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We report an experimental investigation on the effects of fluorine codoping on the radiation response of Ge-doped Optical Fibers (OFs) obtained by three different drawing conditions. The OFs were irradiated with 10 keV X-rays up to 300 Mrad and studied by online Radiation-Induced-Attenuation (RIA) measurements. Confocal Micro-Luminescence (CML) and Electron Paramagnetic Resonance (EPR) were also employed to investigate the permanent radiation-induced-defects. The variation of the Germanium-Lone-Pair-Center (GLPC) and Non-Bridging-Oxygen-Hole-Centers (NBOHC) concentration with the radiation dose is investigated by CML, whereas the ones of the induced Ge(1), Ge(2) and Eʹ centers by EPR. No relevant differences are found in the RIA of the three fibers, as well as in the induced concentrations of Ge(1) and Ge(2) and in the decrease of the GLPC, showing minor relevance of changing the drawing conditions. We found that fluorine codoping does not affect the RIA and that, unexpectedly, the fluorine co-doped zones of the OFs show an enhanced photoluminescence of the radiation induced NBOHC enabling to suggest the presence of both Si and Ge variants. Moreover, an overall increase of the radiation induced Eʹ(Ge) centers is registered in relation to the presence of fluorine showing that this codopant has relevant effects.
© 2014 Optical Society of America
1. Introduction
The interest for several technological applications led to an extended investigation of the response of SiO2-based optical glasses in harsh environments. In this context optical fibers (OFs) are certainly among the most studied silica based devices [1]. They are employed for data transmission and as sensing components in civil nuclear applications, as power plants and waste repositories, in space applications, high energy physics, fusion facilities and for military purposes [2]. Therefore OFs are subject to several radiation environments that differ from one to another for operating temperature, type of radiation, total radiation dose and radiation dose-rate [3]. In any case harsh environments cause material damaging and three phenomena have been found to affect and degrade the performances of OFs: Radiation-Induced Absorption (RIA) [2, 3], Radiation-Induced light Emission (RIE) [4] and compaction or change of the refractive index (observed for example at high neutron fluences) [5]. Generally RIA is the most impacting factor because it limits the transmission efficiency of light and signals, whereas RIE is mainly observed for radiation resistant OFs (small RIA) and it originates from “parasitic” emitted light that lowers the quality of the transmitted signals [2].
Many previous studies have shown that, at microscopic level, radiation causes the creation of point defects, or color centers, and that many of them can be responsible for the appearance of absorption bands and the RIA phenomenon [1–3,6–11].
One of the most important parameter that influences the generation or the conversion of the point defects is the chemical composition of the glass. For example, the refractive index profiles of OFs are controlled by incorporating dopants (e.g. Ge, P, F, etc.) into the otherwise pure silica preforms. The dopants, as well as the unwanted impurities, represent extrinsic defects that are often silent precursors of radiation induced absorbing defects [1,6–8,12].
In previous years we developed the so-called “canonical approach” in order to study, both experimentally and by simulations, the influence of single dopant on the performances of OFs under irradiation [13–16]. This approach is based on the uniformity of the Modified Chemical Vapor Deposition (MCVD) production process of the samples and on the possibility of designing fibers for specific research purposes. The so-called “canonical samples” are produced by iXFiber SAS, Lannion, France. Ge-doped, F-doped, P-doped and pure silica core OFs have already been studied and they have been found to be suitable for different application in different environments [3]. For instance, considering RIA experiments, F-doped and pure silica fibers have shown to be the most radiation-hard, whereas the P-doped ones are the most sensitive. The next step in the study of canonical samples is extending the investigation to more complex systems, in which more than one dopant is present and where dopants can interact among them. OFs codoping could indeed improve some characteristics of the OFs (e.g. preventing the creation of some defects, improving production processes etc.) [17–19].
