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Abstract
In this work, Er/Al/Ge co-doped silica glasses with different GeO2 content (0–3 mol%), as well as Ge/Al co-doped silica glasses, are prepared by combining the sol–gel and high-temperature sintering methods. Further, the effects of the GeO2 content on the absorptions and emissions properties, and lifetimes of the glasses before and after 1KGy γ-ray irradiation are compared. The Er/Al/Ge co-doped silica fibers are prepared from a preform produced via modified chemical vapor deposition (MCVD) combined with nano sol-doping. The effects of Ge co-doping on the optical loss and amplifier gain of the Er-doped silica fibers (EDFs) before and after irradiation are also investigated. The related mechanism and species of the γ-ray radiation-induced color centers are revealed via radiation-induced-absorption (RIA) and continuous wave electron paramagnetic resonance (CW-EPR) spectroscopies. The results revealed that co-doping with GeO2 considerably improves the radiation resistance of the glass and exerts a slight effect on the spectral properties of the Er/Al/Ge co-doped silica glasses before irradiation. The RIA and CW-EPR spectra revealed that the aluminum–oxygen hole center (AlOHC) defects reduce with increasing GeO2 content because the intermediaries, the Ge-related oxygen-deficient centers (GeODC(I) and GeODC(II)), exhibit stronger abilities to trap the holes compared with the [AlO4/2]− group. This reduces the RIA level in the visible and near-infrared regions of the Er/Al/Ge co-doped silica glass. The irradiation experiment on the fiber further confirmed that the radiation resistance of EDFs can be considerably improved by Ge co-doping.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In the past few decades, extensive studies have been conducted on rare-earth (RE)-doped silica fiber lasers and fiber amplifiers [1,2]. Er-doped fiber lasers and amplifiers (EDFA) have attracted considerable attention because the intra-4f transition (4I13/2- 4I15/2) of Er3+ occurs at 1.54 μm, which corresponds to the lowest signal attenuation windows. The Er-doped fibers (EDFs) are an essential component in fiber-optic communications, as well as in biomedical and aerospace applications (lasers and lidars) [3–5]. During space missions (total cumulative dose does not exceed 1KGy), EDFs get exposed to ionizing radiations (high-energy protons, electrons, gamma rays, etc.), which impact their laser performance and cause the radiation darkening (RD) effect that is mainly manifested as an increased loss and decreased laser slope efficiency or gain performance [6,7]. RD is mainly related to the formation of color centers in the EDF core, which causes a rapid increase in the radiation-induced attenuation (RIA). Thus, it is necessary to investigate the origin of RIA to elucidate the degradation mechanisms and ameliorate the radiation hardening of EDFs.
Al is added to EDFAs to impede the clustering of the RE ions, thus remarkably improving the fluorescence yield, flattening the amplifier gain, and broadening the gain bandwidth of Er3+ [8,9]. Further, the addition of Al and Ge to the silica matrix allows an increase in the refractive index (RI) to obtain the desired numerical aperture (NA) [10–12]. Previous studies confirmed that co-dopants (mainly Al, P, and Ge) are mainly responsible for the high RIA levels and radiation sensitivities of EDFAs in RE-doped silica fibers [6,13–15]. Although Al- or Ge-related defects, which are induced by irradiation, increase RIAs of Al- or Ge-doped fibers, the effects of the dopants on their radiation resistance cannot be independently premeditated.
Most of the reports on Ge-related defects focused on their photosensitivity; only a few of them considered their radiation resistance. Several studies have reported the type of color centers formed by Ge-doped silica glasses and optical fibers during ArF laser irradiation, X- or γ-ray radiation, and fiber drawing [16–19]. Likhachev et al. concluded that GeO2 co-doping is one of the best-suited host glass compositions of radiation-resistant EDFs only from the RIA spectrum perspective [15]. Kobayashi et al. proved that Ge co-doping can suppress the P-related defects [20]. Leon et al. demonstrated that Ge doping can suppress the formation of Al-related defects [21]. However, only a few reports focused on the decomposition of the RIA spectra of Al- and Ge-related defects, analysis of the defects using the deconvoluted continuous wave electron paramagnetic resonance (CW-EPR) spectra, and the systematic analysis of the in-depth mechanism that cause the radiation resistance owing to Ge doping.
Based on this background, Er/Al/Ge co-doped silica glasses with different GeO2 contents are prepared herein. The objective of this work is to systematically evaluate the influence of the GeO2 content on the optical and spectroscopic properties of the glasses before and after irradiation. Two optical fibers containing different dopants were drawn to compare the influences of Ge doping on EDFA before and after irradiation. More specifically, the related radiation resistance mechanisms are disclosed by coupling optical absorption spectroscopy and CW-EPR. This work can contribute to the realization of radiation-hardened Er-doped optical amplifiers for future space applications.
