With the rapid development of technologies such as space exploration and Starlink laser communications, active optical fibers and related devices have become widely used. Due to the unique nature of the space environment, these applications must account for the effects of radiation exposure. As a result, extensive research has been conducted to improve the radiation resistance of optical fibers. The primary focus in this field is to reduce the color center defects that arise in optical fibers after irradiation. Bismuth (Bi) is a heavy metal element with a wide range of valence states, making it particularly promising for enhancing radiation resistance in optical fibers. In this paper, we propose a cladding-doped Bi-ion erbium-doped fiber (EDF), which offers a promising solution to prevent performance degradation or failure of optical fiber devices such as optical fiber amplifiers and optical fiber lasers in irradiated environments. It has significant potential for application in radiation-affected environments.

Using GEANT4 software, the influence of varying doping concentrations of Bi ions doped in the cladding on the radiation resistance of EDFs is theoretically studied. Based on this, two types of erbium-doped fibers (EDF1 and EDF2) are fabricated using modified chemical vapor deposition (MCVD) combined with atomic layer deposition (ALD). EDF1 contains no Bi ions in its cladding, while EDF2 has Bi ions doped in the cladding. An experimental setup is used to investigate the changes in the optical fiber's spectral characteristics before and after irradiation, including radiation-induced absorption (RIA) spectra, fluorescence spectra, fluorescence lifetime spectra, gain characteristics, and laser performance.

The simulation results indicate that doping Bi ions into the cladding of the fiber decreases energy deposition in the core, initially lowering and then increasing as the doping concentration rises, with the optimal result achieved at a doping concentration of 1.0% (Figs. 3 and 4). After irradiation with 1500 Gy, EDF2 exhibits an RIA of 5.03 dB/m at 1300 nm, which is about 37.5% lower than EDF1 (Fig. 5). The fluorescence intensity and lifetime of EDF2 declines more gradually, with smaller decreases (Fig. 6). The fluorescence lifetime decreases by 0.28 ms, representing 97.3% of the pre-irradiation value, and a 5.2 percentage points improvement compared to EDF1 (Fig. 7). The normalized radiation-induced gain variation (RIGV) of EDF2 after irradiation is 1.89 dB/kGy, which is 31.9% lower than that of EDF1 (Fig. 8). In addition, the output power and slope efficiency of the EDF2 laser are relatively higher, with a slope efficiency of 2.74%, showing a decrease of 6.6 percentage points compared to EDF1 laser (Fig. 10). The threshold power shows a decrease of 18 mW, which is a reduction of 26.9% compared to EDF1 (Fig. 11). These experimental results suggest that doping Bi ions in the cladding can mitigate the influence of irradiated particles on the fiber core, thus improving the radiation resistance of the optical fiber.

In this paper, we demonstrate, through both simulation and experimentation, that doping a certain proportion of Bi ions in the cladding of active optical fibers can effectively improve their radiation resistance. The resulting optical fibers show excellent radiation resistance, making them highly suitable for use in optical fiber amplifiers and fiber lasers. The role of Bi ions in the cladding doping is analyzed. As a heavy metal with a large atomic mass, Bi can interact with irradiated particles, providing shielding and buffering effects that protect the fiber core. In addition, due to the rich valence states of Bi ions, they can absorb energy from irradiated particles and undergo valence state changes, reducing the influence of irradiation on the fiber core. However, if the Bi doping concentration becomes too high or the irradiation dose is excessive, an excess of secondary particles may be generated, which could further compromise the performance of the optical fiber core. Therefore, optimal Bi doping concentrations must be carefully selected to balance these effects. The findings suggest that active optical fibers can achieve enhanced radiation resistance through Bi-ion doping within an appropriate concentration range, making them highly promising for applications in harsh irradiation environments, such as space laser communication.