Using a combination of experimental techniques such as optical absorption, Raman scattering, continuous wave and pulse Electron Spin Resonance (ESR), we characterize a set of γ-irradiated Yb3+ doped silica glass preforms with different contents of phosphorous and aluminum. We demonstrate that when P is introduced in excess compared to Al, nearly no radiodarkening is induced by γ-rays. On the other hand, when Al>P, a large absorption band is induced by radiation. Thermal annealing experiments reveal the correlation between the decrease of the optical absorption band and the decrease of the Al-Oxygen Hole Center (AlOHC) ESR signal, demonstrating the main role of AlOHC defects in the fiber darkening. HYSCORE (HYperfine Sublevel CORElation) pulse-ESR experiments show a high Al-P nuclear spin coupling when P>Al and no coupling when Al>P. This result suggests that both AlOHC and POHC creation is inhibited by Al-O-P linkages. Confronting our data with previous works, we show that the well-known photodarkening process, meaning losses induced by the IR pump, can also be explained in this framework.
©2013 Optical Society of America
Ytterbium-doped high power laser fibers, operating near 1 micron, are currently used for industrial material processing and biomedical devices. Such active fibers are also promising candidates to achieve the inertial confinement fusion [1,2] or to operate in space for inter-satellites optical communication [3,4]. However, radiative environments imply the formation of absorbing defects, and the resulting absorption features located in the UV and visible range have long absorption tails that overlap with the pump and emission wavelengths of Yb3+. As a consequence, the radiation-induced darkening can lead to a drastic decrease of the laser performance. Several techniques can be used to improve the radiation hardening like Cerium codoping, O2 or H2-treatment [4–8].
Nevertheless, the role of the codopants (such as Al or P which are incorporated to reduce rare-earths clustering [9,10]) in the radiation response of the fibers remains unclear. Previous works suggest that these dopants are responsible for a strong part of the output losses [11–14], but the absorbing defects, namely in the visible-near IR region, have not been yet clearly identified. Even so, this step remains crucial to understand the mechanisms of radiation-induced absorption and to improve efficiently the fibers performance in harsh environment.
In this paper, using optical absorption and electronic spin resonance (ESR) spectroscopy, it is experimentally found that the Aluminium-Oxygen Hole Center (AlOHC) paramagnetic defect is mainly responsible of the radiodarkening (gamma-rays induced absorption) in Yb-doped silica-based fiber preforms. It is also demonstrated that phosphorous codoping allows to prevent the formation of such absorbing defects. A structural study of the pristine samples, combining Raman and pulse ESR spectroscopy, allows a better understanding of the role of phosphorous and aluminium codopants in the vicinity of ytterbium. Confronting this set of experimental data with previous works, we show that the well-known attenuation induced by high-power IR laser diode pumping, called photodarkening [15–18], can also be explained in this framework.
2.1 Samples synthesis and composition
Ytterbium doped aluminophosphosilicate fiber preforms were fabricated by mean of the modified chemical vapour deposition (MCVD) technique. The set of samples has been designed to investigate the interaction between ytterbium and Al/P dopants, and the respective role of Al and P in the darkening of such fibers under irradiation. Thereby, the different cores present roughly the same content of Yb, Si and Ge, and a variable amount of Al and P. The average composition of the cores has been determined by quantitative chemical analyses using Castaing’s microprobe spectroscopy. Each composition is displayed in the Table 1.
2.2 Gamma irradiation experiment
All samples have been exposed to 60Co γ-radiation at room temperature using an armored cell at CEA Saclay (DSM, SIS2M). The irradiation was performed at a dose-rate of 6.0 Gy.min−1 during 5 hours, implying a cumulated dose of 1800 Gy, which corresponds to the typical accumulated dose for a 10-20 years mission in a space environment [3,4]. The pictures of some of the samples before and after irradiation are presented Fig. 1. It is shown that the core samples exhibit a strong darkening after exposure to γ-rays when Al>P, and almost no coloration appears when P>Al.
3. Defects creation and thermal relaxation in post-irradiated samples
3.1 Optical absorption and continuous-wave ESR
After γ-irradiation, each preform was cut into different adapted geometry. The samples used for absorption spectroscopy were double-face polished (up to an optical grade) thin disks of preform of roughly 2.5 mm diameter and 1 mm thickness. Absorption spectra, displayed Fig. 2(a), were recorded in the 280-1340 nm wavelengths range. Only the core of the preforms was probed, using a hole of the dimension of the core, i.e 1 mm diameter. Figure 2(a) clearly reveals the different behaviors of the fibers under gamma-irradiation and the importance of Al/P ratio. The two post-irradiated preforms A and B, which contain an excess of phosphorous compared to aluminum, present no absorption band in the visible and near IR range (the sharp band around 980 nm corresponds to 2F7/2-2F5/2 absorption band of Yb3+). The other samples display a large absorption feature in the visible region with a tail up to the near IR, and the intensity of this absorption increases with the excess of Al compared to P. In order to identify the colored centers generated by γ-irradiation and responsible of the induced absorption, continuous-wave ESR measurements were conducted at room using an X-band EPR-Brüker spectrometer operating at 9.868 GHz, in the 340-360 mT magnetic field range, with a power of 2mW. The post-irradiated samples used for this experiment were cylinders of 2.5 mm diameter and 3.8 mm height.
