In space environments, the performance and lifespan of active optical fibers are significantly compromised by high-energy particles and radiation rays primarily due to radiation-induced darkening (RD). Consequently, the capabilities of fiber lasers and amplifiers in space environments are undermined. This issue is generally attributed to radiation-induced impacts on the color centers within the active fibers. Numerous types of active irradiation suppression technologies are currently available for enhancing the performance of active optical fibers, and their radiation-resistant effects have been validated. The most commonly used passive irradiation suppression technique is shielding with metallic materials. Although effective, this approach is neither lightweight nor flexible, typically requiring significant mass and space, and cannot conform to the circumference of the fiber. Consequently, this study aimed to explore a lightweight, flexible, and directly wearable passive radiation-resistant film that can be applied to the circumference of optical fibers.

This study proposes a multilayer radiation-resistant composite film fabricated using advanced optical precision deposition techniques, boasting the advantages of being lightweight, flexible for wearable applications, and cost-effective. It can be directly applied to the circumference of optical fibers and employed in conjunction with other radiation protection methods for fibers, such as metal shielding and fiber doping, to provide point-to-point protection for fiber components that are more susceptible to radiation, thereby enhancing the overall radiation resistance of the optical fibers. This study employed electron beam evaporation combined with ion-assisted deposition technology to fabricate an Al+ITO+Kapton composite multilayer radiation-resistant film, the specific structure of which is shown in Fig. 1. After exposure to approximately 85 kGy of radiation in a laboratory setting, the Yb³⁺ doped optical fiber without radiation-resistant film (fiber A), along with three other groups of Yb³⁺ doped fibers (fibers B, C, and D) subjected to different protective measures, was tested and compared across various performance aspects. The grouping of the fiber samples into the four experimental groups is presented in Table 3. Fluorescence, loss, and laser tests were conducted to ascertain the efficacy of the radiation-resistant film in aiding the active fibers in withstanding radiation.

The absorption spectra of the four groups of radiation-resistant optical fiber samples are shown in Fig. 3(a), with all fibers having a length of 1 m. All four groups of fibers exhibit strong absorption in the range of 890?1000 nm, which is due to the two characteristic absorption peaks of Yb³⁺-doped fibers at 915 nm and 975 nm. In the range 1000?1450 nm, fiber A showed the most substantial absorption, followed by fibers B and C, while fiber D had the least absorption. The radiation-induced attenuation (RIA) spectra before and after irradiation were calculated using equation (1), and the results are shown in Fig. 3(b). In the figure, it is evident that fiber A experiences the highest radiation dosage, resulting in a substantial increase in absorption loss of nearly 4 dB, compared to fiber D. Although fibers B and C also exhibit increased absorption losses, the increments are approximately 1.5 dB and 0.7 dB, respectively, which are markedly lower than those observed for fiber A. The findings reveal that the incorporation of a radiation-resistant coating notably mitigates radiation-induced absorption losses, significantly reinforcing the radiation resilience of active optical fibers. Consequently, this underscores the efficacy of the radiation-resistant coating in shielding the fibers against radiation. The emission spectra of the four groups of PM-YSF fibers, all with lengths of 1 m, are shown in Fig. 4. It can be observed that all four groups of fibers exhibit strong fluorescence emission at 1030 nm, which is due to the emission peak of Yb³⁺ at this wavelength; subsequently, as the wavelength shifts towards longer wavelengths, the intensity of the emission spectra gradually decreases. In terms of the pattern, compared to fiber D, fiber A exhibited a decrease in emission intensity by 3.5 dBm. For fibers B and C, the reductions were 2 dBm and 1.4 dBm, respectively. This indicates that the fibers with protective measures received significantly lower radiation doses than those without protection, further confirming the effectiveness of the radiation-resistant film in enhancing the radiation resistance of the active fibers. A continuous-wave, fully polarization-maintaining fiber laser with a central wavelength of 1064 nm was designed. The experimental setup is illustrated in Fig. 5. The output power of the fully polarization-maintaining 1064 nm laser using the four groups of active fiber samples as gain media is shown in Fig. 6. The slope efficiency of the control group, the laser with fiber D , was 7% with a laser oscillation threshold of 44.78 mW, whereas that of the laser with fiber A was only 1.8% with an oscillation threshold of 120.6 mW, indicating a significant decline. For the lasers with fibers B and C, however, their slope efficiencies are 4.5% and 4.6%, respectively, with oscillation thresholds of 65.2 mW and 58.4 mW. Compared to the unshielded bare fibers, these fibers showed a nearly 1.5-fold increase in slope efficiency and halving of the laser oscillation threshold power. This illustrates the outstanding protective capability of the radiation-resistant films when exposed to a radiation environment.

This study conducted comparative experiments on four groups of PM-YSF fiber samples with different protective treatments, evaluating their performance from three aspects: absorption spectra, emission spectra, and laser power curves, thereby preliminarily verifying the radiation resistance capability of the radiation-resistant film. The findings reveal that compared to unprotected fibers, the application of the radiation-resistant film on active fibers reduces radiation-induced attenuation (RIA) by 2.5 dB, enhances the fluorescence spectrum intensity by 1.5 dBm, and increases the slope efficiency of the laser by nearly 1.5 times, while reducing the oscillation threshold by approximately half, indicating that the radiation-resistant film effectively mitigates the radiation dose and its impact on active fibers. Adding a layer of heavy metal armor as secondary protection on top of the radiation-resistant film resulted in a slight improvement in the radiation resistance of the fiber. Thus, the multilayered radiation-resistant film developed in this study serves a primary protective role.

In summary, this study proposes a flexible radiation-resistant film with a three-layer structure designed using optical thin-film preparation technology. Wrapping it around the surface of the active optical fibers can effectively enhance the radiation resistance of the active optical fibers. Unlike conventional large metal protective shells, this radiation-resistant film features excellent ductility and flexibility, and it is lightweight, which can cater to the miniaturization requirements of fiber lasers or amplifiers. Additionally, this radiation-resistant film can be used in conjunction with other radiation protection measures (such as the combination of radiation-resistant thin films and metallic materials utilized in this study) to provide point-to-point protection for core components, further enhancing the radiation resistance of optical fibers.