2. Experimental details
2.1 Sample preparation
0.05Er2O3−2Al2O3−xGeO2−(97.95-x)SiO2 glasses (in mol%, x=0, 1, 2, 4, and 6) as shown in Table 1 were initially prepared using the sol-gel method, and converted subsequently to glasses by high-temperature sintering. ErCl3·6H2O (99.99%, Aladdin), AlCl3·6H2O (99.99%, Aladdin), GeCl4 (99.99%, Alfa), tetraethoxysilane and C2H5OH were applied as precursors. Further, deionized water was introduced to sustain the hydrolysis reaction. Based on the GeO2 content, the glasses were labeled EAG0, EAG1, EAG2, EAG4, and EAG6 (referred as EAG glass series for short). Additionally, the Ge/Al co-doped silica glass was also prepared by the same method. Next, inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis revealed that the contents of Al2O3 and Er2O3 in these silica glasses were within 5% of their mean compositions. Even if parts of GeO2 will volatilize, there are still more than 65% of the theoretical content.
Table 1. Mean compositions of the glass samples.
In this work, two Er-doped silica fibers (#EA and #EAG) were prepared, and their preforms were manufactured via a combination of modified chemical vapor deposition (MCVD) and nano sol-doping [22]. The main characteristics of #EA and #EAG are presented in Table 2. The chemical compositions of the fibers were measured by electron probe microanalysis (EPMA). The main difference between the two fibers was the presence and absence of Ge in the #EAG and #EA cores, respectively. Because the Al and Er contents of #EA and #EAG were similar, the introduction of Ge increased the RI of the #EAG core. To ensure that the core NAs of #EA and #EAG were close enough, a certain amount of La was added to #EA to increase its core NA. The radial refractive index profile was measured using an optical analyzer (IFA-100). The NA was calculated based on the measured RI.
Table 2. Main parameters of studied erbium fibers.
2.2 Analyses of the glass samples
To study the effect of γ-ray irradiation on the optical and spectroscopic properties of the Er/Al/Ge co-doped silica glasses, each glass sample was divided into two pieces: one piece was exposed to γ-rays, and the other, which was utilized as a reference, was not irradiated. The glasses were exposed to γ-rays of up to 1 KGy at a rate of 59 Gy/h from a 60Co γ-source in a normal atmosphere. The bulk glasses were polished into 1.9 mm-thin slices with the diameter of 10 mm to measure of their spectral properties. The powdered samples, which weighed ∼100 mg each, were utilized for the CW-EPR tests.
The absorption spectra were recorded on a Perkin Elmer Lambda 900 UV/VIS/NIR spectrophotometer in the range of 190–1700nm. The emission spectra and the fluorescence lifetime were excited at 980 nm by a laser diode employing an Edinburg FLS 920 type spectrometer. The CW-EPR measurements of the paramagnetic point defects were conducted at room temperature (RT) employing a BRUKE ELEXSYS-II E500 CW-EPR spectrometer operating in the X-band (9. 38 GHz) with a microwave power of 1 mW in the 325–340 mT magnetic field range. The CW-EPR spectra were normalized to the same receiver gain and a sample weight of 1 mg. The line shape and deconvolutions were analyzed by the XSophe software [23]. All the measurements were conducted at RT.
2.3 Analyses of the fiber samples
To study the effect of irradiation on the laser performance of EDFs, their optical loss and amplification performances were recorded. The optical loss of the fibers was tested by the cut-off method. Further, the online amplifier analysis of EDF, which corresponded to the case where EDFs were pumped during the irradiation, was conducted. This model considered almost all the photobleaching phenomena that occur in this system. The online test was performed with an X-ray machine with a photon energy of 40–100 keV. Considering that the measurement instrument can also be affected by X-ray, the fiber was the only amplifier component in the irradiation zone. The dose ratio was fixed at a few Gy (SiO2)/s (4.8×10−3 Gy/s). The total dose for the online test was 50 Gy. Further, the total dose for the optical loss test was 400 Gy. The gain was monitored in-situ by an optical spectrum analyzer equipped with an amplifier operating in a forward pumping configuration (Fig. 1). These fibers were pumped by a 976-nm laser diode, which allowed the efficient amplification of the 1557 nm signal. The powers of the input signal and pump were 1 and 234 mW, respectively. A fiber length of 2.2 m was selected for each amplifier to facilitate the comparisons between the amplifier radiation responses (all the tests were performed at RT).
3. Results and discussion
3.1 Changes in the absorption spectra before and after the irradiation
Figure 2(a) shows the absorption spectra of the EAG glass series with different GeO2 contents before the irradiation. The absorption peaks from the visible to near-infrared mainly originated from the transitions between the different Er3+ energy levels, as shown in Fig. 2(a). Ge doping exerted a minimum effect on the absorption coefficients of the transitions between the Er energy levels. This implies that the doping slightly affected the absorption cross-section of Er3+. In addition, as the GeO2 content increased, the UV absorption edge of the glass red-shifted; this is most likely related to the small bandgap of GeO2 (6.6 eV) compared with that of SiO2 (8.2 eV) [24].
