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Abstract
Three Er-doped fibers (EDFs) with different concentrations of Bi ions doping were fabricated by atomic layer deposition combined with modified chemical vapor deposition. The radiation-induced absorption (RIA) could be dramatically weakened by co-doping Bi. Especially, the RIA of Bi/Er co-doped fiber (BEDF) at 1300 nm was 56.0% lower than that of EDF after a 1500 Gy irradiation treatment. With the increase of the irradiation dose, the fluorescence intensity and lifetime of EDF decreased continuously, while BEDF showed a trend, increasing first and then decreasing, and changed little before and after irradiation. The gain characteristics and laser threshold power of BEDF are less varied than those of EDF before and after irradiation. In addition, an irradiation simulation model of EDF and BEDF fiber was established through GEANT4 simulation toolkit and found that Bi ions are more likely to absorb gamma rays, thereby reducing the impact of irradiation on Er ions in BEDF. These results indicate that Bi co-doped EDF has significant performance improvements in radiation resistance, making it ideal for applications in harsh radiation environments.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Extensive research has been conducted on rare-earth-doped optical fibers over the past few decades [1,2]. Er-doped fiber (EDF) can achieve effective optical transmission and optical amplification in the optimal transmission window of fiber band 1550 nm, so it is widely used in space fiber communication and other fields [3,4]. However, there are a large number of ionizing radiations will cause a sharp increase in the loss of optical fibers, resulting in performance reduction or failure of fiber amplifiers or lasers under long-term space irradiation conditions [5–8]. Recently, there are two main reasons for the degradation of EDF properties after irradiation: First, some Er3+ in the fiber captures electrons and becomes Er2+ when exposed to radiation; Second, after irradiation, defective structures related to aluminum (Al), germanium (Ge) or phosphorus (P) are formed in the fiber [9–12]. Therefore, investigating rare-earth-doped fibers to improve their radiation resistance is crucial for their use in harsh environments. Numerous radiation-hardening technologies for rare-earth-doped fibers have been proposed. One of the most effective methods is loading H2 or D2 into fabricated optical fibers [13,14]. However, a major drawback of this approach is the high volatility of the gas. To overcome this issue, a hole-assisted carbon-coated EDF was developed, but its complex process is not conducive to industrialization [15]. Another approach is to dope heavy ions, such as cerium (Ce) or lead (Pb) ions, to alter the composition of the optical fiber and enhance its radiation resistance [16]. During the irradiation process, heavy ions can capture holes and electrons, inhibit the formation of corresponding hole-type and electron-type defect centers, and provide irradiation buffer space for active ions in the fiber [17,18]. In 2023, N. Yan et al. reported a barium gallo-germanate glass with radiation resistance due to the conversion of Bi ions between low and high prices, which inhibited the formation of Ge-related electron center and non-bridging oxygen hole center defects in the irradiation process [19]. However, there are few studies on how Bi ions affect and improve the irradiation resistance of EDF.
In this paper, we analyze the spectral characteristics of the EDF and Bi/Er co-doped fibers (BEDF1 and BEDF2), which are fabricated by modified chemical vapor deposition (MCVD) combining with atomic layer deposition (ALD). The radiation-induced absorption (RIA) spectra, fluorescence spectra, gain characteristics, and laser threshold power were experimentally studied and compared before and after irradiation. Moreover, the irradiation simulation model is established through GEANT4 simulation toolkit, and the effect of Bi ions doping on the irradiation performance of EDF was studied.
2. Experiment setup
2.1 Sample preparation and irradiation conditions
Three fiber samples with different concentrations of Bi ions doping fabricated by ALD combined with MCVD [20–22]. The main elemental components of the three fiber samples are measured by an electron probe micro-analyzer (EPMA-8050 G, SHIMADZU, Japan), as shown in Table 1. The concentration of Er ions is almost the same among the three fiber samples, approximately 0.090 wt%. The concentrations of Bi ions doped in BEDF1 and BEDF2 fibers are 0.010 wt% and 0.020 wt%, respectively. Additionally, the corresponding diameter parameters of core and cladding layers of all fibers are shown in Table 1. Doping with Ge and P ions can increase the refractive index of the fiber cores. Irradiation was addressed with gamma-ray from a 60Co irradiation source (Radiation chamber, Shanghai Academy of Agricultural Sciences, Shanghai, China) with the dose rate of 800 Gy/h at room temperature. The irradiation doses were set as 300, 500, 800, and 1500 Gy, respectively.