In this work we focus on multimode OFs doped with both Ge and F. The OFs were irradiated up to 300 Mrad by 10 keV X-rays. Furthermore, the results are compared to the corresponding canonical Ge-doped OFs that contain no fluorine. In order to have the most complete view of the radiation effects, three complementary spectroscopic techniques have been employed: online radiation induced absorption, Confocal Micro-Luminescence (CML) and Electron Paramagnetic Resonance (EPR). Possible effects due to the changing of the drawing conditions were also investigated.
2. Experimental details
Three different multimode OFs, core size 62.5 µm, doped with germanium and fluorine were drawn from the same preform adjusting speed and drawing tension. The used samples are identified as: GeFD1, draw speed 22 m/min and draw tension 35 g; GeFD2, draw speed 40 m/min and draw tension 65 g; GeFD3, draw speed 90 m/min and draw tension 166 g. The three fibers (collectively named GeFDi, hereafter) have the same dopant profile shown in Fig. 1, as determined by the Electron Microprobe Analysis technique for the GeFD3. The germanium concentration is almost constant along the diameter of the core of the fiber while the fluorine concentration varies in such a way that two concentric zones are defined in the core of the OFs. The combined effect of the two dopants results in a two-step refractive index profile similar to that of the previously studied canonical OFs GeD1, GeD2, GeD3 (collectively named GeDi) in which only germanium was present [20]. In both GeFDi and GeDi OFs about 0.1 wt.% of chlorine was present in the core and 0.2 wt.% in the cladding of the OFs by the MCVD production process of the related preforms. Note that the overall content of germanium in GeFDi OFs exceeds the one of GeDi by about 30% by design of the fibers.
Fig. 1 Radial distribution of germanium, fluorine and chlorine concentrations obtained by Electron Microprobe Analysis of GeFD3. The reported profile coincides for the used fibers GeFDi.
The OFs were irradiated with 10 keV X-rays at room temperature (26 to 30 °C) by means of an ARACOR machine at CEA-DAM-DIF. For the first experiment a maximum dose of 100 Mrad was reached, whereas for the other two runs the radiation doses vary from 5 krad up to 300 Mrad. Some previously published results on GeDi OFs are considered for comparison with GeFDi [20], and some new results concerning their RIA are presented in this work.
The online RIA technique consists in the evaluation of the losses of an optical fiber during the irradiation. An OF is connected to a light source and a spectrometer and, at the same time part of the OF is irradiated. The time evolution of the induced losses is calculated relatively to the initial transmitted spectrum. In our experiment a Deuterium-Halogen DH2000 light source and an Ocean Optics HR4000 spectrometer have been employed to study the responses in the UV-Visible spectral range. From repeatability tests the error assigned to these measurements is less than 10%.
The CML were carried out with a LabRam Aramis (Jobin-Yvon) confocal spectrometer. The 325 nm line (3.81eV) of a He-Cd laser was used as excitation probe light. The laser is focused upon the sample by means of a 40x UV-objective resulting in an 80 µW beam of roughly 5 µm in size at the waist point. The sample is located on a translational stage whose position can be adjusted in the X, Y and Z directions with micrometric precision. Additionally, it’s possible to reduce the size of the pinhole of the confocal microscope in order to reduce the volume of the sample from which the backscattered light is collected and get, at best, a probed spot size of 1.5 µm. In the present work all the measurements were performed with a regular spacing distance of 3 µm and with a probed spot size of 3 µm. This choice allowed obtaining a satisfying signal to noise ratio and, at the same time, completing the measurement quickly enough to avoid any photo-bleaching effect to take place. The collected light is finally diffracted by one of the four available gratings on a CCD camera. Our experimental conditions assure a spectral resolution of 0.5 nm. In a single experiment the cleaved tip of a 4 mm long OF is probed in several points along its diameter. The recorded set of spectra can then be used to derive the cartography of a particular spectral feature as a function of the distance from the center of the OF.
EPR measurements were also carried out at room temperature on all the studied OF with a Bruker EMX-Micro Bay spectrometer working at 9.8 GHz. This experimental technique allowed estimating the absolute concentrations of the paramagnetic point defects of our samples. Using a known silica bulk sample, the apparatus had been previously calibrated [21]. For these measurements all the reported Ge-related defects concentrations must be regarded as the average density in the core of the OFs, whereas the Si-related defects concentrations represent the average density in the whole OF volume. All the concentration values have an absolute error of 50% and a relative error of 25%.
