Ibrahim El Moussawi

Dinh Ho

Tong Minh

Nicolas Baghdadi

Chadi Abdallah

Jalal Jomaah

Olivier Strauss

Dinh Ho Tong

L-band UAVSAR tomographic imaging in dense forest: AfriSAR results

Keywords: AfriSAR campaign, TomoSAR, Phase Calibration, Lidar, Tropical forest

INTRODUCTION

Tropical forests have major impacts on earth's ecosystem in terms of carbon storage, regulating water and weather. Above Ground Biomass (AGB) is the most important parameter that is related directly to the amount of the carbon in the global ecosystem cycle [START_REF] Hajj | Interest of integrating spaceborne lidar data to improve the estimation of biomass in high biomass forested areas[END_REF]. Uncertainty in balancing the global carbon budget arises from a deficiency in AGB density estimation and carbon stocks in tropical forests. NASA , ESA, AGEOS and DLR have developed united efforts in the implementation of AfriSAR campaign on the Gabon forests. AfriSAR campaign aims to gather airborne and field data that support the establishing of vertical structure of forests and biomass missions. The airborne data provided by the AfriSAR campaign consist of Synthetic Aperture Radar (SAR) data and Light Detection And Ranging (Lidar) data .

Tomography SAR technique is widely used in characterizing the vertical structure of the forests and to estimate the canopy height of the trees. The potential of TomoSAR is in identifying the forests top height which is an essential parameter for the AGB estimation [START_REF] Minh | Sar tomography for the retrieval of forest biomass and height: Crossvalidation at two tropical forest sites in french guiana[END_REF][START_REF] Minh | Relating p-band synthetic aperture radar tomography to tropical forest biomass[END_REF][START_REF] Reigber | First demonstration of airborne sar tomography using multibaseline l-band data[END_REF]. Prior to TomoSAR analysis, there is need to compensate the phase residuals that influence the focusing of the vertical structure of forest in the 3D space. These phase disturbances come from atmospheric propagation delays or an error in allocating the platform position [START_REF] Tebaldini | Phase calibration of airborne tomographic sar data via phase center double localization[END_REF]. In order to eliminate these residuals, iterative phase calibration algorithm which is capable of estimating the phase screens and correct the data stack can be applied.

This study presents tomographic imaging on the Gabon Lope Park using L-band NASA/JPL UAVSAR data collected by AfriSAR campaign 2016 after it was adopted to Tebaldini approach [START_REF] Tebaldini | Algebraic synthesis of forest scenarios from multibaseline polinsar data[END_REF] to eliminate phase distortion from the tomographic data. The obtained vertical profiles were validated using Land, Vegetation, and Ice Sensor (LVIS) and Small Foot Print Lidar (SFL) data set as a reference.

The paper is organized as follows: Section 2 describes study area and presents the data set used in the tomographic imaging. Section 3 is dedicated to explain the tomography processing. The validation of the results is illustrated and interpreted in section 4. Section 5 concludes the paper and state the future works.

STUDY AREA AND DATA SET

The tomographic study was conducted on the Gabon National Lope Park. Lope National Park is one of the largest national parks in central Gabon covering an area of 4913 km 2 . Although the terrain is mostly rain forest, the scene in the north part is composed of the last remnants of grass savanna created in central Africa during the ice age 15000 years ago. The park was the first protected area in the Gabon. It is frequently tracked by the National Park Agency of Gabon which makes it well cited as a perfect scientific research site.

Different data sets have been acquired over the Lope during the AfriSAR campaign. The NASA AfriSAR campaign involved two data sets, UAVSAR data and Lidar data (LVIS and SFL data).

The L-band UAVSAR acquisition parameters and the baseline configuration of each track with respect to master track is shown in

TOMOGRAPHY PROCESSING

Multi-baseline SAR data were acquired over forests from slightly different altitude and incidence angles providing useful information in the vertical direction [START_REF] Minh | Sar tomography for the retrieval of forest biomass and height: Crossvalidation at two tropical forest sites in french guiana[END_REF]. The L-band UAVSAR airborne system provides fully polarimetric mode in the NASA AfriSAR campaign. TomoSAR is a signal processing algorithm used in treating the SAR data to focus the illuminated scatterers in the 3D space. Tomographic techniques composed of power estimation methods applied on the multi-baseline data to get the vertical profile of the forests [START_REF] Minh | Sar tomography for the retrieval of forest biomass and height: Crossvalidation at two tropical forest sites in french guiana[END_REF][START_REF] Minh | Relating p-band synthetic aperture radar tomography to tropical forest biomass[END_REF]. In this paper, Capon beam forming power estimator is applied to represent the vertical profile of vegetated areas. A fundamental issue have been taken into account before carrying on tomographic analysis is the phase calibration of the data. Various techniques were proposed to correct the tomographic data. Some techniques are based on the Permanent Scatterers (PS) in which the calibration performance could be defective by the nature of the scatterers having important limitations when applied on the tropical forests due to decorrelation that is caused by dense vegetation [START_REF] Tebaldini | On the role of phase stability in sar multibaseline applications[END_REF]. Another approaches proposed a phase calibration of mutli-baseline SAR data based on minimum entropy criterion [START_REF] Pardini | Phase calibration of multibaseline sar data based on a minimum entropy criterion[END_REF]. One more technique is based on the separation of the ground contribution and volume contribution in order to isolate the ground phase using Sum Kronecker Product (SKP) structure in presenting the data covariance matrix [START_REF] Tebaldini | Algebraic synthesis of forest scenarios from multibaseline polinsar data[END_REF]. Tebaldini et al [START_REF] Tebaldini | Phase calibration of airborne tomographic sar data via phase center double localization[END_REF] proposed recent strategy for calibrating the tomographic data based on the double localization of target position and sensor position errors responsible for the phase disturbances in the data stack. In our tomographic analysis over the Lope, we adopted phase calibration proposed in Tebaldini [START_REF] Tebaldini | Algebraic synthesis of forest scenarios from multibaseline polinsar data[END_REF].

TomoSAR calibration

The TomoSAR data phase calibration is done by eliminating the ground phase contributions from the tomographic Single Look Complex (SLC) data. The exploitation of MultiPolarimetric and MultiBaseline (MPMB) data allows the identification of different Scatterer Mechanisms (SM) [START_REF] Tebaldini | Algebraic synthesis of forest scenarios from multibaseline polinsar data[END_REF]. The separation of the ground contribution and the vegetation from MPMB data is performed by presenting the data covariance matrix through SKP [START_REF] Tebaldini | Algebraic synthesis of forest scenarios from multibaseline polinsar data[END_REF]. The structure model of the data covariance matrix, where each scatterer mechanism is presented by Kronecker Product (KP) of two matrices, is addressed as follows:

W = E[Y Y H ] = C g ⊗ R g + C ν ⊗ R ν ( 1
)
Where Y is the MPMB data vector, H is the Hermitian conjugate, W denotes the data covariance matrix, K is the total number of SMs that contribute to SAR signal, C g,v and R g,v are the polarimetric signature and structure matrix corresponding to the ground and volume contribution, respectively.

After the estimation of the ground covariance matrix, Phase linking algorithm is applied to estimate ground phases. After that, the calibration is carried out by removing ground phases from the original SLC data. Finally, Capon beam forming power estimation is applied to get the tomographic profiles.

TOMOGRAPHIC RESULTS OVER LOPE FOR L-BAND ACQUISITIONS

In this part we present the estimation of L-band tomographic profiles over a section from the lope in the range direction.

The experiment has been formed based on N=7 L-band fully polarimetric SAR images. The baseline spacing is 20 m, resulting a total baseline of 120 m. Capon beam forming power estimation have been applied on sliding window of 50x50 (pixels) for each position in the azimuth direction. The tomogram at different polarimetric channels has been validated by small foot print Lidar (SFL).

