Raphael Romary
email: raphael.romary@univ-artois.fr

Remus Pusca
email: remus.pusca@univ-artois.fr

Thierry Jacq
email: thierry.jacq@edf.fr

Univ Artois

Identification of sensitive feature in the stray magnetic field to detect rotor short-circuit fault in synchronous generators

Keywords: Stray flux, synchronous generators, rotor short circuit fault I

Identification of sensitive feature in the stray magnetic field to detect rotor short-circuit fault in synchronous generators

INTRODUCTION

In the context of pressure on energy resources, associated with an increasing demand for electrical energy due to the electrification of uses (such as electric vehicles), the availability of electric power plants is becoming a major issue. A way to increase the availability is to improve the maintenance plan of the electric generators by surveying incipient degradation of the machine components to avoid an undesirable shutdown. This requires monitoring the machines with invasive or non-invasive sensors, but depending on the age of the machine, its rated power, or its position in the energy delivery plan of the grid manager, machines are more or less equipped with sensors. In France, most of the medium power hydroelectric plants have fewer instrumented machines. The usual faults that can occur on these machines are mainly eccentricity or rotor inter-turn short-circuit fault. Stator winding faults rarely appear because the insulation is generally re-enforced, which limits its degradation. Rotor inter-turn short circuit fault and eccentricity do not generally lead to sudden shut down and they do not prevent the machine to operate normally. Hydroelectric generators have a high number of poles and having a weaker pole due to inter-turn fault in the field winding is not a great concern. However, in these faulty conditions, the vibratory level of the machine increases and in the extreme case, the operating conditions in the (P, Q) diagram may have a limited area. The inter-turn fault in the field winding can be evaluated with an offline test that consists in verifying the voltage equipotentiality over all the poles when all the winding poles, put in series, are flowed the same AC or DC current, but this test can be done only during maintenance periods. For online detection, in absence of specific instrumentation, the use of an external flux sensor can be a good alternative. The exploitation of the external stray flux has proven to be efficient for fault detection in electrical machines [START_REF] Demian | A frequency-domain detection of stator winding faults in induction machines using an external flux sensor[END_REF]. It has been shown that stator inter-turn short circuit can be better detected with the stray flux than with line current, especially by using several sensors placed around the machine [START_REF] Irhoumah | Detection of the Stator Winding Inter-Turn Faults in Asynchronous and Synchronous Machines Through the Correlation Between Harmonics of the Voltage of Two Magnetic Flux Sensors[END_REF]. This is true for both induction and synchronous machine. Stray flux sensors can also be used to detect broken rotor bars in induction machines [3][4], eccentricity [START_REF] Chernyavska | Analysis of Air-gap Eccentricity in Inverter Fed Induction Motor by Means of Motor Current Signature Analysis and Stray Flux of Motor[END_REF], or demagnetized magnet in synchronous machines [START_REF] Goktas | Comprehensive Analysis of Magnet Defect Fault Monitoring Through Leakage Flux[END_REF]. Bearing faults [START_REF] Bessous | Rolling Bearing Failure Detection in Induction Motors using Stator Current, Vibration and Stray Flux Analysis Techniques[END_REF][START_REF] Minervini | A Novel Investigation on Multi-Sensor Signal Signatures for Induction Motors Diagnostics[END_REF][START_REF] Frosini | Induction Machine Bearing Fault Detection by Means of Statistical Processing of the Stray Flux Measurement[END_REF][START_REF] Zamudio-Ramirez | Detection of corrosion in ball bearings through the computation of statistical indicators of stray-flux signals[END_REF] but also other mechanical faults [START_REF] Biot-Monterde | Effect of the Misalignment Level on the Analyses of Current and Stray Flux Signals in Induction Motors[END_REF][START_REF] Rastegar | The effect of the mechanical behavior on the stray flux in an induction machine based electromechanical system[END_REF][START_REF] Zamudio-Ramirez | Detection of Uniform Gearbox Wear in Induction Motors Based on the Analysis of Stray Flux Signals Through Statistical Time-Domain Features and Dimensionality Reduction Techniques[END_REF] have also a significative influence on the stray flux. Concerning large electric generators, [START_REF] Platero | Diode Monitoring by Field Winding Axial Stray Flux in Brushless Synchronous Machines[END_REF] deals with winding field diode monitoring with stray flux, [START_REF] Bechara | Non-Invasive Anomaly Diagnosis for Hydro Electrical Generators Rotor Inter-Turn Short-Circuit Detection Using Stray Flux and the VAE[END_REF] uses classification method to detect field winding inter-turn fault, [START_REF] Cuevas | Non-invasive Detection of Winding Short-circuit Faults in Salient-pole Synchronous Machine with squirrel-cage damper[END_REF] show enhances the correlation between winding faults and vibration. [START_REF] Ehya | Pattern Recognition of Interturn Short Circuit Fault in a Synchronous Generator Using Magnetic Flux[END_REF] combines finite element simulation and experimental to detect winding field inter-turn fault in hydroelectric generators using an external flux sensor. However, all the papers about detection of rotor field winding inter-turn faults with stray flux sensor do not consider the influence of the machine operating condition.