3. Results
In Fig. 2 we report the RIA growth kinetics at 360 nm of the six studied OFs (three GeDi and three GeFDi).
Fig. 2 RIA at 360 nm for the fibers GeDi and GeFDi. The dose rate is 5krad/s and the maximum dose reached is 100 Mrad. In the two insets the ratio of the full spectra GeD1/GeD2, GeD3/GeD2 and GeFD1/GeFD2, GeFD3/GeD2 are also shown for two different doses: 200 krad and 100 Mrad. The straight dashed line is a guide to the eye describing the linear law.
The irradiation dose rate was 5 krad/sec and the fibers were irradiated for 20000 seconds, i.e. up to 100 Mrad. For all the samples the RIA grows almost linearly until about 100 krad. The deviation from linearity has already been observed in studies for irradiation induced infrared absorption in Ge-doped fibers [22]. Subsequently, the RIA kinetic slows down and reaches a steady level for doses higher than about 3 Mrad. No significant differences are detected in the behavior of the six OFs either due to different drawing conditions or different chemical compositions. Also in Fig. 2, two insets are shown in order to compare the shapes of the RIA spectra of differently drawn OFs. More precisely, in one inset the RIA spectra of GeD1 and GeD3 are compared to GeD2 at the two doses 200 krad and 100 Mrad and analogously in the other inset the samples GeFD1 and GeFD3 are compared to GeFD2. In both cases the ratios are constant and close to 1 meaning that changing drawing conditions doesn’t affect the spectral response of the studied OFs.
The transmitted spectrum was recorded for several hours after the end of the X-ray irradiation. In Fig. 3 the recovery process at 360 nm is reported. All the samples show a remarkable recovery probably due to photo-bleaching and thermal effects induced by the injected light power (tens of µW) and ambient temperature (26 to 30 °C). In less than one day 98% of the maximum RIA value at 360 nm is bleached out, similarly with the recovery recently reported for irradiation induced infrared absorption of Ge-doped fibers [22]. In the inset of Fig. 3 all the curves have been normalized to the maximum value in order to better highlight the possible differences in the recovery rates. In this case the behaviors of the three GeDi are essentially identical while GeFDi show certain variability. It is important to mention that all the following post-irradiation measurements, such as the CML and EPR discussed in this work, were recorded for the leftover stable defects. In Fig. 4 we report a typical photoluminescence spectrum obtained with the CML system described above. The laser excitation wavelength is 325 nm and the reported spectrum clearly shows two emission bands peaked around 400 and 650 nm. These bands are attributed to the triplet to singlet emission of the twofold coordinated Ge excited at 325 nm [23–26], indicated as Germanium Lone Pair Center (GLPC), and to the Non-Bridging Oxygen Hole Center (NBOHC) photoluminescence [27], respectively. The microscopic structure of GLPC consists of a germanium atom linked to two oxygen atoms by single bonds and with a pair of electrons not involved in any bond [23–25]. At variance, NBOHC structure is constituted by the dangling bond of an oxygen atom linked to a silicon (or germanium) atom by single bond [27,28].
Fig. 3 Recovery at 360 nm of the six OFs after a total irradiation dose of 100 Mrad. In the inset the same kinetics are normalized to their maximum value in order to compare their recovery rates.
Fig. 4 Typical photoluminescence spectrum of the core of an irradiated GeFDi OF excited at 325 nm. The two bands located around 400 nm ad 650 nm are assigned to GLPC and NBOHC defects, respectively.
Our apparatus allows us to probe a cleaved OF in several points of the diameter and to reconstruct the radial profile of each spectral feature. In the insets of Fig. 5 and Fig. 6, the cartographies are reported for three different radiation doses and for both emission bands reported in Fig. 4. It is worth noticing that for each radiation dose the three GeFDi samples share the same cartography regardless of the different drawing conditions. Furthermore, we note that the profile of the cartography does not depend significantly on the dose.
