronger signal of the P2 defects.

ix In order to evaluate the concentration of P1 a calibration of the second harmonic detection mode should be performed as in the case of P2 defects. Unfortunately, the P1 EPR signal was not detectable in first harmonic detection mode.

Chap. 3: Study of Radiation effects on P-, Ge-and N-doped OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ In previous studies [101] it was pointed out that one of the post-irradiation recovery processes taking place in P-doped OFs involves the conversion of POHC in P1 defects. In our experiments, by increasing the irradiation temperature, we observe a strong decrease of the concentration of the first type of defect (especially the metastable POHC) and an increase of the second one. Our results are consistent with the mechanism suggested in [101]. In particular, it is likely that a high irradiation temperature promotes the conversion of the metastable-POHCs in P1 which clearly displays a higher thermal stability.

Another interesting remark about the P1 defect concerns the corresponding absorption band in the NIR spectral domain. As shown in 3.1.1.4, the P1 defects absorb light around 1.6 µm. Although the EPR study of P1 was particularly difficult because of the weak signal, we can suggest that a P-doped OF working at high temperature would show higher NIR RIA values. For instance, we can guess that an irradiation of 100 Mrad at 280 °C would cause an absorption around 1.6 µm which is twice the one caused at room temperature. Such effects must be certainly taken into account in those practical applications that might involve P-doped OFs working in the infrared domain in radiative environment and at high temperatures.

3.1.3 Ce-codoping of P-doped optical fibers 95 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Ce-codoping of P-doped optical fibers

In this section we present the results concerning the effects of Ce-codoping on the radiation response of a P-doped optical fiber. RIA experiments were performed at the same time on the PCe and PD1 OF during X-ray irradiation. Moreover, EPR investigation has been carried out on samples that were γ-irradiated together to assure the same irradiation conditions and the deposition of the same radiation doses. Finally, we present some results obtained by CML on the PCe optical fiber which can provide complementary information to understand the basic microscopic mechanisms.

RIA study

In Figure 3.29, we report the RIA spectra of the PCe and of the PD1 OFs X-ray irradiated at room temperature and with a dose-rate of 5 krad/s up to 100 Mrad. The presence of the cerium co-dopant causes the pristine fiber to have a very high optical absorption in the UV domain [START_REF] Stroud | [END_REF]77]. As a consequence, the RIA spectra cannot extend much on the low wavelength side in Figure 3.29(a). However, we observe that Ce-codoping drastically impacts the response of the optical fiber in the investigated visible region. First of all, at all doses PCe shows a lower RIA with respect to undoped fiber (Figure 3.29(b)). Secondly, the irradiation causes the Chap. 3: Study of Radiation effects on P-, Ge-and N-doped OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ appearance of a strong absorption band in the UV domain and of a band around 550 nm. The spectra of PCe have been decomposed by means of two gaussian bands while the ones of PD1 by four gaussians using a best-fit procedure as already reported in the previous paragraphs. Figure 3.30 shows the results of the decomposition of the spectra corresponding to a total radiation dose of 600 krad. In Table 3 We notice that the spectral characteristics of the absorption band in the visible domain of the PCe OF are very similar to the ones of the stable POHC defects and for this reason it is natural to assign it to the radiation induced POHC defect.

To further investigate the effects of Ce-codoping, we performed the same kind of decomposition of the spectra as a function of the dose. For the PCe OF the POHC band was allowed to change only 3.1.3 Ce-codoping of P-doped optical fibers 97 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ in amplitude while the spectral parameter of the UV tail and its amplitude could change freely.

Similarly, for PD1 the spectral parameters of the two POHC bands could change only in amplitude whereas the P2 band and the UV tail could change also spectroscopically. The variability of the spectral parameters of the UV tails and P2 defects does not allow a meaningful study of the related radiation induced defects. On the contrary, the low energy part of the spectrum is reliably decomposed at all doses. In Figure 3.31, we report the growth kinetics of the amplitude of the bands employed for the decomposition of this part of the spectrum. Although both fibers reveal to be very sensitive to the radiation in the visible domain, at low doses the RIA caused by POHC in the PCe is much lower than in PD1. It is also found that for doses higher than 1 Mrad, the growth of m-POHC in PD1 slows down, almost reaching a saturation level, and the behaviors of the POHC bands in PCe and PD1 become similar, although in PCe the concentration of this defect is always lower than in PD1 in the investigated dose range. This result suggests a positive effect of Ce codoping to reduce the induced attenuation during irradiation of fibers doped with P in the visible spectral range.

As already mentioned in § 3.1.1, the RIA of P-doped fibers in the visible range could be exploited for the design of radiation dosimeters [89,90,91]. Our study concerning Ce-codoping suggests that a P-Ce-doped OF could also be a valuable candidate for such applications. In fact, we have shown that the PCe OF is enough sensitive to the irradiation as well as the PD1. Moreover, the radiation growth Chap. 3: Study of Radiation effects on P-, Ge-and N-doped OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ kinetic of Figure 3.31 shows that, around 2.2 eV, the RIA does not have any saturating tendency, whereas a P-doped OF does. Finally, in § 3.1.2 we have reported on the strong effects of changing the irradiation temperature on the radiation response of P-doped OFs due to the presence of the metastable POHC. In PCe OF such defects are absent (in the next paragraph this aspect is supported by EPR investigation) and it can be guessed that the PCe OF would be less affected by such issue. In order to support this hypothesis further investigations are needed.

EPR study

In this section we present the EPR study of the effects of irradiation on the PCe OF and we compare them to those obtained on the PD1 OF. For this investigation the samples were γ-irradiated at four different doses from 100 to 1000 Mrad.

In Figure 3.32, we present the EPR signals that were recorded for the study of the POHC centers. In To evaluate such concentration the contribution of E'(Si) and H(I) was subtracted. It can be observed that at all doses the concentration of POHC in PCe is about 3.5 to 5 times lower than in PD1, in qualitative agreement with the results obtained with the RIA analysis. This finding, strengthened by the quantitative estimation of POHC content by EPR, shows the increased radiation resistance induced by Ce codoping. The last EPR study that we present concerns the P1 and P2 defects, which are not accessible by RIA analysis of the PCe OFs.

In Figure 3.35 we report the second harmonic EPR spectra of the PCe and PD1 OFs for each irradiation dose investigated. The spectra were normalized by the masses of the samples in order to allow a quantitate comparison among them. It is evident that Ce-codoping causes a drastic reduction of the signal associated to the P2 defects.

However, the corresponding absorption band is located in the UV domain, where the Ce-related absorption bands are the strongest. On the other hand the P1 defect content is not reduced by Cecodoping.

In Figure 3.36 and Figure 3.37, we present the growth kinetics of P2 and P1 defects, respectively, in the PCe and PD1 OFs as a function of the dose.

x For the samples PD1 irradiated at 100 Mrad a spurious signal was detected in first harmonic detection mode around 3000 G. Its presence is also found on the second harmonic measurements of Figure 3.36(d). For the decomposition of the spectra only the high field lines were then employed. To evaluate the concentration of P2 and the evolution of P1 signal in the samples, the spectra of The most striking results regarding our EPR investigation concern the absence of an impact of Cecodoping on the concentration of P1 defects in the PCe OF with respect to the PD1. In fact, the P1 defect has been shown to absorb light around 1.6 µm, therefore the RIA in the NIR domain, which is mainly due to the P1, should not be affected by the Ce-codoping. However, in previous studies performed on this topic [104,105,106], it was shown that Ce-codoping greatly improves the radiation hardness of P-doped OFs in the infrared. Although the samples employed in [104,105,106] are very different from ours, such results are clearly in contradiction to one another and further investigations should be done to clarify this point. However, it is worth to mention that in our work we have investigated radiation doses higher than 100 Mrad whereas in ref [106], in which the samples are more similar to ours, the radiation dose did not exceed 150 krad.

CML study

In this section we report the results concerning the PL properties of the PCe OFs which were γirradiated in the 100 to 1000 Mrad dose domain.

The pristine PCe OF shows a luminescence band in the visible domain which is excited by UV light.

Such luminescence is attributed to the incorporation of Ce 3+ ion as discussed in § 1.2.3. Chap. 3: Study of Radiation effects on P-, Ge-and N-doped OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ In Figure 3.38 we show the CML spectra, excited at 325 nm, of the pristine and irradiated PCe samples. These measurements were performed assuring the same experimental conditions in order to be able to compare the intensities of the PL. As we can observe in Figure 3.38, and as reported in Figure 3.39, the irradiation causes a drastic decrease of the luminescence of the Ce 3+ centers. In particular, a very strong reduction of the associated emission band is visible between the pristine sample and the one irradiated at 100 Mrad. At higher doses, from 100 to 550 Mrad, the PL still decreases but in a weaker way. A further enhanced decrease is found between 550 Mrad and 970 Mrad. The reduction of the PL of Ce 3+ is attributed to the conversion process of Ce 3+ ion into the Ce 4+ form which has been already reported in literature [77]. The electron released by the Ce 3+ ion under irradiation can then be captured by a hole defect such as the POHC. Such process justifies the reduced production of POHC centers observed by EPR in the PCe OF under irradiation. However, it is worth to mention that the release of electrons from Ce 3+ ions was thought to cause an increase of P2. In fact, as reported in § 1.2.3 [99], the P2 defect is a tetra-coordinated P atom which had captured an electron. On the other side, according to ref. [99], the P1 originates from a threecoordinated P atom which loses one of the electrons not involved in the bounds with the oxygen atoms. As a consequence, Ce-codoping was thought to decrease the number of radiation induced P1 3.1.3 Ce-codoping of P-doped optical fibers 105 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ defects. The results obtained by EPR measurements (Figure 3.36) show that this is not the case: the P2 defects are drastically reduced in the PCe OF, whereas the P1 is almost unaffected by the codoping. Therefore, we must conclude that the incorporation of Ce in the P-doped OF, not only introduces a new radiation activated microscopic mechanism, the conversion of Ce 3+ in Ce 4+ , but also changes the quantity of precursors of P-related defects as P1 and P2.

Conclusions of Chapter 3.1:

Radiation effects on P-doped OFs

As first result we have demonstrated that changing the drawing conditions of a P-doped OF in the typical range of specialty OFs does not affect its radiation response or significantly impact the radiation induced defects. Consequently this type of OF can be drawn at maximum speed, which is certainly advantageous for lowering its production costs. Moreover, our results show that the radiation hardness of this OF is robust against the variation of drawing conditions, which make it exploitable for radiation dosimetry. In regard of this latter application, it is important to mention that for doses higher than 100 krad the radiation induced losses (defects) tend to saturate. This fact obviously limits the use of this type of OF as RIA-based dosimeter at higher doses.

The radiation response of the P-doped OFs was studied at several irradiation temperatures. We found that increasing the irradiation temperature from 25 to 100 °C slightly increases the radiation induced losses in the UV-visible domain. However, increasing the irradiation temperature beyond 150 °C mitigates the RIA in the same spectral range. Although it was impossible to perform RIA experiments in the NIR domain, we were able to detect, by EPR, the P1 defect in our irradiated samples and we found that the creation of this type of defect is promoted by the increase of temperature up to 280 °C (the highest temperature achievable by our experimental setup). This fact is particularly relevant because the P1 defect is associated with an absorption band peaked at 1.6 µm which is the main source of optical losses in the NIR domain in irradiated P-doped OFs. It is worth to mention that our post mortem EPR and OA measurements on γ-irradiated OFs had confirmed the association of the absorption band at 1.6 µm and the EPR signal of the P1 defect. Such Chap. 3: Study of Radiation effects on P-, Ge-and N-doped OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ correlation has not received many confirmations (or denials) since it was first proposed by Griscom et al. in 1983 [99]. As a general remark, we must point out that the possible change of the irradiation temperature must be taken into account in any application involving the use of an OF in a radiative environment. The radiation response of such OF should be characterized against the combined effects of radiation and temperature. For instance, in the case of a RIA-based dosimeter (P-doped OF it is clear that a calibration valid at room temperature could not be exploited to determine the irradiation dose at temperatures over 150 °C.

We performed a room temperature study of the radiation induced effects on a P-doped OF whose core was codoped with cerium. The confocal microluminescence studies have shown that part of the Ce codopant is incorporated in the form of Ce 3+ . After irradiation this ion is converted to (Ce 3+ ) + and contextually a lower concentration of POHC center is induced in this OF with respect to the Cefree sample. These two phenomena are jointly explained by the captured of the released cerium electron by some of the radiation induced POHC center. By EPR measurements we have also observed a significant reduction of the radiation induced P2 defect, which is an electron center, with respect to the simply P-doped OF. Such effect is tentatively attributed to a lower initial content of the tetra-fold coordinated P atoms (PO 4 ) + , which is the precursor of P2, in the Ce-codoped sample as compared to the other one. Finally, our measurements have shown that in the Ce-codoped and Ce-free OFs a high dose γ-irradiation induces similar concentrations of P1 centers. The P1 is considered usually a hole center (E'-like) and the obtained result was probably the most unexpected.

In fact, in previous investigations on Ce-codoped phosphosilicate OF the authors had reported an improvement of the radiation resistance in the NIR domain due to the suppression of the P1-related absorption band. Further investigations are certainly needed to clarify this point.

On the basis of the RIA experiment it is possible to imagine that an OF codoped with P and Ce can also be used as a radiation dosimeter. This kind of OF shows a high sensitivity to radiation in the visible spectral domain. Moreover, our experiments have shown that at room temperature the radiation induced POHC absorption bands increases monotonically up to 100 Mrad, which make it potentially interesting for application in radiative environment characterized by high doses. Similarly to what we have done for the P-doped OF, the effects due to the variation of irradiation temperature should be investigated for the P-Ce-doped OF.

GeDi and

Radiation effects on Ge-doped OFs

In this section all the results related to the Ge-doped optical fibers named GeDi, GeFDi and GeCeDi are reported. As introduced in the previous § 2.5.1, these fibers differ by their drawing conditions and for the co-doping. Among the principal interests of the study carried out there are the comparison of the radiation sensitivity in connection to the drawing speed, and the evaluation of the interactions between the co-dopants and their overall effect on the optical transmission in radiation environment in view also of potential applications.

First the effects of radiation on the OFs doped with Ge, GeDi OFs, and with Ge and F at the same time, GeFDi, will be presented. The reported results will show minor differences between the radiation responses of these two OF typologies evidencing a minor interaction between the F and Ge co-dopants in determining the radiation sensitivity. Then the results concerning Ce-codoping of the Ge-doped OF will be reported and discussed. The effects of changing the drawing condition also are investigated contextually.

All the irradiations were carried out with 10 keV X-rays, as explained in § 2.3.1, unless otherwise noticed.

GeDi and GeFDi OFs

RIA study

In Figure 3.40 we report the RIA spectra of the GeD2 and GeFD2 OFs irradiated at several doses.

During the experiments the temperature inside the irradiation chamber varies between 26 and 30 °C.

The dose-rate was 5 krad/s and the irradiation reached the maximum dose of 100 Mrad. For both the OFs the irradiation causes a strong increase of the absorption of light in the UV-Visible domain.

Chap. 3: Study of Radiation effects on P-, Ge-and N-doped OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

The investigated spectral domain is not wide enough to allow a reliable decomposition in gaussian bands. This is due to the high OA of the pristine Ge-doped OFs in the UV region. However, on the basis of the results reported in literature, it can be suggested that the recorded RIA is mainly due to the generation of Ge(1) defects as discussed in the § 1.2.2, their absorption band tail being responsible for the observed RIA increase. From Figure 3.40 we notice that the shapes of the RIA spectra do not undergo major modifications by increasing irradiation dose, either in the case of GeD2 or GeFD2. Furthermore, almost no changes occur above 1.7 Mrad, both in RIA amplitude and profile. This result suggests that from a microscopic point of view a point defect generation process could be associated to the RIA and, in particular, a mechanism involving a conversion of precursors that are exhausted, or balanced by a bleaching process, explaining the leveling of the absorption coefficient.

In order to compare the spectral profiles of the RIA of GeD2 and GeFD2, in The two ratios reported as a function of the wavelength are essentially flat. It can be concluded that the radiation induces the generation of the same defects absorbing in the spectral domain under study. Moreover, the ratio curves have values between 0.8 and 0.9, which shows that the two OFs have very similar radiation responses. In Figure 3.42, we report the RIA growth kinetics of the GeD2 and GeFD2 OFs at 350 nm. For both the samples the RIA grows almost linearly up to 200 krad. Subsequently, the RIA growth slows down and saturates for doses higher than ~3 Mrad. At high doses the RIA value of GeFD2 is 20% lower than GeD2. Considering the 10% error due to the repeatability of the measurements these differences can be neglected. In Figure 3.43 we compare the radiation response at different drawing conditions. No significant differences are observable in the RIA spectra of GeDi OFs. At variance, GeFD1 displays a 15% higher RIA than GeFD2 and GeFD3, such difference however falls within the repeatability of the measurements and cannot be easily discussed. Furthermore, as can be guessed by the analogy of the spectra shown in Figure 3.43, the same trend is found in the growth kinetics of all the six studied OFs so it is concluded that neither the changed drawing conditions nor the changed chemical compositions induce relevant differences in the RIA.