This study presents an experimental comparison of two 55 MW synchronous real hydroelectric generators. One is assumed to be healthy and the other one has a rotor inter-turn short circuit. Especially designed stray flux sensors have been installed on the machines, and the aim is to highlight what are the signal parameters that are sensitive to a rotor inter-turn short-circuit. The analysis focuses on the spectrum but also the time signal delivered by the sensor for different operating conditions as far as active and reactive power are concerned. A vibratory analysis also brings complementary information.

The paper is organized as follows: section II gives theoretical issues about the external magnetic field and the sensitive spectral lines tied to usual faults, section III presents the machines, the experimental environment with sensor and data acquisition system and the test protocol, section IV deals with results analysis taking into account the operating condition at different active and reactive power. In the last section, a conclusion highlights the main findings of the paper.

II. THEORETICAL ISSUES

A. External magnetic field

The external magnetic field can be classified according to its direction; thus, the notions of axial and radial fields are used. The axial magnetic field is geometrically oriented in a longitudinal direction, parallel to the axis of the machine, and is rather sensitive to the currents in the stator and rotor end windings. The radial magnetic field is in a plane perpendicular to the longitudinal axis and is the image of the air gap induction which is attenuated by the stator laminations but also by the external frame of the machine. The radial magnetic field has 2 components, a normal direction, and a tangential direction. Therefore, the external magnetic field has three components that can be distinguished experimentally by an appropriate choice of a flux sensor position as shown in Fig. 1.

B. Sensitive components

An Analytical modelling based on airgap permeance and magneto motive force generated by stator and rotor armature is very convenient to describe precisely the harmonic content of airgap flux density in terms of time or space harmonics. Such an approach has a lack of accuracy concerning the magnitude of the airgap flux density harmonic component, but it enables us to define the change of the harmonic content of the airgap flux density in faulty conditions and to highlight frequencies of sensitive harmonic components. In [START_REF] Cuevas | Non-invasive Detection of Winding Short-circuit Faults in Salient-pole Synchronous Machine with squirrel-cage damper[END_REF], analytical modelling of electrical machines with stator and rotor inter-turn short circuit, eccentricity leads to define the following sensitive frequencies: For a rotor inter-turn short circuit fccr=2krf+hfr

For a stator inter-turn short-circuit fccs=2krf+f

For dynamic eccentricity fexc= f±hfr

kr varies from -∞ to +∞ and h is a positive integer, f is the supply frequency, fr is the rotating frequency of the machine. After attenuation by the stator lamination and the external frame, these sensitive components will be found in stray flux.

III. EXPERIMENTAL SETUP

A. Machine presentation

Hydraulic synchronous machines are low speed machines and present a high number of poles. This high number of pole require to manufacture the rotor with salient poles, wound as shown in Fig. 2. Due to winding ageing, inter-turn shortcircuit can occur, which make some pole weaker than others, leading to decrease the whole phase flux linkage at given rotor currents. Due to the rotor rotating, the three phases of the machine are affected similarly and the machine stays balanced. Therefore, the rotor fault does not influence the stator electric variable but as the rotor flux reduction has to be compensated by a rotor current increase, some operating conditions where high rotor currents are needed could be excluded. Moreover, due to an irregular force repartition between rotor poles and stator core along the airgap, the level of vibration may increase. The power plant that has been investigated in this study is composed of three synchronous generators which main characteristics are presented in Table I. From the knowledge of the power plant manager, it is known that two of these machines are healthy and one machine have a rotor inter-turn short-circuit in at least one pole. However, the fault severity is unknown. This paper will investigate the faulty, and one healthy machine. The sensors have been instated on the tested machines and it was possible to set up any operating conditions in terms of active power P, and reactive power Q.

B. The sensor

The used sensor is a tri-axial wound flux sensor (Fig. 3) specially sized to be adapted for high power machines. It is composed of 3 windings of axis NOR, AXI, and TAN. It has 4 tabs with magnets so that it can be easily installed against the external frame of the machines. This sensor has the advantage to be directly connected to a data acquisition system without any filter or amplifier. In this study, we consider only the signal from the TAN axis, equivalent to position pos.3 in Fig. 1. In this position, the sensor is supposed to be insensitive to the axial field generated by the current flowing through the end windings of the machines. The obtained signal is an image of the airgap flux Inter turn short circuit density whose spectral content includes faulty frequencies given by ( 1), ( 2), (3) but with a derivative action. An accelerometer is also used to provide a complementary information about the machine's state. It is placed on the external frame of the machine, near the flux sensor.