Fig. 5 Relative concentration variation of GLPC (as determined through the 400 nm CML band) as a function of the dose. The first point represents the pristine fiber. In the inset the cartographies at three different doses are shown for GeFD2.
Fig. 6 Relative concentration variation of NBOHC (as determined through the 650 nm CML band) as a function of the dose. The first point represents the pristine fiber. In the inset the cartographies at three different doses are shown for GeFD2. The blue dashed line represents the square root of the dose curve.
In the same figures the amplitude of the signals are plotted as a function of the dose. Each point of these curves has been constructed by integration of the emission profiles of the corresponding irradiated sample. Figure 5 shows that the pristine fiber is rich in GLPCs and that high irradiation doses cause a slight overall decrease of this defect (about 30%). Despite the fact that the germanium concentration is almost constant along the OF diameter, the cartography shows a higher concentration of GLPC at the center of the fiber and at the core-cladding interface. This profile appears to be comparable to the one reported in previous studies for the GeDi fibers made by two steps with different Ge doping levels [20], with particular regards for the maximum of the signal in the center of the core and value of the integrated profile.
Figure 6 shows that pristine fibers have a low content of NBOHC and that they are efficiently created by irradiation with a sublinear growth prevailing at high doses. In the inset of Fig. 6, it is also evident that the highest signal of NBOHC is induced in the more external part of the core, where the fluorine co-doping level is higher.
In order to highlight the effect of fluorine co-doping, in Fig. 7 the cartographies of NBOHC in the GeD2 and GeFD2 OFs are plotted [20]. The two cartographies are normalized with respect to the value at the center of the core where the chemical compositions of the two fibers are very similar. After the normalization has been done, the signals of NBOHC in the claddings are automatically equal. This shows that the ratios of the PL signals in the center of the core and the cladding are the same for GeD2 and GeFD2. We also note that both samples share the same pure silica cladding and in this region it is expected that the same amount of NBOHC is induced by the irradiation. On the other hand, the most striking effect concerns the different amplitude of NBOHC in the outer zone of the core. In this region the GeD2 fiber has a germanium concentration of around 4.5 wt.% [20], about half the inner zone concentration, and the NBOHC signal is somehow intermediate between the core and the cladding values with peaks at the interfaces separating different zones [20]. By contrast, in the GeFD2 fiber, the same region is characterized by a concentration of fluorine around 1wt. % and a concentration of germanium around 8 wt. %, similar to the inner zone concentration as shown in Fig. 1. Nevertheless, in this case, the NBOHC signal is about 3 times higher.
Fig. 7 Normalized cartographies of NBOHC luminescence in GeD2 (from ref [20].) and GeFD2 (present work) 100 Mrad irradiated fibers. The normalization has been done dividing by the respective PL values at the center of the OF core.
In Fig. 8 the EPR spectra of the GeFDi fibers are reported for three different doses. Each spectrum has been normalized by the mass of the corresponding sample and some experimental parameters (acquisition time, power, etc.) in order to compare the absolute concentration of the paramagnetic defects induced by the irradiation. Figure 8(a) shows the EPR signals of the pristine samples; in this case only one EPR signal is present which is known to belong to the Eʹ(Ge) center [29,30], whose initial concentration is about 1016 defects/cm3. The Eʹ(Ge) center consists of a germanium atom linked to three oxygen atoms and with a remaining unpaired electron [6]. Figures 8(b) and 8(c) show a remarkable modification of the spectral shape due to the creation of other paramagnetic defects. As a matter of fact, these spectra contain two more Ge-related EPR signals corresponding to Ge(1) and Ge(2). The microscopic structure of Ge(1) center consists of an electron trapped on a substitutional germanium atom linked to four oxygen atoms [6,31,32]. Concerning Ge(2) center two microscopic models were supposed: one of these is believed to be a ionized GLPC, in which one of the electrons has been removed from the lone pair [31], whereas the other is hypothesized to be similar to the Ge(1) defect but with a different energetic configuration [32]. No differences are detected among GeFD1, GeFD2 and GeFD3 at any radiation dose. In Fig. 9, the EPR spectrum of GeFD2, normalized by the double integral, is compared to the spectrum of GeD2, analogously normalized, for a single irradiation dose of 2 Mrad.