Fig. 2. (a) Absorption and (b) RIA spectra of the EAG glass with different GeO2 contents of silica glasses. Inset: photographs of the EAG0 and EAG6 glasses before and after the irradiation.
Figure 2(b) shows the RIA spectra of the EAG glass series with different GeO2 contents. The inset shows the pictures of the EAG0 and EAG6 glasses before and after the irradiation. RIA can be obtained by subtracting the spectral absorption coefficient of the pristine samples from that of the measured irradiation samples. RIA is an essential characteristic parameter for evaluating the performance of radiation-resistant materials. As shown in Fig. 2(b), the RIA level, which can be related to the aluminum–oxygen hole center (AlOHC), was the highest among the EAG0 glasses. It reduced dramatically as the GeO2 content increased. The EAG6 glass achieved the lowest RIA level, indicating that it exhibited the best radiation-hardening performance, as further confirmed by the slight coloration after the irradiation (inset of Fig. 2(b)).
3.2 Changes in the photoluminescence (PL) intensity and fluorescence lifetime before and after the irradiation
The effects of the Ge contents on the fluorescence properties of the EAG samples before and after the irradiation are shown in Fig. 3. Figures 3(a) and (c) show the PL intensities of Er3+ at 1530 nm. Figures 3(b) and (d) show the fluorescence lifetimes of Er3+ at 1530 nm.
Fig. 3. (a) Fluorescence spectra and (c) luminescence integral intensity as a function of the GeO2 concentrations of the Er /Al/Ge co-doped silica glasses before (the solid curve) and after (the dash curve) 1 KGy γ-ray irradiation. (b) Decay curves of Er3+ of EAG0 and EAG6 and (d) excited state lifetime values of Er3+ at 1530 nm before and after 1 KGy irradiation as a function of the GeO2 content.
Figure 3(a) shows the PL spectra of the typical samples (EAG0 and EAG6) before and after the irradiation. For comparison, all the spectra were normalized to the integral intensity of the EAG0 glass. Figure 3(c) shows the relative PL integral intensities of the Er/Al/Ge co-doped silica glasses as a function of the GeO2 concentration at a wavelength of 1530 nm. Before the irradiation, the intensity and shape of the PL peak did not change with increasing GeO2. After the irradiation, the PL intensity of EAG0 decreased sharply, while the change in the PL intensity decreased gradually as the GeO2 content was increased. For the EAG6 sample, there was almost no change in the PL integral intensity before and after the irradiation, as shown in Fig. 3(c).
Figure 3(b) shows the decay curves of the 4I13/2→4I15/2 excited state of Er3+ under 980 nm excitation at 1530 nm versus the GeO2 concentrations before and after the irradiation. The curves of the samples were approximated by the exponential decay, and the fitted lifetime values are shown in Fig. 3(d). The trends of the fluorescence lifetime and PL intensity almost corresponded. Before the irradiation, the change in the GeO2 content exerted a minimum effect on the fluorescence lifetime of the 4I13/2 level of Er3+, indicating that the co-doping with a certain amount of GeO2 did not change the fluorescence properties of Er3+. After the irradiation, the fluorescence lifetime of Er3+ in the EAG0 glass decreased significantly. The degradations of the PL intensity and lifetime of the irradiated samples were mainly attributed to the following two reasons: (a) the absorption of the 980 nm pump light by the defect centers, which reduced the pump power available for population inversion [25]; and (b) the energy transfer from the excited Er3+ to the defect centers, which was facilitated by the reduced lifetime of the excited state of Er3+. Figure 3(d) presents that the irradiation also reduced the lifetime significantly. Apart from the role of the defect centers, the reduction of Er3+ to Er2+ was partly responsible for the decrease in the PL intensity of Er3+, as proposed by Hari Babu et al. [26]. The change in the fluorescence lifetime caused by the irradiation reduced gradually with increasing GeO2 concentration, as shown in Fig. 3(d). Regarding the EAG6 sample, there was almost no change in the PL integral intensities before and after the irradiation (Fig. 3(c)), and the change in its fluorescent lifetime before and after the irradiation was the smallest among those of all the glasses in Fig. 3(d). This indicated that the radiation resistance of the Er/Al co-doped glass was greatly improved by Ge co-doping.
3.3 Effects of Ge co-doping on the optical loss and amplifier gain of erbium fibers before and after the irradiation
Figures 4(a) and (b) show the optical loss spectra of #EA and #EAG before and after the irradiation in the spectral region of 1150–1400 nm. The optical losses of #EA at 1200 nm in the pristine, 400 Gy, and 1100 Gy irradiated fibers were 0.02, 5.56, and 10.93 dB/m, respectively. The optical losses of #EAG at 1200 nm in the pristine, 400 Gy, and 1100 Gy irradiated fibers were 0.01, 0.74, and 1.36 dB/m, respectively. The loss, which was induced by X-ray, increased with increasing radiation dose. Compared with #EA fiber, #EAG exhibited a lower background loss after 400 and 1100 Gy irradiations. This indicates that the co-doping of EDF with Ge can lower the background loss level after the irradiation, thereby greatly improving the radiation resistance of #EAG compared with that of #EA.