C. Data acquisition

For both investigated machines, 15 operating points are be considered: three points for the active power (0%, 50%, 100% of Pn), combined with five points for the reactive power (-100%, -50%, 0%, 50%, 100% of Qn). For each operating point, we pick up the flux sensor signal and the accelerometer signal on two acquisition systems: A pulse system from Bruel & Kjaer that compute directly the FFT of the signals and a PXI data acquisition system with LabVIEW that records the signal. The acquisition duration is 5s with a sampling frequency at 20 khz. Therefore, the recorded length is 10000 points. Fig. 4 shows a picture of the whole acquisition system. IV. RESULTS ANALYSIS Four types of analysis will be performed on the recorded signals. For the stray flux signal, a spectral qualitative analysis, a spectral quantitative analysis, and a time domain analysis will be performed. The vibration measured with the accelerometer will just be used to confirm the presence of the fault with a spectral analysis.

A. Qualitative analysis

Fig. 5 shows the spectra over the 0-100 Hz frequency range of the signal measured by the flux sensors at the operating point P=100%, Q=100% for the healthy and the faulty machine. We can clearly see the fundamental at 50 Hz, and the presence of spectral lines every 3.33 Hz, corresponding to the rotation frequency of the machine. Therefore, there are 15 lines between 0 and 50Hz, and 15 others between 50 Hz and 100 Hz. All these spectral lines are related to sensitive frequencies given by ( 1) and ( 2) relative to the rotor faults and dynamic eccentricity. The lines at 3.33 Hz, 6.66 Hz, and 10 Hz (kr=0, h=1,2,3 in (1)) can be accounted for the rotor short-circuit fault, and the lines at 47.66 Hz, 43.33 Hz, and 40 Hz (h=-1, -2, -3 in (3)) to eccentricity. As a first observation, we can see that the sensitive spectral lines appear even for the healthy machine and the difference between the healthy and the faulty machine is quite small. In real machines, there is always minor imperfection due to tolerances in the manufacturing process. Therefore, a pure healthy machine is not a reality.

However, further information can be highlighted if we compare the first rank of the rotor fault-sensitive components at 3.33 Hz, (kr=0, h=1, in (1) and the second rank at 6.66 Hz (kr=0, h=2, in (1)). Table II and Table III gives the magnitudes of the 3,33 Hz and the 6,66 Hz spectral lines for the 15 operating points for the Healthy and for the faulty machine and the variation direction between both frequencies. For the healthy machine, we can see that the variation is mainly positive (14  and 1 ) and for the faulty machine, the variation is mainly negative (11  and 4 ). This is a logical result. Indeed, when the machine has a short circuit fault, the main sensitive spectral line at 3.33 Hz characteristic of the fault (rank kr=0, h=1, in ( 1)) is higher than the second rank at 6.66 Hz (kr=0, h=2, in (1)), so we have a downward trend. On the other hand, in absence of fault, the main sensitive line at 3.33 Hz is lower than the second one 6,66 Hz. The main reason of this property is that the main rank harmonic has generally a higher magnitude than the second rank one.

 B. Quantitative analysis

The quantitative analysis focuses on the main rank harmonic at 3.33 Hz. Moreover, the real magnitude may be not a convenient feature when two machines are compared. Therefore, we will consider here the relative magnitude in the percentage of the fundamental. Tables IV and V gives these relative magnitudes for the healthy and the faulty machine for the 15 operating points.

According to Table IV and Table V, it can be observed that for each operating point, the relative magnitude of the faulty machine is roughly twice the magnitude of the healthy one. Tables III and IV also highlight the need of considering the operating point in the analysis, because independently of the operating point we can find magnitudes for the healthy machine higher than those for the faulty machine. It can also be observed that the operating points at maximum active and reactive power correspond to the cases where the relative amplitudes are the highest, similarly as in Tables I and II for the real magnitude. In these operating conditions, the rotor excitation current is maximum and then the effect of the magnetic unbalance generated by a rotor fault is maximum, which leads to enhancing the spectral lines sensitive to the fault. Therefore, based on these matters, it can be recommended to consider the operating conditions at high active and reactive power.

C. Time domain analysis

For a healthy machine, all the rotor poles produce the same magnetic effect that will be found outside of the machine too. Therefore, the linkage flux of the sensor will present a similar waveform for each pole-pass over one rotation of the machine. In the case of a rotor inter-turn short circuit, a pole will become weaker than the others and there will be a change in the magnitude of the sensor linkage flux when the faulty pole will come in front of the sensor. In this section, the flux signal is used to check these time domain properties. Fig. 6 shows the linkage flux of the sensor for one revolution of the healthy and faulty machines. The flux is obtained by integration of the voltage signal delivered by the flux sensor. In both flux signal, one can find 15 positive poles (north pole) and 15 negative poles (south pole), but it can be observed that the flux magnitude are different for each polepass even for the healthy machine, which means that this machine is probably not perfectly health. This is in accordance with section IV.A about the frequency domain qualitative analysis where the presence of eccentricity was also mentioned. Actually, both eccentricity and rotor fault may cause a change in the magnetic effect of one rotor pole, but it can be expected that the eccentricity produces a smooth change of the flux magnitude over one rotation whereas shorted turns produce a sharp drop of the linkage flux of the sensor as shown in [START_REF] Ehya | Pattern Recognition of Interturn Short Circuit Fault in a Synchronous Generator Using Magnetic Flux[END_REF]. In addition to eccentricity and rotor shorted turn, another phenomenon may cause flux magnitude variations. Actually, due to manufacturing a positioning imperfection of the rotor poles, the airgap between the rotor pole and stator may be not perfectly constant which can make some magnitude variation of the airgap flux density and also magnitude variation of the sensor flux linkage.

