1. Introduction

Since the first demonstration of NIR broadband luminescence in bismuth-doped aluminosilicate glass, intensive research has been devoted to Bi-doped optical devices for the broadband applications in the range of 1100-1800 nm. Up to now, a significant number of Bi-doped optical fibers (BDFs), associated with various bismuth active centers (BACs) such as aluminum (BAC-Al), germanium (BAC-Ge), and silica (BAC-Si), have been developed and employed in many applications such as tunable fiber lasers, fiber amplifiers, and medicine [1–6]. In recent years, the potential applications of Bi-doped fibers in space missions and astrophysics, including intra-satellite communications, fiber optic gyroscopes and fiber amplifiers, have attracted enormous attention. For optical fibers in such harsh irradiated (γ-rays, x-rays, electrons, etc.) environments, high radiation resistance is required. Therefore, it is of great interest to investigate the radiation effects on the spectral properties of BDFs for the further understanding of nature of BACs and to evaluate their potential as the practical applications in space and astrophysical applications. In fact, quite a few studies have addressed the effect of ionizing irradiation on the optical properties of BDFs. The first results of the study of the effect of electron irradiation on the optical properties of BDFs has been presented in [7], it was shown that the concentration of BAC-Ge was substantially decreased, while the resonant absorption of BAC-Al was mildly increased, suggesting the high susceptibility of BACs to electron irradiation and the considerable influence of the glass matrix. Later on, the effect γ-irradiation on the optical properties was intensively investigated. In one study, it was shown that the absorption and emission bands of BAC-Ge were insignificantly affected by the γ-irradiation (~8 kGy) in Bi-doped germanosilicate (GeO₂-SiO₂) fibers, and the bismuth contribution was supposed to be the main contributor for the radiation-induced absorption (RIA) [8]. In another study associated with Bi-doped aluminosilicate (Al₂O₃-SiO₂) fibers, it was revealed that the luminescence of BAC-Al was enhanced upon 980 nm pumping (~0.6 mW) with a dose of 3 kGy [9]. The authors suggested that the formation of subvalent Bi ions (Bi³⁺, Bi⁺) after γ-irradiation was the main cause for these changes. Despite these inspiring efforts, there still remain some problems and ambiguities which need further clarification. Firstly, the effect of γ-radiation is only characterized through the absorption and luminescence measurements of BDFs. It is evidently insufficient because its impact on the gain characteristics has not yet been demonstrated. Moreover, the radiation stability of BACs has not been discussed in terms of strong irradiation doses (tens of kGy). Furthermore, the thermal stability of BACs after γ-irradiation has not been studied in Bi-doped optical fibers.

Therefore, in this paper, with the purpose of understanding the radiation resistance of BACs in Bi-doped fibers, we have performed a series of experiments to investigate the effect the strong γ-irradiation (2-72 kGy) on the spectral properties of Bi/Er codoped aluminosilicate (Al₂O₃-SiO₂) fibers (BEDFs). Furthermore, the possible underlying mechanism of radiation-induced loss is discussed based on the obtained experimental results of the irradiated BEDFs.

2. Experimental schemes

The BEDF was fabricated by conventional modified chemical vapor deposition (MCVD) method. Si, Ge, and a low level of phosphorus (P) were deposited in the soot process, while the incorporation of Al, Bi and Er were accomplished by means of in situ doping. To demonstrate the possible effect of Er³⁺, an erbium doped fiber (EDF) was fabricated with the same concentration of Er³⁺ as the BEDF. The drawn BEDF has an outer cladding diameter of 125 μm, core diameter of 3 μm, and a second mode cut-off wavelength of 920 μm. The concentrations of the core compositions were estimated as [Si] ~28.19, [P] ~0.62, [Ge] ~4.24, [Er] ~0.006, [Bi] ~0.1 and [Al] ~0.1 at% by the energy dispersive X-ray (EDX) analysis. It is worth noting that, in order to stress the influence of concentration, the content of bismuth in our homemade BEDF is several times higher than the optimal number (0.02 at.%) [10]. The γ irradiation experiment was then performed in the ⁶⁰Co-source chamber. Four consecutive sections (1 meter for each) of one BEDF were coiled at 2 cm diameter and were irradiated with the total radiation doses of 2, 12, 36, and 72 kGy, at a constant radiation dose rate of 4.4 kGy/h. For comparison, the EDF with the same length of 1 m was also irradiated up to 72 kGy.

