# Influence of Pre-Radiation and Photo-Bleaching on the Yb-Doped Fiber Laser Radiated with Gamma-ray
## 4 Results and Discussions of Photo-Bleaching
### Spectral Properties  
To further investigate the photo-bleaching mechanism, the absorption spectra of a radiated Yb-doped fiber were measured. The 13 m long Yb-doped fiber was radiated with 7 krad(Si) gamma-ray, and the radiated Yb-doped fiber was subjected to multiple accumulated photo-bleaching treatments. The absorption spectra of this fiber at different states were recorded in Figure 9. Before radiation, the pristine fiber demonstrated a typical absorption spectrum of Yb-doped fiber with a broadband absorption at 915 nm and an intense absorption at 976 nm. After radiation, a broad radiation-induced absorption centered at 640 nm could be observed. With the injection of photo-bleaching laser, the absorption intensity at 640 nm gradually decreased, meaning part of the radiation-induced defect was bleached by the 532 nm laser. After a total photo-bleaching treatment of 7.25 h at 1.5 W and 4 h at 5 W, the absorption intensity at 640 nm no longer changed. The radiation-induced absorption had been bleached to its saturation state.  
A quantitative relationship between the photo-bleaching energy and radiation-induced attenuation at 640 nm was investigated, as shown in Figure 10a. After radiation, the absorption intensity at 640 nm rapidly increased to 0.32 dB/m, caused by the formation  
Figure 8: Photo-bleaching treatment of the pre-radiated fiber laser with 10 krad radiation, (**a**) experimental setup, (**b**) output power performance, hollow symbols for the output power and the corresponding error bars.  
Figure 9: Absorption spectra of radiated Yb-doped fiber with 7 krad radiation photo-bleached by injecting different bleaching lasers.  
of radiation-induced defects in the fiber core. With the augment of bleaching energy at 532 nm injected, the absorption intensity at 640 nm gradually decreased originating from a gradual reduction of gamma-radiation-induced defects by the bleaching effect of the 532 nm laser. Finally, the loss at 640 nm reached its saturation, and approximately 45.2% of radiation-induced absorption could be bleached with the 532 nm laser. The bleaching efficiency (absorption intensity bleached per 532 nm energy) after different bleaching treatments was plotted in Figure 10b. Generally, the bleaching efficiency gradually decreased with the gradual reduction of gamma-radiation-induced defects from the bleaching effect, and, finally, it was close to 0 at the saturation state.  
In summary, a laser diode at 532 nm was applied to bleach the radiated fiber laser and radiated Yb-doped fiber. The output power of the fiber laser partly recovered by 26 W, and the corresponding slope efficiency also recovered from 72.1% to 74.8%. The simultaneous reduction of radiation-induced attenuation from the photo-bleaching treatment also leads to an improvement in the MI threshold. In this work, approximately 45.2% of radiation-induced absorption could be bleached by the 532 nm laser, reaching the bleaching saturation state. The quantitative relationship between the photo-bleaching energy and radiation-induced absorption at 640 nm was also investigated.