Fig. 4. Optical loss spectra of (a) #EA and (b) #EAG fibers before (pristine) and after (400 and 1100 Gy) γ-rays irradiation.
Figure 5 shows the evolution of the amplifier gains of #EA and #EAG fibers irradiated for up to a dose of 50 Gy. To allow a better direct comparison of the radiation effects on the devices in different amplifiers, the output powers were normalized to their maximum values. The results indicated that the radiation sensitivity of the amplifiers strongly depended on the dopant. In amplifier #1 employing #EA fiber, we observed a 21% decrease in the gain at 1557 nm after a 50 Gy X-ray irradiation. This degradation was strongly inhibited in amplifier #2, which was designed with #EAG fiber. It had a small gain degradation level (7%) after 50 Gy irradiation. This result further confirmed that Ge co-doping with an active fiber can greatly improve the radiation resistance behavior of the amplifier.
Fig. 5. Dose dependence of the normalized gains and gain degradations of the two amplifiers (using #EA and #EAG active fibers) during the 50 Gy X-ray irradiation (the gains were normalized to their maximum values).
3.4 Nature of the point defects produced by γ-ray irradiation
Previous studies confirmed that an increase in the RIA of active fibers is associated with the valence variations in the RE ions and the formation of matrix-related point defects [25,27–29]. The point defects were primarily responsible for the RD effects in the active fibers. The RIA and CW-EPR spectra aid the identification of the nature of the point defects. Compared with bivalent RE ions (e.g., Yb2+ and Er2+), the absorption cross-sections of the point defects are larger, and their absorption peaks are closer to the absorption and emission wavelengths of trivalent RE ions (Yb3+, Er3+, etc.) [26]. Sils et al. reported that the absorption of Er2+ primarily occurs in the ultraviolet–visible (UV–Vis) region [30]. One Er-free glass (the Ge/Al co-doped silica glass) was prepared to eliminate the influence of the absorption of Er2+ on the deconvolution of the RIA spectra. No CW-EPR signal of Er3+ was detected at temperatures of >100 K because of their fast spin-lattice relaxation time. Therefore, RT-CW-EPR spectroscopy is employed to detect the paramagnetic point defects in samples containing Er.
Figures 6(a) presents the RIA spectrum of the irradiated Ge/Al co-doped silica glass. The cumulative fitted peak was consistent with the observed RIA. The RIA spectrum of the Ge/Al co-doped sample was decomposed into nine Gaussian components, which peaked at 2.2, 2.9. 4.2, 4.6, 5.0, 5.1, 5.6, 5.9, and 6.4 eV. These bands were attributed to the Al-OHC (a fourfold coordinated Al with a hole trapping an oxygen atom, denoted as ${\equiv} $Al-O$^\circ $, where “${\equiv} $” represents bonds with three separate oxygen) (2.2 and 2.9 eV) [31], the Al dangling band ($\textrm{Al-E}^\prime$, an unpaired electron trapped in an Al atom that is coordinated with three oxygen atoms, donated as ${\equiv} $Al•) (4.2 eV) [31], Ge(1) (a fourfold coordination of the Ge atom trapping an electron, denoted as: =Ge•=, where “=” represents two bands with two different oxygen atoms) (4.5 eV) [32], the Al oxygen deficiency center (Al-ODC, denoted as ${\equiv} $Al••R${\equiv} $) (5.1 eV), The Ge oxygen deficient centers (Ge-ODC(II), denoted as: [ = Ge••]0) (5.1 eV) [33], Ge(2) (the defect possesses a similar structure to that of Ge(1), although it possessed a Ge atom as a second neighbor. It is denoted as${\; } \equiv $Ge•-O-Ge) (5.6 eV) [34], the Si dangling band ($\textrm{Si-E}^\prime$, donated as ${\equiv} $Si•) (5.8 eV) [35] and Ge dangling band ($\textrm{Ge-E}^\prime$, a threefold coordinated Ge atom with an unpaired electron, express as ${\equiv} $Ge•$^\circ $Ge${\equiv} )$ (6.4 eV) [36], respectively. The defects and their characteristics are listed in Table 3.
Fig. 6. (a) Gaussian decomposition of RIA and (b) CW-EPR spectra of Ge/Al co-doped samples after 1 KGy γ-ray irradiation; (experimental (exp.) and simulated (sim.)); (c) CW-EPR spectra of the 1 KGy γ-ray irradiated Er/Al/Ge co-doped silica glasses with different GeO2 contents. Inset: the characteristics of the EPR signals that are associated with AlOHC. (d) EPR spin concentrations of the various paramagnetic defects as function of the GeO2 concentration.
