# Influence of Pre-Radiation and Photo-Bleaching on the Yb-Doped Fiber Laser Radiated with Gamma-ray
## 1 Introduction  
In recent years, due to the superior characteristics of low quantum defect, the favorable band structure of trivalent Ytterbium (Yb\({}^{3+}\)) ions, and good beam quality, Yb-doped silica fibers have been considered the ideal amplifying medium for high-power fiber lasers and amplifiers [1; 2; 3; 4; 5; 6]. Yb-doped fiber lasers have rapidly expanded their market shares in the commercial laser region and have been widely used in scientific research and industrial processing areas [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20]. For different working environments, the Yb-doped fiber lasers should be specially designed, such as satisfying the demands of anti-interference and anti-vibration requirements [15]. When operating in an ionizing radiation environment, such as nuclear power plants and radiotherapy facilities, the Yb-doped fiber lasers should be designed with an anti-radiation ability [21; 22; 23]. For high-power Yb-doped fiber lasers, which benefited from the tremendous improvement in the material component design and fabrication technology, the output power scale has exponentially increased to tens of kilowatts [11; 12; 13]. However, the photodarkening (PD) effect is inevitable for Yb-doped fiber lasers operating at high output power. PD would induce additional absorption in the UV and visible wavelength range with an absorption tail overlap with the pump wavelengths of Yb\({}^{3+}\), caused by the formation of defects [24]. When operating in an ionizing radiation environment, the durability of high-power Yb-doped fiber lasers would be faced with more serious challenges [22; 23]. Besides the PD effect, high-power Yb-doped fibers would suffer from a more severe output power decrease and the additional absorption, known as the radiation-darkening (RD) effect [22; 23; 24; 25; 26; 27; 28; 29]. [PERSON] et al. found that the additional absorb intensity of rare-earth-doped fiber is approximately three orders of magnitude higher than that of non-rare-earth-doped passive fibers [22]. Therefore, improving the radiation resistance of high-power Yb-doped fiber lasers is of research and practical significance. Numerous approaches to improve the radiation resistance of Yb-doped fibers have been proposed, which were mainly separated as the anti-radiation design before ionizing radiation and the bleaching recovery method after ionizing radiation. The material design of Yb-doped fibers to improve the radiation resistance is the most promising approach. For the Yb-doped aluminophosphosilicate ternary fiber, [PERSON] et al. discovered that changing the P/Al ratio would influence the degree of RD effect and the kinds of radiation-induced defects [22]. Keeping the elemental concentration of P > Al benefits the radiation resistance and lightens the reduction of Yb\({}^{3+}\) ions affected by radiation, according to their research. For the Yb/Ce-codoped aluminosilicate binary fiber, [PERSON] et al. found that doing Ce\({}^{3+/4+}\) ions could greatly improve the radiation resistance against gamma-ray of Yb-doped glasses without obvious impact on the absorption properties [29]. However, the negative effects of Yb-doped fibers' material design are not neglectable. When the concentration of P is larger than Al, the absorption and emission cross-section would be decreased, leading to a degradation in the laser performance of Yb-doped ternary fibers [22]. For Yb/Ce-codoped binary fibers, the fluorescence intensity and lifetime of Yb\({}^{3+}\) ions would be significantly decreased by a high concentration of Ce\({}^{3+/4+}\) ions [29], and the interaction between Yb\({}^{3+}\) and Ce\({}^{3+/4+}\) ions would induce additional thermal load. Therefore, the material design approach still needs abundant experiments and data accumulation. Moreover, pre-radiation and photo-bleaching methods, considered to be key techniques in the relevant research field, are effective and easy to achieve [30; 31; 32; 33; 34]. The pre-radiation technique was proven to be quite effective at low output power [30; 31].