The backscatter SLC image where we had chosen the studied section at slant range equal to 500 is illustrated in the top of Fig. 1. The tomographic profiles from the capon beam forming power estimator is normalized between 0 and 1. It was presented at different polarization channels: HH and HV.

The results in Fig. 1 demonstrate that after the isolation and compensation of the ground contribution from the data stack, the tomographic profiles achieved a good correlation with the CHM and DTM from SFL data and clearly the canopy layer and ground level are detected at the two channels. A slight difference was observed between the two channels tomograms, at azimuth range between 2000 and 3000 (pixels), the HV volume scattering seems to be stronger than HH.

Qualitative comparison between the UAVSAR Capon profile (HH and HV) and LVIS waveform is illustrated in Fig. 2. The LVIS level1B has been used to plot the vegetation profiles. The difference between the shapes of LVIS and Capon profiles may be originated from the difference in penetration capabilities or a difference of profile resolution between the two platforms (UAVSAR and LVIS). When observing the Capon profiles, it is worth noticing the correlation between the width of the lobes (vegetation and ground ) and the distribution of the SFL data, this enables the estimation of the forest top height at the Lope. By comparing the HV Capon profile with HH, it can be shown clearly that the ground contribution is stronger regarding the volume contribution of HH channel than for the HV channel concerning the analyzed Region of Interest COL1, while at OKO1 the HH and HV profiles are unexpectedly similar. After observing The penetration of L-band into the vegetation in Fig. 1 and Fig. 2 , we can generalize a conclusion that L-band tomography can be feasible in dense forests.

CONCLUSION

In this paper, the TomoSAR analysis have been applied for estimation of the forest canopy height and terrain using Lband UAVSAR AfriSAR data collected over the Gabon Lope Park. We have shown the importance of eliminating the phase screens residuals from the data stack in order to improve the performance of TomoSAR analysis in illuminating correctly the scatterers in complex dense forests. The tomographic capon profiles at different sections in the forest are validated in a well correlation with SFL Lidar data after the calibration. Our perspectives for this work is to report for the status of using double localization method and SKP method to perform phase calibration of the tomographic data.

Fig. 1 .

 1 Fig. 1. HH and HV. Capon spectrum after phase calibration of the data and eliminating the phase disturbances. The black line and white line corresponding to DTM and CHM from SFL respectively.

Fig. 2 .

 2 Fig. 2. Regions of interest named OKO1 and COL1 in the Gabon Lope Park. HH (in red), HV (in green) Capon profiles after phase screen correction. Capon power and LVIS waveform power are normalized between 0 and 1. The histograms of SFL canopy and ground elevations are presented respectively in blue and in brown. The blue curve corresponding to LVIS waveform..

Table 1 .

 1 Table. 1 and Table. 2 respectively. LVIS is Large Footprint Lidar with full-waveform capability. The SFL data set was collected by the Sassan Saactchi's team JPL in July 2015, with foot print diame-L-band UAVSAR acquisition parameters.

	Acquisition Parameters
	Acquisition Mode	PolSAR
	Look Direction	Left looking
	Pulse Length	40 (µ s)
	Steering Angle	90 (deg)
	Bandwidth	80 (MHz)
	Ping-Pong or Single Antenna Transmit	Ping-Pong
	Air craft speed	224 (m/s)
	Range of look angle	21-65 (deg)
	Antenna Length	1.5 (m)

ter of 10 cm. It provides a uniform information for vegetation characterization such as: Canopy Height Model (CHM) and Digital Terrain Model (DTM).

Table 2 .

 2 Baseline configuration of Multi-baseline acquisition.

	Track Relative baseline
	1	master
	2	20m
	3	40m
	4	60m
	5	80m
	6	100m
	7	120m

ACKNOWLEDGMENT

This research was supported by IRSTEA (National Research Institute of Science and Technology for Environment and Agriculture), the National Council for Scientific Research Lebanon (CNRS-L), University of Montpelier and L'Agence universitaire de la Francophonie.


Dr Maria Rita Cicconi
email: maria.rita.cicconi@fau.de

W Blanc

D De Ligny

D R Neuville

The influence of codoping on optical properties and glass connectivity of silica fiber preforms

Keywords: REE, Raman, Photoluminescence, glass structure, cations structural role REE, Raman, Photoluminescence, glass structure, cations structural role

REE-doped glasses have applications in many fields, and for example, Al and Ce codoping in SiO2 fiber preforms is usually done to attenuate the optical degradation (photodarkening) of e.g. Yb and Tm doped silica optical fibers. In this study, we investigated a SiO2 fiber preform having a gradual increase of the CeO2 + Al2O3 content from 1.5 to 6.3 mol% and a CeO2/Al2O3 molar ratio varying from 0.04 to 1.4. Raman and photoluminescence spectroscopies have been used to evaluate changes in the short-and medium-range order both in the cladding and in the core portions, and the evolution of the glass optical properties, respectively. The variations of the excitation and emission bands have been linked to modifications in the vibration bands of the Raman spectra, and in turn, the relationship between optical properties and glass connectivity has been carefully considered. Our results show that there is a strong positive linear relationship between the increase of the Raman bands in the high-frequency range and the red-shift of the excitation maxima. Therefore, the change of <Ce-O> distances and covalency character influences, in the same way, both the photoluminescence behavior of the core, and the connectivity of the amorphous network.