The small signal absorption and luminescence spectra were measured in both the pristine and irradiated BEDFs, as illustrated in Fig. 1(a). Specifically, the fibers under test (FUTs) were prepared with a “three section” structure, namely, the two ends of a short piece of BEDF (~10 cm) were spliced with passive SMF 1550 fiber. The splice losses of T_s,1 and T_s,2 were determined by the fusion splicer (˂1 dB) and compensated for all the measurements. Firstly, The small signal absorption was measured by the insertion method in the range 650-1600 nm using a halogen lamp as the probe light source, the optical signal was transmitted by the monochrometer, and then detected and transferred to electrical signal by InGaAs (Δλ ~900-1600 nm) or Si (Δλ ~500-1000 nm) detector, the chopper (f = 133Hz) was used to modulate the signal so that only the signal in phase could be extracted and amplified by the lock-in amplifier. Finally, the transmitted signal was collected by the PC for data processing (configuration 1 in Fig. 1(a)). Meanwhile, the unsaturable loss levels of the pristine and irradiated fibers were determined by the pump absorption at various pumping wavelengths, including 830, 980 nm and the spectral range from 1260 to 1600 nm. The luminescence was measured by the backward pumping scheme shown in configuration 2 (Fig. 1(a)), where the luminescence spectra were collected by an optical spectral analyzer (OSA, Agilent 86140B) in the range 900-1600 nm with the commercially available laser diodes operating at 830 and 980 nm. In regards to the thermal treatment, the absorption and luminescence properties of the fibers under test (FUTs) were measured by placing them into the isothermal zone of the furnace (configuration 3) while keeping the same setups for configuration 1 and 2. Besides, The on-off and net gain spectra were also measured to evaluate the radiation effect on the gain properties of BEDFs using a scheme shown in Fig. 1(b) (similar to [11]). The on-off gain (g(λ)) is defined as the ratio of the transmitted signal power with pump on (P₁) and off (P₂), defined as $g\left(\lambda \right)=\frac{10}{L}\times \log (P_1/P_2)$, where L is the length of BEDF. In comparison, the net gain (G(λ)) of BEDF includes the small signal absorption (α(λ)) and defined as $G\left(\lambda \right)=\frac{10}{L}\times \log (P_1/P_0)$. Clearly, the true amplification of the BEDF can only be achieved when the G(λ) value is positive in the desired wavelength range.

 

Fig. 1 (a) Experimental setup for spectral characterization of the pristine and irradiated BEDFs. Configurations 1and 2 were applied for the small signal absorption and backward luminescence measurements correspondingly. The thermal treatment (configuration 3) was directly applied based on the absorption or luminescence setup. (b) Schematic diagram for on-off and net gain measurement using the co-propagation scheme.

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3. Experimental results and discussion

The small signal absorption spectra α(λ) in the spectral region 650-1600 nm of FUTs are shown in Fig. 2(a), several Bi- and Er-related absorption bands are highlighted in different color bars: BAC-Si for absorption bands at 816 and 1400 nm (overlap with OH^- overtones) [12–14]; and BAC-Al for absorption bands at 700 and 1000 nm [15–17], which is partly overlapped with the ⁴I_15/2→⁴I_11/2 transition of Er³⁺ at 980 nm. It is seen from Fig. 2(a) that the shape of the absorption spectra of BEDFs changes insignificantly, whereas the magnitude of base loss of the irradiated fibers increases with the increase of irradiation dose. The same results were obtained for the irradiated EDF with a dose of 72 kGy, as shown in the inset of Fig. 2(a).

 

Fig. 2 (a) Small signal absorption spectra of pristine and irradiated fibers, the absorption spectra of EDF before and after irradiation with 72 kGy are also shown for comparison. (b) RIA spectra of BEDFs irradiated to doses of 2-72 kGy. The inset compares the RIA spectra between BEDF and EDF with the same irradiation dose of 72 kGy.