In Fig. 6 it can be seen that the flux magnitude presents regular smooth change that may be caused by the eccentricity or manufacturing tolerances for the healthy machine (Fig. 7.a) whereas a sharp drop appears for the machine with rotor shorted turns (Fig. 6.b). Therefore, time domain analysis can be complementary to the frequency domain analysis to detect rotor faults, even if this analysis is not so easy as the spectral analysis.

D. Vibratory analysis

The vibratory analysis is not at the heart of the paper, but in this section, we can try to evaluate the consequence of rotor shorted turns on the mechanical behaviour of the machine. The accelerometer is placed on the external frame of the machine, close to the flux sensor. Fig 7 shows the spectrum of vibration (speed information) for both machines in a 0-400 Hz frequency range. Firstly, it can be observed that the spectra are roughly similar. The spectral lines at 100 Hz, 200 Hz, and 300 Hz which originate from electrical excitation of the stator frame clearly appear. They are due to radial forces between the stator and the rotor. In a very low part of the spectrum, the main finding concerns the spectral line at 3.33 Hz that goes from 78µm/s for the healthy machine up to 366µm/s for the faulty machine, for an operating condition with P=100%, Q=100%. A deeper analysis should be taken with care because of the complex mechanical structure of the machine. The presented study has highlighted the interest in the use of the external field to evaluate the health state of an electrical machine, more particularly to detect rotor short-circuit fault in synchronous machines. The flux sensor is simple, cheap, noninvasive and easy to install, even on operating machines. It can be an interesting alternative for the monitoring of noninstrumented machines. The analysis focuses on one healthy and one faulty machines of 55 MW in a hydroelectric powerplant. We were able to appreciate the consequences of the fault on various parameters obtained from real measurements. The originality consists in the possibility to analyze the behavior of the external magnetic field for different operating points characterized by different powers P and Q. The analysis allows us to state that it is better to work with the values reduced in percentage to be able to compare the healthy state of different machines. The obtained results are surely not enough sufficient to give general conclusions, but they allow us to give trends to define analysis and diagnosis procedures that will help to improve the maintenance and thus to increase the life-time of hydroelectric machines. In future work, we will be interested in estimating the severity of the fault. A finite element analysis will be used for that. We can also use the sensors to evaluate operating conditions.

Fig. 1 .

 1 Fig. 1. Radial and axial flux decomposition

Fig. 2 .

 2 Fig. 2. Salient pole rotor with inter-turn short circuit

Fig. 3 .

 3 Fig. 3. The tri-axial stray flux sensor

Fig. 4 .

 4 Fig. 4. Data acquisition system

Fig. 5 .

 5 Fig. 5. Stray flux spectrum for the healthy (a), and the faulty machine (b)

Fig. 6 .

 6 Fig. 6. Time domain signal, healthy machine (a), faulty machine (b)

  Fig. 7. Vibratory spectrum V. CONCLUSION

TABLE I .

 I MACHINE CHARACTERISTICS

	Rated active power	Pn=55 MW
	Rated reactive power	Qn=35 MVAR
	Number of pole	30
	Rotational speed	200 rpm
	Supply voltage	10000V
	Frequency	50Hz
	Rated stator current	3700 A

TABLE II .

 II

	oscilloscope	Stray flux acquisition
		system
					(a) Healthy machine
	Vibration
	acquisition
	system
					(b) Faulty machine
		SPECTRAL COMPONENT. HEALTHY MACHINE
				3.33 (µV) Hz	(µV) 6.66Hz	Variation
			Q=-100%	165	260	
			Q=-50%	246	410	
		P=0%	Q=0	321	506	
			Q=50%	467	540	
			Q=100%	595	586	
			Q=-100%	234	278	
			Q=-50%	281	365	
		P=50%	Q=0	323	454	
			Q=50%	512	607	
			Q=100%	539	693	
			Q=-100%	281	322	
			Q=-50%	300	372	
		P=100 %	Q=0	342	450	
			Q=50%	443	514	
			Q=100%	448	515	

MAGNITUDE, IN µV OF THE 3.33 HZ AND THE 6.66 HZ

TABLE III .