Table 1. Main parameters of fiber samples
2.2 Experiments methodology
The absorption spectra of the erbium-doped fiber samples were measured by the conventional truncation method, and the spectral characteristics of the three fiber samples before and after the radiation were measured using a white light source (AQ4305) and a spectrum analyzer (OSA, YAKOGAWA AQ-6315A), and the measurement wavelength range was selected. is 400–1700nm, and the OSA resolution is set to 10 nm. The optical fiber lengths were approximately 10-200 cm. Fluorescence spectra were measured with a forward-direction pump system with a 980 nm laser and an OSA (YOKOGAWA AQ-6370C) in the 600-1700nm wavelength region, and resolution was 2 nm. The Er3+ fluorescence lifetime was measured by FLS980 steady-state fluorescence spectrometer from Edinburgh, UK. The excitation wavelength was 980 nm and the emission wavelength was 1535 nm. To further investigate the laser output power variation based on three fiber samples before and after radiation, a backward pumped linear distributed Bragg reflector (DBR) laser system was established using wavelength division multiplexer (WDM), high-reflectivity fiber Bragg grating (HR-FBG), and low-reflectivity fiber Bragg grating (LR-FBG), as shown in Fig. 1. The WDM was utilized to inject the pump light and generate the laser at 1551 nm. A fiber isolator (ISO) was used to ensure unidirectional light transmission. The length of three fiber samples was 80 cm.
Fig. 1. Schematic diagram of fiber laser system. The insect picture is the transmission spectra of the HR-FBG and LR-FBG.
3. Experimental results and discussion
3.1 Absorption spectra
The absorption spectra of three fiber samples under different radiation doses are shown in the insert of Fig. 2. Irradiation causes an increase in the absorption coefficient of the three fiber samples, and the absorption coefficient increases with increasing irradiation dose (note the different vertical coordinates in the figures). In addition, the absorption coefficient of the short wavelength band is higher than that of the long wavelength band.
Fig. 2. RIA spectra under different irradiation doses for the: (a) EDF, (b) BEDF1, and (c) BEDF2. Insert: the absorption spectra of three fiber samples under different irradiation doses. (d) RIA spectra of three fiber samples at 1300 nm vs. different irradiation doses.
The RIA spectra were obtained by subtracting the absorption spectra of the pristine samples from those of the irradiated samples. RIA is suitable for describing the additional absorption of optical fiber, and it is an essential characteristic parameter to measure the radiation resistance of optical fiber. The RIA spectra of the three fiber samples with different Bi contents at different doses in the range 700-1600 nm are shown in Fig. 2. Note that the RIA of three fiber samples increase with the increase of radiation 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 1000-1400 nm. However, a sharp rise takes place at 1100 nm and the growth trend of RIA become steeper towards the shorter wavelength range. Such RIA shape of the irradiated three fiber samples, 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 [9,23]. The RIA of BEDF is generally smaller than that of EDF. BEDF2 exhibits the best radiation resistance at the same exposure level. By comparing the RIA of EDF and BEDF1, we find that high Bi doping can significantly improve the radiation tolerance of EDF in high-dose environments. In addition, we find a relatively clear absorption band of Er ions in the RIA spectra, which may be due to the fact that a portion of Er3+ traps electrons and produces Er2+ when exposed to radiation [12]. The experimental results of BEDF1 and BEDF2 show that the radiation resistance can be further improved through Bi co-doping. In particular, RIAs of the three fibers were illustrated at 1300 nm after irradiation with different doses, as shown in Fig. 2(d). The RIA of the EDF has a faster growth rate than that of the BEDF. Especially, after radiation doses of 1500 Gy treatment, compared with EDF, the RIA of BEDF1 and BEDF2 are reduced by 32.6% and 56.0%, which shows that higher Bi ions co-doping can significantly improve the radiation resistance of EDF.