Introduction

Since the 1960s, when the first commercially available silica-based optical fiber was developed, the applications of this material have greatly widened from telecommunication to sensors, to biomedical applications [START_REF] Ballato | Glass and Process Development for the Next Generation of Optical Fibers: A Review[END_REF][START_REF] Dragic | Materials for optical fiber lasers: A review[END_REF]. In a recent review, Ballato and coauthors [START_REF] Ballato | Glass and Process Development for the Next Generation of Optical Fibers: A Review[END_REF] report an overview of the main characteristics of optical fibers, and about the development of new materials and processes that will enable the design of advanced optical fibers suited to further uses. Above and beyond the development of new bulk chemistries or of composite materials (e.g. nanoparticles), researcher have first tried to enhance our understanding of SiO2-based optical fibers by studying the occurrence of defects under various conditions, or by investigating the effect of doping on the formation or suppression of defects e.g. [START_REF] Griscom | Nature Of Defects And Defect Generation In Optical Glasses[END_REF][START_REF] Girard | Overview of radiation induced point defects in silica-based optical fibers[END_REF][START_REF] Griscom | A Minireview of the Natures of Radiation-Induced Point Defects in Pure and Doped Silica Glasses and Their Visible/Near-IR Absorption Bands, with Emphasis on Self-Trapped Holes and How They Can Be Controlled[END_REF]. Many other studies were focused on the core composition, in order to avoid the clustering of Rare Earth Elements (REE), and enhance REE solubility, e.g. [START_REF] Arai | Aluminum or phosphorus co-doping effects on the fluorescence and structural properties of neodymium-doped silica glass[END_REF][START_REF] Ishii | Preparation of Cerium-Activated Silica Glasses: Phosphorus and Aluminum Codoping Effects on Absorption and Fluorescence Properties[END_REF]. Al and/or P and/or Ce codoping, besides increasing REE solubility, has been recognized as a very effective way to mitigate Yb-or Tmbased fibers optical degradation (aka photodarkening) [START_REF] Shao | Suppression mechanism of radiationinduced darkening by Ce doping in Al/Yb/Ce-doped silica glasses: Evidence from optical spectroscopy, EPR and XPS analyses[END_REF][START_REF] Lupi | Steady photodarkening of thulium alumino-silicate fibers pumped at 107 µm: quantitative effect of lanthanum, cerium, and thulium[END_REF]. For instance, a combination of Al or P, or the codoping of Al, P and Ce is able to reduce the photodarkening of Si-based fibers. This mitigation, in Ce-codoped fibers have been attributed to the coexistence of cerium with two stable oxidation state (trivalent and tetravalent), and the occurrence of two processes: either Ce 3+ that by trapping a hole may lose the 4f electron to form Ce 4+ species or Ce 4+ trapping an electron to form Ce 3+ species [START_REF] Shao | Suppression mechanism of radiationinduced darkening by Ce doping in Al/Yb/Ce-doped silica glasses: Evidence from optical spectroscopy, EPR and XPS analyses[END_REF][START_REF] Lupi | Steady photodarkening of thulium alumino-silicate fibers pumped at 107 µm: quantitative effect of lanthanum, cerium, and thulium[END_REF][START_REF] Engholm | Improved photodarkening resistivity in ytterbium-doped fiber lasers by cerium codoping[END_REF][START_REF] Jetschke | Efficient Yb laser fibers with low photodarkening by optimization of the core composition[END_REF][START_REF] Jetschke | Photodarkening kinetics as a function of Yb concentration and the role of Al codoping[END_REF]. Significantly fewer studies have been devoted to the understanding of how elements enter into the silica network, and how the different properties of the SiO2 optical fibers are modified e.g. [START_REF] Arai | Aluminum or phosphorus co-doping effects on the fluorescence and structural properties of neodymium-doped silica glass[END_REF][START_REF] Sen | Structural role of Nd3+ and Al3+ cations in SiO2 glass: a 29Si MAS-NMR spin-lattice relaxation, 27Al NMR and EPR study[END_REF]. Because of the strong link between glass structure and properties, an understanding of the cations speciation would lead not only to the understanding of the mitigation of the photodarkening, but also to the understanding of the glass fibers properties on a broader scale. Indeed, this lack of knowledge hampers the development of new materials and applications. Consequently, we started an investigation of silica optical fiber preforms codoped with cerium and/or aluminum, focusing on the element structural role and glass connectivity. In a recent study, we assessed the role of aluminum in influencing Ce speciation (valence, coordination, bond character) in aluminosilicate glasses and Ce-doped or Ce/Al codoped silica fiber preforms, collapsed under different conditions [START_REF] Cicconi | Cerium/aluminum correlation in aluminosilicate glasses and optical silica fiber preforms[END_REF]. The addition of aluminum prompts the stabilization of reduced Ce species, and induces variations of the Ce local environment, causing a blue shift of the photoluminescence emission band. Furthermore, it was established a Ce redox ratio limit (Ce 3+ /Cetot > 0.85 ± 0.07) that marks the clear occurrence of additional bands in the UV region. Based on the results, it was evaluated that the photodarkening attenuation is most probably due to to the ability of trivalent Ce to trap hole centers. Since Ce redox ratio and bond covalency change depending on Al concentration/structural role, cerium photoluminescence properties were correlated by considering the glass bulk composition (in terms of optical basicity), and Al average coordination [START_REF] Cicconi | Cerium/aluminum correlation in aluminosilicate glasses and optical silica fiber preforms[END_REF]. Despite the remarkable results obtained, codoping at the dilute scale may influence differently the response of the properties.

In the present study, by using a SiO2 fiber preform having a gradual change of the Al2O3/CeO2 molar ratio codoping, we correlate cerium photoluminescence properties to the variations occurring in the glass connectivity, and in turn on the codopant structural role (valence, bond distances, and coordination). Raman spectroscopy was used to identify changes in the short-and medium-range order of cladding and codoped cores, and these results were associated to the variations observed in the luminescence properties, and in turn, to the Ce species structural environment.

In addition to photodarkening attenuation, the luminescence properties of cerium (and REE in general) are quite important in many other fields, including phosphors, scintillators and generation of fiber gratings [START_REF] Chiodini | SiO2-based scintillating fibers for x-ray detection, in: Hard X-Ray Gamma-Ray Detect[END_REF][START_REF] Bettinelli | Optical spectroscopy of Ce3+, Tb3+ and Eu3+ in new scintillating glasses[END_REF][START_REF] Dong | Photosensitivity in Ce^3+-doped optical fibers[END_REF][START_REF] Morey | Photoinduced Bragg Gratings in Optical Fibers[END_REF][START_REF] Blasse | Investigation of Some Ce3+-Activated Phosphors[END_REF]. Hence, understanding the evolution of cerium (REE) valence and local environment and the effects on the host structure has a key role in the development of new materials and technical applications.

Experimental Methods

Preform was prepared by the conventional modified chemical vapor deposition (MCVD) technique. In order to obtain a gradual change of the codopant molar ratio along the preform, aluminum and cerium were incorporated with the gradual solution doping technique [START_REF] Lupi | Gradual-Time Solution Doping for the Fabrication of Longitudinally Varying Optical Fibres[END_REF]. Two doping solutions were prepared: solution A contains 1 mol/l of AlCl3:6H2O; in solution B, CeCl3:6H2O and AlCl3:6H2O concentrations were 1 and 1 mol/l, respectively. The pure silica core porous layer was soaked first with solution A. Then, solution A is removed and the solution B is injected in the preform and slowly drained until the bottom (draining time: 20 minutes) to induce a change in cerium concentration. After drying of the solvent, the core layer was sintered down to a dense glass layer in O2 flux. Then, the tube was collapsed under the same flux into a solid rod, referred to as preform, at an elevated temperature higher than 1800 °C.

Small disks (1 mm thickness) of the solid rod (9 mm diameter) have been cut at different positions along the preform. Each disk represents a different CeO2/Al2O3 molar ratio in the codoped core, starting from the sample cut at 4 cm (CeO2/Al2O3 = 0.04), up to the sample cut at 32 cm (CeO2/Al2O3 = 1.4) as well as a variation of the CeO2 + Al2O3 content from 1.5 to 6.3 mol% (see Table 1). The surrounding cladding is composed almost exclusively by SiO2 (99.98 -99.89 mol%), with small impurities of Al2O3. Disks are labeled according to the distance, and then to the increasing CeO2/Al2O3 molar ratio. A schematic drawing (Figure 1) reports sample labels, and codopant molar ratios, whereas the core composition is reported in Table 1. Images and chemical analyses have been obtained by Energy dispersive X-ray (EDX) and scanning electron microscope (SEM) analysis at the Institute de Physique du Globe de Paris (France).

The SEM-EDX mapping allowed checking the homogeneous distribution of the dopants in the core portions. Photoluminescence spectra of each disk core have been collected with a spectrofluorometer equipped with double monochromators (Czerny-Turner) in excitation and emission (Fluorolog3, Horiba Jobin Yvon), using a 450 W Xe-lamp as the excitation source (excitation and emission spectral resolution ≤ 1.5 nm). Spectra have been normalized to the maximum intensity. Raman spectra have been collected both with laser excitations at 488 nm and 532 nm at the Department of Materials Science and Engineering (WW3, FAU Erlangen-Nürnberg, Germany). The Thermo Scientific Nicolet TM Almega Raman spectrometer (532nm) coupled with a high-quality Olympus visible microscope (objective 100x), and a highresolution grating (2400 lines/mm) provides a very good spatial and frequency resolution, respectively of ~1 µm and 1 cm -1 . All spectra have been normalized to the total area, after removal of a polynomial background. Moreover, Raman spectra were also collected with a different excitation line (488nm) in order to check the presence of any contribution arising from luminescence centres.