 III MAGNITUDE, IN µV OF THE 3.33 HZ AND THE 6.66 HZ SPECTRAL COMPONENT. FAULTY MACHINE

			3.33Hz	6.66Hz	Variation
			(µV)	(µV)
		Q=-100%	553	532	
		Q=-50%	594	515	
	P=0%	Q=0	811	290	
		Q=50%	1017	611	
		Q=100%	1340	1470	
		Q=-100%	453	275	
		Q=-50%	614	258	
	P=50%	Q=0	807	367	
		Q=50%	1110	977	
		Q=100%	1261	1368	
		Q=-100%	602	453	
		Q=-50%	654	465	
	P=100%	Q=0	814	674	
		Q=50%	1053	1178	
		Q=100%	1086	1247

TABLE IV

 IV

	.	RELATIVE MAGNITUDE (IN %) OF THE 3.33HZ SPECTRAL
			COMPONENT. HEALTY MACHINE
				P
			10%	50%	100%
	-100%	0,13	0,19	0,27
	-50%	0,18	0,23	0,30
	Q	0%	0,25	0,28	0,38
		50%	0,41	0,42	0,45
	100%	0,61	0,55	0,44

TABLE V

 V

	.	RELATIVE MAGNITUDE (IN %) OF THE 3.33 HZ SPECTRAL
			COMPONENT. FAULTY MACHINE
				P
			10%	50%	100%
		-100%	0,30	0,24	0,36
		-50%	0,32	0,34	0,39
	Q	0%	0,47	0,48	0,54
		50%	0,66	0,83	0,81
		100%	1,17	1,09	0,86


Wilfried Blanc

Bernard Dussardier
email: bernard.dussardier@unice.fr

Gérard Monnom

Romain Peretti

Anne-Marie Jurdyc

Bernard Jacquier

Marie Foret

Ann Roberts

Erbium emission properties in nanostructured fibers

A new route was recently proposed to modify some spectroscopic properties of rare-earth ions in silica-based fibers. We had shown the incorporation of erbium ions in amorphous dielectric nanoparticles, grown in fiber preforms. Here we present the achieved stabilization of nanometric erbium-doped dielectric nanoparticles within the core of silica fibers. We present the nanoparticle dimensional characterization in fiber samples. We also show the spectroscopic characterisation of erbium in preform samples with similar nanoparticle size and composition. This new route could have important potentials in improving rare-earth doped fibre amplifiers and laser sources.

Introduction

2 Doping optical fibers with rare-earth ions has allowed the demonstration, development and marketing of numerous devices such as lasers and amplifiers. The most prominent example is the erbium-doped fiber amplifier (EDFA), developed more than twenty years ago [START_REF] Mears | Low noise erbium-doped fibre amplifier operating at 1.54 µm[END_REF]. This silicabased fiber is a key device of wavelength division multiplexing (WDM) telecommunication systems [START_REF] Desurvire | Erbium-Doped fiber Amplifier, Device and System Developments[END_REF]. To increase the transmission capacity, one of the requirements is to broaden the amplification bandwidth. As shown earlier for various glass compositions, the linewidth of the 4 I 13/2 -4 I 15/2 stimulated emission depends on three main parameters i.e. Stark splitting, homogeneous and inhomogeneous broadenings of transitions [START_REF] Bigot | Resonant fluorescence line narrowing measurements in erbium-doped glasses for optical amplifiers[END_REF]. Then, the full width at half maximum (FWHM) of the 1.5 µm erbium emission is found to vary from 40 nm in silica to 80 nm in tellurite or multicomponent silicate glasses [START_REF] Yang | Effect of Bi 2 O 3 on the spectroscopic properties of erbium-doped bismuth silicate glasses[END_REF]. Several other applications of rare-earthdoped fibers would benefit from a modification of their spectroscopic properties. Many studies have been carried out that aimed at developing suitable silicate and non-silicate rare-earth doped glass hosts to improve their lasing and amplification properties. From a spectroscopic point of view, these glasses are more interesting than pure silica but they don't have all the advantages of the latter: low cost, reliability, connectivity, and many others.

Other approaches exist, like materials doped with semi-conductor or metallic nanoparticles, and ceramized materials. The former use the photoluminescence intensity enhancement through non-radiative energy transfer from semi-conductor or metallic nanoparticles (absorbing incident light thanks to their large cross-section) to rare-earth ions. The other interesting approach that is considered here is the ceramization of doped materials, usually made of oxides or oxy-fluorides [START_REF] Wang | New transparent vitroceramics codoped with Er 3+ and Yb 3+ for efficient frequency upconversion[END_REF]. In this case, the rare-earth or metallic luminescent ions are isolated from the matrix and located in a host that provides desired luminescent properties. Over the past several decades considerable work has been carried out on the incorporation of rare-earth oxide nanocrystallites into different glass hosts. Different techniques have been developed such as co-sputtering [START_REF] Fujii | Photoluminescence from SiO2 films containing Si nanocrystals and Er: effects of nanocrystalline size on the photoluminescence efficiency of Er 3+[END_REF], pyrolysis [START_REF] St | Synthesis and characterization of discrete luminescent erbiumdoped silicon nanocrystals[END_REF], ion implantation [START_REF] Chryssou | Evidence of energy coupling between Si nanocrystals and Er 3+ in ion-implanted silica thin films[END_REF], laser ablation [9] and sol-gel processes [START_REF] Yeatman | Sol-Gel Fabrication of Rare-Earth Doped Photonic Components[END_REF]. Aside from the spectroscopic advantages of these routes, the issue of optical transmission imposes that particles be of enough small size to minimize light scattering to an acceptable value. This is usually obtained for dimensions less than several tens of nanometers [START_REF] Tick | Are low-loss glass ceramic optical waveguide possible?[END_REF] in the case of dielectric nanoparticles, depending on their composition and on the expected application. In most cases the fiber core glass is a non-silica or a very low silica material made by the double crucible furnace melting procedure [START_REF] Samson | Efficient neodymium-doped glass-ceramic fiber laser and amplifier[END_REF]. However these materials are not as reliable as compared to silica based rare earth-doped fibres.