The here reported results shows that changing the drawing conditions in the studied drawing range, typical of specialty fibers, does not impact the fiber response to radiation. On one hand this shows that it is not possible to improve the radiation response of the OFs by varying the drawing conditions of the preform produced by MCVD. On the other hand, the results show a good 3.2.1 GeDi and GeFDi OFs 111 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ hardness: if a fiber is acceptable for a given application, it will be robust against the production process changes. Moreover, the possibility to draw an OF at higher speed can be of great interest for the production of the OFs. Analogous results were found for the P-doped optical fibers studied in the previous section, suggesting that for preparation aims the explored drawing speeds are equivalent in view of application of fibers in radiation environments. Interestingly, it is found that F codoping is not able to affect the radiation response in the UV-Visible of Ge-doped fibers, independently on drawing speed.

EPR study

In this section the irradiation effects on GeFDi OFs are studied by EPR. The obtained results are compared with the ones of ref. [62], where analogous investigations on the GeDi OFs were done. In Figure 3.44, the EPR spectra of the GeFDi fibers irradiated at three different doses are reported.

Chap. 3: Study of Radiation effects on P-, Ge-and N-doped OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ Each spectrum has been normalized by the mass of the corresponding sample in order to compare the absolute concentration of the paramagnetic defects induced by the irradiation. Once more the different drawing conditions do not produce appreciable differences among the OFs and the same spectral profiles are found at each dose. Figure 3. 44(d) shows the EPR spectra of the pristine samples; in this case only the EPR signal of the E'(Ge) center is found, whose initial concentration is estimated to be about 10 16 defects/cm 3 . Figure 3.44(a), (b) and (c) show remarkable modifications of the spectral shape on increasing the irradiation dose due to the creation of other paramagnetic defects. A detailed analysis of these spectra (see further) shows that they contain two more Gerelated EPR signals corresponding to Ge(1) and Ge(2), described in the § 1.2.2, and the distorted signal of E'(Si). In Figure 3.45, the EPR spectrum of GeFD2, normalized by the signal double integral, is compared to the spectrum of GeD2, analogously normalized, for a single irradiation dose of 2 Mrad. Some differences are here visible suggesting different relative populations of the induced defects. More information on the induced defect concentrations can be gained decomposing the EPR spectra by means of the characteristic EPR lines of the Ge-related defects Ge(1), Ge(2) and Eʹ(Ge) reported in ref. [65]. In Figure 3.46 we report a typical decomposition carried out for the GeFD2 OF irradiated at 100 Mrad. We observe that around 3462 G, where all the EPR signals overlap, the quality of the fitting is quite poor. This is probably due to two factors. Firstly, the EPR lines characteristic of the 3.2.1 GeDi and GeFDi OFs 113 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ Ge-related defects show a certain variability with the dopant concentration [65] and for the present study, in which the Ge concentration is around 8 wt%, the EPR lines relative to silica doped with 12 wt% of Ge have been employed [65]. Secondly, the EPR signature of the Ge(2) defect in ref. [65] was obtained with the help of simulations since it was not possible to experimentally separate its signal from the others. However, as far as we are concerned with the determination of relative concentration of different Ge-related defects, we consider the decompositions of our experimental data reliable. 1), E'(Ge), Ge( 2) and E'(Si) reported in ref. [65].

The results of the decompositions of the EPR spectra, at all the irradiation doses, are shown in Figure 3.47 for GeD2 and GeFD2. In details, the concentrations of the radiation induced defects in GeFD2 OF are compared to the results obtained for the GeD2 OF reported by A. Alessi et al. [62].

The growth kinetics of Ge(1) and Ge(2) defects are very similar for the two types of fibers: for doses lower than 200 krad the concentration grows linearly and reaches the constant values 3x10 17 Ge(1)/cm 3 and 4x10 17 Ge(2)/cm 3 , respectively, at doses higher than 2 Mrad. This finding agrees with the reported RIA at 350 nm (due mainly to Ge(1)) which evidenced the absence of a great improvement of the radiation hardness of the GeFDi OFs with respect to the simple Ge-doped ones. On the other hand the E'(Ge) defects grow with a sub-linear law without reaching any Chap. 3: Study of Radiation effects on P-, Ge-and N-doped OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ saturation level in the investigated radiation dose range. It is also interesting to note that GeFD2 fiber displays about 2.5 times higher concentration of E'(Ge) at all the doses compared to GeD2.

Considering the full overlap of the spectra at each dose reported in Figure 3.44, this result can be extended to all the drawing conditions investigated. Basing on this spectral analysis it is concluded that F codoping somehow enhances the generation of E'(Ge). This effect could be related to the presence of precursor sites involving the F or this latter element induced changes on the amorphous matrix. On the other side, the F is not able to affect the generation of Ge(1) and Ge(2) as well as their possible correlated formation. Finally, considering the E'(Si) defects, we must say that their concentration in GeFDi OF is somewhat harder to estimate with respect to the Ge-related centers. The mass of our samples are not large enough to allow the detection of E'(Si) in first harmonic detection mode. As a consequence, the 3.2.1 GeDi and GeFDi OFs 115 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ E'(Si) are usually recorded in second harmonic detection mode. However, in the case of Ge-doped samples, the EPR signal of E'(Si) overlaps with the signal coming from the paramagnetic Ge-related defects and in particular the E'(Ge) centers. A quite qualitative analysis of the spectra show that at high doses E'(Si) grow with a sub-linear law and reach a maximum value of about 3x10 16 defects/cm 3 at 300 Mrad.

CML Study

In this section the results obtained by CML measurements on the GeFDi OFs are reported. The data are compared to the ones of the GeDi OF reported in [62].

In Figure 3.48, we report a typical photoluminescence spectrum obtained with the CML system in the optical fibers cores. The laser excitation wavelength is 325 nm and the reported spectrum clearly shows two emission bands peaked around 400 and 650 nm. As described in the first chapter, these bands are assigned to the triplet to singlet emission of the GLPC, and to the NBOHC photoluminescence, respectively (as described in § 1.2.1 and 1.2.2). Our apparatus allows us to probe a cleaved OF in several diametral points and to reconstruct the radial profile of each spectral feature. In Figure 3.49(a) and Figure 3.50(a), the cartographies for three Chap. 3: Study of Radiation effects on P-, Ge-and N-doped OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ different radiation doses and for both emission bands identified in Figure 3.48, are reported. Despite the fact that the germanium concentration is almost constant along the OF diameter, the cartography shows a higher concentration of GLPC at the center of the fiber and at the core-cladding interface.

This profile appears to be compatible to the one reported in previous studies for the GeDi fibers made by two steps with different Ge doping levels [65]. It can be observed that the profile of the cartography does not depend significantly on the dose, and that the GLPC content is reduced by irradiation whereas the NBOHC one is increased. It is worth noticing that for each radiation dose the GeFD1, GeFD2 and GeFD3 samples share the same cartography (not shown) regardless of the different drawing conditions. In Figure 3 Figure 3.50 shows that pristine fibers have a low content of NBOHCs and that they are efficiently created by irradiation. In Figure 3.50(a), it is also evident that the highest signal of NBOHC is induced in the more external part of the core, where the Fluorine co-doping level is higher. [62]) and GeFD2 (present thesis) in the 100 Mrad irradiated OFs. The normalization has been done dividing by the respective PL values at the center of the OF core. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ In order to highlight the effect of Fluorine co-doping, in Figure 3.51 the cartographies of NBOHC in the GeD2 and GeFD2 OFs are shown using results of ref. [62]. The two cartographies are normalized with respect to the value at the center of the core where the chemical compositions of the two fibers are very similar. After the normalization has been done, the signals of NBOHC in the claddings are automatically equal. This shows that the ratios of the PL signals in the center of the core and in the cladding are the same for GeD2 and GeFD2. We also note that both samples share the same pure silica cladding and in this region it is expected that the same amount of NBOHC is induced by the irradiation. On the other hand, the most striking effect concerns the different amplitude of NBOHC in the outer zone of the core. In this region the GeD2 fiber has a Germanium concentration of ~4.5 wt.%, about half the inner zone concentration, and the NBOHC signal is somehow intermediate between the core and the cladding values with peaks at the interfaces separating different zones [62]. By contrast, in the GeFD2 fiber, the same region is characterized by a concentration of Fluorine around 1wt. % and a concentration of Germanium around 8 wt. %, similar to the inner zone concentration. Nevertheless, in this case, the NBOHC signal is about 3 times higher.

GeDi and

The CML experiments show a strong effect of Fluorine co-doping. This is indeed clear whether we compare the NBOHC cartographies of GeDi and GeFDi OFs or better if we compare the relative signal of NBOHC in the two core zones of the GeFDi samples. The NBOHC signal of the outer zone (~1wt.% F) is almost twice the signal in the inner zone. The dopant concentration profile of GeFDi (Figure 2.15) clearly shows an almost constant concentration of Germanium in the whole core and the only parameter that considerably varies between the inner and the outer parts of the core is the concentration of Fluorine. At present no hypothesis has been suggested on the nature of the NBOHC we recorded. However, it is known that both Si-related and Ge-related NBOHCs can exist in Ge-doped silica glasses. The two types have a quite similar red emission band: Si-NBOHC band peaked at ~1.92eV and FWHM about 0.16 eV; Ge-NBOHC band peaked at ~1.86 eV and FWHM about 0.18 eV. The small relative displacement of the two bands can be exploited to gain some additional information regarding the NBOHC in the here studied samples. To further investigate the results three normalized PL spectra recorded at different radial positions at the dose of 300 Mrad, are shown in Measurable spectral differences are observed in Figure 3.52(a). At 39 µm from the center, i.e. in the cladding, the PL spectrum agrees with the data from literature on Si-related NBOHC [22]. In the outer zone of the core, 27 µm from the center, the emission band is red-shifted and broadened, reaching spectral characteristics that are closer to the ones of the Ge-NBOHC [22]. However, we don't exclude the coexistence of the two types of defects. Finally in the inner zone, 12 µm from the center, the PL band is quite similar to the one in the outer part but with a small blue-shift and a slightly smaller FWHM. This suggests that the Si-NBOHC component is relatively more important than the Ge-NBOHC one if compared with the previous case. The NBOHC spectrum of the OF center is also an intermediate case. However the Fluorine concentration here is not constant and well defined. Since the Ge content is essentially constant for radii lower than ~32 µm, these results suggest that the Fluorine co-doping seems to facilitate the creation of Ge-NBOHC, or to improve their stability, or to enhance their emission. To summarize, Figure 3 It is interesting to notice that the displacement of the maximum of the emission band is higher in the outer zone of the core where the Fluorine concentration is the greatest.

Our data evidence a complex scenario regarding NBOHC, in particular the Ge-NBOHC, and Eʹ(Ge) as emerges comparing the results of the present paragraph and of the previous one. First of all, the sub-linear growth kinetics with the dose of both defects suggest that they could be generated by the breaking of the Ge-O-Ge(Si) linkages, as previously suggested in ref. [151]. Anyway, since in the GeFDi OFs the Ge-NBOHCs are generated with higher concentration in the co-doped zone with respect to the inner part of the core (see Figure 3.50(a)) and since the Ge contents of these parts are the same, a role of Fluorine in the generation or in the increase of the post irradiation stability of NBOHCs should be guessed and need to be deeper investigated. In addition, it must be said that if the excess of Eʹ(Ge) were related to the NBOHC excess located in the co-doped region, the Eʹ(Ge) concentration in this zone would increase with respect to the calculated one, that considered the defects uniformly distributed in the Ge doped zone. This means that the Eʹ(Ge) concentration would be higher than a factor 3 in the co-doped volume with respect to that reported in Figure 3.47. In this context, if the role of F is related to Ge-bonded atoms, it would be important to improve the knowledge regarding the presence (or the content) of Ge-F linkages nearby Ge-O-Ge(Si) linkages, their radiation response and the behaviour of the F atoms eventually released by a broken Ge-F bond in terms of diffusion and reaction with other defects or sites (Ge-O-Ge, Ge-O-Si and Si-O-Si) of the network.

Ce-codoping of Ge-doped OFs

In this section we present the results concerning the effects of Ce-codoping on the radiation response of a Ge-doped optical fiber. In this study the RIA experiments were performed at the same time on the GeCeD2 and GeD2 OFs to compare their radiation sensitivity. Moreover, the effects due to changing the drawing conditions were investigated and EPR measurements were carried out.

The samples prepared for the EPR measurements were γ-ray irradiated together to assure the same irradiation conditions and the deposition of the same radiation doses.

Ce-codoping of Ge-doped OFs 121 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Finally, we present some results obtained by CML on the GeCeD2 OF which can provide complementary information to understand the microscopic radiation induced mechanisms.

RIA study

In Figure 3.53 we report the RIA spectra of the GeCeD2 and of GeD2 OFs irradiated at room temperature with a dose-rate of 5 krad/s up to 100 Mrad. As in the case of the PCe OF, the presence of the Cerium co-dopant causes the pristine fiber to have a very high optical absorption in the UV domain. As a consequence, the RIA spectra cannot extend much on the high energy side, as shown in Figure 3.53(a). However, by comparing Figure 3.53(a) and (b) it can be observed that Ce-codoping changes the radiation response of the optical fiber in the investigated visible region. In details, at all irradiation doses the GeCeD2 OF shows a higher RIA than the GeD2. The irradiation does not cause the appearance of any well-defined absorption band in the explored range and no reliable decomposition is here suggested. In order to highlight the differences of the radiation responses of GeCeD2 and GeD2 OFs, in The greatest differences in the RIA spectra are observed at the high wavelength side of the investigated spectral region, in which the GeCeD2 OF is very sensitive to the radiation compared to GeD2. Moreover, from the comparison of RIA reported in Figure 3.55, it is seen that the RIA of GeCeD2 grows linearly with the dose below 1 Mrad, then the growth rate decreases without reaching a level RIA value, at variance to GeD2 OF, and keeps growing with a sublinear dependence on dose up to the maximum explored dose of 100 Mrad. Finally, in Figure 3.56 the results obtained in the study of the effects on radiation sensitivity of changing the drawing conditions of the OFs doped with Ge and Ce are reported. The RIA spectra of the GeCeD1, GeCeD2 and GeCeD3 OFs at 600 krad and 36 Mrad overlap in the investigated wavelength domain. As in the cases of PDi, GeDi and GeFDi no significant differences are induced by the different drawing conditions. These results evidence that independently of the doping and codoping the drawing speeds investigated are absolutely equivalent concerning the optical radiation sensitivity of the produced fibers.

EPR study

In this section the EPR study of the effects of irradiation on the GeCeD2 OF is reported and compared to the one on the GeD2 OF. For this investigation the samples were γ-ray irradiated at four different doses from 100 to 1000 Mrad. It is worth to note that this dose range limitedly overlaps with that investigated by RIA.

In Figure 3.57, we show the EPR spectra recorded to detect the presence of the Ge(1), Ge(2) and E'(Ge) centers in the irradiated GeCeD2 and GeD2 OFs at different doses.

Chap. 3: Study of Radiation effects on P-, Ge-and N-doped OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ The content of defects of the pristine sample is negligible compared to the induced ones in both GeCeD2 and GeD2. Major differences are observed in the shapes of the EPR signals corresponding to the samples irradiated at 100 Mrad, whereas at higher doses the spectra become more alike apart from the spectral region above 3472 G. In order to understand the origin of such differences we performed a decomposition of the spectra as in § 3.2.1.2. First of all, in Figure 3.58 we show a detail of the spectra of GeCeD2 and GeD2 corresponding to the maximum dose of 970 Mrad. The feature at 3480 G, which is observable in the spectrum of the GeD2 OF, is an isolated spectral feature belonging to the Ge(2) defect [65]. Such feature is not visible in any of the spectra of the GeCeD2 irradiated samples, so it must be concluded that no Ge(2) is induced in GeCeD2 or their concentration is below the instrumental detection sensitivity. Therefore, the decomposition of GeCeD2 EPR spectra were performed without employing the EPR line of Ge(2).