Results and discussion

Ce 3+ , contrary to the oxidized Ce 4+ species, is optically active and has fluorescence emission that originates from a transition from one (or more) of the 5d levels, to the 2F ground state [START_REF] Ishii | Preparation of Cerium-Activated Silica Glasses: Phosphorus and Aluminum Codoping Effects on Absorption and Fluorescence Properties[END_REF][START_REF] Das Mohapatra | A spectroscopic study of cerium in lithium-alumino-borate glass[END_REF][START_REF] Reisfeld | Spectroscopic properties of cerium in glasses and their comparison with crystals[END_REF]. Trivalent cerium has a [Xe] 4f 1 ground electronic state, which is split into two sublevels (doublet 2 F5/2 and 2 F7/2), separated by about 2200 cm -1 , and the excited levels 5d lie about 40000 cm -1 above the ground state [START_REF] Reisfeld | Spectroscopic properties of cerium in glasses and their comparison with crystals[END_REF][START_REF] Fasoli | Effect of reducing sintering atmosphere on Ce-doped sol-gel silica glasses[END_REF]. Despite the well-shielded inner electronic shell, the f-d transitions are strongly dependent on the chemical and structural environment.

Hence, both Ce 3+ excitation (Ex) and emission (Em) spectra might shift depending on Ce content, temperature, glass polymerization and connectivity [START_REF] Ishii | Preparation of Cerium-Activated Silica Glasses: Phosphorus and Aluminum Codoping Effects on Absorption and Fluorescence Properties[END_REF][START_REF] Cicconi | Cerium/aluminum correlation in aluminosilicate glasses and optical silica fiber preforms[END_REF][START_REF] Reisfeld | Spectroscopic properties of cerium in glasses and their comparison with crystals[END_REF][START_REF] Paul | Ultraviolet absorption of cerium(III) and cerium(IV) in some simple glasses[END_REF][START_REF] Herrmann | Spectroscopic properties of cerium-doped aluminosilicate glasses[END_REF]. For example, in a previous study on Na aluminosilicate glasses we observed that both Ce 3+ excitation and emission bands shift depending on the Al/Na molar ratio, and drastically change for peraluminous compositions [START_REF] Cicconi | Cerium/aluminum correlation in aluminosilicate glasses and optical silica fiber preforms[END_REF].

Figure 2 shows the photoluminescence spectra for all codoped core samples. Excitation spectra were collected at the maximum of the emission, and vice versa. By increasing Ce content, and in turn, the CeO2/Al2O3 molar ratio, there is a gradual shift of both Ex and Em bands toward lower frequencies. The excitation band is an asymmetric broad band that can be deconvoluted in three contributions, with the first two, respectively at ~ 35000 cm -1 (~ 285 nm; G3Ex), and ~ 32300 cm -1 (~ 310 nm; G2Ex), remaining nearly at the same frequency, despite the different compositions (Table 2). On the contrary, the third band is a very small feature centered at ~ 30900 cm -1 (~ 324 nm; G1Ex) in Ce-poor preform (sample 4), which represents only 11% of the whole excitation band area (Figure 2). By increasing the CeO2 content, this feature gradually shifts toward lower frequencies and increases its intensity, becoming the stronger contribution in sample 32 (representing ~ 50% of the whole excitation band). The Ce-rich preform (sample 32) has this contribution centered at ~ 29600 cm -1 (~ 339 nm) (Table 2).

The asymmetric luminescence is related to the allowed electronic transition 5d → 4f, and two sublevels of Ce 3+ transitions from the 5d to 2F7/2 and 2F5/2 states are clearly discernible (Figure 2). The emission bands have been analyzed and deconvoluted in two Gaussians that are split by ~2300 cm -1 apart, in agreement with the L-S splitting of Ce 3+ ions [START_REF] Reisfeld | Spectroscopic properties of cerium in glasses and their comparison with crystals[END_REF][START_REF] Fasoli | Effect of reducing sintering atmosphere on Ce-doped sol-gel silica glasses[END_REF]. Sample 9 presents the minimum split of ~2180 cm -1 (see Table 2), and has the lowest Stokes shift (~ 5100 cm -1 ). By increasing the Ce content, there is a general decrease of the Em asymmetricity, and an increase of both Stokes shifts and ground level splitting. Additionally, the emission shows a red shift, with the Em maximum moving from 27400 cm -1 to 24000 cm -1 (~ 366 and 416 nm, respectively), by increasing Ce content (Figure 2). Band position and results from the bands' deconvolution of both excitation and emission spectra are reported in Table 2. Absorption bands of Ce ions in glasses, attributed to the 4f1 → 5d1 transition, have been reported in the wide-ranging region 28500 -50000 cm -1 , depending on the bulk chemistry. In this region, two broad contributions related to charge transfer (CT) transition of Ce 4+ overlap with the Ce 3+ ions 4f1 → 5d1 bands [START_REF] Ishii | Preparation of Cerium-Activated Silica Glasses: Phosphorus and Aluminum Codoping Effects on Absorption and Fluorescence Properties[END_REF][START_REF] Reisfeld | Spectroscopic properties of cerium in glasses and their comparison with crystals[END_REF][START_REF] Reisfeld | Spectroscopic properties of cerium in solgel glasses[END_REF][START_REF] Brandily-Anne | Specific absorption spectra of cerium in multicomponent silicate glasses[END_REF][START_REF] Ebendorff-Heidepriem | Formation and UV absorption of cerium, europium and terbium ions in different valencies in glasses[END_REF]. The position and the intensity of Ce 4+ CT absorption bands change remarkably depending on glass (host) composition [START_REF] Paul | Ultraviolet absorption of cerium(III) and cerium(IV) in some simple glasses[END_REF][START_REF] Duffy | Ultraviolet absorption and fluorescence spectra of cerium and the effect of glass composition[END_REF][START_REF] Bei | Optical properties of Ce 3+-doped oxide glasses and correlations with optical basicity[END_REF]. Ce 3+ emission band as well may cover a wide range of frequencies, depending on glass composition, and in particular, the presence of aluminum causes a marked blue shift of the luminescence band in Ce-doped glasses (e.g. [START_REF] Ishii | Preparation of Cerium-Activated Silica Glasses: Phosphorus and Aluminum Codoping Effects on Absorption and Fluorescence Properties[END_REF]), along with a change in the cerium average oxidation state [START_REF] Cicconi | Cerium/aluminum correlation in aluminosilicate glasses and optical silica fiber preforms[END_REF]. Based on the previous investigations on the influence of bulk chemistry (optical basicity) and Al content on Ce redox state [START_REF] Cicconi | Cerium/aluminum correlation in aluminosilicate glasses and optical silica fiber preforms[END_REF][START_REF] Baucke | Redox reactions between cations of different polyvalent elements in glass melts. An optical basicity study[END_REF][START_REF] Paul | Cerous-ceric equilibrium in binary alkali borate and alkali silicate glasses[END_REF], we do not expect a very high amount of oxidized species in the preform cores. Because of the low basicity of the glass (very high amount of SiO2) and a CeO2/Al2O3 molar ratio > 0.1, we estimate a Ce 3+ /Cetot ratio ≤ 0.8 for all glasses, except for the Ce-poor one (sample 4). The latter has > 98 mol% of SiO2, and a CeO2/Al2O3 ratio of 0.04, and an expected Ce 3+ /Cetot ratio ≥ 0.9. In addition, since the excitation bands of samples 32 to 9 are very similar to those previously reported for aluminosilicate and (Ce, Al) codoped silica fiber preforms [START_REF] Cicconi | Cerium/aluminum correlation in aluminosilicate glasses and optical silica fiber preforms[END_REF], we are confident that they should have a similar average estimated Ce 3+ /Cetot ratio. In contrast, sample 4 excitation has the two strongest contributions at higher frequencies: In particular, the contribution at ~ 35700 cm -1 (~ 280 nm), has a frequency compatible with the occurrence of band(s) related to charge transfer transitions. In peraluminous silicate glasses and (Ce, Al) codoped silica fibers this band has been observed to become predominant for high amount of reduced Ce species [START_REF] Cicconi | Cerium/aluminum correlation in aluminosilicate glasses and optical silica fiber preforms[END_REF]. Since sample 4 has an expected Ce 3+ /Cetot ratio ≥ 0.9, following optical basicity considerations, we might associate as well the high-frequency band in the excitation signal to trapped hole centers, and in turn, to trivalent cerium that may lose the 4f electron: Ce 3+ + h + = Ce 3++ + e -.