The cw-ESR spectra Fig. 2(b) present a composite and intense feature between 352 and 356 mT, characteristic of the presence of Ge-related paramagnetic point defects [19,20]. E’-Ge, Ge(1) and Ge(2) defects are induced by γ-irradiation, but could not be responsible of the darkening, due to their absorption located respectively at 6.3, 4.4 and 5.8 eV, i.e in the UV range [19,20]. The inset of Fig. 2(b) focuses on the weak signal at 348 mT (g = 2.025) which is the signature of AlOHC point defect [21,22]. To better visualize this AlOHC signal, ESR spectroscopy has been performed on the sample G at different powers, in the range 0.1mW-50mW (Fig. 3). Figure 3(a) diplays the evolution of the ESR signal and Fig. 3(b) the intensity of the AlOHC signal in function of the power. Because saturation is observed above P = 2mW, this power value has been choosing as the working power.
The AlOHC ESR band is nearly missing in the A and B samples, whereas it appears for the others and grows with the excess of Al compared to P. Comparing optical absorption and cw-ESR measurements, a correlation seems to exist between the generation of AlOHC point defects and the large absorption band in the visible-near IR region in Al-rich preforms (we note however a poor correlation concerning the sample C which displays no AlOHC ESR signal and visible absorption, probably due to experimental errors concerning absorption measurements) In order to confirm this assumption, thermal annealing measurements follow by absorption and cw-ESR characterization have been performed.
3.2 Thermal annealing experiment
The sample G, which contains the higher Al excess, has been chosen to investigate the effect of thermal annealing on the AlOHC point defects using ESR and optical absorption. The irradiated G preform was annealed during 10 min at temperatures ranging from 20°C to 650 °C. The Fig. 2 displays the optical absorption recorded after heating at each temperature. A gradual decrease of the large absorption band from 300 nm up to the IR region is observed when the annealing temperature increases.
Figure 4 shows the intensity of the absorption at 550 nm and the intensity of the AlOHC ESR signal in function of the annealing temperature. The visible absorption remains stable from Tamb to 150-200 °C. Beyond this temperature, the absorption band decreases rapidly up and almost entirely vanishes above 450 °C. Taking into account the experimental errors, the evolution of the AlOHC defect probed by ESR follows the evolution of the optical absorption, putting in evidence the correlation between the visible-near IR range absorption and the AlOHC defect (Fig. 5).
This result unambiguously reveals that the AlOHC point defect is the main colored center responsible of the darkening in post-irradiated fibers.
4. Glass structure and Yb3+ environment in pre-irradiated samples
In order to better understand the formation of AlOHC colored centers induced by γ-irradiation, a structural study of the pristine samples has been performed using Raman scattering and pulse-ESR spectroscopy. Raman spectroscopy allows to probe the global glass structure, namely the insertion of Al and P in the silica network, and the pulse-ESR spectroscopy gives direct information concerning the local environment of the Yb3+ paramagnetic ions.
4.1 Raman spectroscopy: Al and P codopants insertion
Raman analyses were performed using a Renishaw spectrometer in backscattering configuration, with the 532 nm excitation line provided from a Nd3+-YAG laser. Figure 6 shows the Raman spectra acquired on the A, C, D and F preforms as well as the pure a-SiO2 Raman spectrum. The Raman spectra display a strong broad band around 440 cm−1 and weaker features at 490, 600 and 800 cm−1. All these vibrational modes are presents in pure SiO2 glass Raman spectrum and so provides from SiO2 network vibrations. The 440 cm−1 main band, assigned to Si-O-Si bending mode, has been used to normalize the spectra. The two sharp bands around 500 and 600 cm−1 come from the breathing modes of 4 and 3-member rings respectively, and the weak high-frequency bands at 1060 and 1190 cm−1 arise from TO and LO antisymmetric stretching vibrations of Si-O-Si linkages respectively . This high-frequency range is largely modified by Al and/or P addition.
