1. Introduction

Since the first demonstration of broadband near-infrared (NIR) luminescence from bismuth-doped silicate glass in 1999 [1], intensive research has been devoted to the Bi-doped optical fibers (BDFs) for numerous practical applications (e.g. superluminescent sources, fiber lasers and broadband amplifiers) in the spectral range 1.15 to 1.5 and 1.6 to 1.8 µm [2,3]. The ultrabroadband properties of Bi-doped fibers benefit from the existence of multiple BACs associated with various codopants [4–6]. However, it turned out that the laser efficiency of BDF-based lasers to date is still lower compared to the rare-earth-doped analogues, especially for Bi-doped aluminosilicate fibers emitting from 1.15 to 1.3 µm (η∼25%) [7,8]. This is due to the limited knowledge of the nature of NIR emitting BACs. Consequently, various post-treatment approaches have been proposed to modify the spectral properties of BACs. In particular, laser-induced effects have been repeatedly reported to shed some light on the nature of BACs [9–11]. Recently, Peng et al. demonstrated a novel Bi-functionalized grating inside the borosilicate glasses with the direct femtosecond laser writing method, paving a new way towards extending the amplification bandwidth of an optical waveguide in the micro-region [12]. On the other hand, growing attention is paid to the effects of ionizing (x-rays, γ-rays) and particle irradiation (electrons, neurons) since the applications of BDFs have extended to the astrophysical field such as space amplifiers and fiber gyroscopes [13,14]. For fibers operating in such environments, high radiation resistance should be maintained. In fact, a few papers have addressed the influence of γ-irradiation on the optical properties of BDFs with various glass hosts [15–17]. However, it turns out that BDFs with the electron beam (E-beam) irradiation, as an alternative particle radiation source with low penetration, has not been systematically studied. E.M. Dianov et al. firstly reported the effect of E-beam irradiation on the optical attenuation of Bi/Al and Bi/Ge silicate fibers, revealing the high susceptibility of BDFs to the E-beam irradiation [18]. However, the radiation resistance of BACs (as an independent unit) is not evaluated. The underlying mechanism for radiation-induced absorption has not been discussed, especially the contribution of each of the radiation-induced defects. Moreover, the effect of E-beam irradiation on the gain properties of BACs has not been presented, and the thermal stability of E-beam irradiated BACs has not been investigated.

Therefore, in this paper, we systematically investigated the effect of E-beam (5-140 kGy) on the spectral behaviors of BACs in Bi/Er codoped aluminosilicate fibers (BEDFs) through a series of experiments. Based on the experimental results, the radiation resistance of multiple BACs is evaluated. The underlying mechanism of the radiation-induced absorption (RIA) is elucidated. Furthermore, the thermal stabilities of the pristine and irradiated BEDFs are studied. These results bring new insights into the nature of BACs and the operating conditions of BEDFs as the practical gain medium.

2. Experimental section

The BEDF sample was fabricated by conventional modified chemical vapor deposition (MCVD) method. The Bi, Er and Al were incorporated into the silica preform by in situ solution doping. Thereafter, the silicate preform was drawn into a single mode fiber with a core diameter of 3.5 µm with a cut-off wavelength of 954 nm. The refractive index profile of the drawn BEDF was measured using the 3D fiber index analyzer, as shown in Fig. 1. The equivalent refractive index difference was about 0.02 with a sharp dip in the center, which is mainly due to the depletion of bismuth (T_Bi-boiling ∼1560 °C) during the collapse. The core concentrations of main dopants of the BEDF were estimated as: [Bi] ∼0.1, [Al] ∼0.1, [Ge] ∼4.24, [Er] ∼0.006 at% by the energy dispersive X-ray (EDX) analysis (inset of Fig. 1). Subsequently, four consecutive sections (L ∼0.7 m for each) from the same BEDF were prepared for E-beam irradiation. E-beam irradiation was performed at INFLPR “travelling-wave” linear accelerator, operating at about 5.5 MeV, for a beam current of 3.5 to 5 µA. During the radiation exposure, the optical fiber samples were located at 60 cm from the accelerator exit window, while the dose rate was 26.7 Gy/s, as measured with a graphite calorimeter. The fibers under test (FUTs) were prepaid by splicing the two ends of the electron irradiated BEDF (10 cm; radiation dose: 5, 25, 75 and 140 kGy) with two single-mode fibers 1550 (SMF1550). In this way, the FUTs were kept consistent for all the experiments. The two splice losses (T_s1 and T_s2) were monitored by the fusion splicer (< 1 dB) and compensated for all the experiments. The luminescence was measured by the backward pumping scheme, where the luminescence signal was 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.

 

Fig. 1. Refractive index profile of the pristine BEDF. The inset shows the concentration distribution of dopants across the fiber core region.

Download Full Size | PDF

3. Experimental results and discussion

Firstly, the insertion absorption spectra of the FUTs in the range 500-1600 nm before and after electron irradiation were measured by the single photon analyzer (shamrock SR-500), and is shown in Fig. 2.