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The measurements of absorption spectra of the pristine and irradiated fibers allowed us to determine the RIA spectra as a function of irradiation dose. Typical RIA spectra in the range 700-1600 nm are illustrated in Fig. 2(b). One can see that the RIA level becomes higher with increasing irradiation dose in the whole spectral region of measurements. Besides, it is observed that the RIA firstly slowly increases with the decrease of wavelength in the range 1100-1600 nm. However, a sharp rise takes place at 1000 nm and the growth trend of RIA become steeper towards the shorter wavelength range. Such RIA shape of the irradiated BEDFs is similar to that of irradiated EDFs [18–20], which has been confirmed to be related to the band tail of the radiation-induced aluminum oxygen hole center (Al-OHC) point defects peaking around 540 nm [21,22]. In order to determine the change of background loss from the RIA spectra, the unsaturable loss (α_uns) levels of the pristine and irradiated samples were measured at the resonant wavelengths of BACs (830 and 980 nm), and in the spectral range 1260-1600 nm where absorption of most BACs are absent (except for BAC-Si at 1400 nm), as shown in Figs. 3(a) and 3(b). It is seen that the increase of irradiation dose results in the increase of α_uns at 830 nm (Δα_uns ~44.5 dB/m with 72 kGy irradiation). More specifically, it turns out that, after γ-irradiation, the unsaturable loss levels of all irradiated BEDFs increase by the same value as the small signal absorption at 830 nm (Fig. 2(b)), similar behavior is observed at 980 nm (the inset of Fig. 3(a)). This strongly suggests that the absorption bands of BACs are not affected, whereas the RIA originates from the increased background loss at these two wavelengths We further investigate the change of unsaturable loss levels of irradiated BEDFs in the spectral range 1260-1600 nm where BACs are absent. As expected, the increase of the unsaturable loss levels of all the fibers corresponds to the RIA levels (Fig. 2(b)) for all the measured wavelengths. However, it is worth noting that there is an unexpected increase of unsaturable loss levels at wavelengths of 1380 and 1400 nm. It may be possibly explained by the excessive absorption overtones of OH^- group [7,12], which contributes to the additional unsaturable loss in this region.

 

Fig. 3 (a) Pump absorption as a function of input power at 830 nm between pristine and irradiated samples. The inset shows the unsaturable loss levels at 980 nm of BEDFs before and after irradiation to different doses (2-72 kGy). (b) Unsaturable loss level as a function of pump wavelength for all the BEDFs in the range 1260-1600 nm.

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On the basis of the absorption data, it is suggested that the observed growth of small signal absorption (RIA) in irradiated BEDFs is caused by the induced background loss, and the absorption bands assigned to multiple BACs are reasonably insensitive to the high γ-irradiation dose (~72 kGy). Still, it is of great interest to investigate the underlying mechanism of the induced background loss. It is known that the multiple defects (SiE’, GeE’, Al-OHC point defects, etc.) and Bi-/Er-related radiation-induced color centers (RCC) are generated upon γ-irradiation [8,9,20]. Typically, the contribution Er-related RCC, in particular with a low concentration, is negligible (0.44 dB/m at 1300 nm with a high concentration of 0.23 mol.%) and overshadowed by the RCC associated with the host glass [19,20,22]. In our BEDF, the Er’s concentration (0.006 at.%) is nearly 20 times lower than that of Al and Bi. Thus, the effect of Er-related RCC can be neglected. In addition, as seen from the inset of Fig. 2(b), the contribution of Bi-related RCC can be obtained by subtracting the RIA spectrum of EDF from that of BEDF. It is observed that the RIA level induced by Bi-related RCC is 2-5 dB/m in the spectral range of measurements, which is nearly 10 times lower than that induced by Al-OHC point defects at 900 nm. Thus, based on the analysis of the absorption data, we suggest that the background loss is mainly caused by the Al-OHC point defects instead of Bi- or Er-related RCC.

The results of the changes of absorption coincide with the luminescence data, typical luminescence spectra of pristine and irradiated BEDFs are shown in Figs. 4(a)–4(d). Upon 980 nm excitation, two dominant emission bands centered at 1190 and 1536 nm arise, which are attributed to BAC-Al and Er³⁺, respectively. Meanwhile, it is observed that the peak intensities of both BAC-Al and Er³⁺ decrease monotonically with the increase of radiation dose. Specifically, the peak emission intensity of BAC-Al and Er³⁺ have been reduced by 30 and 8% upon 72 kGy irradiation, suggesting that the Er³⁺ exhibits a higher radiation-resistance than BAC-Al. Furthermore, the dependences of peak luminescence intensity of BAC-Al on the excitation intensity for pristine and irradiated BEDFs are illustrated in the inset of Fig. 4(b). It is seen that the obtained slope decreases moderately from 0.09 to 0.06 nW/mW after γ-irradiation with a dose of 72 kGy. Similar behavior is observed upon 830 nm pumping (P ~50 mW), as illustrated in Figs. 4(c) and 4(d). However, one can notice that the peak emission intensity of BAC-Si (λ ~1410 nm) saturates to a certain level (~1.6 nW) towards the maximal dose of 72 kGy, this may be possibly explained by a lower concentration of BAC-Si in our homemade BEDF, which is further evidenced by the early saturation (P_sat ~8 mW) of luminescence of BAC-Si under 830 nm pumping (Fig. 4(d) inset). It should be noted that the luminescence results of BAC-Al obtained in this paper is inconsistent with the data presented in [7], where the luminescence intensity of BAC-Al was mildly enhanced upon electron-irradiation. The discrepancy may be explained by the inherent difference in the concentration of bismuth. It is suggested that a low concentration of bismuth (<0.02%) is favorable for the formation of BACs [10]. Hence, the excessive bismuth (in this work) in the host glass after high γ-irradiation may lead to unexpected high background loss due to the existence of Bi-related RRC and other forms of bismuth, which significantly lowers the luminescence of BAC-Al in the heavily doped BEDFs. On the basis of the luminescence performance upon excitation at 980 and 830 nm, we conclude that the excessive background loss, which is induced by the strong γ-irradiation, does adversely lead to the low luminescence efficiency of BACs and Er³⁺ in BEDFs.