Raman spectra of samples C, D and F present similar features at high frequency: a broad band centred at 1135 cm−1 and a shoulder at 1240 cm−1. A sharp band at 1330 cm−1, well attributed to P = O stretching vibration in SiO2-P2O5 glasses [24,25], is also observed in C sample. On the other hand, significant differences are observed in the Raman spectrum of F preform. First, the 1330 cm−1 vibration is much more intense. Secondly, the shape of the broad high frequency band is rather different, and its maximum is located at 1145 cm−1, revealing P-O-P linkages . These linkages are confirmed by the presence of the 730 cm−1 P-O-P stretching mode .
A precise quantification of the vibrations is rather complicated because many vibrational bands due to different types of linkages occur in the 1000-1300 cm−1 wave numbers region. Moreover, the Raman bands frequency and width depend of bond angle distribution, and the Raman cross-section, linked to the bonds polarizability, depend also of the linkages type. All these reasons rule out the Raman spectra deconvolution. However, some characteristics can be extracted from high frequency Raman spectra. For Al/P>1, the shape of the 1000-1300 cm−1 broad band looks like AlPO4 molecular sieves spectra, indicating the presence of AlPO4-like units . For Al/P<1, AlPO4 units vibrations contribute to the 1000-1300 cm−1 broad band, as well as P-O-P vibrations. Concerning the 1330 cm−1 band, a transition can be observed at Al/P = 1, which clearly indicate the presence of P = O linkages for P≥Al glasses.
No detail about ytterbium connectivity can be extracted from Raman spectroscopy. The next section, which displays pulse-ESR measurements, will bring such information.
4.2 Pulse-ESR spectroscopy: Yb3+ local environment
In order to compare the local environment of Yb3+ as a function of Al and P contents, two-dimensional hyperfine sublevel correlation (2D-HYSCORE) ESR spectra have been performed recorded at 4K, using the pulse sequence π/2-τ- π/2-t1- π-t2- π/2-τ-echo with a time delay τ = 136 ns and at a static magnetic field of 350 mT where all the samples present significant spin echo absorption (the two-pulsed echo filed swept spectra of these samples can be found in ). This type of experiment, based on the coupling between electronic spin and nuclear spin (hyperfine interaction) allows to probe the local environment of the Yb3+ paramagnetic centers via the spin echo modulation at Larmor frequency of the neighbour nuclei. Oxygen atoms that fully occupy the first coordination shell of Yb3+ are not observed (I = 0). The different 2D spectra are displayed Fig. 7.
The three diagonal peaks at 3.0 MHz, 3.9 MHz and 6.0 MHz correspond to the Larmor frequencies of 29Si, 27Al and 31P nucleus respectively. We note that when P>Al, the intense pattern at 6.0 MHz indicates that phosphorous formed a cage (or solvation shell) surrounding Yb3+ , also demonstrated in the case of Er3+ [27,28] that Additional off-diagonal cross-peaks at (3.9-6.0) MHz and (6.0-3.9) MHz are observed for samples A and B, i.e when P>Al. These cross-peaks revealed the Al-P coupling, demonstrating the proximity of Al and P nuclei in the vicinity of the Yb3+ paramagnetic centers when phosphorous is introduced in excess compared to aluminium. On the other hand, no Al-P correlation is observed for samples D, E, F and G where Al>P.
5.1 AlOHC defect and glass structure
From the results of thermal bleaching combining optical absorption and cw-ESR, it was demonstrated that AlOHC point defects are mainly responsible of the darkening in post-irradiated preforms (see section 3). However, it is known that P-related point defects, namely Phosphorous-Oxygen Hole Center (POHC), can be induced by γ irradiation [12,29]. The P = O link, observed by Raman spectroscopy in samples where Al/P<1 (Fig. 6), is yet a precursor of the POHC paramagnetic defect . Nevertheless, no POHC species have been detected by cw-ESR in the whole set of samples, in particular in the preform A where P is largely introduced in excess compared to Al. The absence (or the weak concentration) of POHC is also confirmed by the lack of absorption band in the visible range of this post-irradiated preform (Fig. 2).
A surprising result is that the level of AlOHC induced by radiation does not depend linearly on the absolute concentration of Al incorporated in the glass matrix, but rather on the Al/P ratio. Indeed, the post-irradiated samples A and B which present non negligible Al concentration, exhibit rather no darkening phenomenon and no AlOHC formation. This means that AlOHC are created only when phosphorous is not introduced in excess compared to Al. Raman spectroscopy measurements reveal that Al and P formed preferentially AlPO4 units. When P>Al, the excess of phosphorous formed P-O-P and P = O linkages. In this case, the absence of AlOHC after irradiation shows that AlPO4 structures inhibit such defects creation. On the other hand, when Al>P, the Al excess, which is not located into AlPO4 linkages, is ionized by γ-rays.