 

Fig. 2. Insertion absorption spectra of the FUTs before and after electron irradiation in the range 500-1600 nm. Inset: the enlarged part of the absorption spectra from 500 to 750 nm for all the FUTs.

Download Full Size | PDF

As seen in Fig. 2, multiple Bi-related absorption bands are observed and can be classified into two categories: (i) BAC-Al for 500 (A),700 (B) and 1000 nm (D) [19], (ii) BAC-Si for 816 (C) and 1400 nm (E) [20]. It is seen that the shape of the BAC absorption bands is mildly affected by irradiation. However, one can notice a remarkable increase in the optical absorption level after irradiation. Interestingly, there appears an additional absorption band peaking at 560 nm after strong irradiation (∼140 kGy) (inset of Fig. 2), and it becomes more pronounced with increasing irradiation dose. This additional absorption band has been confirmed to be associated with Al-related oxygen hole center (Al-OHC) point defects [16,21]. Apparently, the incorporation of Al into the BEDFs greatly enhances the radiation sensitivity. To clearly interpret the rise in the optical absorption, the RIA spectra are derived from the optical absorption spectra between the pristine and irradiated fibers (5-140 kGy), as shown in Fig. 3. It is seen that the RIA level is wavelength dependent and reaches the maximum at 560 nm due to the presence of Al-OHC defects. Also, the RIA level becomes higher with the increase of irradiation dose.

 

Fig. 3. RIA spectra in the range 700-1600 nm of BEDFs irradiated to doses of 5-140 kGy. Inset: the RIA spectra of the BEDF and EDF irradiated to 140 kGy and the RIA difference between them.

Download Full Size | PDF

To determine the change of background loss from the RIA spectra, the unsaturable losses at wavelengths of 830 (BAC-Si) and 980 nm (BAC-Al) were measured in BEDFs before and after E-beam irradiation. It should be noted that there is a negligible effect of Er³⁺ at 980 nm which we cannot rule out. However, the effect of Er³⁺ can be reasonably neglected due to the extremely low concentration of our fiber and the fact that the Er-related defects contributing to RIA are very small (the Er-associated RCC at λ = 1300 nm is 0.18 dB/m with a doping level of 0.04 mol%) [22]. Typical results are shown in Figs. 4(a) and 4(b). As seen in Fig. 4(a), the plateau for each curve represents the unsaturable loss level (α_uns) at 980 nm. With increasing radiation dose to 140 kGy, αuns increases from 36.6 to 48.1 dB/m. The increase of α_uns (∼11.4 dB/m) is nearly the same as the increased small signal absorption (α) at 980 nm, suggesting that the observed growth of RIA at this wavelength should arise from the induced background loss instead of the change of BAC-Al. Similarly, the increase of α_uns at 830 nm is also identical to the growth of small signal absorption at different radiation doses, whereas the active absorption (referred as saturable loss α_s, α_s+α_uns=α) remains unchanged (α_s ∼12 dB/m) (Fig. 4(b)), indicating the high radiation resistance of BAC-Si. However, the useful pump absorption, namely the pump efficiency (Figure of Merit, defined as the ratio of α_s to α) has been reduced notably from 21.6 to 12.8%, indicating the detrimental effect of electron irradiation on the pump efficiency. Based on the results of the unsaturable loss, it is clear that the observed RIA accounts for the background loss which originates from the multiple defects (SiE’, GeE’, Bi/Er-related color center (RCC), Al-OHC).

 

Fig. 4. (a) Pump absorption of the BEDFs before and after irradiation at λex = 980 nm. (b) Unsaturable loss, saturable loss and figure of merit as a function of radiation dose at 830 nm.

Download Full Size | PDF

In order to clarify and determine the contribution of each of the defects for the observed RIA, an erbium-doped fiber (EDF) having the same Er³⁺ and Al concentration as the BEDF was also irradiated with 140 kGy as a reference (inset of Fig. 3). Firstly, it is well known that the contribution of Er-related RCC is negligible [22] and usually overshadowed by the presence of RCC with the host glass (e.g. P-OHC, Al-OHC), especially considering the fact that the Er³⁺ concentration in our BEDFs is extremely low (∼0.006 at%). Therefore, the significant RIA (inset of Fig. 3) of the EDF should primarily arise from the presence of Al-OHC. By subtracting the RIA spectrum of EDF from that of BEDF under 140 kGy, the Bi-related RCC can be obtained. It is seen that the RIA level induced by Bi-related RCC is much lower (4 times lower at 540 nm) compared to Al-OHC, suggesting the dominant contribution of Al-OHC to the RIA in our irradiated BEDFs. However, it is noticed that the RIA level from 1200 to 1600 nm is slightly increased and mainly caused by the Bi-related RCC in this region. Similar behavior is also observed in Bi-doped germanosilicate fiber [23].

The results of optical absorption and unsaturable loss are also in good agreement with the luminescence data. Under excitation at 980 nm (Fig. 5(a)), the peak luminescence intensities of BAC-Al (∼1190 nm) and Er³⁺ (∼1536 nm) decrease monotonically with increasing radiation dose. Similar behavior is observed for BAC-Si at 1430 nm under excitation at 830 nm (Fig. 5(b)). Obviously, the degraded luminescence of BACs results from the induced background loss instead of the reduction of BACs, especially the radiation-induced darkening effect of Al-OHC point defects [16]. Further to the luminescence characterization, the effect of E-beam irradiation on the gain properties of BEDFs was also investigated.