Here we investigate the growing of dielectric (oxide) nanoparticles (DNP) in silica-based optical fibres and the modification of spectroscopic properties of erbium ions when incorporated into DNP. Through this route, we keep the advantages of the mechanical properties of silica glass and spectroscopic properties can be engineered through the choice of the DNP chemical composition. To the best of our knowledge only a few studies on nanostructured silica fibres have been dedicated to oxide DNP doped with luminescent ions [START_REF] Yoo | Optical properties of the optical fiber containing Co 2+ doped ZnO-Al 2 O 3 -SiO 2 glass-ceramics[END_REF]. To obtain DNP-doped fibers, several routes have been proposed. The one of interest here is the spontaneous phase separation mechanism [START_REF] Zarzycki | Glasses and the vitreous state[END_REF]. In some glass systems, one observes co-existence of two liquid and/or solid phases. Silicate systems have a strong stable immiscibility when they contain divalent metals oxides, such as SrO, CaO, FeO, ZnO and MgO. As an example, if a silicate glass containing few mol% of CaO is heated during the manufacture process, it may decompose in two phases, one silica-rich and one CaO-rich (present as spherical particles).

The role of calcium on the preparation of the DNP in preforms has already been discussed [START_REF] Blanc | Er-doped oxide nanoparticles in silica-based optical fibers[END_REF]. In the present paper, we present the preparation and characterization of fibers containing nanoparticles, obtained by drawing Ca-and Er-codoped preforms. Then, we introduce spectroscopic measurements of erbium emission obtained from preform samples containing DNP of the same composition and approximately the same size as in the fibers.

Experimentals

Nanostructured preforms and fibers preparation

Silica preforms were prepared by the MCVD process [START_REF] Nagel | Optical Fiber Telecommunications: Fiber Fabrication[END_REF] ; calcium and erbium ions have been incorporated through the solution doping technique [START_REF] Townsend | Solution-doping technique for fabrication of rare-earth-doped optical fibers[END_REF]. A detailed description of the technique adapted to DNP growing was reported earlier [START_REF] Blanc | Er-doped oxide nanoparticles in silica-based optical fibers[END_REF]. The concentration in calcium in the soaking solution was a key parameter: it was varied from 0 to 1 mol/l, whereas the concentration in erbium was the same for all samples. The Er 2 O 3 concentration in the sample with no calcium was estimated at about 220 ppm mol by absorption spectroscopy. As for standard telecom fibers, germanium was added in all preforms to raise the refractive index of the core, while a small amount of phosphorous was incorporated to ease the fabrication. Fibers were obtained by stretching preforms in a drawing tower at temperatures higher than 2000°C under otherwise normal conditions.

Characterization of the erbium-doped dielectric nanoparticles

Preforms and fibers were characterized through various techniques: scanning electron microscopy (SEM), transmission electron microscopy, energy dispersive X-ray spectroscopy, atomic force microscopy, quantitative phase microscopy (QPM), extended X-ray absorption fine structure. In this paper only results obtained for the first time on fibers by SEM and QPM are presented below. SEM pictures were realized on the exposed core section of freshly cleaved fibers. QPM measurements have been performed transversally on fiber samples to observe the DNP in the fiber core [START_REF] Barty | Quantitative optical phase microscopy[END_REF]. This method derives quantitative phase measurements from images captured using a bright-field microscope without phase or interference contrast optics. It is a performant and non destructive imaging technique allowing the direct observation of refractive index variations across a transversal section of objects such as optical fibers [19].

Mapping of erbium fluorescence in fiber core was investigated by a confocal microscope. The cleaved end-face of a fiber was excited at 514 nm with an argon laser through a microscope objective and the Er 3+ green fluorescence (~540 nm) was detected through the same microscope objective. The cleaved sample was scanned along both transverse axes under the microscope to obtain a fluorescence intensity image. The resolution of this technique is however limited by diffraction at around 1 µm in size. However this is much smaller than the core typical size (8 µm) and gives a qualitative distribution of erbium ions across it.

Er 3+ emission spectroscopy

The series of Ca-and Er-codoped fibers studied here had high losses from scattering. As show below, the DNP average size in fiber was still to high to allow acceptable transmission.