 

Fig. 4 NIR luminescence spectra of BEDFs before and after γ irradiation. (a) Upon 980 nm pumping (P~80 mW), (b) Peak intensities of BAC-Al and Er³⁺ versus irradiation dose, the inset shows the peak intensity of BAC-Al versus input power. (c) Upon 830 nm pumping (P ~50 mW). (d) Peak intensities of BAC-Al and BAC-Si as a function of radiation dose, the inset shows the peak intensities of BAC-Al and BAC-Si versus input power.

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Further to the absorption and luminescence characterization of irradiated fibers, the gain performance of irradiated BEDFs was investigated under 830 and 980 nm pumping, as shown in Figs. 5(a) and 5(b). Upon 830 nm pumping (P ~50 mW), broadband excited state absorption (ESA) is seen from 900 to 1300 nm, which is caused by the presence of BAC-Al (~3.6 dB/m at 1030 nm), whereas the on-off gain g_FUT peaking at 1400 (BAC-Si, g ~1.7 dB/m) and 1536 nm (Er³⁺, g ~4.7 dB/m) are observed over the range 1400-1600 nm in all the BEDFs. Obviously, the change of the shape of on-off gain spectra of irradiated fibers is not observed, and the gain/ESA coefficients of BACs remain unchanged in response to the γ irradiation. It strongly reveals that the population of BACs at metastable levels remains stable after exposure to γ-irradiation with different doses, which further supports the viewpoint that the spectral properties of BACs are insensitive to high radiation. However, as expected, seen from the inset of Fig. 5(a), the net gain G_FUT ascribed to BAC-Al at 1030 nm degrades from −35 to −60 dB/m with increasing radiation dose up to 72 kGy, which is apparently due to the considerable increase of background loss over the spectral range of measurements (see Fig. 2(b)). The analogous trend is also observed at 980 nm excitation, where the on-off gain (Er³⁺ at 1536 nm) and ESA (BAC-Al at 1120 nm) of BEDF remain unchanged while the net gain of BAC-Al decreases significantly (from −30 to −50 dB/m) when exposed to γ-irradiation (Fig. 5(b)). By analyzing the results of gain measurements for irradiated BEDFs, one can see that the reduction of luminescence of BACs is not associated with the decrease of concentration of BACs, but due to the significant rise of background loss induced by Al-OHC point defects. Since it has been reported that it is aluminum that is responsible for the high radiation sensitivity [19,20], it is suggested the content of Al should be adjusted in a proper regime. More importantly, when it combines with bismuth to form BAC-Al in Bi-doped aluminosilicate fibers, the concentration of bismuth should be strictly controlled because high contents of BAC-Al introduce significant ESA or up-conversion, which is due to the existence of higher-lying levels of BAC-Al [16,23]. The ESA of BAC-Al adversely impacts the gain and laser efficiencies in BDFs. As shown in Figs. 5(a) and 5(b), no net gain of BACs is shown in the range 1000-1600 nm due to the extremely high concentration of bismuth (0.1 at%) in the pristine BEDF.

 

Fig. 5 On-off gain spectra of BEDFs before and after γ irradiation (up to 72 kGy) under (a) 830 nm pumping at P ~50 mW and (b) 980 nm pumping at P ~80 mW. Insets illustrate the on-off and net gain coefficient of BAC-Al (at peak wavelength) as a function of irradiation dose under 830 and 980 nm pumping, respectively.