Table 3. Absorption peak position, full width at half maximum (FWHM) of the RIA spectra (eV) and g values of the CW-EPR spectra of the various defect centers in radiated samples
Figure 6(b) shows the experimental and simulated CW-EPR spectra of the irradiated Ge/Al co-doped silica glass. The spectrum revealed a composite structure from 325 to 340 mT. The spectrum was decomposed into five components by the reference line shapes of the $\textrm{Si-E}^\prime$ Al-OHC, $\textrm{Ge-E}^\prime$, Ge(1), and Ge(2) centers. However, Al-ODC and Ge-ODC were the diamagnetic centers (no CW-EPR signal), which could not be detected in the CW-EPR spectra. Although the $\textrm{Al-E}^\prime$ center was a paramagnetic, it was only detected when CW-EPR equipment was performed in the dispersion mode and with a very low microwave power, because the $\textrm{Al-E}^\prime$ centers exhibited longer spin-lattice relaxation times compared with the $\textrm{Si-E}^\prime$ and Al-OHC centers [35]. Therefore, the $\textrm{Al-E}^\prime$ centers were not detected in this work because of the experimental limitation (the absorption mode) even though they were present in the irradiated glass samples, as proven by the RIA spectra (Fig. 6(a)).
In Fig. 6(c), the EPR spectra of the 1 KGy γ-ray irradiated Er/Al/Ge co-doped silica glasses with different GeO2 contents shows a composite and intense feature in the range of 325–340 mT, which is characteristic of the Al- and Ge-related paramagnetic point defects. With a comparable Al concentration, the total content of Ge-related defects increased with increasing Ge concentration. However, the concentration of the Al-related defects (AlOHC) decreased considerably (the inset of Fig. 6(c)), and this is consistent with the change in RIA (Fig. 2(b)). Further, it indicated that the GeO2 content affected the total Ge-related defect contents and the Al-related defect contents.
Figure 6(d) shows the EPR spin concentrations of the various paramagnetic defects as a function of the GeO2 concentration. The absolute concentrations of the Al- and Ge-related defects were determined by using the SpinCount software and the double integral of each component in the EPR-simulated spectra. Considering the experimental errors due to the addition of GeO2, the spin concentration of AlOHC decreased gradually, and the spin concentrations of the Ge(1), Ge(2), and $\textrm{Ge-E}^\prime$ centers increased gradually, thus confirming its consistency with the RIA result (Fig. 2(b)).
Table 3 summarizes the characteristic values of the color centers that were identified in the RIA and CW-EPR spectra. The g values were calculated from the experimental spectra with an accuracy of ±0.0001. In this work, the parameters employed for the decomposition of RIA and the simulation of EPR, were quantitatively consistent with the data reported in Refs. [37] and [34], and each cumulative fitted spectrum was consistent with the experimental result.
3.5 Suppression mechanism of the radiation induced darkening by GeO2 co-doping
The results in Sections 3.1 and 3.2 indicated that Ge co-doping exerted a slight effect on the spectral properties of the EAG glass before the irradiation and that it can significantly improve the radiation resistance of the EAG glass. Similar to the results obtained for the EAG series glass, Section 3.3 indicated that Ge co-doping can significantly improve the radiation resistance behavior of #EAG fiber. In section 3.4, the RIA spectral peak fitting and the simulation of RT-CW-EPR were adopted to systematically reveal the natures of the color centers in the EAG glass series and their evolution with the GeO2 content. In this section, the inhibition mechanism of Ge co-doping on the RD effect of the EAG glass is explained from a microstructural viewpoint.
Figure 7 shows the formation model of the primary Al- and Ge- related color centers that were generated by the γ-ray irradiation of the Er/Al/Ge co-doped silica glasses. Regarding the Al-related color centers, previous studies [31,38] have reported that the [AlO4/2]− and [AlO3/2]0 groups are the precursors for generating the AlOHC and $\textrm{Al-E}^\prime$ color centers, respectively. Under irradiation, the [AlO4/2]− group becomes an Al-OHC color center by trapping holes, and the [AlO3/2]0 group becomes an $\textrm{Al-E}^\prime$ color center by trapping electrons. Their formation processes are shown in Figs. 7(a) and (b), respectively, and can be expressed as follows [31,38]:
Regarding the Ge-related color centers, previous studies [39] have reported that the Ge-ODC color center is the precursor of $\textrm{Ge-E}^\prime$ color center. There are two types of Ge-ODC color centers (Ge-ODC(I) and Ge-ODC(II)), and their structural models can be expressed as ${\equiv} \textrm{G}{\textrm{e}^{ {\bullet}{\circ} }}\textrm{Ge} \equiv$ and ${\equiv} \textrm{Ge - Ge} \equiv$, respectively. Under irradiation, the preexisting Ge-ODC(II) and Ge-ODC(I) color centers can be transformed into one other: Ge-ODC(I) becomes the $\textrm{Ge-E}^\prime$ color center by trapping holes, as shown in Fig. 7(c), as expressed by the following equation [39]:
The [GeO4/2]0 group is the precursor of Ge(1) and Ge(2). Under irradiation, the [GeO4/2]0 group becomes Ge(1) and Ge(2) by trapping electrons, as shown in Fig. 7(d), and expressed by the following equation [21]:
As the GeO2 content was increased, the defect contents of GeODC(I) and GeODC(II) also increased, as reported by Essid et al. [39]. As the ability of GeODC(I) to capture holes is stronger than that of the [AlO4/2]− group, an increase in the GeO2 content would ensure the capture of more holes by GeODC(I), thus increasing the irradiation-induced $\textrm{Ge-E}^\prime$ content; meanwhile, as the number of holes trapped by [AlO4/2]− group is reduced, thus reducing the AlOHC content (Fig. 6(d)). This explanation was further supported by the RIA spectrum. Figure 6(a) shows that the GeODC(II) content decreased significantly after the irradiation. As GeODC(I) and GeODC(II) can be transformed into one other, it also indicated that irradiation could reduce the GeODC(I) content.