Then erbium ion distribution and surroundings were spectroscopically analysed in preform samples at room and very low temperatures (1.5 K). Broadband luminescence was obtained by exciting samples at 980 nm, whereas resonant fluorescence line narrowing (RFLN) was performed at very low temperature under the excitation from a tunable external cavity laser diode around 1535 nm, around the peak of the erbium absorption. RFLN measurements were aimed at explaining the shape of the emission band [START_REF] Bigot | Resonant fluorescence line narrowing measurements in erbium-doped glasses for optical amplifiers[END_REF]. With this technique the fluorescence at the same wavelength as the excitation wavelength at low temperature indicates the zero-phonon transition (transition between the lowest levels of the 4 I 13/2 and 4 I 15/2 manifolds). It is then possible to identify different sites. A narrow line infrared laser at wavelength resonant with the first stark component of the electronic transition around 1.53µm was used.

Decay curves of the low-energy emission band (around 1540 nm) were recorded under two different excitation wavelengths: 980 and 1536 nm. The sample was a >4-mm long parallelepiped extracted from a preform, containing a cylindrical Er-doped core surrounded by pure silica. The pump beam was modulated in time with a 1-ms "square shape". It was directed into the core along its longitudinal axis. The emitted light was collected transversally from two different points relative to the coupled pump: 1) close to the excitation point and 2) at 4 mm far from this point. The fluorescence decay signals were collected with a infrared sensitive photomultiplier Hamamatsu R 5509 (response time of the system: 100 µs) and displayed on an oscilloscope.

Results and discussion

In Figure 1 are shown images obtained by the Quantitative Phase Microscopy technique.

The core of the fiber corresponds to the central darker part. Calcium concentration was varied from 0 to 1 mol/l (from bottom to top, in Figure 1). We clearly observe some dark spots in the core which are ascribed to nanoparticles. As it was reported in preforms, DNP are observed only for Ca concentration of 0.1 mol/l or higher [START_REF] Acapito | Role of CaO addition in the local order around erbium in SiO 2 -GeO 2 -P 2 O 5 fiber preforms[END_REF]. The absence of DNP for low concentration can be explained by the fact that, according to the SiO 2 -CaO binary phase diagram, the immiscibility gap occurs for a CaO concentration between 2 and 26 mol%. The DNP average size can not be deduced from this technique due to the diffraction limit. However, we can clearly observe that DNP are always almost spherical and the volumic fraction increases with calcium concentration.

In the following, we focus our attention on samples doped with a solution containing 0.1 mol/l of calcium. SEM pictures from such a fiber is shown on Figure 2. The fiber core corresponds to the light gray disk. Its diameter is ~8 µm. The central part of the core is almost dark due to the evaporation of germanium and calcium elements; this is a common artifact of the MCVD technique. The inset of Figure 2 shows typical spherical DNP. The DNP mean size was determined by averaging DNP diameter across the whole core sections for several SEM pictures.

It was found around 100 nm and no particles bigger than 200 nm were observed., whereas in preform samples, the average size was 250 nm. Therefore although the drawing of the preform into fiber reduced the core diameter by a factor ~70, DNP average diameter stayed within the 100 nm range. This shows that DNP of this particular composition remain stable in the material although it was submitted to a strong deformation at high temperature. Other phase separating agents with a high immiscibility gap may give a better DNP size stablility. The phenomenon still needs further investigations, using molecular dynamics simulations for instance.

Mapping of the Er 3+ emission intensity by confocal microscopy is reported on Figure 3.

Intensity is not homogeneously distributed in the core. A maximum clearly appears in a region of less than 1 µm in diameter (due to diffraction limit). This result indicates that erbium ions are located inside or very close to the DNP as it was observed in preforms through EDX analyses [START_REF] Acapito | Role of CaO addition in the local order around erbium in SiO 2 -GeO 2 -P 2 O 5 fiber preforms[END_REF]. Note that no erbium fluorescence is detected outside the DNP. This result was expected due to the low solubility of silica (or even germano-silicate) for rare-earth ions, whereas strongly modified and amorphous silicates, such as in the DNP, have a high solubility for these ions.

At room temperature, it was observed that erbium emission depends on calcium concentration. For the lowest concentration (0.01 mol/l), when no DNP are observed, erbium emission is similar to that from a standard silica erbium-doped preform when no calcium is added to the core composition (Figure 4). However, when calcium concentration is 0.1 mol/l, erbium emission spectrum is broadened [START_REF] Blanc | Er-doped oxide nanoparticles in silica-based optical fibers[END_REF]. At very low temperature, the same was observed (not shown here). It is assumed that the change of local order around the erbium ions embedded into the calcium rich phase is responsible of the observed broadening. This luminescent broadening is the signature of a sensible modification of the close environment of rare earth ions in DNP-doped fibers, compared to standard silica fibers. To investigate this, RFLN spectroscopy was performed. Spectra obtained for two excitation wavelengths (1539.6 and 1534.6 nm) are presented on Figure 5. At low energy excitation (1539.6 nm on Figure 5), one main fluorescent peak is observed, resonant with the excitation wavelength. A second emission band exists at 1545 nm too, and is present for any excitation wavelength. Its origin is still unclear and is not discussed in this paper. At higher energy excitation (1534.6 nm on Figure 5), we observed unusual spectra because, additionally to the resonant fluorescence peak, a high and broad emission band appears at lower energy (~6500 cm -1 ). Such a band has never been observed in this kind of samples. However, the same was reported with neodymium doping and interpreted as the result of energy transfer [START_REF] Yen | Optical Energy Transfer[END_REF].