Fig. 8 EPR spectra normalized by mass and spectrometer parameters of the GeFDi samples for three different doses: a) pristine (nIRR), showing 100% E’-Ge signal, b) 2 Mrad, showing ~34% Ge(1), ~54% Ge(2) and ~12% Eʹ(Ge), c) showing 300 Mrad, showing, ~17% Ge(1), ~23% Ge(2), ~58% Eʹ(Ge) (a ~2% of signal of Eʹ(Si) has been taken into account).
Fig. 9 EPR signal comparison between GeD2 and GeFD2 at the same radiation dose of 2 Mrad. Spectra are normalized by the double integral.
Some differences are here visible. More information on the induced defects concentrations can be gained decomposing the EPR spectra by means of some characteristic EPR lines of Ge-related defects Ge(1), Ge(2) and Eʹ(Ge) [29]. The results of such decomposition are shown in Fig. 10 where the concentrations of the radiation induced defects in GeFDi OFs are compared to the results obtained for the GeDi OFs by applying a similar analysis [29].
Fig. 10 Absolute concentration curves of paramagnetic defects as a function of the total dose obtained by spectral decomposition of the EPR signals. Both the results on GeD2 and GeFD2 are here plotted for comparison.
The growth kinetics of Ge(1) and Ge(2) defects are very similar for the two types of fibers: for doses lower than 200 krad the concentration grows linearly and reaches saturation values for doses higher than 2 Mrad: 3x1017Ge(1)/cm3 and 4x1017Ge(2)/cm3. On the other hand the Eʹ(Ge) defects grow with a sub-linear law without reaching any saturation level in the investigated radiation dose range. It is also interesting to note that GeFDi fibers display about 2.5 times higher concentration of Eʹ(Ge) at all the doses compared to GeDi. Finally, we must take into account the Eʹ(Si) defect. It consists of a dangling bond of a Silicon atom bonded to three oxygen atoms [1]. The Eʹ(Si) concentration in GeFDi OF is somewhat harder to estimate as their EPR signatures overlap with the signal coming from Eʹ(Ge), when suitable experimental parameters are employed. However, at high doses a sub-linear concentration growth has been recorded with a maximum value around 3x1016 Eʹ(Si)/cm3 at 300 Mrad.
4. Discussion
Previous studies have shown that the radiation induced absorption at 360 nm (3.4 eV) is mainly due to the Ge(1) defect [13], whose absorption band is centered at about 4.4 eV (280 nm) with a FWHM greater than or equal to 1.3 eV [2,6,32]. Therefore we can consider the curves in Fig. 2 and Fig. 3 to be proportional to the concentration of Ge(1) defects during and after the irradiation. The similarity of the Ge(1) content is also supported by the EPR data in Fig. 10.
The 10% uncertainty associated with the repeatability of the online RIA experiments doesn’t allow us to detect and clearly ascribe any effect to the change of drawing conditions both in the case of Ge- and Ge/F- doped OFs. Moreover, as far as we consider the RIA at 360 nm, fluorine co-doping doesn’t certainly produce major changes. It is worth noticing that in Fig. 2 and Fig. 3 the three fibers containing fluorine as co-dopant are systematically below the other three at high doses. However, the differences here involved do not exceed 25%. These findings allow concluding that, if present, minor effects on RIA at 360 nm are connected to drawing or doping. We note that the similarity of the RIA for Ge and Ge/F co-doped fibers agrees with the data reported in [33] in the near infrared spectral domain.