In order to exclude any possible contribution from the silica glass, the photoluminescence signals of some of the cladding portions have been measured as well, by using the same experimental conditions used for the codoped cores. It was observed the occurrence of some bands in the outer silica claddings, with visible intensity variations across the portion analyzed. The broad contributions observed have maxima centered at ~ 27000 and ~ 22700 cm -1 (excitation and emission maxima, respectively). These bands may be related to some impurities, and most likely to the occurrence of Si defects in the cladding. The most intense bands observed have been compared to the signals collected in the core (Figure 3).

Photoluminescence spectra of the codoped cores have been collected at the maximum of the excitation or of the emission signals, indicated by the vertical black arrows in Figure 3. The signal intensity increases by increasing the Ce molar content, both in the excitation and in the emission. The strongest excitation signal observed in the different cladding (grey solid line) consists of a broad asymmetric band centred at ~ 370 nm and a minor contribution at ~ 300 nm (orange vertical arrow). Two emission spectra are reported for the cladding, one collected with an excitation wavelength of 300 nm (orange solid line, a position almost overlapping the maximum of sample 4) and one collected at 350 nm (grey solid line). Even if the emission of Ce-doped samples (9 to 32) overlaps the one of the cladding, the contribution that the cladding might provide is at least 6 times smaller and shifted at lower frequencies. The Ex signal of the cladding is partially overlapping the cores Ex bands, and may contribute to the signal only on the tail at lower frequencies, thus without influencing the three main contributions (Figure 2). Therefore, the influence of the silica cladding on Ce bands is very limited. In order to better understand the frequency shifts and the photoluminescence evolution depending on the Ce content, we have taken into account the possible occurrence of different cerium populations. Figure 4 shows the excitation spectra of samples 4 and 32 (black solid lines), and the three Gaussian functions used for signal deconvolution (dotted lines). Emission spectra have been collected for different excitation wavelengths (vertical arrows) for both samples. When the excitation increases from 280 to 335 nm (~ 35700 to ~ 29900 cm -1 , respectively) the emission of sample 32 barely shifts (~ 620 cm -1 ), and the two Emission bands have a very similar shape. Therefore, it is reasonable to consider that we are exciting the same electronic level. On the contrary, in the Ce-poor glass (sample 4) the Em maximum drastically shifts (up to ~3900 cm -1 ) for the same excitation wavelengths reported above. Furthermore, besides the change of the Em maximum depending on the excitation wavelength, there is also a variation of band symmetry (Figure 4). In the case of crystalline materials, it has been reported that the distortion of Ce 3+ sites affects the crystal field splitting of the 5d configuration, and that the occurrence of vacancies might contribute in changing Ce charge distribution. Consequently, both would lead to red shift emission and changes in band symmetry [START_REF] Dorenbos | 5d-level energies of Ce 3+ and the crystalline environment[END_REF][START_REF] Wang | Luminescence properties and its red shift of blue-emitting phosphor Na3YSi3O9 :Ce3+ for UV LED[END_REF][START_REF] Dorenbos | Ce3+ 5d-centroid shift and vacuum referred 4f-electron binding energies of all lanthanide impurities in 150 different compounds[END_REF]. In case of amorphous materials, stronger crystal field splitting would lower the energy level of the 5d configuration. Moreover, the increase of bond covalency character as well might affect the 5d orbital. Hence, both covalency and crystal field increases induce a red shift. Following these considerations, we might expect that the different emission signals of sample 4 derive from the distribution of Ce in different sites, and in turn, to the different chemical environments surrounding Ce 3+ ions (i.e. <Ce-O> distances and bond covalency). In particular, we could consider that the shorter wavelength emitting site is less covalent in nature, whereas the longer wavelength emitting site is less ionic. The increase of Ce content induces a red shift of the excitation because the longer wavelength emitting site becomes predominant. The Raman spectra of silicate glasses can be divided in different major regions, and we will consider two of them: the high and the low-frequency regions. The low-wavenumber region from ~ 350 to 650 cm -1 , gives information on the T-O-T rocking, bending mode and ring distributions (see [36] and references therein). The high-wavenumber portion extending from ~ 860 to 1300 cm -1 (also referred as Q-range) contains stretching motions of tetrahedral cations T (e.g. T = Si 4+ , Al 3+ ) [36]. The short-range order of silicate glasses can be described through the abundance of the Q n species, where n is the number of bridging oxygen atoms (BO) and Q represents the four-fold coordinated cation (i.e. fully polymerized = Q 4 ). A fully polymerized SiO2 glass network has only weak bands in the frequency range ~ 950 -1300 cm -1 , whereas it has a strong asymmetric band in the low-frequency region (350 -500 cm -1 ). Moreover, highly polymerized glasses have two well-pronounced peaks, usually referred to as defect lines, D1 (~ 490 cm -1 ) and D2 (~ 600 cm -1 ), associated, respectively, with the breathing modes of (SiO)4and (SiO)3-rings [START_REF] Sharma | Raman investigation of ring configurations in vitreous silica[END_REF][START_REF] Pasquarello | Identification of Raman Defect Lines as Signatures of Ring Structures in Vitreous Silica[END_REF]. In general, by increasing the number of non-bridging oxygen (NBO), the main band in the low-frequency region decreases in intensity and shifts to higher wavenumbers, whilst the Q-range shifts toward lower frequencies and increases its intensity because other bands (related to other Q species, such as Q 3 , Q 2 ) appear in the 850-1300 cm -1 portion ( [36] and references therein). SiO2 glass vibrations in the Q-range can be deconvoluted with three main contributions that represent different types of stretching vibration of the SiO4 tetrahedra [START_REF] Mcmillan | The structures and vibrational spectra of crystals and glasses in the silica-alumina system[END_REF][START_REF] Mcmillan | A study of SiO2 glass and supercooled liquid to 1950 K via high-temperature Raman spectroscopy[END_REF][START_REF] Galeener | Comparison of the neutron, Raman, and infrared vibrational spectra of vitreous SiO2, GeO2, and BeF2[END_REF][START_REF] Seifert | Three-dimensional network structure of quenched melts (glass) in the systems SiO2--NaAlO2, SiO2--CaAl2O4 and SiO2--MgAl2O4[END_REF], and here we use the same terminology adopted in our previous work (see [START_REF] Cicconi | Cerium/aluminum correlation in aluminosilicate glasses and optical silica fiber preforms[END_REF] and reference therein). Indeed, to describe the vibration of the SiO4 tetrahedra in silica glass we use Q 4,I , and Q 4,II species (at frequencies higher than 1100 cm -1 ), and the T2 stretching mode at ~1050 cm -1 (the latter related to two oxygen atoms that move closer to the central T atom, while the other two oxygens of TO4 tetrahedra move away) [START_REF] Pasquarello | Dynamic structure factor of vitreous silica from first principles: Comparison to neutron-inelastic-scattering experiments[END_REF].

Because of the very low doping level of

The cladding portion of all samples is nearly completely composed of SiO2 (Al2O3 impurities ≤ 0.13 mol%), thus the Raman spectrum of a pure SiO2 glass (QSIL®), collected with the same experimental conditions, has been used as reference (dashed line in Figure 5A).