To confirm and analyze the existence of an energy transfer in our sample, we study the results from fluorescence lifetime measurements. It was observed that when the light is collected far from the excitation point, the measured lifetime is increased compared to that measured on the decay curve corresponding to collection of light at the excitation point (Figure 6). This was observed for two different excitation wavelengths. Note that the lifetime measured at the excitation point is similar to that from a standard erbium-doped fiber. We assign this behavior of the 1540-nm emission band to a self absorption phenomenon within erbium ions. Self-absorption is a radiative transfer from a donor to an acceptor of the same nature. We will show in the following that the hypothesis of transfer shown in RFLN can be explained by self absorption. At low temperature, self absorption by similar Stark level cannot induce the apparition of additional band in the RFLN spectrum. This is why self absorption occurs between two different Er 3+ ions having energy-coincident Stark levels, namely the lowest Stark level of the donor Er 3+ ions, and the second level of acceptor Er 3+ . The energetic structure of those two Er 3+ ions sites are so different that it can be concluded that their environments are different. This prooves that radiative transfer phenomenon occurs between to different classes of sites. This hypothesis is reinforced by the full sites distribution of erbium in this material shown in ref [START_REF] Peretti | Saturation spectrale de gain dans les amplificateurs à fibres dopées erbium : largeur homogène et inhomogène et approche des nano matériaux[END_REF], which is the sum of two Gaussians.

Usually, radiative energy transfer is observed in heavily doped materials [START_REF] Bensalah | Spectroscopic properties and quenching processes of Yb 3+ in fluoride single crystals for laser applications[END_REF][START_REF] Stoita | Measurement of the fraction of reabsorbed light in an Er 3+ -doped glass[END_REF][START_REF] Guy | Modelization of lifetime measurement in the presence of radiation trapping in solid-state materials[END_REF].

Although the concentration is low in our samples, a self absorption effect is observed when the emitted light comes across a highly absorbant and/or many absorbing centers on its free path.

Two factors can simultaneously cause this self absorption effect. On one hand, scattering of light is enhanced due to the presence of DNP, hence increasing the mean free path of light in the core and the self absorption probability. On the other hand, if erbium ions are located into (or close to) the DNP, absorption cross-section could be modified because DNP are chemically very different compared to the silica host..

Conclusion

In this paper, we have demonstrated that it is possible to obtain dielectric nanoparticles doped fibers directly through the MCVD process. These DNP are used to embed erbium ions. Isolated from silica glass, rare-earth ions spectroscopic properties are then modified by the DNP chemical compositions. We have then shown, with Er-DNP-doped preforms, the existence of two types of sites for erbium ions and evidenced a radiative energy transfer between them. Also, an increase of the erbium emission bandwidth is measured at room temperature. These observations open

Figure Captions

Figure1:Figure 2 :

 2 Figure1: Quantitative Phase Microscopy measurement on Ca-doped fibers. The bright region is

Figure 3 :

 3 Figure 3 : Mapping of Er 3+ emission intensity in a Ca-doped fiber (0.1 mol/l) under a confocal

Figure 4 :

 4 Figure 4 : Room temperature erbium emission spectra in standard preform (dashed line) and

Figure 5 :

 5 Figure 5 : Resonant fluorescence of erbium emission for two excitation wavelengths.

Figure 6 :

 6 Figure 6: Fluorescence decay at 1.5 K from a Ca-doped preform sample at 1540 nm, measured

Figure 1 FIGURE 2 :Figure 3 Figure 4 Figure 5

 12345 Figure 1

Figure 6 :

 6 Figure 6:

Acknowledgements

The authors acknowledge support from CNRS and the Ministère des Affaires Etrangères (France) (P2R project # P2RI 04-11), and from the Australian Research Council. The authors also thank Michèle Ude and Stanislaw Trzésien for samples preparation, and Luan Nguyen (CNRS-CRHEA, Valbonne) for SEM images.

interesting perspectives in various fields of application for rare-earth-doped optical fibers.

Particularly, the sensible broadening of the erbium luminescence around 1.55 µm is very interesting for optical telecommunications or short pulsed laser sources, where intrinsically broad and flattened gain must be implemented in reliable silica-base optical fibers. Generally speaking, this new generation of fibers would allow new applications for silica-based amplifying optical fibers.