Concerning the results of the CML experiments we could not detect any significant difference due to the different drawing conditions except for a slightly higher relative concentration of radiation induced NBOHC in the GeFD3 OF (highest drawing speed) at the two lowest radiation doses, 5 krad and 50 krad. This result could be explained by a slightly higher percentage of strained bonds in the pristine GeFD3 fiber compared to the other two. In this context, the analysis of the EPR measurements do not help clarifying this aspect because the signal of the NBOHC has a small amplitude and it overlaps with other signals [1,34,35], increasing the uncertainties on its estimation.
Regarding the PL of the GLPC defects, we observed roughly an overall 30% decrease due to the irradiation. Moreover, a detailed study of the cartographies at different radiation doses shows no significant differences in the profiles and shapes of the different spectra. Such result suggests a homogeneous effect of the X-rays on the local initial concentration of GLPCs along the cross section of the OF. Furthermore, the amplitude of the GLPC PL in the center of the core and its dose dependence in GeDi and GeFDi are similar. These findings indicate that the GLPC content in the two types of fiber is comparable despite the differences in the doping profiles. This similarity, whose origins could be deeper clarified, is relevant for the use of fibers under irradiation since GLPC content is an important factor in determining the radiation response of Ge-doped silica, as it was evidenced by previous investigations and it appears to be confirmed here by RIA and by the EPR data of the Ge(1) and Ge(2) [1,32, 36–39]. At variance, the CML experiments show a strong effect of fluorine co-doping, as evidenced by Fig. 7. This is indeed clear whether we compare the NBOHC cartographies of GeDi and GeFDi OFs, or better if we compare the relative signal of NBOHC in the two core zones of the GeFDi samples. The NBOHC signal of the outer zone (~1wt.% F) is almost twice the signal in the inner zone. Figure 1 clearly shows an almost constant concentration of germanium in the whole core and the only parameter that considerably varies between the inner and the outer parts of the core is the concentration of fluorine.
At present no hypothesis has been suggested on the nature of the NBOHC we recorded. It is known that both Si-related and Ge-related NBOHC can exist in Ge-doped silica glasses [28]. The two types have a quite similar red emission band: Si-NBOHC peaked at ~1.92eV, FWHM about 0.16 eV and lifetime 10-20 μs; Ge-NBOHC peaked at ~1.86 eV, FWHM about 0.18eV and lifetime ~5 μs [28]. The small relative displacement of the two bands can be exploited to gain some additional information regarding the NBOHC of our samples by investigating the spectral features of the observed emission as a function of the radial position in the fibers. Figure 11 shows the cartography of the spectral position of the maximum of the NBOHC emission band in the GeFD2 OF irradiated at the maximum dose, 300 Mrad. The cladding contains only pure silica and here the NBOHCs are the Si-related ones. Moving toward the center of the OF causes a redshift of the maximum which is compatible with the appearance of a Ge-related NBOHC band contribution [28]. Note that the displacement of the maximum of the emission band is higher in the outer zone of the core where the fluorine concentration is the greatest. To better illustrate these results three different normalized PL spectra recorded at the dose of 300 Mrad, are shown in the inset of the same figure. Measurable modifications are observed. At 39 µm from the center, i.e. in the cladding, the Si-related NBOHC spectrum agrees with the data from literature. In the outer zone of the core, 27 µm from the center, the emission band is red-shifted and broadened, reaching spectral characteristics that are closer to the ones of the Ge-NBOHC [28]. However, we don’t exclude the coexistence of the two types of defects. Finally in the inner zone, 12 µm from the center, the PL band is quite similar to the one in the outer part but with a small blue-shift and a slightly smaller FWHM. Since the Ge content is essentially constant for radii less than ~32 µm, these results suggest that the fluorine co-doping seems to facilitate the creation of Ge-NBOHC or to improve their emission.
Fig. 11 Spectral position cartography of the maximum of the NBOHC emission band in the GeFD2 OF irradiated at the maximum dose, 300 Mrad. In the inset the normalized NBOHC PL recorded at three different distances from the center of the fiber are shown: 12 µm ([Ge] = 8wt.%, [F] = 0.02wt.%), 27 µm ([Ge] = 7.8wt.%, [F] = 1wt.%), 39 µm ([Ge] = 0wt.%, [F] = 0wt.%). For each spectrum the low energy tail of the GLPC emission band (400 nm) of a pristine GeFD2 fiber was subtracted.