3.2 Fluorescence intensity and lifetime
Figure 3(a)-(c) depict the fluorescence intensities of EDF, BEDF1, and BEDF2 before and after irradiation, while their corresponding fluorescence lifetimes are shown in the inserts of Fig. 3(a)-(c). The fluorescence intensity of the three fibers at 1535 nm changed with different irradiation doses, as shown in Fig. 3(d).
Fig. 3. Fluorescence spectra before and after 1500 Gy irradiation treatment for the: (a) EDF, (b) BEDF1, and (c) BEDF2. Insert: decay curves of Er3+ at 1535 nm. (d) Fluorescence intensity of three fiber samples vs. different irradiation doses at 1535 nm.
Bi ions doping promotes the fluorescence intensity and excited state lifetime of Er3+ for pristine samples since co-doping Bi ions in EDF can effectively broaden the Er3+ ion-related absorption band [24]. After 1500 Gy irradiation treatment, EDF shows significant reductions in both Er3+ fluorescence intensity and excited state lifetime, while the change is negligible for BEDF1 and BEDF2. This behavior is correlated to the RIA and is likely due to the interaction between the Er3+ ions and point defects produced under gamma-ray irradiation [25]. Additionally, the decrease in Er3+ fluorescence intensity is related to the absorption of 980 nm pump by the defect center, which reduces the pump power available for population inversion, and the reduction in the density of Er3+ ions by irradiation [26]. With the increase of irradiation dose, the fluorescence intensity and lifetime of EDF decreased continuously, while BEDF showed a trend, increasing first and then decreasing, and changed little before and after irradiation treatment. After low-dose irradiation treatment, the RIA in BEDF is low, and the Bi-related center transfer energy to Er ions, thus enhancing the fluorescence intensity and lifetime of Er ions and increasing the fluorescence intensity to a certain extent. After high-dose irradiation treatment, the RIA in BEDF increases sharply, resulting in a decline in the fluorescence intensity. At the same time, the defect structure generated in the fiber after irradiation treatment will absorb more pump energy, resulting in a decrease in pump conversion efficiency and a decrease in fiber fluorescence lifetime. Among the three fiber samples, the fluorescence intensity and fluorescence lifetime of BEDF2 changed the least before and after irradiation treatment, indicating that the appropriate high concentration of Bi ions co-doping significantly enhanced the radiation resistance of EDF.
3.3 Gain characteristics and laser threshold power of three fiber samples
The variation of radiation-induced gain variation (RIGV) spectra for the three fiber amplifiers at different irradiation doses, as shown in Fig. 4(a)-(c). RIGV is calculated by subtracting the irradiated gain from the un-irradiated gain. The signal at 1550 nm was generated by a C-band broadband light source with a power of -20 dBm.
Fig. 4. RIGV spectra under different irradiation doses for the: (a) EDF, (b) BEDF1, and (c) BEDF2 with 600 mW pumping. (d) Normalized gain degradation as a function of the total accumulated doses for the EDFA designed with three fiber samples at 1550 nm.
The RIGV of the three fiber amplifiers also increased with increasing irradiation doses. BEDF2 amplifier exhibited the smallest change in RIGV, which was consistent with the results of the RIA. As the irradiation dose increased, EDF showed a severe gain degradation, with RIGV reaching 3.48 dB at 1550 nm after 1500 Gy irradiation treatment. On the other hand, BEDF1 and BEDF2 demonstrated excellent radiation resistance at both low and high doses, with RIGV of 2.51 dB and 1.30 dB at 1550 nm after 1500 Gy irradiation treatment, respectively. In order to facilitate direct comparison of the influence of irradiation on the gain characteristics of the three optical fibers, the output power is normalized, as shown in Fig. 3(d). After 1500Gy irradiation treatment, EDF's gain at 1550 nm decreased by 62.0%, while BEDF1 and BEDF2 only decreased by 34.1% and 13.0%, respectively. The experimental results show that the gain characteristics of highly doped Bi ions EDF are relatively stable before and after irradiation.