Similarly, the number of [GeO4/2]− groups increased with increasing GeO2 content. Therefore, additional electrons were captured by the [GeO4/2]− groups to form the Ge(1) and Ge(2) color centers. The decrease in the number of electrons that were captured by the [AlO3/2]0 groups also decreased the number of $\textrm{Al-E}^\prime$ color centers, as shown in Fig. 6(d).
In this work, the radiation-induced AlOHCs exhibited a visible optical absorption band in which the tail affected the fiber attenuation of the pump wavelength of EDFA at ∼980 nm. Ge co-doping effectively suppressed the intensity of the AlOHC band and increased the Ge-related defects. Further, the absorption of the Ge-related color centers in the UV range improved the radiation resistance, implying that the radiation resistance of EDF improved upon Ge co-doping in the core glass. Thus, EDF with Er/Al/Ge co-doping exhibits prospects for future space applications.
4. Conclusions
To summarize, we systematically studied the effect of GeO2 on the spectral properties and radiation resistance behaviors of Er/Al/Ge co-doped silica glass and fiber samples. The nature of the color centers generated by γ-ray irradiation, were identified, and the mechanism of radiation hardening was discussed. Based on the absorption, emission, and fluorescence lifetime results, it was confirmed that Ge co-doping would not affect the spectral properties of Er3+ and that it can notably enhance the radiation resistance of the glass and fiber samples. The online amplifier test of EDFs indicated that the gain behavior degraded very rapidly in EDF without Ge co-doping. The gain degraded by 21% in #EA fiber and 7% in #EAG fiber during the online test under the same radiation condition. The degradation degree of the above-mentioned properties was inversely related to the GeO2 contents of the glasses and fibers.
Moreover, we fitted the RIA spectra and simulated the CW-EPR spectra. Further, the inhibition mechanism of the RD effect of the EAG glasses and fibers from microstructure by Ge co-doping, were proposed. The formation and transformation processes of the Al- and Ge-related color centers in the EAG glass series and their evolution with the GeO2 content were also proposed. It was found that the Ge content is vital to the suppression of the growth of the Al-related defects through the formation of Ge-related color centers. This work offers an essential reference for the optimization and design of the core-glass composition of radiation-hardening Er-doped silica fibers for future space EDFA applications.
Funding
National Natural Science Foundation of China (Grant Nos. 61775224, Grant Nos. 61875216, Grant Nos. 62005297); Key Technologies Research and Development Program (2020YFB1805900).
Acknowledgements
This work was financially supported by National Key R&D Program of China (2020YFB1805900), National Natural Science Foundation of China (Grant Nos. 62005297, 61875216, 61775224)
Disclosures
The authors declare no conflicts of interest.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
References
1. Q. Wang, N. Dutta, and R. Ahrens, “Spectroscopic properties of Er doped silica glasses,” J. Appl. Phys. 95(8), 4025–4028 (2004). [CrossRef]
2. Z. Zhang, C. Guo, L. Cui, Y. Zhang, C. Du, and X. Li, “All-fiber few-mode erbium-doped fiber amplifier supporting six spatial modes,” Chin. Opt. Lett. 17(10), 100604 (2019). [CrossRef]
3. M. Storm, D. Engin, B. Mathason, R. Utano, and S. Gupta, “Space-based erbium-doped fiber amplifier transmitters for coherent, ranging, 3D-imaging, altimetry, topology, and carbon dioxide lidar and earth and planetary optical laser communications,” in EPJ Web of Conferences, vol. 119 (EDP Sciences, 2016), p. 4.