In Figure 3.59 the growth kinetics of the Ge(1), Ge(2) and E'(Ge) are shown. With the increase of the irradiation dose we observe a similar increase of E'(Ge) for both the samples with systematically larger concentration in GeCeD2. In fact, E'(Ge) becomes the dominant recorded defect in these fibers in the explored dose range, explaining the similarity of the EPR lineshapes corresponding to the radiation dose of 970 Mrad.

Moreover, increasing the dose causes a slight decrease of the Ge(1) and Ge(2) defects in GeD2 and of Ge(1) defect in GeCeD2. This tendency suggests that these defects are converted into other centers, an aspect that needs to be further investigated.

All the EPR results obtained on the GeCeD2 OF are shared by GeCeD1 and GeCeD3, confirming that the drawing speeds employed have no relevant radiation sensitivity differences.

CML study

In this section we report the results obtained by CML measurements on the GeCeD2 OFs.

In Figure 3.60, four normalized PL spectra are shown corresponding to the center and external part of the core of the pristine OF and the one irradiated at 970 Mrad. The laser excitation wavelength is 325 nm and the reported spectra show an asymmetric emission band peaked around 500 nm. This emission is detected only in the core of the OF (the corresponding cartographies are shown in Figure 3.61(a)), where the Ge and Ce are present. In order to compare the shapes of the curves the spectra were normalized at their maxima. No shape variation of the PL emission is detected either in different parts of the core of the fiber or in the irradiated samples. As a consequence we have to consider that either the emission band belongs to a single defect or that the observed PL is due to more than one defect whose relative concentrations are not affected by the irradiation. 128

Chap. 3: Study of Radiation effects on P-, Ge-and N-doped OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ Figure 3.61(a) shows the cartographies of the GeCeD2 samples. We notice that the intensity of the emission band is higher in the external part of the core, where the concentration of Ce is lower. This phenomenon can be due to clustering tendency of cerium already observed in pure silica [82]. Moreover, in Figure 3.61(b) we report the change induced by irradiation of the intensity of the PL band. For doses lower than 500 Mrad the band decreases monotonically, reaching about 40% of the value corresponding to the pristine sample. At higher doses, the PL intensity is essentially constant.

If we consider that the PL band at 500 nm is related to the Ce 3+ centers present in the Ge-doped silica matrix, as can be hypothesized on the basis of literature data [81,107], the reduction can be assigned to the radiation induced conversion of Ce 3+ in Ce 4+ ions. The electron lost by the Ce 3+ ion can then create an electron defect or recombine with a hole defect.

On the basis of the CML experiments, we can make few considerations regarding the GLPC centers.

In fact, in the previous section we have shown that the GLPC have an emission band at 400 nm which is excited by the 325 nm laser light. We could imagine that such emission should be present in the spectra of the GeCeD2 OFs too. However, this band cannot be isolated and, in addition, the shape of the PL does not change in different parts of the core of the OFs even if the relative concentration of the dopants does. Furthermore, the irradiation does not produce any variation of the shape of the PL emission which means that if the GLPCs were present in the OF, their relative concentration changes exactly as the relative concentration of the Ce 3+ ions. On the other hand, we know by EPR measurements that no Ge(2) defects are induced in the GeCeD2 OFs and that the proposed microscopic model of Ge(2) identifies the GLPC defect as its precursor [START_REF] Leone | Silicon-based Materilas and Devices[END_REF]. On the basis of all these evidences it is reasonable to deduce that the Ce-codoping of Ge-doped OF strongly impacts the creation of GLPCs in the pristine OF during the production process, , since an unbalanced change of the CML spectral profile should be observed if GLPCs were unchanged by irradiation and Ce 3+ emission not. As a further study carried out by CML measurements, in Figure 3.62 we report a typical NBOHC spectrum detected in the GeCeD2. In Figure 3.63(a), we show the cartographies of NBOHC PL band at several radiation doses and that of the pristine samples featuring a negligible content of this defect compared to the irradiated ones.

Chap. 3: Study of Radiation effects on P-, Ge-and N-doped OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ Moreover, the shape of the cartographies shows that the irradiation causes a much higher increase of NBOHC in the pure silica cladding of the OFs than in the core. This effect can be well explained by the presence of the Ce co-dopant in the core of the OF. In fact, the conversion of Ce 3+ ions in Ce 4+ can provide electrons that can bleach some of the radiation induced NBOHC. Finally, in Figure 3.63(b) we report the relative growth kinetics of NBOHC in the GeCeD2 OF as a function of the dose in the core and in the cladding of this OF. At all doses the intensity of the radiation induced NBOHC is twice larger in the cladding of the optical fiber than in the core.

Conclusions of Chapter 3.2:

Radiation effects on Ge-doped OFs

As in the case of the P-doped OF, we have demonstrated that changing the drawing conditions in the typical range of specialty OFs does not affect the radiation response of Ge-doped, Ge-F-doped and Ge-Ce-doped OFs. Therefore the drawing process can be performed at highest speed without impacting the radiation response of the OF.

We have investigated the effects of ionizing radiation at room temperature on germanosilicate OF codoped with fluorine. In order to highlight the effects of the codopant we have also performed complementary studies on the well-known codopant-free Ge-doped OFs and reported some previous results for comparison. The addition of fluorine has not affected significantly the RIA in the UV-visible domain. The radiation response of this fiber seems to be primarily controlled by the native concentration of GLPC centers, which is similar in the F-codoped and F-free samples and that decreases with radiation dose. Only two irradiation-related effects can be attributed to Fcodoping. The first one is a higher induced concentration of E'(Ge) centers. The second is a higher CML signal related to NBOHC defects in the core region where the concentration of fluorine is maximal. A correlation between these two effects can be suggested. However, since the EPR measurements of E'(Ge) are not local, we could not prove it strictly.

We have also investigated the effects of ionizing radiation at room temperature on germanosilicate OF codoped with cerium. Our measurements have shown that Ce-codoping increases the RIA in the 3.2.2 Ce-codoping of Ge-doped OFs 131 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ visible range. At the same time, by EPR, we have found that in the Ce-codoped samples no Ge [START_REF] Origlio | Properties and Radiation Response of Optical Fibers: Role of Dopants[END_REF] defects are induced by irradiation, whereas the concentrations of E'(Ge) and Ge(1) are similar to the ones induced in the Ce-free samples. This result confirms some previous studies reported in literature which indicated that Ge(2) is a hole center. The CML study has shown a photoluminescence band in the visible range which is assigned to the Ce 3+ ions. We notice that the intensity of this band decreases in the region where the concentration of the codopant is the highest.

Such phenomenon is probably due to the reported tendency of cerium to form clusters at "high" concentrations. Under irradiation the intensity of this PL decreases which corresponds to the known conversion of the Ce 3+ ions to (Ce 3+ ) + . The released electrons are probably involved in the creation of the Ge(1) centers. We have remarked that no shape differences can be observed in the PL band of the irradiated samples with respect to the pristine ones. This fact suggests that luminescent band associated with GLPC (potentially overlapped to the one of Ce 3+ ) is negligible compared to the PL of Ce 3+ and maybe absent. In the latter case the creation of Ge(2) would be forbidden in the microscopic model of this defect which consider the GLPC as its precursor. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Radiation effects on N-doped doped OFs

In this section we report and discuss the experimental results obtained for the ND1, ND2 and ND3 OFs. RIA, EPR and CML measurements were performed in order to characterize the responses of the OFs under ionizing irradiation, and to identify the presence of specific point defects and their formation mechanisms. The effects of changing the drawing conditions of the OFs are also investigated.

Study of Radiation effects vs drawing

In Figure 3.64 we report the RIA spectra of the ND2 OF X-rays irradiated at several doses. During the irradiation the temperature inside the irradiation chamber varied between 26 and 30 °C. The dose-rate was 5 krad/s and the irradiation time was 20000 s long, i.e. the fiber was irradiated up to 100 Mrad. The transmitted spectrum of the pristine fiber does not extend below 280 nm probably because of the strong absorption band related to the ODC(II) that this kind of OF usually has [123].

Consequently, the reported RIA spectra cannot reach wavelength shorter than 290 nm. It is evident that the irradiation causes a strong increase of the absorption of light in the UV-Visible domain. The microscopic structure of the radiation induced defects for N-doped OFs are still debated [123].

However, it is important to mention that our sample, as reported in § 2.5.3, has a high content of Cl impurities and the RIA spectra in the investigated spectral domain are probably the result of the absorption due to both N-related and Cl-related radiation induced centers [START_REF] Pacchioni | Defects in SiO2 and Related Dielectrics[END_REF]. In order to study the effects of changing the drawing conditions of the OFs, the RIA experiment was performed on the ND1 and ND3 samples as well, irradiating them simultaneously to ensure the same irradiation conditions. In Figure 3.65 we show the RIA of the three OFs at two different doses: 600 krad and 100 Mrad. In the case of the N-doped OF we can observe significant differences among the ND3 OF and the other two. xi Beside the fact that all other types of OFs have shown to be insensitive to the change of drawing conditions, the results here reported are even more remarkable if we consider that for this fiber the drawing speed varied only from 30 m/min (ND1) to 20 m/min (ND3) with a corresponding tension variation from 85 to 45 gr (see § 2.5.3). The fiber which has been drawn slowly shows a slightly higher radiation resistance. In detail, at low doses the RIA of the ND3 OF is 80% lower than the other two in the studied spectral domain. At high doses, the ND3 OF shows a 50% lower RIA around 300 nm and a 80% lower RIA around 500 nm. In Figure 3.66 we show the RIA growth kinetics at two wavelengths, 300 and 450 nm, for the three studied OFs. All the fibers are characterized by a first sublinear increase of the RIA up to ~100 krad, a subsequent plateau for doses in the range from 200 krad to 2 Mrad and again a sublinear increase for higher doses. In agreement with the results of Figure 3.65, the ND3 OF shows lower RIA values at all doses.

xi A repetition of the experiment confirmed the results reported in this section.

Chap. 3: Study of Radiation effects on P-, Ge-and N-doped OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

EPR study

In this section we present the results of the EPR measurements on the γ-ray irradiated ND1, ND2 and ND3 OF samples. For this study the measurements were carried out a few weeks after the irradiation. As in the case of the previously studied OFs the paramagnetic defects detected by EPR should be considered as the stable ones leftover by the recovery process which takes place after the irradiation.

In Figure 3 In Figure 3.68 we show the relative concentration of the =N-Si • = in the ND1, ND2 and ND3 OFs irradiated at four doses. We observe that no saturation level is reached in the investigated dose range.

Moreover, in contrast with the RIA results obtained during the irradiation, the concentrations of these stable defects in the three types of samples are comparable. On one hand, we need to point out that =N-Si • = is not necessarily a defect absorbing light in the spectral domain we investigated with our experimental setup. On the other hand, the post-irradiation recovery processes can be responsible for the similarity between the residual defect concentrations in the three differently drawn OFs. In fact, it should be necessary that the most irradiation sensitive fibers could also feature Chap. 3: Study of Radiation effects on P-, Ge-and N-doped OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ a faster recovery. This latter aspect could be clarified by recording the RIA recovery in future experiments and deserves more investigations.

In Figure 3.69(a) we report the EPR spectra of the =Si-N • defects in the γ-irradiated ND2 samples.

In order to record such spectra we had to perform the measurements at 77 K and with a sufficiently high microwave power to saturate and neglect the overlapping EPR signal of H(I) centers. In order to compare the relative concentration of =N • we have evaluated the area of the negative peak around 3445 G after subtracting an appropriate baseline. In Figure 3.69(b) the growth kinetics of this defect is reported. The concentration of =N • increases with the dose without saturating. The measurements performed at room temperature on the ND1 and ND3 OFs (not shown) did not present significant differences with the one of ND2 and, consequently, the growth kinetics of Figure 3.69(b) is representative of the radiation induced =N • in all these three fibers. Basing on this result the differences observed by the RIA measurements for the three drawings are not confirmed by these paramagnetic defects, as already noticed for the =N-Si • =. In this context it is worth to underline that EPR measurements of N-related defects and RIA measurements are both related to the core region of the fibers, so the found differences are reliable and should be further studied.

EPR study 139 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

In Figure 3.70(a), we report the EPR-signals of E'(Si) centers in the irradiated ND2 OF. For these samples we preferred to perform the measurements in the first harmonic mode since, at high doses, the signal of the E'(Si) centers looks quite broadened. In order to highlight the lineshape differences, in Figure 3.70(b) we report the spectra of the four irradiated samples of the ND2 OF after normalizing them for the value of the corresponding double integral. Furthermore, the spectrum of the E'(Si) recorded in the F2D3 OF is reported for comparison. xii It is observed that on increasing the dose a gradual broadening occurs and that the lineshape in the N-doped samples is always broader than in the F-doped one. xii The study of the F2D3 OF is reported in § 4.1.2 regarding the O2 loading treatment. However, the corresponding EPR spectrum of E'(Si) is here reported to provide an example of a non-distorted E'(Si) signal. As found for the other studied defects in these fibers, the growth kinetics of E'(Si) do not reach a leveling. Moreover no significant differences are noticed among the ND1, ND2 and ND3 OFs, considering the experimental errors. It is worth to observe that the highest concentration value is about 7·10 17 defects/cm 3 , not large enough to explain a dipolar broadening arising from E'(Si) centers interaction alone.

The broadened lineshape of the E'(Si) signals can however be due to the dipolar interaction between E'(Si) and all the other paramagnetic defects induced by irradiation in the sample. Usually such condition can be realized only if high defect concentrations are produced [152]. To this end, it must be said that no broadening of E'(Si) signal was observed in any of the other studied OFs (P-doped or Ge-doped) in which a total concentration of paramagnetic defects as high as 2·10 18 defects/cm 3 can be reached. However, we must remind that the EPR measurements only provide an average estimation of defects in the whole OF (or in the core of the OF when we deal with dopant-related defects) and it is always possible that a major fraction of the defects is found in a small region of the OF in which, as a consequence, very high defect concentrations are reached justifying the dipolar broadening. Alternatively, we can imagine that, for structural reasons, in the N-doped zone of the OFs the E'(Si) are created relatively close to other paramagnetic defects so that a dipolar interaction between them can take place.

Attenuation of the γ-rays irradiated ND2 OF

In this section we present the results concerning the attenuation in the IR domain of the ND2 OF γrays irradiated at four different doses. In this spectral range, the attenuation of the pristine OF is small enough that we can consider it negligible compared to the attenuation induced by γ-rays in the investigated dose range.

In Figure 3.72 we report the IR attenuation spectra of the irradiated ND2 OF obtained by means of the cut-back technique. We observe that all the spectra are composed by an absorption tail at short wavelengths and an unresolved broad absorption at longer wavelengths. In In Figure 3.74, we show the correlation curves between the attenuation around 1.5 µm and the EPR growth kinetics of >N-Si • =, =N • and E'(Si). Our results do not highlight a clear association of the IR absorption band with a specific paramagnetic defect of the N-doped OFs, as it was possible doing in the case of the P1 defect in the P-doped OFs.

CML study

In this section we report the results obtained by CML measurements on the ND3 OF, which is the fiber that has shown the minimum RIA values. The data refer to two samples, the pristine fiber and the γ-rays irradiated sample at 970 Mrad.

In Figure 3.75(a) we show the CML spectra of the core of the ND3 OF excited at 325 nm, before and after the γ-rays. The pristine sample shows a PL band around 400 nm which was previously observed by Dianov et al. [123], and tentatively associated to a Si-ODC center (like the ODC(II))

neighboring a nitrogen atom. The cartography associated to the PL emission at 400 nm is shown in whereas the intensity of the PL changes by at least a factor 5. As a consequence, it could be suggested that the luminescence originates from a microscopic structure involving two N atoms or ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ from a strain-related type of defect. Since the samples have similar concentration profiles of N and Cl (see Figure 2.17) it is also possible that the PL originates from a defect in which an N atom and a

Cl atom are involved. A spectrum of the core of the irradiated ND3 sample is shown in Figure 3.75(a). The irradiation causes the appearance of a new emission band around 560 nm which is excited at 325 nm and which will be discussed later on. Concerning the emission band around 400 nm, we observe that in the sample irradiated at 970 Mrad by γ-rays the PL increases, in particular, by a factor 4 in the outer part of the core and a factor 2.5 in the inner one, as shown in Figure 3.75(b). As a consequence the ratio between the intensity of the PL in the inner and outer parts of the core is about 3.4 in the irradiated ND3 OF. The irradiation is expected to reduce the number of strain-related defects, converting them in some others, and consequently the PL around 400 nm should be due to some other kind of defect involving nitrogen or chlorine or both.