In the cladding portions analyzed, Raman spectra show some small variations compared to the reference SiO2, mainly on the intensity of the defect lines, and on the relative intensity of the bands in the Q-range. In SiO2-rich amorphous materials having less than 4 mol% Al2O3, NMR data indicates the occurrence of Al 3+ mostly 4-fold coordinated ( [4] Al 3+ ) with small contributions from distorted [6] Al 3+ (6-fold coordinated) [START_REF] Sen | High-resolution multinuclear NMR structural study of binary aluminosilicate and other related glasses[END_REF]. Al, as network former, mainly enters the more distorted Q 4,II sites causing an intensity increase and a small negative shift of this band [START_REF] Cicconi | Cerium/aluminum correlation in aluminosilicate glasses and optical silica fiber preforms[END_REF][START_REF] Navrotsky | The Tetrahedral Framework in Glasses and M e l t s -Inferences from Molecular Orbital Calculations and Implications for Structure , Thermodynamics , and Physical Properties[END_REF]. These considerations fit well with the signal collected in the cladding, and thus, we can relate the small variations observed among the different claddings to the inclusion of Al 3+ in the tetrahedral sites, which induces different average <T-O> distances and small changes in the ring statistic (insets in Figure 5A). The mathematical average of the seven different claddings (hereafter called 𝐶𝑙𝑎𝑑 %%%%%% ) was done in order to have a reference spectrum that takes into account the chemical variability found in the outer cladding. The 𝐶𝑙𝑎𝑑 %%%%%% spectrum is reported in Figure 5B (solid black line) and compared to the spectra collected in the core areas.

In the core portions analyzed, the (Ce, Al) codoping induces much more visible changes in the glass network, both in the short-and medium-range order. At low frequencies, the main T-O-T band and the defect bands decrease in intensity and the latter become broader, compared to SiO2 and 𝐶𝑙𝑎𝑑 %%%%%% (Figure 5B). Moreover, there is a small shift (~ 2-3 cm -1 ) toward higher frequencies of the maximum of the low-frequency region. These changes indicate a lower quantity of smaller (SiO)4-and (SiO)3-rings with the increase of the doping, and the shift to higher wavenumbers of the Main Band suggests a small decrease of the ring intertetrahedral angles T-O-T. The strongest variations observed in the cores are in the 860 -1300 cm -1 frequency region, where new contributions systematically rises (between ~ 860 and 1050 cm -1 ) by increasing the CeO2 content (inset Figure 5B). In order to evaluate the modifications occurring in the glass structure upon incorporation of Ce and Al oxides, and the influence of the different Ce contents, we studied in detail the high-frequency region by means of two approaches. The first one is a subtractive method where the signal of the cladding has been subtracted to the signal collected in the core, and the difference spectrum has been deconvoluted by mathematical functions. The second approach involves the more classical deconvolution of the whole high-frequency portion.

In order to separate the contributions to the Raman spectra due only to the Al/Ce codoping, the signal collected into the cladding has been subtracted to the one collected in the core region, for each sample. The resulting difference spectra highlights the distinct portions where the vibrational bands are affected by the incorporation of Al2O3 and CeO2 (Figure 6A).

In the low-frequency region, the relative intensity of the defect lines changes depending on the amount of codoping. Furthermore, the shift of the main band at ~ 434 cm -1 is evident (Figure 6B). In the high-frequency portion, the contribution in the Q-range, 860-1050 cm -1 , strongly increases by increasing the Ce and Al content (Figure 6C). The modifications observed in the asymmetric band centred at ~ 800 cm -1 will not be discussed here, preferring to focus on changes in the Q-range. Figure 6D reports an example of the subtractive method used here. The cladding and core spectra collected for sample 32 are shown in Figure 6D, along with the difference spectrum obtained (symbols). The latter has been fitted, and four contributions can be distinguished for high codoping: two well defined bands that are related to the two main peaks (named P2 and P3), and two smaller side bands associated with the shoulder observed for each of the main peaks (P1 and P4, respectively at lower and higher frequencies in Figure 6D).

The difference spectrum of sample 4 presents a single contribution centred at 1120.2(2) cm -1

(Figure 6E). The difference spectrum is well fit with a Lorentzian function with a full-width half-maximum FWHM = 45.5(3) cm -1 (the use of a Gaussian for this sample would imply the use of other two side bands, and we prefer to use the minimum number of bands to achieve a satisfactory fit). The difference spectrum obtained between pure SiO2 and a reference glass having 98.8 mol% SiO2 and 1.2mol% Al2O3 indicates a residual contribution with a nearly identical shape and position (1120.9(10) cm -1 ; unpublished data). Since sample 4 core has 1.5 mol% of Al2O3, and only 0.05mol% of CeO2 we can affirm with confidence that this contribution is only due to the Al 3+ cations entering the tetrahedral sites. By increasing the Ce content to ~1 mol%, another main contribution appears, which represents 50% of the whole area. Indeed, sample 9 difference spectrum requires two Gaussian bands, named P2 and P3, respectively centered at 988.5(2) cm -1 and 1118.7(4) cm -1 to fit the two dominant contributions (see Figure 6E). Additionally, a small shoulder is visible in the high-frequency portion (P4 band). Further addition of CeO2 induces the increase of the P2 band, and the appearance of the shoulder at lower frequencies (P1). Thus, these two bands (P1+P2) contribute to more than 60% of the total difference spectra (see Table 3).

The subtractive method here used allows underlining that the main variations in the Qrange are related to the appearance of the P1 and P2 contributions (respectively at ~ 917 and ~ 985 cm -1 ) and their rise by increasing the CeO2 and Al2O3 content. Based on this data analysis, we propose the following assignment for the two main contributions: P3 to Al-O stretching modes and P2 to the presence of cerium ions. The second method used to study the variations occurring in the Raman signals involves the more classical fit of the whole high-frequency region. The deconvolution has been done for all samples, both in the cladding and codoped core portions. As a reference, the analysis of the Raman signals of the SiO2 glass and of the average cladding 𝐶𝑙𝑎𝑑 %%%%%% was done as well in the range 950-1300 cm -1 . As reported above, pure SiO2 glass weak bands in the Q-range can be deconvoluted with three main contributions related to Q species with four bridging oxygens. In the cladding portions, these three bands are assigned to Q 4,I , Q 4,II and T2 stretching modes (see Figure 7). A small negative frequency shifts of the Q 4,II species is probably due to Al 3+ cations entering the tetrahedral sites. The results of the fit of SiO2 and of the average cladding (𝐶𝑙𝑎𝑑 %%%%%% ) Q-range Raman signals are reported in Table 4. In the codoped core, besides these three main contributions, other two bands appear in the lower part of the Q envelope (< 1050 cm -1 ), and increase in intensity by increasing the (Ce, Al) doping (see Figure 7). These two contributions below 1050 cm -1 are not related to electronic transitions (luminescence) because they have the same intensities and Raman shift position even when collected with a different excitation line.

In order to give stable fits, the full-width half maximum (FWHM) of the Gaussian functions should be constrained below 60 cm -1 [START_REF] Bancroft | Lorentzian dominated lineshapes and linewidths for Raman symmetric stretch peaks (800-1200 cm-1) in Qn (n = 1-3) species of alkali silicate glasses/melts[END_REF], thus, a further contribution between 1090 -1110 cm - 1 should be added. This supplementary band would take into account the occurrence of Q 3 species possibly due to both Ce ions and higher coordinated Al 3+ species acting as network modifiers. However, due to pure fit stability, we decided to avoid including this further contribution. Therefore, only five Gaussian functions were used to deconvoluting core Raman spectra: the three related to TO4 tetrahedra (hereafter called G1 to G3) and two additional contributions at lower frequencies (hereafter called G4 and G5).

By increasing the codoping, the bands G1 to G3 show a minor increased intensity compared to the cladding, and a shift of the component at ~ 1150 cm -1 (G2). On the other hand, the two additional bands at lower frequencies (G4, G5) strongly increase by increasing the cerium content. Thus, particular attention will be given to the frequency and intensity evolution of the bands at ~ 925 cm -1 (G 4 ) and ~ 980 cm -1 (G 5 ). This increase in G4 and G5 intensities leads the whole Q envelope to move toward lower frequencies. To represent this tendency we used the center of mass C (or centroid) of the analyzed range, calculated as (see (where Atot is the total area of the Q-range; see Table 4) resulting in ~ -60 cm -1 shift of the highfrequency band by increasing the Ce, Al codoping from 1.5 to 6.3 mol%. As reference, the center of mass of pure SiO2 has been estimated at ~ 1119.7 cm -1 , whereas the one related to the average cladding signal is ~ 1116 cm -1 (see Table 4). Sample 32 core has a very high amount of CeO2 (~3.7 mol%) and does not follow the trend observed for the other codoped cores. A possible explanation takes account of the clustering taking place because of the high Ce content.