The threshold powers of fiber lasers with three fiber samples before and after irradiation, as shown in Fig. 5(a)-(c). Under different gamma irradiation doses of 0, 300, 500, 800, and 1500 Gy, the threshold power of EDF was 9.53, 26.72, 26.72, 61.25, and 87.20 mW, respectively. The threshold power of BEDF1 was 9.53, 11.72, 11.72, 17.98, and 31.04 mW, respectively. Similarly, the threshold power of BEDF2 was 9.53, 11.72, 11.72, 13.64, and 22.34 mW, respectively. After three fiber samples were irradiated, it was found that the threshold power of EDF increased significantly compared with BEDF, while the threshold power of BEDF2 doped with more Bi ions increased slowly compared with BEDF1, as shown in Fig. 5(d). Therefore, it is obvious that the laser output performance of EDF doped with highly doped Bi ions is relatively stable compared with EDF before and after irradiation and the doping strategy of appropriate highly doped Bi ions is an effective solution for improving the radiation resistance of EDF in practical applications.
Fig. 5. The laser threshold power under different irradiation doses for the: (a) EDF, (b) BEDF1, and (c) BEDF2. (d) Threshold power of three fiber samples vs. different irradiation doses.
4. Theoretical calculation and discussion
In silica fibers, each silicon atom forms a stable tetrahedral structure with four oxygen atoms, where each oxygen atom is shared by two silicon atoms, known as bridging oxygen. However, doping the silica fiber with Ge and Al oxides changes the ratio of silicon to oxygen, resulting in the formation of non-bridging oxygen where oxygen atoms start to bond with silicon atoms, thus breaking the silicon-oxygen network. Er and Bi ions, being in low concentrations, are distributed in the gaps between the tetrahedrons, and their positions in the tetrahedral gaps need to follow a specific coordination structure while maintaining local electrical neutrality. Previous research suggests that rare earth ions have multiple coordination forms in silica optical fibers, with coordination numbers ranging from 6 to 9 [27]. Er3+ ion, the activator in the active optical fiber, has a coordination number of 6 and is located in the gap outside the silicon-oxygen network, connected to the surrounding non-bridging oxygen by ionic bonds. An irradiation simulation model of EDF and BEDF fiber was established through GEANT4 simulation toolkit to study the characteristics of Bi co-doping under irradiation. The model of gamma-ray irradiating different fiber samples, as shown in Fig. 6.
The irradiation source was set to gamma photons with an energy of 1.24 MeV per particle, and gamma photons and matter interact primarily through three processes: the photoelectric effect, the electron pair effect, and the Compton effect. The core radius and cladding radius of the four groups of fibers are 4.5 µm and 62.5 µm. Four groups of fibers (1#, 2#, 3# and 4#) were constructed with different core compositions and concentrations, and each group had ten fibers. The ions with the same mass proportion in the four groups of fiber cores include silicon (Si), oxygen (O), Ge, Al and P, while the ions with different mass proportion include Bi and Er. The mass ratios of Bi and Er ions in the four groups of optical fibers are shown in Table 2. The mass proportion of Er ions of the ten fibers in group 1# and 2# varies the same, and is in the range of 0∼2.0 wt %. The mass proportion of Bi ions of the ten fibers in group 3# and 4# varies the same, and is in the range of 0∼2.0 wt %. The same amount of gamma photons emitted by the fiber was irradiated by simulation experiments. The energy deposition in the four groups of fiber cores was calculated, and the energy deposition results were shown in Fig. 7. The results show that the energy deposition in the 1# and 2# groups of fiber cores increases with the increase of Er ions mass ratio. With the increase of Bi ions mass ratio, the energy deposition in the 3# and 4# fiber cores also increased.
Fig. 7. Simulation result: (a) Energy deposition in the group 1# and 2# optical fiber cores. (b) Energy deposition in the group 3# and 4# optical fiber cores.