In Figure 3.76(a), we again show the CML spectra of the pristine and irradiated ND3 samples. The spectrum corresponding to the pristine sample has been rescaled by an appropriate factor in order to match the peak amplitude at 400 nm. Finally, the spectrum of the pristine ND3 OF has been subtracted from the one of the irradiated ND3 in order to isolate the PL band peaked at 560 nm. It The PL band at 560 nm is present only in the N-doped region of the irradiated OF. Similarly to the band at 400 nm, the intensity of the emission at 560 nm in the inner part of the core is 3.3 times higher than in the outer zone. However, this two emission bands cannot belong to the same point defect since the pristine OF shows only one of them. Anyway, it could be reasonably stated that also the band at 560 nm is related to some N related defect.

In Figure 3.77(a), we report the CML spectra of the ND3 OF irradiated at 970 Mrad and excited at 442 nm. This sample shows a bright emission band centered at 680 nm whereas the pristine one has none. After converting the wavelength spectrum to photon energy spectrum and correcting for the spectral dispersion xiv , the resulting PL band is centered at 1.82 eV with a FWHM of 0.44 eV. The cartography corresponding to this emission is reported in Figure 3.77(b).

xiii The spectrum was not corrected for the instrument response. xiv The spectrum was not corrected for the instrument response. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ As in the case of the previously reported emission bands, the diametral cartography of the PL band peaked at 680 nm is related to the N dopant (as well as the Cl impurities). Once again the intensity of the PL in the inner part of the core of the OF is 3.3 times higher than in the outer one. In order to compare the cartographies among them and with the concentration profile of nitrogen reported in Figure 2.17, in Figure 3.78 we report all of them after having performed an opportune normalization. The cartographies of the emission bands at 400, 560 and 680 nm overlap very well. This fact suggests the existence of a single basic mechanism that involves the creation of the three luminescent defects but their origin has not been clarified yet.

Conclusions of Chapter 3.3:

Radiation effects on N-doped OFs

In this thesis we have also investigated the radiation effects on nitrogen doped OFs at room temperature. We characterized the RIA up to 100 Mrad in UV-visible range. By EPR we could detect two N-related defects (=N-Si • = and =N • ) in the γ-irradiated samples. On the other hand, the known EPR signal associated with an electron trapped on tetra-fold coordinated nitrogen was not observed in our samples. The CML study has revealed several luminescence bands. The pristine samples show a luminescence at 400 nm when excited at 325. This PL band was first observed by Dianov et al. and tentatively assigned to a ODC(II) center neighboring a nitrogen atom [123]. After irradiation two more emission bands can be detect. One is peaked at 560 nm and is excited at 325. The second one can be excited at 442 nm and is peaked at 680 nm. The spatial correlation between the nitrogen concentration profile and the CML cartographies enable us associate these two PL bands to Nrelated centers. It is worth to mention that, to the best of our knowledge, such PL bands have not been reported in literature yet. As a final remark concerning the N-doped OFs, we must mention that for this type of OF, whose preform was produced by SPCVD, a detectable difference in the RIA was observed due to different drawing conditions. In particular, we have noticed that the OF drawn at the lower speed shows a roughly 70% lower RIA at all the radiation doses in the investigated spectral domain. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Chapter 4 4 Study of O 2 -loaded OFs

In this chapter we report and discuss the results obtained on the OF samples that were loaded with molecular oxygen at high temperature (390 °C) and high pressure (~90 atm) with the treatment apparatus described in § 2.4.

Tomashuk et al. have recently produced and studied some single-mode OFs with pure-silica or Fdoped silica core and F-doped silica cladding that were synthesized by MCVD-method varying the O 2 and SiCl 4 molar flow rates of the vapor-gas mixture during the fabrication process of the preform core [153]. They found that OFs produced in O 2 excess conditions are more radiation resistant in the NIR spectral range (from 1 to 1.7 μm) than the ones produced in standard conditions. Furthermore, Yoo et al. have reported that Yb-doped aluminosilicate OFs designed for lasing application and suffering from photo-darkening effects are considerably improved by O 2 loading because of the suppression of Yb-related oxygen deficient centers [154].

In the first section of this chapter we report evidences of the effectiveness of the loading treatment on the whole set of OFs. In the subsequent parts we discuss the specific influence that the O 2loading treatment has on the radiation response of the different fiber typologies. In this chapter we also present some results concerning the investigated near infrared radioluminescence of O 2 molecules loaded in PSC and F-doped OFs. After the O 2 -loading treatment the appearance of a narrow band around 1550 cm -1 is observed, which is due to the vibrational motion of the interstitial oxygen molecules that diffused into the OF.

No other spectral modifications are detectable, suggesting that no changes are induced in the amorphous matrix by the applied treatment.

As previously mentioned, the same O 2 -loading was performed on several OFs: SiD3, F2D3, PD2 and GeD2. In none of these fibers the O 2 -Raman band was detected before the treatment. In In all the panels of Figure 4.2, we report, as an inset, the magnified spectral region where the presence of the O 2 -Raman band is clearly visible. All the Raman spectra were recorded using the excitation wavelength of 325 nm. As a consequence, the graph in Figure 4.2(d), corresponding to the GeD2-O2 OF, is affected by the presence of the tail of the PL band of the GLPC centers, which is peaked around 400 nm [155]. For this reason, in the corresponding inset a straight baseline was subtracted in order to highlight the presence of the O 2 Raman band.

On the basis of literature data [START_REF] Norton | [END_REF]141], the thermal treatment conditions (temperature, pressure and duration) were chosen with the intent to load as much O 2 as possible, and to reach an almost constant profile of O 2 concentration along the fiber diameter. To verify this latter condition, in As expected, the concentration of O 2 molecules along the diameter of the OFs is fairly constant.

Finally, the problem of the permanence of O 2 into the OF was addressed [START_REF] Francesca | Studio del processo di diffusione di O2 a temperatura ambiente in nanoparticelle di silice[END_REF]156]. Unlike molecular hydrogen, which at room temperature can diffuse out from the fibers in few days [START_REF] Doremus | Diffusion of reactive molecules in solids and melts[END_REF], the studies on the diffusion coefficient of O 2 suggest that no relevant outgassing of molecular oxygen could take place at room temperature in the lengthscale of the fibers. In Since the doping levels of the studied samples are relatively low (<8%mol), the results obtained on the SiD3 OF can be considered well representative of the outgassing process in all the O 2 -loaded OF.

Irradiation effects in O 2 -loaded PSC and F-doped OFs

In this section we report the study of the radiation response of the O 2 -loaded PSC and F-doped OFs with respect to the corresponding unloaded ones. In fact, these two fiber typologies are very similar and are considered among the most radiation resistant for many applications including those at high doses of radiation [48]. The F-doping of the core does not introduce new absorption bands in the investigated UV-visible range with respect to the PSC OFs. We observe a continuous increase of the RIA in the UV-Visible range for both OFs, with higher efficiency in the case of the O 2 -loaded one. In order to compare the growth kinetics in the UV domain, in Figure 4.6 we show the RIA of the two OFs at the specific wavelength of 240 nm. At all the irradiation doses, the SiD3-O2 OF shows at least 2.5 higher RIA than the SiD3 OF. Chap. 4: Study of O2-loaded OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ Similarly to the previous case, at all doses, the F2D3 OF shows lower RIA spectra than the corresponding O 2 -loaded F2D3-O2 even if the growth kinetics are very alike. Figure 4.8 shows the RIA growth kinetics at 240 nm of the two OFs. As in the previous case, the F2D3-O2 OF shows a 2.5 higher RIA than F2D3. In order to compare the radiation responses of the four tested OFs, in Figure 4.9(a) the RIA spectra at the maximum dose are reported evidencing the larger RIA of the O 2 loaded OFs. In Figure 4.9(b), we report the attenuation differences between the O 2 loaded OFs and their corresponding unloaded ones. From these measurements, it is clear that the O 2 pre-treatment in its current form increases the two fibers radiation sensitivities in the investigated spectral domain. The RIA measurements were further studied by band decomposition of the spectra. In the following we report the results of such analysis for the F2D3 and F2D3-O2 OFs. At each irradiation dose, the fitting of the data was performed by a least square linear regression based on a set of absorption bands, known from literature, with fixed shape and position and with opportune amplitude. xv Required and used only for the fitting of F2D3-O2.

xvi From ref. [20].

xvii The spectral parameters fall within the variability of the values reported in literature. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ 4.1) are collectively named "Other defects".

In Figure 4.11, the growth kinetics of the amplitudes of the absorption bands of E'(Si), NBOHC and O 3 , obtained by the above reported decomposition, are reported as a function of the dose. In F2D3-O2 the concentration of NBOHC is 4.8 times higher than in F2D3 at all doses. The E'(Si) centers, on the other hand, have a quite different evolution in the two cases. At low doses the O 2 -loaded optical fiber shows a faster generation than the unloaded one. Around 10 Mrad, the concentration of E'(Si) centers is the same in the two OF and above this value the F2D3-O2 sample has less E'(Si) ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ than F2D3. In particular, at the maximum dose of 100 Mrad, F2D3 has 1.5 times more E'(Si) than F2D3-O2; moreover, its growth kinetics above 10 Mrad is proportional to the dose 1/2 , whereas for F2D3-O2 the growth follows a dose 1/3 law. Finally, it is worth noticing that the concentration of ozone molecules, present only in the case of the O 2 -loaded OF, does not reach a saturation level in the investigated radiation dose range.

To further investigate the role of interstitial O 2 in the irradiation effects on optical fibers, -rays irradiations have been carried out and the samples have been studied by EPR measurements.

In Figure 4.12, we report the EPR spectra relative to the F2D3, F2D3-O2, SiD3 and SiD3-O2 OFs γ-rays irradiated at 1 Grad. Each spectrum has been normalized by the mass of the respective sample in order to compare the intensities of the detected signals which are then proportional to the defect concentrations.

In Figure 4.12(a) and In Figure 4.12(c) and Figure 4.12(d), the signals due to the NBOHC and POR are reported (see § 1.2.1). In order to detect these two centers the measurements were performed at 77 K. The intensity of the signal of F2D3 and SiD3 is roughly ten times lower than in F2D3-O2 and SiD3-O2. In Figure 4.12, the spectra of the former fibers have been multiplied by a factor 12 for better representation.

At low magnetic field values a feature of the POR signal is present [START_REF] Pacchioni | Defects in SiO2 and Related Dielectrics[END_REF], highlighted in the insets of Chap. 4: Study of O2-loaded OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ Fig. 4.12 c) and d), whereas at large field the features associated to NBOHC and POR overlap. The experimental conditions we used should have enabled the detection of interstitial HO 2  molecules embedded in the silica matrix [157,158]. However, the EPR signature of such defect has not been detected, suggesting that HO 2  is either absent or below our detection limit. This finding evidences a low availability of H related species in the used fibers, in agreement with the dry typology of used silica. Both RIA and EPR spectra show a higher concentration of NBOHC in the O 2 -loaded samples. By contrast, it must be noticed that EPR spectra reveal also a considerable increase of POR defects in the O 2 -loaded samples which has no correspondence in the analysis by decomposition of the RIA spectra. In fact, the amplitude of the band at about 5.3 eV in Figure 4.10, associated to POR, is the same independently on the presence of molecular oxygen in the matrix. Such difference could arise from the different types of irradiation and different irradiation temperatures (as mentioned in the previous chapter). Furthermore, it must be taken into account that EPR measurements were performed two months after the γ-irradiation and, during this period, recovery and conversion processes could have taken place. As a consequence, a strict correspondence between post mortem γ-rays irradiated samples EPR measurements and online RIA measurements could not apply, in particular for the overall content of POR. A dedicated study is required.

To microscopically explain the observed results it is worth to consider that, as reported in previous studies for bulk silica, the O 2 molecules, loaded into the fibers, could interact with the radiation induced defects [START_REF] Kao | Proc. IEE[END_REF]10]. In particular, the decrease of E'(Si) centers can arise from several processes.

First of all, the O 2 treatment could modify the content of oxygen deficient centers of the pristine optical fiber that are precursors of E'(Si) defect, as the oxygen vacancy ≡Si-Si≡. Furthermore, during the irradiation, E'(Si) centers could interact with the available O 2 molecules by the following processes [START_REF] Pacchioni | Defects in SiO2 and Related Dielectrics[END_REF]:

O 2 → 2 O 0 4.1 ≡Si·+O 0 → ≡Si-O· 4.2 or ≡Si-O • +O 0 → ≡Si-O-O • 4.3
The first mechanism well agrees with the experimental evidences. Indeed, a higher concentration of NBOHC was expected and confirmed both by reported RIA and EPR measurements. The second one, on the other hand, would lead to a higher concentration of POR defects in the O 2 -loaded fibers 4.1.3 O2 loading of P-doped OFs 163 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ and, as a consequence, a higher intensity of the associated absorption band, which was not observed in the RIA, as above reported, but it was for EPR data. As already stated, this aspect deserves further investigation, especially considering the not fully clarified attribution of the absorption band of POR. Furthermore, the RIA spectra of Figure 4.10 clearly show a higher absorption at about 3.5 eV of Clrelated defects in the F2D3-O2 sample compared to the F2D3. Whether the absorption band is assigned to Cl 0 or Cl 2 [42,56], the mechanisms suggested for the generation of Cl related species are: variance, some applications (diagnostic for example) require the optical fiber to be sensitive to radiation in certain spectral domains. In these cases, depending on the OF application, O 2 -loading could be exploited to increase the radiation sensitivity.

O 2 loading of P-doped OFs

In this section we report the results concerning the effects of O 2 loading on the radiation response of the P-doped OFs. First, we present the RIA measurements performed on the PD2-O2 OF.

Successively, we report the results concerning the EPR measurements performed on the PD2-O2

Chap. 4: Study of O2-loaded OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ OF γ-irradiated at 1 Grad. In order to highlight the differences between the O 2 -loaded samples and the unloaded ones, all the experiments were carried out in parallel on the PD2 OF as well.

In Figure 4.13, we report the RIA spectra of the PD2-O2 and the PD2 OFs X-rays irradiated at room temperature and with a dose-rate of 5 krad/s up to 100 Mrad. The RIA spectra of the PD2-O2 and the PD2 are very similar and, to highlight the differences between the samples, in Figure 4.14 we show two spectra of each fiber corresponding to 200 krad and 13 Mrad. RIA spectrum of PD2-O2 is not higher than the one of PD2 at all wavelengths), it is reasonable that the O 2 presence has impacted the absorption bands of some other defects, such as P2 and E'(Si), making them decrease with respect to the unloaded OF.

At the higher dose of 13 Mrad, the PD2-O2 OF shows a higher RIA in the whole investigated spectral domain. In particular, the presence of O 2 clearly causes a larger concentration of POHC centers in the visible range. It must be noticed, however, that there is not a large change of the shape of the RIA in this spectral range suggesting an almost proportional increase of metastable and stable ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ POHC. By contrast, in the UV range remarkable spectral modifications can be observed, suggesting an even stronger impact of the oxygen-excess related defects on the RIA at high radiation doses. In Figure 4.15, we report the growth kinetics of the RIA of the two samples at three specific wavelengths. At low doses (<500 krad), the RIA curves of Figure 4.15 overlap. Moreover, in Figure 4.14 we show no significant spectral differences in the visible domain. We can deduce that, at low doses, the creation rate of POHC centers is the same in the O 2 -loaded and in the unloaded OFs.

However, at high doses, an excess of POHC centers is detected in the PD2-O2 compared to PD2.

The origin of such excess is discussed after the EPR measurements are presented. Regarding the RIA at 270 nm, small information can be extracted from the growth kinetics. As supported by Figure 4.14, at low doses the RIA is governed by the P-related defects which are present in both OFs, whereas at high doses the RIA increase is caused by the oxygen-excess related ones.

In Figure 4.16, we report the EPR spectra of the PD2-O2 and PD2 OF irradiated at 1 Grad by γrays. In particular, the signals of the POHC, normalized by the sample masses, are reported. Again a P1 defect is converted into a m-POHC. However, it must be noticed that the shapes of both RIA and EPR spectra show that both types of POHC increase in the PD2-O2 OF, and that their increase is almost proportional. The suggested models are not sufficient to justify the increase of the stable POHC and further mechanisms must take place. Since the precursor of the stable POHC is a P atom linked to four O atoms by two simple bonds and two double bonds [99], we should imagine a possible reaction that could increase this type of defect in presence of molecular oxygen. According to Griscom et al. [99], the precursor of the P4 defect is a negatively charged P atom linked to two oxygen atoms by simple bonds and with two lone pair electrons. Although we have never observed the formation of P4 defects by EPR measurements in the irradiated samples, it is natural to imagine that such a center could react with an oxygen molecule as follows:

=P+O 2 → =P(=O) 2 4.9

In this way the precursor of P4, which for unknown reasons has not been observed, is converted to a precursor of the stable POHC.

