Defect management via compositional design for superior photobleaching efficiency in Yb-doped fibers after low-dose gamma radiation

Rare-earth-doped fiber lasers are widely used in radiation environments such as space and nuclear facilities, where ionizing radiation induces defects in the gain fiber and causes performance degradation. In this study, we investigate the role of the P/Al ratio on the photobleaching performance of Yb-doped fibers. An online photobleaching platform was constructed to assess the bleaching efficiency at two wavelengths (450 nm and 638 nm) by measuring the radiation-induced absorption spectra of fibers with different P/Al ratios. Gaussian fitting of the spectra reveals that Al-rich fibers are dominated by Al-OHC and Yb 2+ defects, while fibers with higher P/Al ratios exhibit P-related defects such as l-POHC and r-POHC. Regardless of the P/Al ratio, 450 nm bleaching light consistently provides higher bleaching efficiency than 638 nm, and the sample with a P/Al ratio near 1 showed the greatest photobleaching efficacy. These findings clarify how fiber composition and photobleaching wavelength jointly govern radiation recovery and provide guidance for designing radiation-resistant fiber lasers.