Table 2. Mass ratio of Bi and Er ions in four groups fiber samples
The relationship between the energy deposited on Er ions in group 2# and the mass ratio of Er ions in the fiber can be calculated by subtracting the energy deposited in the fiber core with Bi ions mass ratio of 0.5 wt % in group 3# from the energy deposited in each fiber core of group 2#. For comparative analysis, the energy deposited in single-mode fiber core can be subtracted from the energy deposited in group 1# fiber core to obtain the relationship between the energy deposited on Er ions in group 1# fiber core and the mass ratio of Er ions in fiber, as shown in Fig. 8(a). The results show that the effect of gamma rays on Er ions in Bi co-doped EDF is less than that in EDF. When Er ions doping concentration is less than 0.05 wt% in Bi co-doped EDF, the energy deposited on Er ions is more than 90.0% less than that in EDF. This indicates that Bi ions are more likely to absorb gamma rays in the Bi ions co-doped EDF, thus reducing the effect of gamma rays on Er ions and enhancing the radiation resistance of the fiber. By subtracting the energy deposited in the optical fiber core with the same Bi ions mass ratio in group 4# and group 3#, the relationship between the energy deposited on Er ions in group 4# and the Bi ions mass ratio in the fiber can be obtained, as shown in Fig. 8(b). The results show that with the increase of Bi ions mass ratio in group 4# fibers, the energy deposited on Er ions in fiber decreases gradually. When the mass proportion of Bi ions was 0.3 wt %, the energy deposited on Er ions was the smallest, but when the mass proportion of Bi ions continued to increase, the energy deposited on Er ions began to increase gradually.
Fig. 8. The relationship between the energy deposited on Er ions and: (a) mass proportion of Er ions in group 1# and group 2#. (b) the mass proportion of Bi ions in group 4# fiber core.
The interaction between gamma photons and ions in the fiber core generates secondary particles whose trajectories were extracted to analyze the phenomenon discussed above. By analyzing the statistics of secondary particle trajectories, we find that the addition of Bi ions increases the number of secondary particles in the fiber core. However, most of these particles fly out of the fiber core. When Bi ions are minimally doped into the core, the resulting secondary particles remaining in the core are insignificant and have minimal effect on Er ions. Conversely, increasing the number of Bi ions in the core results in an increase in the number of secondary particles in the fiber core. This, in turn, increases the likelihood of secondary particles interacting with Er ions, leading to increased energy deposition on Er ions and ultimately increased radiation resistance of EDF. Therefore, it is necessary to doping proper concentration of Bi ions in EDF to achieve the best irradiation resistance effect of the fiber.
5. Conclusion
In conclusion, we fabricate three fiber samples with various Bi concentrations via ALD combined with MCVD. The RIA can be significantly reduced by co-doping Bi ions. After 1500 Gy irradiation treatment, RIA of BEDF1 and BEDF2 at 1300 nm is 32.6% and 56% lower than EDF, respectively. With the increase of irradiation dose, the fluorescence intensity and lifetime of EDF decreased continuously, while BEDF showed a trend, increasing first and then decreasing, and changed little before and after irradiation. The gain characteristics and laser threshold power of BEDF are relatively stable before and after irradiation. At the same time, an irradiation simulation model of EDF and BEDF fiber was established through GEANT4 simulation toolkit to study the characteristics of Bi co-doping under irradiation. With the increase of Bi ions mass ratio, the energy deposited on Er ions in BEDF decreased after the same dose irradiation. This indicates that Bi ions are more likely to absorb gamma rays, reducing the impact of irradiation on Er ions. The experimental and simulation results demonstrate that the appropriate concentration of Bi doping can effectively improve the irradiation resistance of EDF. The excellent performance achieved with the Bi co-doped EDF is of great significance for applications in harsh radiation environments.
Funding
National Natural Science Foundation of China (61935002, 61975113); Shanghai professional technical public service platform of advanced optical waveguide intelligent manufacturing and testing (19DZ2294000); 111 Project (D20031).
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.
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