Independently on the microscopic interpretation, combining these results with the previously reported correlation of the P1 EPR signal and the infrared RIA, suggests that producing P-doped optical fibers in oxygen excess conditions can improve their radiation resistance in the infrared domain.

4.1.3 O2 loading of P-doped OFs 171 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Near infrared radio-luminescence of O 2 loaded OFs

In this section, we report an experimental investigation of the infrared Radio-Luminescence (iRL) emission of interstitial O 2 molecules loaded in PSC and F-doped OFs. In the previous paragraphs, we showed that the O 2 loading treatment accomplished to dissolve high concentrations of oxygen molecules into the silica matrix. As described in § 1.2.1, the interstitial molecular oxygen exhibits an emission band in the NIR domain centered at 0.975 eV (1272 nm), arising from its first excited singlet state, with a FWHM of 0.011 eV and decay time ~0.8 seconds for silica glasses with low-OH content. In this study we investigate the possibility to detect this emission under ionizing environment.

The two OFs that we employed for this investigation are F2D3-O2 and SiD3-O2. The pristine and unloaded OFs do not contain any detectable concentration of interstitial O 2 molecules and no infrared luminescence signature is traceable under X-ray irradiation. At variance a sharp luminescence at 1272 nm is detected when the O 2 treated OFs were irradiated with 10 keV X-rays at room temperature. In Figure 4.19, the iRL spectra are reported for both the studied fibers. In order to detect such signals, 2.5 cm of the tested OFs were transversally irradiated at the maximum radiation dose-rate of Chap. 4: Study of O2-loaded OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ 5 krad/s. The integration time of each recorded spectrum was set to one minute. The spectra clearly evidence the presence of a band peaked at 1272 nm with FWHM of 18 nm. These spectral features are in good agreement with the emission band from the interstitial O 2 molecules in silica detected under Vis and NIR excitation and enables to identify the band with the O 2 PL [START_REF] Carvalho | [END_REF]37].

The radiation exposure is expected to degrade the intensity of the iRL through at least two main processes: the radiation induced conversion of interstitial O 2 molecules into other types of defects (oxygen excess related centers) and the RIA at 1272 nm. To evaluate the total impact of these two effects, the fibers were irradiated at room temperature with a dose rate of 5 krad/s for several hours until the dose of 100 Mrad was reached. In Figure 4.20, we report the evolution of the integrated iRL intensity, estimated by the area of the emission band reported in Figure 4.19, as a function of the deposited dose. In the case of the F2D3-O2 fiber the experiment was performed in one run without any irradiation interruption. The results show a 19% reduction of the signal. . Variation of the integrated area of the O2 iRL at 1272 nm with the X-rays irradiation dose. The dose-rate is 5 krad/s. SiD3-O2 OF was irradiated in two steps with a 65 hours break in between, whereas a single irradiation was performed on F2D3-O2 OF.

In the case of the SiD3-O2 fiber the experiment was performed in two days, with a 65 hours long interruption period, and the overall intensity exhibits a reduction of about 16%. We must point out that during the irradiation the temperature inside the irradiation chamber varied slowly from 26°C to 31°C. Furthermore, it is relevant to underline that at the end of the first irradiation of the SiD3-O2 4.1.3 O2 loading of P-doped OFs 173 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ fiber the temperature was close to 31 °C, whereas 26°C were recorded at the beginning of the second irradiation step.

To complete the study of the observed iRL, we performed short irradiation times (~10 minutes) experiments to underline and characterize both the dose-rate and the temperature influences. In The iRL dose-rate dependence is linear, as evidenced by the straight line, and no saturation phenomenon was observed over the investigated dose-rate range. It is worth noticing that basing on the found linearity, for lower dose rates, both the detection system and the fiber itself (core size and fiber length) can be optimized to extend the range of monitoring. The influence of the temperature on the iRL signal is reported in Figure 4.22. For this study the dose-rate was fixed at 5 krad/s and for each temperature the iRL was monitored for ~10 minutes.

The temperature was varied between an irradiation and the subsequent one. The data show that increasing the temperature strongly reduces the intensity of the iRL and, at 280 °C, this latter falls below the sensitivity of our experimental setup and detection system. After the temperature dependence study, in order to check the O 2 permanence inside the fibers, we performed a new measurement for each fiber at room temperature. As shown in Figure 4.22, the iRL is restored and the signal fully recovers its value before the heating. This proves that the loaded OF performances are not affected by temperature variation up to 280 °C for at least tens of minutes long periods. First, no O 2 outgassing mechanism takes place efficiently for such thermal changes and duration. Second, the possible thermally activated defect-related processes (conversion and recovery)

do not impact the O 2 iRL intensity. The iRL decrease with temperature can be attributed to the intrinsic non-radiative relaxation from the O 2 excited singlet state originating the NIR signal [160],

and to a possible reduction of the population efficiency of this latter state. Further studies are needed to clarify this aspect. It is worth to mention that, on the basis of the temperature dependence of the iRL reported in Figure 4.22, the dose dependence shown in Figure 4.20 is even weaker, taken into account that a 3% reduction of the iRL is due to the temperature increase (from 26 to 31 °C) inside the irradiation chamber between the beginning and the end of the irradiation.

We notice that the iRL band has some very interesting properties that could be exploited for radiodosimetry applications in radiative environments characterized by high dose-rate and high doses as shortly highlighted in the following. The spectral location of the signal emitted is within the high transparency region of the pristine OFs; the infrared domain of interest presents minimum values of ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ RIA in both fluorine-doped and Pure-Silica optical fibers; Cherenkov radiation contribution generated only by high energy photons (> 1 MeV) affects the NIR domain weakly as opposed to the UV-visible one [161,162]; similarly to the other systems that exploit radiation induced luminescence, the NIR emission from O 2 allows for real-time measurements of the radiation dose-rate and, by integration over the irradiation time, of the total radiation dose. Moreover, the sensitivity of this technique can be greatly improved by employing especially designed OFs (with larger cores), longer coiled up lengths and more sensitive detection systems. The results reported suggest that the efficiency of the emission, the spectral range and selectivity should enable safe monitoring of hazardous environment from remote locations.

Conclusions of Chapter 4:

Study of O 2 -loaded OFs

The last part of this thesis concerns the effect of O 2 -loading treatment of some OFs and the subsequent modifications of radiation response. This study was focused on three principal OFs types (P-doped, Ge-doped and N-doped) whose radiation responses were characterized against the variation of some constructive parameters and irradiation conditions. In greater details, we have evaluated the impact of changing the drawing conditions (speed and tension) of the OFs in the range of variation which is characteristic of specialty OFs. Furthermore, we have investigated the effects due to the presence of a codopant in the core of the OFs and the "interaction" of this latter with the point defects usually induced by radiation in the corresponding codopant-free OF. In the case of the P-doped OFs, we have studied the changes in the radiation response and defects generation due to X-ray irradiations performed at temperatures ranging from 25 to 280 °C. Finally, we have reported on the effects of an O 2 -loading treatment that, to the best of our knowledge, was first performed on OFs in the context of this thesis. The response to radiation of some O 2 -loaded OFs was investigated. In contrast to the rest of the thesis, in this last part we have mainly worked on pure-silica-core and F-doped OFs in order to collect that basic information exploitable for future studies on otherwise doped OFs.

In the following we summarize schematically the most relevant results obtained during our experimental investigation.

 In none of the studied OFs, whose preform was produced by MCVD method, we observed any significant difference in the radiation response due to the different drawing conditions. On one hand, our results show that changing the drawing conditions within the range employed for specialty fibers cannot improve the OF radiation resistance. On the other hand, the radiation response of these OFs can be considered robust against the variability of the ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ response of this fiber seems to be primarily controlled by the native concentration of GLPC centers, which is similar in the F-codoped and F-free samples and that decreases with radiation dose. Only two irradiation-related effects can be attributed to F-codoping. The first one is a higher induced concentration of E'(Ge) centers. The second is a higher CML signal related to NBOHC defects in the core region where the concentration of fluorine is maximal. A correlation between these two effects can be suggested. However, since the EPR measurements of E'(Ge) are not local, we could not prove it strictly.

 We have also investigated the effects of ionizing radiation at room temperature on germanosilicate OF codoped with cerium. Our measurements have shown that Ce-codoping increases the RIA in the visible range. At the same time, by EPR, we have found that in the Ce-codoped samples no Ge(2) defects are induced by irradiation, whereas the concentrations of E'(Ge) and Ge(1) are similar to the ones induced in the Ce-free samples. This result confirms some previous studies reported in literature which indicated that Ge(2) is a hole center. The CML study has shown a photoluminescence band in the visible range which is assigned to the Ce 3+ ions. We notice that the intensity of this band decreases in the region where the concentration of the codopant is the highest. Such phenomenon is probably due to the reported tendency of cerium to form clusters at "high" concentrations. Under irradiation the intensity of this PL decreases which corresponds to the known conversion of the Ce 3+ ions to (Ce 3+ ) + . The released electrons are probably involved in the creation of the Ge(1) centers. We have remarked that no shape differences can be observed in the PL band of the irradiated samples with respect to the pristine ones. This fact suggests that luminescent band associated with GLPC (potentially overlapped to the one of Ce 3+ ) is negligible compared to the PL of Ce 3+ and maybe absent. In the latter case the creation of Ge(2) would be forbidden in the microscopic model of this defect which consider the GLPC as its precursor.

 In this thesis we have also investigated the radiation effects on nitrogen doped OFs at room temperature. We characterized the RIA up to 100 Mrad in UV-visible range. By EPR we could detect two N-related defects (=N-Si • = and =N • ) in the γ-irradiated samples. The known EPR signal associated with an electron trapped on tetra-fold coordinated nitrogen was not observed in our samples. The CML study has revealed several luminescence bands.

The pristine samples show a luminescence at 400 nm when excited at 325. This PL band was first observed by Dianov et al. and tentatively assigned to a ODC(II) center neighboring a nitrogen atom [123]. After irradiation two more emission bands can be detected. One is peaked at 560 nm and is excited at 325. The second one can be excited at 442 nm and is peaked at 680 nm. The spatial correlation between the nitrogen concentration profile and the CML cartographies enable us associate these two PL bands to N-related centers. It is worth to mention that, to the best of our knowledge, such PL bands have not been reported in literature yet. As a final remark concerning the N-doped OFs, we must mention that for this type of OF, whose preform was produced by SPCVD, a significant difference in the RIA was ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ observed due to different drawing conditions. In particular, we have noticed that the OF drawn at the lower speed shows a roughly 70% lower RIA at all the radiation doses in the investigated spectral domain.

 The last part of this thesis concerns the effect of O 2 -loading treatment of some OFs and the subsequent modifications of radiation response. First we have shown by micro-Raman spectroscopy measurements that the treatment succeeded to incorporate high quantities of molecular oxygen into the samples. We have estimated a concentration of 2•10 18 molecules/cm 3 . The concentration profile of the O 2 molecules is almost uniform along the cross section of the OF and we have shown that no outgassing phenomenon took place at room temperature in more than 1.5 years long time. RIA experiments were performed on O 2 -loaded Pure-silica-core and F-doped OFs as well as on the corresponding unloaded samples. The O 2 -loaded samples have shown a higher concentration of radiation induced NBOHC defects and the presence of radiation induced O 3 molecules. Moreover, the presence of molecular oxygen increases the absorption band of the chlorine related defects and a microscopic mechanism that explains this effect has been suggested. On the other hand, by EPR we have verified a lower concentration of radiation induced E'(Si) centers in the O 2 -loaded samples than in the unloaded ones. RIA and EPR studies were also performed on an O 2 -loaded P-doped OF and the corresponding unloaded one. The presence of O 2 increases the generation of POHC under irradiation. The P2 and P1 defects are drastically reduced. Since the P1 defect absorbs light in the NIR domain, our result suggests that producing P-doped OF in oxygen-excess conditions could improve their radiation resistance in the NIR spectral domain. Finally, in the last paragraphs of the thesis we report on the radiation induced-luminescence of O 2 in the PSC and F-doped OFs. We have shown that under X-rays irradiation, 2.5 cm of O 2 -loaded fiber generate a detectable emission at 1272 nm which is known to be related to O 2 molecules. The results reported emphasize the potentialities of the O 2 -loaded OFs for real-time remote monitoring of radiation environments characterized by high doses and high dose-rates.

On the basis of the experimental studies we carried out, some further research lines can be suggested. It would be interesting to extend the online RIA measurements in the infrared domain, especially for those fibers that are supposed to work in this spectral range. At the same time it would be interesting to perform dedicated experiments in order to reliably decomposing the RIA spectra of all OF types. From an applicative point of view, it is also necessary to extend the study of the combined effect of irradiation and temperature that we performed only on the P-doped OFs, to all types of OFs. As a matter of fact, the results reported in [149] and in this thesis have shown that this kind of investigation is mandatory in order to characterize the radiation response of OFs that are irradiated at variable temperature. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ Concerning the effects related to the dopant of the core of the OFs, it is certainly important to further investigate the radiation effects on the N-doped OFs, for which very few results have been reported in literature. At the same time, the study of the interactions of several codopants inside the core of an OF could be extended in order to involve new elements and maybe more than two codopants.

Since we have proven that an O 2 loading treatment can strongly impact the radiation response of an OF, we think that it can be important to deeply investigate the effects of this treatment. Adjusting the oxygen excess could help improving certain properties of an OF and, at the same time, give some insight on the created defects and the physical phenomena going on under irradiation. Finally, it is important to remember that one of the actual research purposes of investigating canonical samples is to implement a multi-scale simulative approach. Such approach will help overcoming the difficulties that arise from the large variety of radiative environments and working conditions that OF can be subjected to, and the impossibility to qualify the OF in all the radiative conditions that they can be exposed to in future applications.

  Introduction ........................................................................................................................................................ 1 Background on optical fibers and point defects ................................................................................... 1.1 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Figure 1 . 1 :

 11 Figure 1.1: A basic OF structure.

Figure 1 . 2 :

 12 Figure 1.2: Silica refractive index change as a function of dopant concentration. Adapted from ref. [3].

Figure 1 . 3 :

 13 Figure 1.3: Schematic representation of the refraction and reflection of a light ray on an interface dividing two media of refractive indexes n 1 and n 2 .

1 )

 1 equation 1.1 has no solutions and no refracted ray can exist. The incident ray is therefore totally reflected. In an OF the light rays impinging on the core/cladding interface with an angle greater than the critical angle arcsin ( 𝑛 𝑐𝑙𝑎𝑑𝑑𝑖𝑛𝑔 𝑛 𝑐𝑜𝑟𝑒

Figure 1 . 4 :

 14 Figure 1.4: Example of a multistep-index profile.

1. 1 . 3

 13 Figure 1.5: Typical attenuation spectrum of a silica-based OF. Adapted from [4].

Figure 1 . 8 :

 18 Figure 1.8: Tetrahedral structure of silica.

Figure 1 . 11 :

 111 Figure 1.11: Emission spectrum of NBOHC and UV excitation spectrum. Adapted from [23].

Figure 1 . 13 :

 113 Figure 1.13: Summary of the main optical absorption bands of the Ge-related defects in semi-logarithmic plot [48].

Figure 1 . 14 :

 114 Figure 1.14: Evolution of the PL signals associated with GLPC induced by 4.66 eV laser irradiation. The inset shows the OA spectra acquired before and after the irradiation. Adapted from ref. [61].

Figure 1 .

 1 Figure 1.15: EPR signals associated to the Ge(1) defect in samples with different Ge contents. Adapted from ref. [65].

Figure 1 .

 1 Figure 1.16: EPR signal associated to the E'(Ge) defect in Ge-doped silica samples. Adapted from ref. [65].

Figure 1 . 18 :

 118 Figure 1.18: Absorption spectra of SiO2:Ce 0.01 mol% sintered in oxidizing (dashed red line) and reducing (solid black line) atmospheres. The inset shows the dependence of the absorption coefficient at 3.3 eV on Cerium nominal concentration for samples sintered in reducing conditions. Adapted from ref. [79].

Finally

  , concerning Ce-codoping, it is important to mention that Ce 3+ is an efficient luminescent center in the visible range. The characteristics of the emission band are sensitive to the environments of Ce 3+ as shown in Figure1.19. To this reason, Vedda et al. have shown that the drawing process of a Ce-doped silica preform can induce significant differences between the radio-luminescence (RL) of the fiber and the one of the original preform[81].