Among the different trapped-electron centers which could be created with AlOHC during the pair generation process (for a complete review, see ), one can namely mention Si-E’, Ge-E’, Al-E’ or P-related electron-trapped paramagnetic defects. Unfortunately, the high magnitude of Ge-related defects in the post-irradiated ESR spectra (Fig. 2) does not allow to correlate the AlOHC creation with a particular electron-trapped defect. Another potential candidate is the Yb2+ specie, which can be formed by Yb3+ reduction mechanism. This possibility seems highly probable considering the proximity between Al and Yb3+, revealed by several recent theoretical and experimental studies [10,31–33], and confirmed by our pulse-ESR measurements (Fig. 7). Indeed, the HYSCORE spectra of D, E, F and G samples show important Al pattern without Al-P coupling. This reveals that the Al atoms in the vicinity of Yb3+ are not bounded to P atoms and do not form AlPO4 units, allowing AlOHC and Yb2+ pair generation. We note that ytterbium doped aluminosilicate have a charge transfer band at 230 nm (at the origin of Yb2+ and colored centers formation under high energy irradiation), which could be at the origin of the photodarkening process [17,34].
5.2 AlOHC and photodarkening
The complex mechanism of photodarkening process, meaning the IR pump induced losses in Yb3+ doped fibers, is a subject of a long and controversy debate [15–18,34–45]. In our present study, the purpose is not to bring new insights concerning the energy transfers and cooperative effects leading to UV emission at the origin of colored centers formation. However, a comparison of previous photodarkening studies with the present results seems to indicate that AlOHC defects are mainly responsible of the coloration in both cases: different recent studies show the dependence of the output losses in function of the Al and P codopants content [35,42], and reveal that the addition of phosphorous in aluminosilicate matrix reduced the photodarkening effects . This result is coherent with our measurements showing the formation of AlPO4 units and the decrease of AlOHC formation with P adding. Moreover, results of thermal bleaching in a photodarkened Al-rich glass with a composition close to the preform G  display also similar behaviour than the thermal treatment of post γ-irradiated G sample (Fig. 4). This reinforced the hypothesis of AlOHC defects creation/annihilation during the photodarkening/thermal bleaching process.
However, one can note that in Al-rich preforms, meaning when Al>P, gamma irradiation induced more darkening as Al excess increases (see Fig. 2) while for similar preforms, the photodarkening is reduced as Al excess increases [35,46]. This apparent contradiction probably finds its origin in the different radiation ways (between radiodarkening and photodarkening) leading to the defects creation. In the case of radiodarkening experiments, all the samples have received the same gamma energy in the whole material. More complex is the photodarkening process, for which the probability of the emission of a high energy photon (able to create defects) highly depends on the matrix composition and its atomic structure.
A set of gamma-irradiated Yb3+ doped aluminophosphosilicate glass preforms have been extensively investigated using several kinds of spectroscopic measurements. The results show a high dependence of the Al/P ratio in the darkening process. For Al/P<1, a very slight coloration is observed whereas Al/P>1 samples revealed an important darkening after irradiation. The combination of optical absorption and cw-ESR measurements at different annealing temperatures demonstrates that the creation of AlOHC colored centers is the main process at the origin of the darkening in Al-rich glasses, while these defects are not favored in P-rich glass in which Al atoms are in AlPO4 units as it has been shown by Raman spectroscopy. Pulse-ESR spectra reveal that Al-O-P linkages are in the vicinity of Yb3+ ions when P>Al, but not when P<Al.
Moreover, several similarities between radiodarkening and photodarkening have been highlighted, suggesting that AlOHC defects are at the origin of the darkening in both gamma and high-power IR irradiations. However, the rate of AlOHC created after irradiation highly depends on the kind of irradiation sources (γ-rays or IR light).
As perspectives, several measurements are indispensable to clarify the different mechanisms involved in situ during the irradiation process, namely the dynamics of transient defects [4, 47], or even annihilation via thermalbleaching  or photobleaching [37,48]. Additionally, the way to reach, considering photodarkening, high energy photons that create the defects will also be a key point. Other points remain obscured: does Yb2+ is the electron-trapped specie conjugated to AlOHC, or other species are involved? The contribution of spectroscopy to probe the atomic organization to correlate with optical properties is, in our opinion, a key issue to better understand radiation induced darkening in optical waveguides.
This project was funded by the CEA transversal programm “matériaux avancés”. The authors gratefully acknowledge O. Cavani (Prysmian group) for fiber preforms synthesis and S. Esnouf (CEA-Saclay) for gamma irradiations.
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