4. Q. He, F. Wang, Z. Lin, C. Shao, M. Wang, S. Wang, C. Yu, and L. Hu, “Temperature dependence of spectral and laser properties of Er3+/Al3+ co-doped aluminosilicate fiber,” Chin. Opt. Lett. 17(10), 101401 (2019). [CrossRef]
5. A. T. Gursel, Fiber Lasers and Their Medical Applications (IntechOpen, 2018), Chap. 5.
6. S. Girard, B. Tortech, E. Regnier, M. Van Uffelen, A. Gusarov, Y. Ouerdane, J. Baggio, P. Paillet, V. Ferlet-Cavrois, and A. Boukenter, “Proton-and gamma-induced effects on erbium-doped optical fibers,” IEEE Trans. Nucl. Sci. 54(6), 2426–2434 (2007). [CrossRef]
7. Q. Du, J. Michon, B. Li, D. Kita, D. Ma, H. Zuo, S. Yu, T. Gu, A. Agarwal, and M. Li, “Real-time, in situ probing of gamma radiation damage with packaged integrated photonic chips,” Photonics Res. 8(2), 186–193 (2020). [CrossRef]
8. R. Quimby, W. Miniscalco, and B. Thompson, “Clustering in erbium-doped silica glass fibers analyzed using 980 nm excited-state absorption,” J. Appl. Phys. 76(8), 4472–4478 (1994). [CrossRef]
9. A. Saitoh, S. Matsuishi, C. Se-Weon, J. Nishii, M. Oto, M. Hirano, and H. Hosono, “Elucidation of codoping effects on the solubility enhancement of Er3+ in SiO2 glass: Striking difference between Al and P codoping,” J. Phys. Chem. B 110(15), 7617–7620 (2006). [CrossRef]
10. J. W. Fleming, “Dispersion in GeO2–SiO2 glasses,” Appl. Opt. 23(24), 4486–4493 (1984). [CrossRef]
11. B. Malo, F. Bilodeau, J. Albert, D. C. Johnson, K. O. Hill, Y. Hibino, and M. Abe, “Photosensitivity in optical fiber and silica-on-substrate waveguides,” in Photosensitivity and self-organization in optical fibers and waveguides, vol. 2044 (International Society for Optics and Photonics, 1993), pp. 42–54.
12. M. Z. Amin, K. K. Qureshi, and M. M. Hossain, “Doping radius effects on an erbium-doped fiber amplifier,” Chin. Opt. Lett. 17(1), 010602 (2019). [CrossRef]
13. T. S. Rose, D. Gunn, and G. C. Valley, “Gamma and proton radiation effects in erbium-doped fiber amplifiers: active and passive measurements,” J. Lightwave Technol. 19(12), 1918–1923 (2001). [CrossRef]
14. T. Deschamps, H. Vezin, C. Gonnet, and N. Ollier, “Evidence of AlOHC responsible for the radiation-induced darkening in Yb doped fiber,” Opt. Express 21(7), 8382–8392 (2013). [CrossRef]
15. M. E. Likhachev, M. M. Bubnov, K. V. Zotov, A. L. Tomashuk, D. S. Lipatov, M. V. Yashkov, and A. N. Guryanov, “Radiation resistance of Er-doped silica fibers: effect of host glass composition,” J. Lightwave Technol. 31(5), 749–755 (2013). [CrossRef]
16. V. Neustruev, “Colour centres in germanosilicate glass and optical fibres,” J. Phys.: Condens. Matter 6(35), 6901–6936 (1994). [CrossRef]
17. J. Nishii, K. Kintaka, H. Hosono, H. Kawazoe, M. Kato, and K.-I. Muta, “Pair generation of Ge electron centers and self-trapped hole centers in GeO2− SiO2 glasses by KrF excimer-laser irradiation,” Phys. Rev. B 60(10), 7166–7169 (1999). [CrossRef]
18. M. Fujimaki, T. Watanabe, T. Katoh, T. Kasahara, N. Miyazaki, Y. Ohki, and H. Nishikawa, “Structures and generation mechanisms of paramagnetic centers and absorption bands responsible for Ge-doped SiO2 optical-fiber gratings,” Phys. Rev. B 57(7), 3920–3926 (1998). [CrossRef]
19. A. Alessi, S. Girard, M. Cannas, S. Agnello, A. Boukenter, and Y. Ouerdane, “Evolution of photo-induced defects in Ge-doped fiber/preform: Influence of the drawing,” Opt. Express 19(12), 11680–11690 (2011). [CrossRef]
20. Y. Kobayashi, E. H. Sekiya, K. Saito, R. Nishimura, K. Ichii, and T. Araki, “Effects of Ge Co-doping on P-related radiation-induced absorption in Er/Yb-doped optical fibers for space applications,” J. Lightwave Technol. 36(13), 2723–2729 (2018). [CrossRef]
21. M. Leon, M. Lancry, N. Ollier, B. Babu, L. Bigot, H. El Hamzaoui, I. Savelii, A. Pastouret, E. Burov, and F. Trompier, “Ge-and Al-related point defects generated by gamma irradiation in nanostructured erbium-doped optical fiber preforms,” J. Mater. Sci. 51(22), 10245–10261 (2016). [CrossRef]
22. F. Lou, L. Hu, C. Yu, M. Wang, L. Zhang, X. Xu, and D. Chen, “Photo-darkening resistence ytterbium-doped silica optical fiber and preparation method,” C. N. patent 109502961A (22 March 2019).