 

Fig. 5. Luminescence spectra of the BEDFs irradiated to doses of 5-140 kGy upon excitation at (a) 980 nm and (b) 830 nm.

Download Full Size | PDF

The typical on-off gain (g_FUT, defined as the pump-induced transmission change over the FUT without considering the ground state absorption, see [24,25] for more details) spectra of BEDFs before and after irradiation upon excitation of 980 and 830 nm are illustrated in Figs. 6(a)–(d). Under 980 nm pumping (Fig. 6(a)), only one predominant on-off gain ascribed to Er³⁺ peaking at 1536 nm is observed. In contrast, the broadband ESA from 1000 to 1400 nm is observed and attributed to BAC-Al. Still, it is seen that the shapes of on-off gain spectra are insignificantly affected by the E-beam irradiation, further confirming the high radiation resistance of BACs.

 

Fig. 6. On-off gain spectra of the FUTs before and after electron radiation (5-140 kGy) under 980 and 830 nm pumping, respectively (a, c) and (b, d) The on-off gain and net gain values at 1100 and 1536 nm versus radiation dose under 980 and 830 nm pumping, respectively.

Download Full Size | PDF

However, as seen in Fig. 6(b), the net gain (G_FUT, true amplification including the ground state absorption) of BAC-Al and Er³⁺ decrease drastically with increasing radiation dose. Similar phenomena are also shown under 830 nm pumping (Figs. 6(c) and (d)). Although the amount of BACs involved in the stimulated emission for amplification doesn’t change, however, the electron irradiation does have detrimental effects on the amplification and lasing properties of BEDFs because the induced excessive background loss leads to a high lasing threshold and lower slope efficiency.

At last, the thermal annealing (∼500 °C) effect on the luminescence and absorption properties of irradiated BEDFs (∼140 kGy) was also investigated, as shown in Figs. 7(a) and 7(b). As seen in Fig. 7(a), during the heating process, the luminescence of BAC-Al in both pristine and irradiated fibers reduce significantly with the increasing temperature, and almost vanishes at 500 °C. However, it is observed that the luminescence of the pristine fiber degrades more quickly than the irradiated fiber. During the cooling process, the luminescence at 1190 nm for both fibers shows a recovery trend with the decrease in temperature. Still, a larger percentage of luminescence is recovered (∼71.4%) in the irradiated fiber than the pristine one (∼42.4%). This coincides with the trend of Δα_T, where the Δα_T level is two-times higher in the pristine fiber upon recovering to room temperature (inset of Fig. 7(b)).

 

Fig. 7. (a) Evolution of luminescence at 1190 nm of pristine and irradiated (140 kGy) BEDFs during heating and cooling. (b) Heat induced loss Δα_T at 1300 nm of pristine and irradiated fibers versus temperature. Inset: heat induced loss spectra of pristine and irradiated fibers after heating and subsequent cooling to room temperature.

Download Full Size | PDF

More specifically, there is noticeable thermal bleaching in the electron irradiated BEDF at a relatively low temperature (∼300 °C), whereas the thermal darkening is aggravated with increasing temperature in the pristine fiber (Fig. 7(b)). The thermal bleaching phenomenon strongly confirms the annihilation of Al-OHC point defects, which has been reported in the irradiated Yb-doped aluminosilicate fibers [26,27]. On the other hand, it is known that the high annealing temperature (600 °C) leads to the precipitation of metallic bismuth nanoparticles [28], introducing strong thermal darkening effect which contributes to the background loss in Bi-doped fibers. Thus, it is reasonable to deduce that the decrease in luminescence results from the thermal-induced background loss and a reduced number of active bismuth ions [26]. Nevertheless, due to the alleviation of thermal bleaching of Al-OHC defect points in the irradiated fiber, it features a slower decrease rate and a higher recovery ratio compared to the pristine one. This strongly implies that the thermal stability of our homemade BEDF is enhanced after high E-beam irradiation.

4. Conclusion

In conclusion, the effect of electron irradiation (5-140 kGy) on the spectral characteristics of BEDFs (Bi ∼0.1 at%) has been systematically investigated. Firstly, it was revealed that the NIR emitting BACs exhibit high radiation resistance, evidenced by the negligible change of active absorption and on-off gain of BACs after electron irradiation. Secondly, the radiation-induced absorption is found to be wavelength dependent and mainly caused by the generation of Al-OHC point defects. Finally, the thermal effect on the luminescence and absorption properties of BAC-Al of the irradiated (140 kGy) fiber has been investigated, revealing the higher thermal stability of the irradiated BEDF. Our findings bring new insights for the understanding of nature of NIR emitting BACs and are of interest for: (i) the fundamental study of structural modification and glass-network rearrangement initiated by different irradiation sources and (ii) the potential applications for Bi-doped aluminosilicate fibers in harsh environments.