Figure 1 .Figure 1 . 20 :

 1120 Figure 1.19: (a) Radio-luminescence spectrum Ce-doped fiber, (b) radio-luminescence spectra of bulk Ce-doped silica and powder. Adapted from ref. [81].

Figure 1 . 21 :

 121 Figure 1.21: Main phosphorus related paramagnetic defects induced by radiation in P-doped silica. The supposed precursor structures are also shown. Adapted from ref. [99].

Figure 1 .

 1 Figure 1.22: EPR spectrum at 100 K of the P1, P2 and P4 defects. Adapted from ref. [99].

Figure 1 . 23 :

 123 Figure 1.23: EPR spectrum at 100 K of POHC defect. Additional peaks indicated by the arrows are due to the metastable POHC variant. In the central part of the spectrum the lineshape related to E'-Si(P) is also visible. Adapted from ref. [99].

Figure 1 . 24 :

 124 Figure 1.24: Radiation induced optical absorption spectra in a P-doped silica sample in the UV-Visible range (a), and NIR range (b). Adapted from ref. [99].

Figure 1 . 25 :

 125 Figure 1.25: Attribution of the main P-related absorption bands showed in Figure 1.24 in irradiated phosphosilicate glasses. Adapted from ref. [99].

Figure 1 .

 1 Figure 1.26: Semi-logarithmic plot of the absorption bands, from UV to NIR domain, associated with the known Prelated point defects. Adapted from ref. [48].

Figure 1 .

 1 Figure 1.27: Emission (a) and excitation (b) spectra of the tetra-coordinated phosphorous. Adapted from [102].

Figure 1 .

 1 Figure 1.28: PL spectra of phosphate glass obtained after 10, 20, 60, and 200 s total exposure time to 10 mW, 488 nm light. The peaks at roughly 590, 615, and 640 nm are artifacts of the 488 nm dichroic filter transmission profile. Adapted from [103].

Figure 1 . 29 :

 129 Figure 1.29: Normalized excitation (a) and emission (b) spectra of Ce-P-codoped SiO2 glass. Adapted from ref. [107].

Figure 1 . 30 :

 130 Figure 1.30: Absorption spectrum of a multimode N-doped OF. Adapted from ref. [108].

For

  the results discussed in this PhD thesis, it is relevant to report on the known radiation induced effects in N-doped OFs. As shown in Figure 1.31, in 1995 Dianov et al. have shown that, in the NIR domain, the radiation induced losses in N-doped OFs are close to the ones of F-doped OFs in the case of steady state γ-irradiation up to 10 KGy (and much lower than Ge-doped OFs) [112].

Figure 1 .

 1 Figure 1.31: γ-rays radiation induced losses in F-(----), Ge-(--) and N-(-) doped OFs. Adapted from ref. [112].

Figure 1 . 32 :

 132 Figure 1.32: EPR spectra at 300 K of N-related defects in silica. Three EPR components are highlighted: (a) ( 29 Si-O)-Si • = (not due to N), (b) =Si • -(N<) and (c) ≡Si-N • -Si≡. Adapted from ref. [115].

Figure 1 .

 1 Figure 1.33: Experimental, (a), and simulated, (b), EPR signal of the ≡Si-N • -Si≡. In (b) the central lines due to other paramagnetic centers, marked by arrows in (a), are absent. Adapted from ref. [115].

1. 2 . 4 N

 24 Figure 1.34: Experimental, (a), and simulated, (b), EPR signal of the =N • = defect at room temperature. In (b) the central lines due to other paramagnetic centers are absent. Adapted from ref. [118].

Figure 1 .

 1 Figure 1.35: A typical experimental UV absorption spectrum of N-doped OF preform (silicon oxynitride) with decomposition in gaussian bands. Adapted from ref. [123].

Figure 1 .

 1 Figure 1.38: Mechanisms that lead to the creation of defects for the different types of irradiation (UV, X, γ, neutrons, ions). Adapted from ref. [127].

  main types of experiment: Radiation Induced Attenuation (RIA); Electron Paramagnetic Resonance (EPR); Confocal Micro-Luminescence (CML).

Figure 2 . 1 :

 21 Figure 2.1: Schematic representation of the experimental techniques employed in this thesis work in order to characterize the effects of radiation on the studied OFs.

Figure 2 . 2 :

 22 Figure 2.2: Jablonski diagram of the excitation and relaxation pathways involving the ground and the first excited electronic levels of a molecule. S0, S1, S2 represent spin singlet states, and T1 a spin triplet state. Continuous arrows highlight absorption transitions and dashed lines emission ones. ISC represents the intersystem crossing process of spin state change. Typical time scales of the given processes are reported for reference. Extracted from [130].

Figure 2 . 3 :

 23 Figure 2.3: Schematic energy levels scheme for Rayleigh and Raman scattering. The arrows mark the photon related processes.

Figure 2 . 4 :

 24 Figure 2.4: Typical EPR spectrum of powders or amorphous materials containing paramagnetic centers. (a) Derivative spectrum; the positions relative to the principal g tensor values (g1, g2, and g3) are highlighted. (b) Integral spectrum corresponding to the spectrum reported in (a). Adapted from [60].

Figure 2 . 5 :

 25 Figure 2.5: EPR derivative spectrum showing the three lines characteristic of the hyperfine interaction between an electron spin l/2 and a nuclear spin 1. The line splitting in field units is also evidenced. Extracted from [60].

Figure 2 . 6 :

 26 Figure 2.6: Experimental setup employed for the online RIA measurements. The maximum irradiated OF size and the xray beam size are 2.5 cm. A two fibers mounting is sketched with the relative light sources (DH2000) and employed spectrometers (HR4000), see text. The yellow square represents the heating plate used in some temperature controlled experiments.

52 Chap. 2 :Figure 2 . 7 :

 52227 Figure 2.7: Experimental setup of the OA measurements by the cut-back technique.

2. 2 . 3 Figure 2 . 8 :

 2328 Figure 2.8: Experimental setup for the detection of the radiation induced emitted light.

Figure 2 . 10 :

 210 Figure 2.10: Scheme of micro-luminescence and micro-Raman analysis set-up.

Figure 2 . 11 :

 211 Figure 2.11: Block scheme of the EPR reflection spectrometer. Adapted from [135]

Figure 2 . 13 :

 213 Figure 2.13: Schematic representation of the PARR reactor employed for the O2 loading treatments. 1 is the vessel, 2 is a gasket, 3 is the cover of the vessel, 4 and 5 are the input and output valves for the gases, 6 is a pressure transducer, 7 is the well in which a thermocouple is placed, 8 and 9 are two steel half-rings for closing, 10 is a safety steel ring, 11 is a analogical pressure gauge.

Figure 2 . 15 :

 215 Figure 2.15: Concentration profiles of Ge, F and Cl in the GeFDi OFs obtained by EMPA measurements.

Figure 2 . 16 :

 216 Figure 2.16: Concentration profiles of P and Cl in the PDi OFs obtained by EMPA measurements.

Figure 2 . 17 :

 217 Figure 2.17: Concentration profiles of N and Cl in the NDi OFs obtained by EMPA measurements.

Figure 2 . 18 :

 218 Figure 2.18: Concentration profiles of F and Cl in the F2D3 OF obtained by EMPA measurements.

3. 1 . 1

 11 Study of Radiation and Drawing effects by X-ray irradiation 67 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Figure 3 . 1 :

 31 Figure 3.1: RIA of the PD2 OF under X-ray irradiation at several doses at room temperature. The radiation dose-rate is 5 krad/s.

Figure 3 . 2 :

 32 Figure 3.2: RIA of PD2 OF as a function of the total dose at 270, 400 and 600 nm.

Figure 3 .

 3 3 we show the RIA of the three OFs at two different doses: 140 krad and 3 Mrad. As explained in § 2.5.2, the three OFs differ from one another only for the drawing conditions.

Figure 3 . 3 :

 33 Figure 3.3: RIA spectra of PD1, PD2 and PD3 OFs at two different doses, 140 krad and 3 Mrad.

  .4, the EPR signals of several samples are shown. All the spectra were normalized by the sample masses so that their shapes and intensities can be easily compared. The complexity of the recorded lineshapes evidences that the spectra are the superposition of signals attributable to POHC, m-POHC and E'(Si) (central lines). In Figure3.4(a) the main spectral features specific of each of these defects are highlighted by arrows. The microwave power and the amplitude of the modulation magnetic field were chosen so that the spectra were not distorted (except for the E'(Si) contribution which can be studied separately). No EPR signal due to POHC is detected in the pristine OFs. At low doses, Figure3.4(c), the EPR signal is mainly constituted by POHC centers.Then, increasing the radiation dose, a sharp spectral feature appears at 3460 G, which is the EPR line of E'(Si). The relative intensity of E'(Si) signal grows with the dose and its presence is well visible at the maximum dose of 300 Mrad, Figure 3.4(a). It is interesting to observe that m-POHCs are 3.1.1 Study of Radiation and Drawing effects by X-ray irradiation 71 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ detected even if measurements were carried out a few weeks after irradiation, and maintaining the samples at ambient conditions. This result confirms what was already reported by Origlio et al. in ref.

Figure 3 . 4 :

 34 Figure 3.4: EPR spectra of POHC centers in the PD1, PD2 and PD3 OFs X-irradiated at several doses. The spectra were normalized by masses of the samples in order to compare the relative intensities and the shapes.

2 .Chap. 3 :Figure 3 . 5 :

 2335 Figure 3.5: Estimated concentration of POHC (both stable and metastable) for PD1, PD2 and PD3 OFs X-irradiated at different total doses.

Figure 3 . 6 :Figure 3 . 7 :

 3637 Figure 3.6: EPR spectra of E'(Si) centers in PD1, PD2 and PD3 OFs X-irradiated at several doses. The spectra were normalized by masses of the samples in order to compare the relative intensities and the shapes.

Figure 3 . 8 :

 38 Figure 3.8: EPR signal of the P2 center recorded in the PD2 OF X-irradiated at 10 Mrad. The central lines are due to distorted EPR signals of POHC centers and E'(Si).

Figure 3 . 9 :

 39 Figure 3.9: Estimated concentration of P2 for PD1, PD2 and PD3 OFs X-irradiated at different total doses.

Figure 3 . 10 :

 310 Figure 3.10: Defect growth kinetics of the POHC (both stable and metastable), E'(Si) and P2.

  the center 24 µm from the center 9 µm from the center 0 µm from the center

Figure 3 .

 3 Figure 3.11: CML spectra of the PD2 OF irradiated at 300 Mrad at several distances from the center. The black line represents the spectrum of the cladding of the OF whereas the other curves are different points inside the core of the OF. The excitation wavelength is 325 nm.

  Integral of [645;650] nm

Figure 3 . 12 :

 312 Figure 3.12: Cartography of the area of the PL band of NBOHC in the range [645;650] nm (its maximum) in the PD2 OF irradiated at 300 Mrad.

Figure 3 . 13 :

 313 Figure 3.13: Cartography of the PL band around 600 nm in the PD2 OF irradiated at 300 Mrad

3. 1 . 1

 11 Study of Radiation and Drawing effects by X-ray irradiation 79 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Figure 3 . 14 :

 314 Figure 3.14: OA spectra of the γ-irradiated PD1 OF at four different doses.

Figure 3 . 15 :

 315 Figure 3.15: Second-harmonic EPR spectra of the γ-irradiated PD1 OF at four different doses. The EPR signatures of P2, P1 and E'(Si 29 ) are highlighted by arrows. The highly distorted central line is due to E'(Si) signal. All the spectra were normalized for the samples mass.

Figure 3 .Figure 3 . 16 :

 3316 Figure 3.15 shows the EPR second harmonic detection measurements of the irradiated samples.Three EPR doublets are visible in this figure which are assigned to P2, P1 and the E'(Si 29 ) hyperfine structure. In the pristine sample (not shown) no EPR signals were detected. It is worth noting that the P1 defect is detectable only by second harmonic measurements due to its low concentration per magnetic field unit. A monotonic increase of the P1 signal features is observed. To better clarify this trend a decomposition of the acquired EPR spectra has been carried out.

Figure 3 . 17 : 82 Chap. 3 :

 317823 Figure 3.17: Correlation of the OA at 1.6 µm and the intensity of the EPR signal of the P1 defect in the γ-irradiated PD1.

Figure 3 . 18 :

 318 Figure 3.18: Growth kinetics curves of POHC, P2 and E'(Si) in PD1 for γ and X irradiations.

Figure 3 . 19 :

 319 Figure 3.19: EPR spectra of PD1 OF X and γ irradiated at the dose of 100 Mrad (a) and 300 Mrad (b). The spectra were normalized by the masses of the samples in order to compare the relative intensities of the four spectra

Figure 3 . 20 :

 320 Figure 3.20: RIA spectra of the PD3 OF irradiated at three different doses: 100 krad, 2 Mrad and 100 Mrad. Each graph contains five spectra corresponding to five different irradiation temperatures. At the highest irradiation dose some negligible instrumental artifacts appear around 580 nm.

Figure 3 . 21 :

 321 Photon energy (eV)

3. 1 . 2 Figure 1 .

 121 Figure 1.25 and Figure1.26) the presence of both varieties of POHCs can be recognized and it is found that they represent the most important absorbing defects up to ~3.5 eV (~350 nm) in the investigated temperature range. At higher photon energy also the P2 defect presence can be hypothesized probably overlapped to the NBOHC defect, whose presence is suggested by the CML measurements on the room temperature irradiated samples (see § 3.1.1.3).

Figure 3 . 22 :

 322 Figure 3.22: RIA at the three representative wavelengths of 270 (4.6 eV), 400 (3.1 eV) and 600 nm (2.1 eV) for the PD3 OF irradiated at five different temperatures.

3. 1 . 2 Figure 3 . 23 :

 12323 Figure 3.23: EPR spectra of the POHC centers detected in the PD2 OF irradiated at four different temperatures. The PD2 OF was irradiated at 20 Mrad and the spectra in the figure were normalized by the samples masses. The m-POHC are highlighted by arrows.

Figure 3 . 24 :

 324 Figure 3.24: Concentration of POHC in the PD2 OF as a function of the dose at different temperatures (a), and as a function of the temperature at several doses (b). The estimated concentrations have an error of 30%.

Figure 3 . 25 :

 325 Figure 3.25: Concentration of E'(Si) in the PD2 OF as a function of the dose at different temperatures (a), and as a function of the temperature at several doses (b). The estimated concentrations have an error of 30%.

Figure 3 . 26 :

 326 Figure 3.26: Second harmonic EPR spectra of the P1 and P2 centers detected in the PD2 OF irradiated at three different temperatures. The PD2 OF was irradiated at 300 Mrad and the spectra in the figure where normalized by the samples masses.

Figure 3 .Figure 3 . 27 :

 3327 Figure 3.27 shows the dependence of the concentration of P2 on the dose and the temperature. For all the growth kinetics the initial linear growth is followed by a limit concentration value. However, for temperatures lower than 190 °C the growth kinetics shown in Figure3.27.a are the same within the experimental errors. On the contrary, the curve corresponding to the irradiation at 280 °C is well below the others. It is worth to notice that the P2 defects absorb light around 270 nm and that in the RIA study reported in the previous paragraph (Figure3.20) the absorption spectrum relative to 280 °C is substantially below the others.

Figure 3 . 28 :

 328 Figure 3.28: Growth kinetics of P1 in the PD2 OF as a function of the dose at different temperatures (a), and as a function of the temperature at several doses (b). The data are affected by an error of 40%.

Figure 3 . 29 :

 329 Figure 3.29: RIA spectra of PCe OF (a), and PD1 OF (b), irradiated at room temperature and with a dose-rate of 5 krad/s up to a dose of 100 Mrad.

Figure 3 . 30 :

 330 Photon Energy (eV)

Figure 3 . 31 :

 331 Figure 3.31: Growth kinetics of the amplitude of the bands employed for the decompositions (Figure 3.30 and Table3.2) of the RIA experiments of PCe and PD1 OFs.

Figure 3 .

 3 Figure 3.32(a), we observe the presence of three defects in the PCe OF: POHC, H(I) and E'(Si) (central saturated line).

Figure 3 . 32 :

 332 Figure 3.32: EPR spectra of the POHC centers in PCe (a), and in PD1 (b) OFs at different -irradiation doses. All the spectra were normalized for the masses of the samples. The y-axis of (a) has been magnified by a factor five.

Figure 3 . 33 :Figure 3 . 34 :

 333334 Figure 3.33: Lineshape comparison of the EPR spectra of the POHC centers in the PCe and PD1 OFs -ray irradiated at 970 Mrad. The spectra were normalized at the negative peak of POHC at about 3460 G, highlighted by the circle, the arrow at about 3390 G evidences the other spectral component of POHC.