Concerning the results of the EPR measurements, for GeDi and GeFDi OFs, the fluorine co-doping seems to be responsible for about 2.5 higher concentration of Eʹ(Ge) defects. The slightly higher Ge content in GeFDi cannot simply justify the excess of Eʹ(Ge). Furthermore, we observe that the dose dependence of the other Ge related defects is comparable and Ge(1) and Ge(2) concentrations are essentially the same. This finding agrees with the reported RIA at 360 nm which evidenced the absence of a great improvement of the radiation hardness of the GeFDi OFs with respect to the simple Ge doped ones (GeDi). Furthermore, as mentioned above, the similar behaviours of the Ge(1), Ge(2) and GLPC in the GeDi and GeFDi agree with the previously reported strict relation occurring between the formation of the Ge(1) and Ge(2) and the GLPC content [36]. In fact, independently from the used defect models the GLPC affects the electron donor content [31, 32].
Our data evidence a complex scenario regarding Eʹ(Ge), NBOHC, and in particular for the Ge-NBOHC. First of all, as reported in Fig. 6 and Fig. 10 at doses higher than 104 krad, the sub-linear dose dependence suggests that Eʹ(Ge) and Si and Ge-NBOHC could be generated concurrently by the breaking of Ge-O-Ge(Si) linkages, as previously suggested for similar Si related defects [40]. Furthermore, since in the GeFDi OFs the Ge-NBOHCs are generated with higher concentration in the co-doped zone between 20 and 30 μm from the fiber center, and with lower efficiency in the inner part of the core below 20 μm, and since the Ge contents of these parts are the same, a role of fluorine in the generation or in the increase of the post irradiation stability is guessed. In addition, it must be said that if the excess of Eʹ(Ge) were related to the NBOHC excess located in the co-doped region, the Eʹ(Ge) concentration in this zone would increase with respect to the one calculated from the EPR data that assume the defects population uniformly distributed in the overall Ge doped zone. This means that the concentration increase would be higher than a factor 3, with respect to GeDi fibers, in the co-doped volume suggesting a further effect of F.
5. Conclusion
We experimentally studied the radiation effects on Ge doped and Ge/F co-doped silica optical fibers. For each type of fibers three different drawing conditions were investigated. Their radiation responses were studied by performing X-ray irradiation up to doses of 200 or 300 Mrad. The irradiated fibers were analyzed by acquiring online Radiation Induced Absorption (RIA) measurements in the UV-visible spectral range, post irradiation Confocal Microscopy Luminescence (CML) and Electron Paramagnetic Resonance (EPR) spectra in order to monitor the signals related to (i) the Germanium Lone Pair Centers, (ii) the Non bridging Oxygen Hole Centers, (iii) the Ge related paramagnetic point defects (Ge(1), Ge(2) and Eʹ(Ge) and (iv) the Eʹ(Si).The data indicate that the changes of the drawing parameters, within the values used for production of specialty fibers, do not modify significantly the radiation response of the Ge/F co-doped fibers as was previously reported for the Ge doped ones. By comparing fibers having different chemical composition, but produced in similar conditions, we can conclude that adding fluorine has no great impact on the radiation response in the investigated UV-visible range. Such similar responses of the two types of fibers are attributed to the comparable amount of the native GLPC in the pristine samples and radiation induced Ge(1), Ge(2). However, despite the similar content of germanium, a greater amount of induced Eʹ(Ge) was recorded in the OF co-doped with fluorine and at the same time the CML measurements revealed a much more intense photoluminescence of NBOHC in the region of the fiber where the concentration of fluorine is higher suggesting a prominent role of F codoping. The spectral features of the induced NBOHC together with their radial profile along the fiber enabled to evidence the presence of their Si and Ge variants with larger Ge variant presence in correspondence to the larger F doping.
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