23. G. R. Hanson, K. E. Gates, C. J. Noble, M. Griffin, A. Mitchell, and S. Benson, “XSophe-Sophe-XeprView: a computer simulation software suite (v. 1.1.3) for the analysis of continuous wave EPR spectra,” J. Inorg. Biochem. 98(5), 903–916 (2004). [CrossRef]
24. L. Pajasova, “Optical properties of GeO2 in the ultraviolet region,” Czech. J. Phys. 19(10), 1265–1270 (1969). [CrossRef]
25. Y. Mebrouk, F. Mady, M. Benabdesselam, J.-B. Duchez, and W. Blanc, “Experimental evidence of Er3+ ion reduction in the radiation-induced degradation of erbium-doped silica fibers,” Opt. Lett. 39(21), 6154–6157 (2014). [CrossRef]
26. B. Hari Babu, N. Ollier, M. León Pichel, H. El Hamzaoui, B. Poumellec, L. Bigot, I. Savelii, M. Bouazaoui, A. Ibarra, and M. Lancry, “Radiation hardening in sol-gel derived Er3+-doped silica glasses,” J. Appl. Phys. 118(12), 123107 (2015). [CrossRef]
27. S. Rydberg and M. Engholm, “Experimental evidence for the formation of divalent ytterbium in the photodarkening process of Yb-doped fiber lasers,” Opt. Express 21(6), 6681–6688 (2013). [CrossRef]
28. C. Shao, J. Ren, F. Wang, N. Ollier, F. Xie, X. Zhang, L. Zhang, C. Yu, and L. Hu, “Origin of radiation-induced darkening in Yb3+/Al3+/P5+-Doped silica glasses: effect of the P/Al ratio,” J. Phys. Chem. B 122(10), 2809–2820 (2018). [CrossRef]
29. R. Dardaillon, M. Lancry, M. Myara, C. Palermo, and P. Signoret, “Radiation-induced absorption and photobleaching in erbium Al–Ge-codoped optical fiber,” J. Mater. Sci. 55(29), 14326–14335 (2020). [CrossRef]
30. J. Sils, S. Hausfeld, W. Clauß, U. Pahl, R. Lindner, and M. Reichling, “Impurities in synthetic fluorite for deep ultraviolet optical applications,” J. Appl. Phys. 106(6), 063109 (2009). [CrossRef]
31. J. C. Lagomacini, D. Bravo, A. Martín, F. J. López, P. Martín, and Á. Ibarra, “Growth kinetics of AlOHC defects in γ-irradiated silica glasses,” J. Non-Cryst. Solids 403, 5–8 (2014). [CrossRef]
32. L. Skuja, “Isoelectronic series of twofold coordinated Si, Ge, and Sn atoms in glassy SiO2: a luminescence study,” J. Non-Cryst. Solids 149(1-2), 77–95 (1992). [CrossRef]
33. M. Takahashi, H. Shigemura, Y. Kawamoto, J. Nishii, and T. Yoko, “Photochemical reactions of Ge-related defects in 10GeO2· 90SiO2 glass prepared by sol–gel process,” J. Non-Cryst. Solids 259(1-3), 149–155 (1999). [CrossRef]
34. S. Agnello, A. Alessi, F. Gelardi, R. Boscaino, A. Parlato, S. Grandi, and A. Magistris, “Effect of oxygen deficiency on the radiation sensitivity of sol-gel Ge-doped amorphous SiO2,” Eur. Phys. J. B 61(1), 25–31 (2008). [CrossRef]
35. H. Hideo and K. Hiroshi, “Radiation-induced coloring and paramagnetic centers in synthetic SiO2: Al glasses,” Nucl. Instrum. Methods Phys. Res., Sect. B 91(1-4), 395–399 (1994). [CrossRef]
36. Y. Watanabe, H. Kawazoe, K. Shibuya, and K.-I. Muta, “Structure and mechanism of formation of drawing-or radiation-induced defects in SiO2: GeO2 optical fiber,” Jpn. J. Appl. Phys. 25(Part 1, No. 3), 425–431 (1986). [CrossRef]
37. C. Shao, M. Guo, Y. Zhang, L. Zhou, M. Guzik, G. Boulon, C. Yu, D. Chen, and L. Hu, “193 nm excimer laser-induced color centers in Yb3+/Al3+/P5+-doped silica glasses,” J. Non-Cryst. Solids 544, 120198 (2020). [CrossRef]
38. K. L. Brower, “Electron paramagnetic resonance of AlE′ centers in vitreous silica,” Phys. Rev. B 20(5), 1799–1811 (1979). [CrossRef]
39. M. Essid, J. Albert, J. L. Brebner, and K. Awazu, “Correlation between oxygen-deficient center concentration and KrF excimer laser induced defects in thermally annealed Ge-doped optical fiber preforms,” J. Non-Cryst. Solids 246(1-2), 39–45 (1999). [CrossRef]