This deconvolution method, as the subtractive one, highlights that the higher doping induces strong changes in the glass topology, and the rise of the bands in the lower frequency of the Q-range is associated to the higher content of Ce ions. Despite the increase of codopant amount in the core (and in turn, the decrease of silica), it seems that there is not a strong depolymerization since the Raman Main Band was only slightly affected. A decrease of Q 4 species is anyway restricted since Al 3+ ions tend to enter preferentially the Q 4,II site. The main role of cerium ions is then uncertain. It could be charge-compensator of Al 3+ ions, and/or directly connected to the tetrahedral network. Then, the contributions G4 and G5 could be associated to a weakening of the stretching of the bond Si-O when Ce is nearby, by analogy of recent assignments done by Nesbitt et al. [START_REF] Nesbitt | Factors affecting line shapes and intensities of Q3 and Q4 Raman bands of Cs silicate glasses[END_REF]. In order to assess and link structural changes to macroscopic properties in the codoped cores, we correlated the variations observed in the glass connectivity to the cerium photoluminescence properties, by taking into account the center of mass and the integrated areas of the deconvoluted bands in the Q-range, and the frequency position of Excitation and Emission signals.

By changing the codopant amount in the core portions, it is possible to observe a common trend for all the parameters taken into account, related both to the glass structure and to the optical properties. The decrease of the Q-range center of mass is very well correlated with the red shift of both Excitation and Emission maxima. Indeed, by increasing the codoping, the center of mass of the Q-range drop, similar to the drop observed in the photoluminescence maxima (Figure 8A). Furthermore, we can observe that sample 4 has the highest value of the Q-range center of mass, which in turn corresponds to the highest values of Ex and Em. Sample 4 presents the most different photoluminescence properties and its C value is very close to the one calculated for the average cladding. A plausible explanation could be that a very low doping level of Ce 3+ ions induces these species to associate not to the aluminosilicate network but to dangling point defects. Instead, when in higher amounts Ce ions tend to associate to the aluminosilicate network before reaching the limit of cerium clusters formation. This could be appreciated by looking at the increase of the integrated area of the bands in the lower portion of the Q-range (Figure 8B), and especially to the practically linear correlation between the sum of the integrated area of the P1 and P2 bands and the frequency of the excitation maxima The correlation observed between glass short-range order and optical properties endorses that Ce species are strongly connected to the glass network, and the small changes in the medium-range order supports the conclusion that Ce 3+ ions partially act as chargecompensator to neighboring Al polyhedral (AlO4 -, AlO5 3-). Consequently, ⟨Ce-O⟩ connectivity changes depending on Al coordination/content, in agreement with previous experimental and molecular dynamic studies [START_REF] Cicconi | Cerium/aluminum correlation in aluminosilicate glasses and optical silica fiber preforms[END_REF][START_REF] Du | Structure of cerium phosphate glasses: Molecular dynamics simulation[END_REF] and references therein.

Conclusions

We investigated a series of silica preforms having different codopant contents in the core, and variations of the CeO2/Al2O3 molar ratios from 0.04 to 1.4. Raman spectroscopy has been used to evaluate changes in glass connectivity and modifications in the short-and mediumrange order both in the cladding and in the core portions. Photoluminescence studies of the codoped cores allowed evaluating the evolution of the glass optical properties.

This work allowed determining a correlation between glass topology and optical properties and to better understand the short-range order variations depending on the silica fiber preform codopant ratios. In particular, we observed that:

-by increasing the CeO2-Al2O3 content, there is a red shift of both excitation and emission spectra, and at the same time, a clear shift toward lower frequencies of the Raman Qrange;

-very low doping level of Ce 3+ ions (0.05 mol%) probably induces these species to associate not to the tetrahedra network but to point defects;

-when in higher amounts Ce ions behave differently, and the positive correlation between glass short-range order and optical properties endorses that Ce species are strongly associated to the aluminosilicate glass network;

-the change of <Ce-O> bonding character influences, in the same way, both the photoluminescence behavior of the core, and the connectivity of the amorphous network.

The combined use of photoluminescence and Raman spectroscopy enabled discriminating the distinct influence of the two codopants depending on the CeO2-Al2O3 content. These results will allow to better design the codopant ratios needed to avoid photodarkening of silica fiber, but also to shed some light on the mechanisms involved in the attenuation of this unwanted optical degradation. Besides application in optical fibers, the understanding of the relationship between macro and micro-scale properties is a necessary step in the way of developing material with ad hoc properties. G 2 (cm -1 ) 𝐴 1 6 G 3 (cm -1 ) 𝐴 1 8 G 4 (cm -1 ) 𝐴 1 :

Tables

G 5 (cm -1 ) 𝐴 1 < Ato t C (cm -1 )

Figures Caption

Figure 1: Schematic drawing of the core portion of the SiO2 preform (silica cladding not represented). Each bold number indicates the position where the disks have been cut (cm) along the preform, which also represents samples labeling. Darker color indicates a higher CeO2 content. On top is reported the CeO2/Al2O3 molar ratio in the codoped core.

Figure 1 :

 1 Figure 1: Schematic drawing of the core portion of the SiO2 preform (silica cladding not represented). Each bold number indicates the position where the disks have been cut (cm) along the preform, which also represents samples labeling. Darker color indicates a higher CeO2 content. On top is reported the CeO2/Al2O3 molar ratio in the codoped core.

Figure 2 :

 2 Figure 2: Normalised excitation (Ex) and emission (Em) spectra for the different samples, and deconvoluted Gaussian bands. By increasing the CeO2 molar content both Ex and Em bands show a red shift, and an increase of Stokes shifts and of the ground level splitting. The luminescence properties of sample 4 are different. See text for details.

Figure 3 :

 3 Figure 3: Photoluminescence spectra collected in the codoped cores are compared to the most intense signals observed in the cladding (grey solid lines). Black vertical arrows indicate the Ex and Em wavelength used to collect the signals in the codoped core samples. For the cladding, two emission bands are reported, one collected with an excitation wavelength (lEm) of 300 nm (close to the Ex maximum of sample 4 -orange vertical arrow), and the other one collected at 350 nm (grey vertical arrow).

  photoluminescence signals are so different.

Figure 4 :

 4 Figure 4: (a) Normalized excitation spectra for samples 4 and 32 (black solid lines) and Gaussian bands used for signal deconvolution (dotted lines). The Emission spectra (colored solid lines) were collected at different excitation wavelengths marked by the vertical arrows.

Figure 5 :

 5 Figure 5: Raman spectra collected in the cladding (A), and in the codoped core (B) of the different disks. A) The Raman spectrum of pure SiO2 glass (dashed black line) is compared to signals from the cladding portions .B) An average spectra of all claddings (𝑪𝒍𝒂𝒅 %%%%%%% , black solid line) is reported as reference and compared to the core signals. Insets provide a magnification of the D2 defect line, and of the Q-range.

Figure 6 :

 6 Figure 6: A-C) Raman spectra collected in the core region and the subtracted spectra obtained by the difference between core and cladding of the same sample. Magnifications of the defect lines and of the Qrange are reported, respectively, in panel B and C. In A, a Cladding spectrum is reported as reference in black. D) Example of the subtractive method used. The Raman signal of each cladding has been subtracted to the one collected into the core region. The difference spectrum obtained has been deconvoluted with mathematical functions, and based on their positions we use the labels P1, P2, P3 and P4. E) Stacked difference spectra and position of the mathematical functions for some of the samples.

