# Influence of Pre-Radiation and Photo-Bleaching on the Yb-Doped Fiber Laser Radiated with Gamma-ray
## 3 Results and Discussions of Pre-Radiation
### Spectral Properties  
Besides the output power reduction, radiation would also influence the spectral properties of the output laser. We measured the output laser's spectra at different output powers pumped by a 976 nm LD and with different radiation doses per the Telecom Optical Spectrum Analyzer (OSA, AQ6370D, from Yokogawa Inc., Musashino City, Japan). Figure 5a presents the output laser spectrum at 946 W of the non-pre-radiated fiber laser with 10 krad(Si) radiation. The central wavelength is 1060.25 nm with a narrow 3 dB band  
Figure 4: (**a**) Time domain and (**b**) frequency domain properties of the output laser in pre-radiated fiber laser.  
Figure 3: Output power of fiber lasers at different radiation doses, black solid line for the non-pre-radiated fiber laser, red dashed line for the pre-radiated fiber laser, hollow symbols for the output power variation, and the corresponding error bars.  
width of 0.1258 nm, and the suppression ratio reached 30.0 dB between the signal peak and the Stimulated Raman Scattering (SRS) peak centered at 1113 nm. As a kind of non-linear effect, the intensity of the SRS peak is related to the fiber length and output power. The insert of Figure 5a is the signal peak with a narrow wavelength range. Figure 5b presents the output laser spectrum at 792 W of the pre-radiated fiber laser with 10 krad(Si) radiation, showing the same central wavelength with a narrow 3 dB bandwidth of 0.1258 nm. The SRS suppression ratio was also 30.0 dB. With the same radiation dose, pre-radiation treatment has little influence on the spectral property of radiated fiber lasers.  
The spectral properties at different cumulated radiation doses were measured, as shown in Figure 6. The central wavelength of both fiber lasers slightly fluctuated within the range between 1060.22 nm and 1060.26 nm. Due to the seed laser not being radiated with gamma ray, the fluctuation mainly originated from a testing error or a tiny change in environmental conditions. With the radiation cumulated, 3 dB bandwidth of the non-pre-radiated fiber laser slightly decreased from 0.1450 nm to 0.1258 nm, and 3 dB bandwidth of the pre-radiated fiber laser decreased from 0.1481 nm to 0.1258 nm. The SRS suppression ratio also changed, within the range of 28.9 dB to 31.4 dB. Considering the measuring accuracy of the spectrograph and the experimental error during the test, it can be concluded that the radiation dose at this level has little influence on the spectral properties, such as the central wavelength, 3 dB bandwidth, and SRS suppression ratio.  
In summary, within the current radiation dose range (less than 10 krad), the main influence mechanism of radiation on fiber lasers is the radiation-in  
Figure 5: The output laser spectra of (**a**) non-pre-radiated fiber laser radiated by 10 krad(Si), (**b**) pre-radiated fiber laser radiated by 10 krad(Si), inserts present the laser spectra within a narrow wavelength range.  
Figure 6: Spectral properties of fiber lasers at different radiation doses, (**a**) the central wavelength and 3 dB bandwidth of the output laser (arrows represent the coordinate axis corresponding to the lines), (**b**) the SRS suppression ratio of the output laser.  
The obvious output power reduction can be observed, while there is no significant change in spectral properties. When the gamma radiation dose is within 10 krad, pre-radiation treatment can slightly strengthen the radiation resistance of high-power fiber lasers against gamma-ray, as proven in Figure 3. The reason pre-radiation enhances the radiation resistance of Yb-doped fibers is partly related to the increasing OH, which originated from the reaction of H+ ions from the fibers' coating layer radiated with gamma-ray and the intrinsic SiO2 units in fiber [38; 39; 40]. Whereas, the mode instability threshold would decrease at high-power output states, due to the cumulated radiation-induced attenuation of pre-radiation and radiation, which needs to be comprehensively considered before the pre-radiation treatment is implemented.