Figure 3 . 35 :

 335 Figure 3.35: Second harmonic EPR spectra of the P1 and P2 centers in PCe and PD1 OFs at four -irradiation doses. All the spectra were normalized by the mass of the corresponding samples. x The spectral features of the two defects investigated are marked by arrows in panel a).

Chap. 3 :Figure 3 . 36 :Figure 3 . 37 :

 3336337 Figure 3.36: Growth kinetics of P2 in the PCe and PD1 OFs as function of the dose. The estimated concentrations have an error of 40%.

Figure 3 .

 3 Figure 3.35 where decomposed as described in § 3.1.1.4. In the investigated dose range the P2 defect has reached its saturation value for the PCe OF, whereas a decreasing trend is observable for the PD1 OF. The P1 defect, on the other hand, increases with the radiation dose for both fibers.

3. 1 . 3

 13 Ce-codoping of P-doped optical fibers 103 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Figure 3 . 38 :

 338 Photon Energy (eV)

Figure 3 . 39 :

 339 Figure 3.39: Intensities of the PL emission of Ce 3+ ions (excitation at 325 nm) in the pristine and irradiated PCe samples. The measurements were performed in the same experimental conditions.

Figure 3 . 40 :

 340 Figure 3.40: RIA spectra of GeD2 (a), and GeFD2 OFs (b), irradiated at room temperature with dose-rate 5 krad/s, up to 100 Mrad.

Figure 3 .Figure 3 . 41 :

 3341 Figure 3.41: Ratio of the RIA spectra between GeFD2 and GeD2 at two irradiation doses, 200 krad and 4.6 Mrad.

Figure 3 . 42 : 110 Chap. 3 :

 3421103 Figure 3.42: RIA at 350 nm for the GeD2 andGeFD2 OFs irradiated at 5 krad/s at room temperature.

Figure 3 . 43 :

 343 Figure 3.43: RIA spectra of GeD1, GeD2, GeD3 (a), and GeFD1, GeFD2, GeFD3 (b), at the two doses of 200 krad and 4.6 Mrad.

Figure 3 . 44 :

 344 Figure 3.44: EPR spectra of the GeFDi samples irradiated at three different doses: a) 300 Mrad, b) 20 Mrad, c) 200 krad, and d) not irradiated. The spectra are normalized by the samples masses.

Figure 3 . 45 :

 345 Figure 3.45: EPR signal comparison between GeD2 and GeFD2 at the same irradiation dose of 2 Mrad. Spectra are normalized by the signal double integral.

Figure 3 . 46 :

 346 Figure 3.46: Decomposition of the EPR spectrum of the GeFD2 OF irradiated at 100 Mrad by means of the EPR signals of Ge(1), E'(Ge), Ge(2) and E'(Si) reported in ref.[65].

Figure 3 . 47 :

 347 Figure 3.47: Growth kinetics curves of Ge(1), Ge(2) and E'(Ge) defects as a function of dose obtained by spectral decomposition of the EPR signals. Both the results on GeD2 and GeFD2 are here plotted for comparison. All the points have an error of 30 %.

Figure 3 . 48 :

 348 Figure 3.48: Typical photoluminescence spectrum of the core of an irradiated GeFDi OF excited at 325 nm. The two bands peaked around 400 nm ad 650 nm are assigned to GLPC and NBOHC defects, respectively.

  photoluminescence signals are plotted as a function of the dose for each irradiated fiber. Each point of these curves has been constructed by integration of the cartography of the corresponding irradiated sample. Figure3.49 shows that the pristine fibers are rich of GLPC and that irradiation causes a slight overall decrease of this defect (about 30%).

Figure 3 . 49 :

 349 Figure 3.49: (a) CML cartographies of GLPC emission at 400 nm in GeFD2 at three different irradiation doses; (b) relative concentration variation of GLPC (as determined through the integration of the cartography of the PL band at 400 nm) as a function of the dose. The first point represents the pristine fiber.

Figure 3 .Figure 3 . 51 :

 3351 Figure 3.50: (a) CML cartographies of NBOHC emission at 650 nm in GeFD2 OF at three different irradiation doses. (b)relative concentration variation of NBOHC (as determined through the integration of the cartography of the PL band at 650 nm) as a function of the dose. The first point represents the pristine fiber. For displaying purposes the cartography corresponding to the pristine fiber has been multiplied by a factor 20.

Figure 3 .Figure 3 .

 33 Photon Energy (eV)

120 Chap. 3 :

 1203 .52(b) shows the cartography of the spectral position of the maximum of the red emission band in the GeFD2 OF irradiated at the Study of Radiation effects on P-, Ge-and N-doped OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ maximum dose of 300 Mrad. The cladding contains only pure silica and therefore the revealed NBOHCs are surely the Si-related ones. Moving toward the center of the OF causes a redshift of the PL maximum which is compatible with the appearance of a Ge-related NBOHC band contribution.

Figure 3 . 53 :

 353 Figure 3.53: RIA spectra of GeCeD2 OF (a), and GeD2 OF (b), irradiated at room temperature with a dose-rate of 5 krad/s up to 100 Mrad.

Figure 3 .Figure 3 . 54 :

 3354 Figure 3.54 we report their RIA spectra at the two radiation doses of 600 krad and 36 Mrad, whereas in Figure 3.55 we report the RIA at 450 nm as a function of the dose.

Figure 3 . 55 :

 355 Figure 3.55: Growth kinetics of the RIA at 450 nm as a function of the dose for the GeCeD2 and GeD2 OFs.

Figure 3 . 56 :

 356 Figure 3.56: RIA spectra of GeCeD1, GeCeD2 and GeCeD3 at two different doses, 600 krad and 36 Mrad.

Figure 3 . 57 :

 357 Figure 3.57: EPR spectra of the GeCeD2 and GeD2 OFs γ-ray irradiated at four different doses. The spectra were normalized by masses of the samples in order to compare the relative intensities and the lineshapes.

Figure 3 . 58 :

 358 Figure 3.58: The isolated EPR feature of the Ge(2) defect in the EPR spectra of the GeCeD2 and GeD2 OFs γ-irradiated at the maximum dose of 970 Mrad.

Chap. 3 :Figure 3 . 59 :

 3359 Figure 3.59: Growth kinetics of Ge(1), Ge(2) and E'(Ge) as a function of the dose obtained by spectral decomposition of the EPR signals. Both the results on GeCeD2 and Ge2 are here plotted for comparison. All the points have an error of 30 %.

Figure 3 . 60 :

 360 Figure 3.60: Normalized CML spectra of the pristine and 970 Mrad γ-irradiated GeCeD2 OF, recorded in the center (0 m) and in the external part (24 m) of the core of the OF. The excitation wavelength is 325 nm.

Figure 3 .

 3 Figure 3.61: (a) CML cartographies of the emission at about 500 nm in the GeCeD2 OF irradiated at different doses. (b) relative amplitude variation of the emission at 500 nm as a function of the dose. The first point represents the pristine fiber.

Figure 3 . 62 :

 362 Figure 3.62: Typical CML spectrum of NBOHC detected in GeCeD2 OFs. The excitation wavelength is 633 nm.

Figure 3 . 63 :

 363 Figure 3.63: (a) CML cartographies of NBOHC in GeCeD2 OF irradiated at different doses. (b) Concentration variation of NBOHC (determined through the 650 nm PL band) as a function of the dose, in the core and in the cladding of the OFs. For display purposes the cartography corresponding to the pristine fiber has been multiplied by a factor 7.

Chap. 3 :Figure 3 . 64 :

 3364 Figure 3.64: RIA of the ND2 OF under X-ray irradiation at several doses. The dose-rate is 5 krad/s and the irradiation was performed at room temperature.

Figure 3 . 65 :

 365 Figure 3.65: RIA spectra of ND1, ND2 and ND3 OFs at two different doses, 600 krad and 100 Mrad.

Figure 3 . 66 :

 366 Figure 3.66: RIA growth kinetics of the ND1, ND2 and ND3 OFs as a function of the total dose at 300 nm (a), and at 450 nm (b).

Figure 3 .Figure 3 . 68 :

 3368 Figure 3.67: (a) EPR spectra of the =N-Si • = centers in the γ-rays irradiated ND2 OFs . The spectra were normalized by the masses of the corresponding samples; the main spectral features of known defects are marked by arrows. (b) The same EPR spectra reported in (a) after normalization to the positive peak amplitude at 3438 G; the arrows mark the features of the =N-Si • = center.

Figure 3 .

 3 Figure 3.69: (a) EPR spectra at 77 K of the =N • centers in the γ-rays irradiated ND2 OFs . The spectra were normalized by the masses of the corresponding samples. The arrows mark the main isolated spectral features of the defect. (b) Relative concentration of = N • in the ND2 OFs. The values are affected by a 30% error.

Figure 3 .

 3 Figure 3.70: (a) EPR spectra of the E'(Si) centers in the γ-rays irradiated ND2 OFs . The spectra were normalized by the masses of the corresponding samples. (b) The same EPR spectra shown in (a) after normalizing by the respective values of the double integral. The EPR signal of the F2D3 OF γ-rays irradiated at 780 Mrad is reported for comparison after applying the same normalization.

Chap. 3 :Figure 3 . 71 :

 3371 Figure 3.71: Concentration of E'(Si) in the ND1, ND2 and ND3 OFs. The data are affected by a 25% error.

3. 3 . 3

 33 Attenuation of the γ-rays irradiated ND2 OF 141 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Figure 3 . 72 :

 372 Figure 3.72: IR attenuation spectra of the ND2 OF γ-rays irradiated at four different doses.

Figure 3 .Chap. 3 :Figure 3 . 73 : 3 Figure 3 . 74 :

 333733374 Figure 3.73: Attenuation values at 1.5 µm as a function of the dose in the ND2 OF.

Figure 3 .

 3 Figure 3.75(b). The cartography resembles the doping profile of the OF and the PL is surely related to the N-doping. However, we must point out that the concentration of N changes by a factor two between the inner and outer parts of the core (the central hole of N concentration can be ignored)

Figure 3 .

 3 Figure 3.75: (a) CML spectra of the N-doped zone (12 µm from the center) of the pristine and γ-rays irradiated (970 Mrad) ND3 OF. The excitation wavelength is 325 nm. (b) CML cartographies of the amplitude of the emission band at 400 nm for the two samples.

3. 3 . 4

 34 CML study145 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ is observed that by this procedure the spectral profile of a single band emerges. After converting the spectrum in eV and correcting for the spectral dispersion xiii the resulting PL band is slightly asymmetrical, it is centered at 2.2 eV and has a FWHM of 0.52 eV. The cartography of such band is reported in Figure3.76(b).

Figure 3 .

 3 Figure 3.76: (a) CML spectra of the N-doped zone (12 µm from the center) of the ND3 OF before irradiation rescaled, γ-rays irradiated (970 Mrad) and their difference. The excitation wavelength is 325 nm. (b) CML cartographies of the emission band at 560 nm of the ND3 OF irradiated at 970 Mrad.

Figure 3 .

 3 Figure 3.77: (a) CML spectra of the N-doped zone (12 µm from the center) of the γ-ray irradiated (970 Mrad) ND3 OF. The excitation wavelength is 442 nm. (b) CML cartography of the emission band at 660 nm of the ND3 OF irradiated at 970 Mrad.

Figure 3 . 78 :

 378 Figure 3.78: Normalized cartographies of the three emission bands investigated in this section. The N concentration profile is reported for comparison.

4. 1 . 1 1 )Figure 4 . 1 :

 11141 Figure 4.1: Raman spectra of the core of the pristine SiD3 OF and SiD3-O2 OF one week after the O2 treatment, a), and a magnification of the spectral range where the O2 Raman band is located, b). The spectra have been opportunely normalized to let the amplitude of the silica R-band at ~440 cm -1 coincide.

Figure 4 . 2 ,

 42 we report the Raman spectra of the cores of the OFs after performing the O 2 -loading treatment , hereafter named SiD3-O2, F2D3-O2, PD2-O2 and GeD2-O2. Each fiber clearly shows the presence around 1550 cm -1 of the O 2 Raman signal. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Figure 4 . 2 :

 42 Figure 4.2: Raman spectra of the cores of the O2-loaded OFs: SiD3-O2 a), F2D3-O2 b), PD2-O2 c) and GeD2-O2 d). In each graph an inset shows a magnification of the spectral range where the O2-Raman band is located. The spectrum in (d) is affected by the presence of the PL of GLPC centers, in the corresponding inset a baseline was subtracted. The arrows highlight the O2 Raman band

Figure 4 .Figure 4 . 3 :

 443 Figure 4.3 we report the cartographies of the O 2 Raman band for both SiD3-O2 and F2D3-O2. The distance between the probed points is 8 µm and the probed spot size was 5 µm.

Figure 4 . 4 , 1 )Figure 4 . 4 :

 44144 Figure 4.4: Raman spectra of the core of the SiD3-O2 OF one week and 1.5 years after the treatment in O2 a), and a magnification of the spectral range where the O2 Raman band is located, b).

Figure 4 .

 4 Figure 4.4(b) clearly shows that the sample, kept at room conditions, is not affected by the outgassing phenomenon.

First, we show

  the results concerning the RIA experiments in the UV-Visible domains during X-rays irradiation; this study includes the decomposition of the RIA spectra of the F2D3 and F2D3-O2 OFs to identify the role of different defects. In the second part we show the results obtained by EPR investigation on the γ-rays irradiated samples. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ In Figure 4.5(a), we show the results of the online RIA measurements for the sample SiD3. The reported spectra correspond to several radiation doses up to the maximum of 100 Mrad. Analogously, in Figure 4.5(b), the results of the RIA measurements of the SiD3-O2 OF are shown.

Figure 4 . 5 :

 45 Figure 4.5: RIA-spectra at several doses for (a) the SiD3 and (b) the SiD3-O2 OFs up to the maximum dose of 100 Mrad. The dose rate is 5 krad/s and the irradiation was performed at room temperature.

4. 1 . 2 Figure 4 . 6 :

 1246 Figure 4.6: RIA growth kinetics of the SiD3 and SiD3-O2 OFs at the specific wavelength of 240 nm as a function of the dose.

Figure 4 . 7 :

 47 Figure 4.7: RIA-spectra at several doses for (a) the F2D3 and (b) the F2D3-O2 OFs up to the maximum dose of 100 Mrad. The dose rate is 5 krad/s and the irradiation was performed at room temperature.

Figure 4 . 8 :

 48 Figure 4.8: RIA growth kinetics of the F2D3 and F2D3-O2 OFs at the specific wavelength of 240 nm as a function of the dose.

4. 1 . 2 Figure 4 . 9 :

 1249 Figure 4.9: Comparison of the RIA-spectra at the maximum dose (100 Mrad) for F2D3, F2D3-O2, SiD3 and SiD3-O2, OFs (a). Differences of the RIA spectra at 100 Mrad of F2D3-O2 and F2D3, and of SiD3-O2 and SiD3 OFs, (b).

Figure 4 . 10 :

 410 Figure 4.10: Spectral decomposition using the bands reported in Table4.1 of the F2D3-O2 (a), and F2D3 (b), OFs irradiated at the maximum dose of 100 Mrad. (c) Spectral decomposition of the difference of RIA between the F2D3-O2 and F2D3 OFs irradiated at 100 Mrad. The bands associated to defects of main interest in this work are indicated in the legends, whereas all other bands (UV-tail, POR, ODC(II), Cl-related reported in Table4.1) are collectively named "Other defects".

Figure 4 . 11 :

 411 Figure 4.11: Growth kinetics of the amplitudes of the absorption bands associated with E'(Si), NBOHC and O3 derived from the spectral decomposition of the RIA of the F2D3 and F2D3-O2 OFs (see text). In the case of the NBOHC band the amplitude is referred to its maximum at 4.8 eV.

Figure 4 .

 4 12(b), the EPR signals of the E'(Si) centers are reported. The measurements were performed in the second harmonic detection mode. From these spectra it is possible to evaluate the concentrations, in defects/cm3 , of E'(Si) in the four samples: SiD3, 1.5•10 17 ; SiD3-O2, 1.5•10 16 ; F2D3, 1.9•10 17 ; F2D3-O2, 3•1016 . The concentrations have an absolute error of 4.1.2 Irradiation effects in O2-loaded PSC and F-doped OFs 161 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ 25%. Nevertheless, the SiD3-O2 and F2D3-O2 OFs show respectively 10 and 6.3 times lower concentrations than the corresponding unloaded OFs. These EPR measurements confirm qualitatively the results inferred by the spectral decomposition of the RIA measurements, somehow supporting this latter analysis, and confirm a role of interstitial O 2 in the generation mechanisms of E'(Si) centers by irradiation.