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At last, the thermal effect on attenuation and luminescence properties of irradiated BEDFs is also studied. For simplicity, we only carried out thermal annealing experiments on the pristine and irradiated (~72 kGy) BEDFs. The upper temperature limit was controlled at 500 °C to prevent possible irreversible effects in the host glass [24,25]. Figures 6(a) and 6(b) illustrate the heated induced loss (Δα) at 1300 nm and peak emission intensity of BAC-Al (~1180 nm) as a function of heating temperature for pristine and irradiated fibers, respectively. As seen in Fig. 6(a), Δα₁₃₀₀ changes insignificantly when the temperature is increased to 400 °C in the pristine fiber. However, a sharp increase of Δα₁₃₀₀ (48 dB/m) is observed when the temperature rises up to 500 °C. It reveals the strong thermal darkening effect at a high temperature, which is in good agreement with the results reported in [10,26]. In the cooling process, Δα₁₃₀₀ decreases linearly with the decrease of temperature. Finally, 5 dB/m of Δα₁₃₀₀ is still present at room temperature. This may be attributed to the small amount of metallic bismuth nanoparticles formation and rearrangement of glass structural defects [10,24,27,28]. In addition, the change of peak emission intensity of BAC-Al in the pristine fiber is in good agreement with the trend of Δα₁₃₀₀ (Fig. 6(a) inset). In contrast, for γ-irradiated BEDF, remarkable thermal induced bleaching is observed upon heating to 300 °C, which may be associated with the degradation of Al-OHC point defects. Similar thermal bleaching behavior has been found previously in γ-irradiated Yb- and Er-doped aluminosilicate fibers [21,29]. However, starting from ~300 °C, Δα₁₃₀₀ shows the opposite trend and thermal induced darkening gradually dominates in the annealing process (~500 °C). Consequently, a small amount of heat-induced loss (Δα₁₃₀₀ ~1.6 dB/m) is retained after cooling to room temperature in irradiated BEDF. It can be seen that, the heat induced losses (upon heating to 500 °C) almost recover when the fibers (pristine or irradiated) cools down to room temperature, which is in good agreement with the results presented in [30,31], in which the temperature range 500-550 °C is recognized as the “threshold” for irreversible change of the spectral properties of BACs. Moreover, the reversibility of the luminescence of BAC-Al at around 1100 nm has been further developed as a temperature sensor based on the fluorescence intensity ratio (FIR) technology [31]. In terms of luminescence performance between pristine and irradiated BEDF, one can see that a larger portion of emission of BAC-Al (~70%, see inset of Fig. 6(b)) is recovered in γ-irradiated BEDF than that (less than 50%) in pristine BEDF upon heating to 500 °C. This may be explained by the annihilation of Al-OHC point defects at a low temperature, which alleviates the radiation-induced darkening effect to some extent. In other words, the γ-irradiated BEDF shows a higher thermal stability than the pristine one.

 

Fig. 6 (a) Evolution of heat-induced loss at 1300 nm of pristine BEDF during heating (~500 °C) and cooling. The inset shows the evolution of luminescence intensity at 1180 nm upon excitation at 980 nm. (b) Evolution of heat-induced loss at 1300 nm of the irradiated BEDF (~72 kGy) heated to different temperatures. The inset illustrates the evolution of luminescence intensity at 1180 nm corresponding to different heating temperatures.

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4. Conclusion

In conclusion, for the first time, the study of the effect of γ-irradiation (2- 72 kGy) induced effect on the spectral characteristics of Bi/Er codoped aluminosilicate fibers (high doping of bismuth ~0.1 at%) has been presented. It has been revealed that the BACs exhibit a high level of radiation resistance, evidenced by the negligible change of absorption, on-off gain bands or ESA of BACs over the spectral range 650-1600 nm. Meanwhile, it is confirmed that the RIA is mainly caused by an induced background loss, evidenced by the simultaneous growth of unsaturable loss levels in the irradiated BEDFs. Moreover, the RIA spectra of the BEDF and EDF have been obtained for the same irradiation dose (~72 kGy). It is shown that the Al-OHC point defects have a significant contribution to the RIA level. The luminescence data of BACs under 830 and 980 nm pumping are also obtained. It is found that the noticeable decrease of luminescence of BACs is caused by the strong radiation-induced background loss, which is associated with the high concentration of bismuth and irradiation induced defects in the host glass. Furthermore, the experimental results of the effect of temperature on the absorption and luminescence properties of irradiated BEDFs have also been obtained. It is found that the radiation-induced darkening of Al-OHC point defects can be alleviated by thermal bleaching at a low temperature (~300 °C), whereas the thermal darkening begins to dominate at a higher temperature (~500 °C) due to the tendency of bismuth ions to form metallic bismuth or clusters. The results of thermal treatment also suggest that the BEDF features higher thermal stability upon the γ-irradiation. We believe that the findings in this work bring new insights into the nature of BACs from the fundamental perspective and will impact the future applications of BEDFs in harsh environments where high irradiation and temperature are present.