Figure 7 :

 7 Figure 7: Details of the Raman spectra and deconvolution of the high-frequency regions of some of the samples investigated. In the codoped core, for CeO2 contents ≥ 1 mol%, two additional vibrations rise, respectively at ~ 916 cm -1 and ~ 970 cm -1 . The chemical compositions of both cladding and core are reported for sake of clarity (as mol% oxide).

(

  Adjusted Coefficient of Determination R 2 = 99.92): y(x) = (-34.5±2.6)*x + 32772±202 This positive relationship means that the change of <Ce-O> species covalency character influences, in the same way, both the photoluminescence behavior of the core, and the connectivity of the amorphous network.

Figure 8 :

 8 Figure 8: Correlations between parameters from Raman data analysis and Photoluminescence studies of the codoped cores. A) the Raman center of mass C vs. the Emission (Em) and Excitation (Ex) maxima. B) the frequency Excitation maxima are related to the sum of the integrated areas of the contributions P1 and P2, obtained following the subtractive method. The latter shows a nearly perfect linear correlation (Adjusted R 2 = 99.92).

Figure 2 :

 2 Figure 2: Normalised excitation (Ex) and emission (Em) spectra for the different samples, and deconvoluted Gaussian bands. By increasing the CeO2 molar content both Ex and Em bands show a red shift, and an increase of Stokes shifts and of the ground level splitting. The luminescence properties of sample 4 are different. See text for details.

Figure 3 :

 3 Figure 3: Photoluminescence spectra collected in the codoped cores are compared to the most intense signals observed in the cladding (grey solid lines). Black vertical arrows indicate the Ex and Em wavelength used to collect the signals in the codoped core samples. For the cladding, two emission bands are reported, one collected with an excitation wavelength (lEm) of 300 nm (close to the Ex maximum of sample 4 -orange vertical arrow), and the other one collected at 350 nm (grey vertical arrow).

Figure 4 :

 4 Figure 4: (a) Normalized excitation spectra for samples 4 and 32 (black solid lines) and Gaussian bands used for signal deconvolution (dotted lines). The Emission spectra (colored solid lines) were collected at different excitation wavelengths marked by the vertical arrows.

Figure 5 :

 5 Figure 5: Raman spectra collected in the cladding (A), and in the codoped core (B) of the different disks. A) The Raman spectrum of pure SiO2 glass (dashed black line) is compared to signals from the cladding portions .B) An average spectra of all claddings (𝐶𝑙𝑎𝑑 %%%%%% , black solid line) is reported as reference and compared to the core signals. Insets provide a magnification of the D2 defect line, and of the Q-range.

Figure 6 :

 6 Figure 6: A-C) Raman spectra collected in the core region and the subtracted spectra obtained by the difference between core and cladding of the same sample. Magnifications of the defect lines and of the Q-range are reported, respectively, in panel B and C. In A, a Cladding spectrum is reported as reference in black. D) Example of the subtractive method used. The Raman signal of each cladding has been subtracted to the one collected into the core region.The difference spectrum obtained has been deconvoluted with mathematical functions, and

Figure 7 :

 7 Figure7: Details of the Raman spectra and deconvolution of the high-frequency regions of some of the samples investigated. In the codoped core, for CeO2 contents ≥ 1 mol%, two additional vibrations rise, respectively at ~ 916 cm -1 and ~ 970 cm -1 . The chemical compositions of both cladding and core are reported for sake of clarity (as mol% oxide).

Figure 8 :

 8 Figure 8: Correlations between parameters from Raman data analysis and Photoluminescence studies of the codoped cores. A) the Raman center of mass C vs. the Emission (Em) and Excitation (Ex) maxima. B) the frequency Excitation maxima are related to the sum of the integrated areas of the contributions P1 and P2, obtained following the subtractive method. The latter shows a nearly perfect linear correlation (Adjusted R 2 = 99.92).

Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6

 123456 Figure 1

Figure 7

 7 Figure 7

Figure 8

 8 Figure 8 754

Table 4 )

 4 : 𝐶 = (𝐺 . * 𝐴 1 2 ) + (𝐺 5 * 𝐴 1 6) + (𝐺 7 * 𝐴 1 8) + (𝐺 9 * 𝐴 1 :) + (𝐺 ; * 𝐴 1 <) 𝐴 =>=

Table 1 : Chemical composition (as mol%) of the codoped core measured by SEM-EDS.

 1

	Sample	SiO2	Al2O3	CeO2	CeO2/Al2O3
	position/label
	32	93.7	2.6	3.71	1.44
	29	94.9	2.4	2.67	1.11
	24	95.7	2.1	2.16	1.02
	19	96.1	2.0	1.87	0.93
	14	96.5	1.9	1.58	0.84
	9	97.4	1.6	1.03	0.63
	4	98.5	1.5	0.05	0.04

Table 2 : Results from the photoluminescence studies. Excitation (Ex) and emission (Em) maxima, and parameters obtained from the deconvolution of the bands with Gaussian (G*) functions are reported. SS: Stokes shifts. All values as cm -1 . The Ce/Al molar ratios are reported for sake of clarity.

 2

	Sample		Excitation			Emission		splitting ground	SS	Ce/Al
									levels
		Ex max	G1Ex	G2Ex	G3Ex Em max	G1Em	G2Em
	32	30024	29498	32160	35095	24017	24741	22370	2371	6007	1.44
	29	30077	29524	32166	34969	24452	25234	22838	2396	5625	1.11
	24	30235	29751	32503	35268	24724	25420	23112	2309	5511	1.02
	19	30512	29721	32235	35003	24643	25363	23064	2299	5869	0.93
	14	30390	29638	32060	34724	24709	25395	23084	2311	5680	0.84
	9	30816	29985	32380	34931	25526	26324	24142	2181	5290	0.63
	4	32969	30958	32531	34931	27360	28164	25734	2381	5609	0.04

Table 3 : Results of the study of the difference Raman spectra reported in Figure 5E. Ce and Al oxides molar contents are reported for sake of clarity. The position of each peak, the total integrated area of the difference spectra, and the integrated area (as %) of the two contributions at lower frequencies (P1 + P2) are reported.

 3

	Sample	Al2O3 cladding mol%	Al2O3 core mol%	CeO2 core mol%	P1 (cm -1 )	P2 (cm -1 )	P3 (cm -1 )	P4 (cm -1 )	Tot Integrated area	% Int. Area P1+P2
	32	0.03	2.58	3.71	907.5	985.2	1121.8	1200.8	3.16	74.5
	29	0.06	2.41	2.67	905.3	980.9	1115.8	1185.5	4.13	78.1
	24	0.02	2.12	2.16	926.8	985.7	1116.5	1163.8	1.44	72.0
	19	0.02	2.00	1.87	912.2	985.6	1116.2	1180.0	2.30	65.6
	14	0.02	1.88	1.58	937.2	988.1	1116.0	1159.5	0.90	64.0
	9	0.11	1.62	1.03		988.5	1118.7	1176.6	0.78	49.5
	4	0.13	1.47	0.05			1120.2*		0.37	0.0

* Single Lorentzian function. See text for details

Table 4 : Results from the Raman signal deconvolution of the Ce/Al codoped cores, in the high-frequency range. Each Gaussian function integrated area (AG), and total integrated area (Atot), along with the Center of mass (C) of the whole Q-range are reported. Data obtained for the average Cladding (𝑪𝒍𝒂𝒅 %%%%%%% ) and reference SiO2 are reported for comparison.

 4

	Sample	Ce/A l	G 1 (cm -1 )	𝐴 1 2

Acknowledgment: SEM-EDS analyses were supported by IPGP multidisciplinary program PARI, and by Region île-de-France SESAME Grant no. 12015908.