Figure 4 . 12 :

 412 Figure 4.12: EPR signals normalized by the samples masses of F2D3-O2, F2D3, SiD3-O2 and SiD3 OFs γ-rays irradiated at 1 Grad: a) and b), second-harmonic room temperature spectra of the E'(Si) centers; c) and d), first harmonic NBOHC and POR spectra recorded at 77 K. In panels c) and d), the curves corresponding to F2D3 and SiD3 have been multiplied by a factor 12 for better representation and the insets show a magnification of the low field part of the spectra.

  On the basis of these generation mechanisms, the production of Cl-related defects implies the creation of E'(Si) defects. These ones are probably involved in the subsequent creation of NBOHC via the reaction with the interstitial O 2 molecules above discussed. By this simple scheme, the main differences in the radiation responses of the loaded and unloaded OFs are explained, since together with the larger production of Cl related defects a larger production of NBOHC is found in F2D3-O2 (the presence of Chlorine in the studied OFs has been verified by EMPA measurements and the relative concentration is reported in § 2.5.4).Although we have shown that high concentration of O 2 enhances the radiation sensitivity in the UV-Visible spectral range of F-doped and PSC fibers, it is important to point out that in our study we tried to maximize the content of O 2 dissolved into the silica matrix in order to amplify the effects of such treatment. A study of the radiation response as a function of O 2 content is necessary to determine if a certain level of interstitial O 2 can improve the radiation resistance of the OFs. At

Figure 4 . 13 :

 413 Figure 4.13: RIA spectra of PD2-O2 OF (a), and PD2 OF (b), X-rays irradiated at room temperature and with a doserate of 5 krad/s up to a dose of 100 Mrad.

Figure 4 . 14 :

 414 Figure 4.14: RIA spectra of PD2-O2 and PD2 OFs X-rays irradiated at 200 krad and 13 Mrad at room temperature.

Figure 4 .

 4 Figure 4.15: a) RIA at 270 nm, and b) at 400 and 600 nm in the PD2-O2 and PD2 OFs. The dose rate is 5 krad/s and the X-rays irradiation has been performed at room temperature.

Figure 4 . 16 :

 416 Figure 4.16: EPR spectra of the POHC centers in PD2-O2 and PD2 OFs γ-irradiated at 1 Grad. All the spectra were normalized by the mass of the corresponding samples. The arrow marks the NBOHC signal.

Figure 4 . 18 :

 418 Figure 4.18: Second harmonic EPR spectra of the P1 and P2 centers in PD2-O2 and PD2 OFs γ-rays irradiated at 1 Grad. All the spectra were normalized by the mass of the corresponding samples. The central part of the spectra has been omitted for clarity.

Figure 4 .

 4 Figure 4.18 clearly shows that the O 2 loading strongly impacts the generation of P1 and P2 defects. From the measurements of Figure 4.18 we can evaluate concentrations of P2 of 3•10 17 and 4.4•10 16defects/cm 3 for the PD2 and PD2-O2 OFs, respectively, showing that the concentration of P2 is 7 times lower in the O 2 -loaded OF compared to the unloaded one. Concerning the P2 defect, its precursor site is a substitutional P atom, =P=[99]. During the irradiation this positively charged structure can trap an electron and become a P2 defect. Since it is not clear how the O 2 loading can impact the precursor site of the P2 defect we imagine that the presence of O 2 in proximity of its precursor can cause a local distortion of the matrix such that the trapping of an electron and the consequent creation of P2 is prevented.

Figure 4 . 19 :

 419 Figure 4.19: iRL of interstitial O2 molecules embedded in the SiD3-O2 and F2D3-O2 OFs. The dose-rate is 5 krad/s, the irradiated zone is 2.5 cm long.

Figure 4 .

 4 Figure 4.20: . Variation of the integrated area of the O2 iRL at 1272 nm with the X-rays irradiation dose. The dose-rate is 5 krad/s. SiD3-O2 OF was irradiated in two steps with a 65 hours break in between, whereas a single irradiation was performed on F2D3-O2 OF.

Figure 4 .

 4 Figure 4.21, we report the iRL intensity evolution as a function of the dose-rate. The experiment was performed at room temperature and the experimental points were normalized at the maximum value.

Figure 4 . 21 :

 421 Figure 4.21: Normalized O2 iRL intensity evolution as a function of the dose-rate for both SiD3-O2 and F2D3-O2 OFs. The measurements were carried out at room temperature. The linear dependence is evidenced by the straight line.

Figure 4 . 22 :

 422 Figure 4.22: O2 iRL intensity as a function of the temperature for both SiD3-O2 and F2D3-O2 OFs at 5 krad/s dose rate, normalized to the maximum iRL intensity.

  First we have shown by micro-Raman spectroscopy measurements that the treatment succeeded to incorporate high quantities of molecular oxygen into the samples. We have estimated a concentration of 2•10 18 molecules/cm 3 . The concentration profile of the O 2 molecules is almost uniform along the cross section of the OF and we have shown that no outgassing phenomenon took place at room temperature in more than 1.5 years long time. RIA experiments were performed on O 2 -loaded Pure-silica-core and F-doped OFs as well as on the corresponding unloaded samples. The O 2 -loaded samples have shown a higher concentration of radiation induced NBOHC defects and the presence of radiation induced O 3 molecules. Moreover, the presence of molecular oxygen increases the absorption band of the chlorine related defects and a microscopic mechanism that explains this effect has been suggested. By EPR we have verified a lower concentration of radiation induced E'(Si) centers in the O 2 -loaded samples than in the unloaded ones. RIA and EPR studies were also performed on an O 2 -loaded P-doped OF and the corresponding unloaded one. The presence of O 2 increases the generation of POHC under irradiation. On the other hand, the P2 and P1 defects are drastically reduced. Since the P1 defect absorbs light in the NIR domain, our result suggests that producing P-doped OF in oxygen-excess conditions could improve their radiation resistance in the NIR spectral domain. Finally, in the last Chap. 4: Study of O2-loaded OFs ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ paragraphs we report on the radiation induced emission of O 2 in the PSC and F-doped OFs. We have shown that under X-rays irradiation, 2.5 cm of O 2 -loaded fiber generate a detectable emission at 1272 nm which is known to be related to O 2 molecules. The results reported emphasize the potentialities of the O 2 -loaded OFs for real-time remote monitoring of radiation environments characterized by high doses and high dose-rates. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ConclusionsIn this experimental thesis we have reported on the effects of ionizing radiation (X-rays and γ-rays) on several types of multimode silica-based OFs up to a maximum radiation dose of 1 Grad. Our investigation aimed to characterize the radiation responses of such samples in terms of radiation induced attenuation and to gain further insight on the point defects generation taking advantage of complementary spectroscopic techniques.

Table 2 .

 2 .1.

		Inner core zone (wt%)	Outer core zone (wt%) Drawing speed (m/min) Drawing tension (g)
	GeD1	Ge 8.6	Ge 4.6	22	33
	GeD2	Ge 8.6	Ge 4.6	40	70
	GeD3	Ge 8.6	Ge 4.6	70	135
	GeFD1	Ge 8.1 ; F 0.05	Ge 8.1; F 1.1	22	35
	GeFD2	Ge 8.1 ; F 0.05	Ge 8.1; F 1.1	40	65
	GeFD3	Ge 8.1 ; F 0.05	Ge 8.1; F 1.1	90	166
	GeCeD1	Ge 4.4 ; Ce 3.9	Ge 4.4; Ce 2	17	50
	GeCeD2	Ge 4.4 ; Ce 3.9	Ge 4.4; Ce 2	17	40
	GeCeD3	Ge 4.4 ; Ce 3.9	Ge 4.4; Ce 2	17	30

1: Chemical compositions of the core zones of the Ge doped OFs and drawing conditions.

In the next figures we show the chemical concentration profiles of dopants obtained by Electron Micro-Probe Analysis (EMPA) v on the GeDi OFs and GeFDi OFs.

v EMPA measurements were performed by BUREAU VERITAS. Figure 2.14: Concentration profiles of Ge and Cl in the GeDi OFs obtained by EMPA measurements.

Table 2 .

 2 .2. 2: Chemical compositions of the core zones of the P doped OFs and drawing conditions.

		Inner core zone (wt%)	Outer core zone (wt%) Drawing speed (m/min) Drawing tension (g)
	PD1	P 7.6	P 3	22	30
	PD2	P 7.6	P 3	35	40
	PD3	P 7.6	P 3	55	84
	PCe	P 12.6; Ce 1.4	P 12.6; Ce 0.7	15	27

Table 2 .

 2 .2.

		Inner core zone (wt%)	Outer core zone (wt%) Drawing speed (m/min) Drawing tension (g)
	ND1	N 2.65; Cl 3	N 1.35; Cl 1.6	30	85
	ND2	N 2.65; Cl 3	N 1.35; Cl 1.6	25	65
	ND3	N 2.65; Cl 3	N 1.35; Cl 1.6	20	45

3: Chemical compositions of the OFs and drawing conditions.

Table 2 .

 2

		Core zone (wt%)	Cladding inner zone (wt%)
	F2D3	F 0.2; Cl 0.1	F 1.4; Cl 0.1
	SiD3	F 0; Cl 0.1	F 1.5; Cl 0.1

4: Chemical compositions of the core zones of the OFs and drawing conditions.

Table 3 .

 3 1: Spectral parameters (peak center, x0 and Full Width at Half Maximum, FWHM) in eV, corresponding to the decomposition with gaussian bands of the spectra reported in Figure3.21.

	x0	FWHM	x0	FWHM	x0	FWHM	x0	FWHM
	30 °C 2.27	0.54	3.0	1.1	4.42	0.80	5.4	2.38
	150 °C 2.25	0.55	3.06	1.17	4.36	0.76	5.35	2.3
	280 °C 2.25	0.62	3.2	1.19	4.44	0.93	5.7	2.0

Table 3 .

 3 .2 the values of the spectral parameters employed for the decomposition are reported. 2: Spectral parameters (peak position, x0, and Full Width at Half Maximum, FWHM) in eV corresponding to the decomposition with gaussian bands of the spectra of Figure3.30.

	Band (POHC) Band (m-POHC)	Band (P2)		UV tail
	x0	FWHM	x0	FWHM	x0	FWHM x0 FWHM
	PD1 2.28	0.54	3.0	1.1	4.45	0.9	6.1	2.7
	PCe 2.26	0.55	/	/	/	/	5.8	2.6

i This type of E'(Si) is the one detected in the OF samples which were investigated in this PhD thesis.

vi The concentrations of the dopant-related defects are evaluated by considering only the mass of the core of the OF. In fact, no contribution can come from the cladding zone of the OF to a P-related EPR signals since no P is there contained.

Acknowledgements

¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

All absorption bands we used are chosen basing on literature data and have a gaussian shape, except for the one associated to NBOHC, which is conveniently described by a linear combination of five gaussian bands, as recently reported in [21]. In Table 4.1 all the employed bands are summarized together with their spectral characteristics and the associated microscopic structures. It is worth to note that a gaussian band centered at 6.25 eV was added to take into account high energy contributions to the RIA. Though such a band has been already reported in literature, no microscopic structure has been associated with it [20]. Furthermore, POR and Cl-related bands have been not definitively isolated. Finally, in the case of the F2D3-O2 sample we needed to add the absorption band relative to the interstitial ozone molecules, already reported in literature [START_REF] Kajihara | [END_REF], in order to fit the related spectra.

In Figure 4.10(a), we report the spectral decomposition of the F2D3-O2 OF at the dose of 100 Mrad. In Figure 4.10(b), the decomposition of the F2D3 OF is reported at the same dose and, finally, in Figure 4.10(c), we report the decomposition of the difference of the two previous spectra.

By comparing the decompositions it is found that the F2D3-O2 OF shows a higher concentration of NBOHC and the generation of interstitial O 3 molecules, the latter being absent in the F2D3 OF.

Both these defects are clearly associated with the oxygen-excess of the O 2 -loaded samples. No significant differences have been measured in the intensity of the band associated with the POR defects at about 5.3 eV. At variance, the O 2 -loaded OF shows a significantly lower generation of E'(Si) centers at the maximum irradiation dose. Finally, in the spectral region around 3.5 eV, where the Cl-related defects are usually detected, higher RIA values are measured in the case of F2D3-O2. These findings are confirmed by the difference spectrum reported in Figure 4.10(c) and its spectral decomposition. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ The concentrations of E'(Si) defects are 1.7•10 17 and 2.4•10 16 defects/cm 3 for PD2 and PD2-O2 OFs, respectively. As in the case of the PSC and F-doped OF the O 2 -loading decreases the number of radiation induced E'(Si), in this case by a factor 7. It must be reminded, by the way, that EPR measurements give us information only regarding the average volume concentration of defects, and in the case of E'(Si), which is a silica intrinsic defect, we cannot distinguish the ones contained in the core, which represent the 25 % of the total mass, from the ones contained in the cladding, which represent the 75% of the total mass. Since the studied PSC, F-doped and P-doped OFs all have a pure silica cladding, it is not surprising to suggest that they have similar total amounts of E'(Si) centers per mass unit. In this context the similarity of results among these fibers can be related in part to the cladding contribution.

In Figure 4.18, the second harmonic EPR spectra of the P1 and P2 defects for the two studied OFs are reported. The spectra were normalized by the masses of the respective samples.

Conclusions ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ drawing conditions. Obviously, from a practical point of view, it is interesting to point out that using the highest drawing speed is not detrimental for the performances of an OF employed in radiative environments.

 The radiation response of the P-doped OFs was studied at several irradiation temperatures.

We found that increasing the irradiation temperature from 25 to 100 °C slightly increases the radiation induced losses in the UV-visible domain. However, increasing the irradiation temperature beyond 150 °C mitigates the RIA in the same spectral range. Although it was impossible to perform RIA experiments in the NIR domain, we were able to detect, by EPR, the P1 defect in our irradiated samples and we found that the creation of this type of defect is promoted by the increase of temperature up to 280 °C (the highest temperature achievable by our experimental setup). This fact is particularly relevant because the P1 defect is associated with an absorption band peaked at 1.6 µm which is the main source of optical losses in the NIR domain in irradiated P-doped OFs. It is worth to mention that our post mortem EPR and OA measurements on γ-irradiated OFs had confirmed the association of the absorption band at 1.6 µm and the EPR signal of the P1 defect. Such correlation has not received many confirmations (or denials) since it was first proposed by Griscom et al. in 1983 [99].

 We performed a room temperature study of the radiation induced effects on a P-doped OF whose core was codoped with cerium. The confocal microluminescence studies have shown that part of the Ce codopant is incorporated in the form of Ce 3+ . After irradiation this ion is converted to (Ce 3+ ) + and contextually a lower concentration of POHC center is induced in this OF with respect to the Ce-free sample. These two phenomena are jointly explained by the captured of the released cerium electron by some of the radiation induced POHC center.

By EPR measurements we have also observed a significant reduction of the radiation induced P2 defect, which is an electron center, with respect to the simply P-doped OF. Such effect is tentatively attributed to a lower initial content of the of tetra-fold coordinated P atoms (PO 4 ) + , which is the precursor of P2, in the Ce-codoped sample as compared to the other one. Finally, our measurements have shown that in the Ce-codoped and Ce-free OFs a high dose γ-irradiation induces similar concentrations of P1 centers. The P1 is considered usually a hole center (E'-like) and the obtained result was probably the most unexpected. In fact, in previous investigations on Ce-codoped phosphosilicate OF the authors had reported an improvement of the radiation resistance in the NIR domain due to the suppression of the P1-related absorption band. Further investigations are certainly needed to clarify this point.

 We have investigated the effects of ionizing radiation at room temperature on germanosilicate OF codoped with fluorine. In order to highlight the effects of the codopant we have also performed complementary studies on the well-known codopant-free Ge-doped OFs and reported some previous results for comparison. The addition of fluorine has not affected significantly the RIA measurements in the UV-visible domain. The radiation

Appendix A

Calibration of the EPR second harmonic detection mode for the P2 defect.

The second harmonic EPR signal of P2 can be exploited to increase the detection sensitivity of this defect. Unfortunately, this detection mode does not by itself let determine the defects concentration.

However, if the P2 signal could be detected at high concentration of defects also by first harmonic EPR measurements a calibration could be carried out for fixed recording parameters of second harmonic detection.

Using the samples of PD2 OF irradiated at room temperature, it has been possible to calibrate the second harmonic detection. ------------------------------------------------------------A -1,02453E15