# Influence of Pre-Radiation and Photo-Bleaching on the Yb-Doped Fiber Laser Radiated with Gamma-ray
## 3 Results and Discussions of Pre-Radiation  
### Output Power  
Output power reduction is the most obvious radiation-induced effect on fiber lasers. We investigated the linear relationship between output power and pump power of both fiber lasers. We also fitted the linear relationship to compare the slope efficiency at different radiation doses, as shown in Figure 2. The linear relationship was presented by the dotted lines, and output power with its error bar was also demonstrated with the hollow symbols in Figure 2. For the non-pre-radiated fiber laser with 0 krad(Si), the output power was about 1065 W with a slope efficiency of 82.6%, as shown in Figure 2a. When the cumulated radiation dose reached 1 krad(Si) and 3 krad(Si) with a dose rate of 0.1 rad(Si)/s, output power of the non-pre-radiated fiber laser hardly decreased, with the corresponding slope efficiency of 83.2% and 82.0%, respectively. When the radiation dose rate increased to 0.4 rad(Si)/s and the cumulated radiation dose reached 10 krad(Si), the output power rapidly decreased to 946 W with a slope efficiency of 74.1%.  
For the pre-radiated fiber laser with 0 krad(Si), the output power was approximately 878 W with a slope efficiency of 79.8%. When the cumulated radiation dose reached 1 krad(Si) and 3 krad(Si), the output power was slightly decreased, with the corresponding slope efficiency of 79.8% and 78.6%, respectively. When the cumulated radiation dose reached 10 krad(Si) with the rate of 0.4 rad(Si)/s, the output power decreased to 792 W with a slope efficiency of 72.1%, as shown in Figure 2b.  
The output power performance at different cumulated radiation doses was shown in Figure 3. When the cumulated radiation reached 10 krad(Si), the output power of the non-pre-radiated fiber laser decreased to 88.8% with the reduction of 119 W, while the pre-radiated fiber laser decreased to 90.2% with the reduction of 86 W. Compared to the non-pre-radiated fiber laser, the output power of the pre-radiated fiber laser was less reduced, which presented a higher radiation resistance. Whereas, the mode instability effect was observed in the pre-radiated fiber laser, caused by the cumulated radiation-induced attenuation of pre-radiation and radiation.  
Figure 2: Output power of fiber lasers at different radiation doses, (**a**) the non-pre-radiated fiber laser, (**b**) the pre-radiated fiber laser, hollow symbols for the output power and the corresponding error bars, black dotted lines for the fitted linear relationship of 0 krad, yellow dotted lines for the fitted linear relationship of 10 krad.  
When the cumulated radiation dose reached 10 krad(Si) and pump power reached the maximum, the output power of the pre-radiated fiber laser presented fluctuation. Measured with a highspeed Si Biased photodetector (DET02 AFC, provided by Thorlabs Inc., Newton, NJ, USA) with a detection range of 400-1100 nm and a mixed domain oscilloscope (MDO3054, provided by Tektronix Inc., Beaverton, OR, USA) with a sampling rate of 2.5 GS/s, the time domain and frequency signals of the output laser were analyzed, as shown in Figure 4. The voltage fluctuation could be observed on the millisecond scale in the time domain, and peaks around 5 kHz could also be recognized in the frequency domain. From the time and frequency domain analysis, the mode instability effect of the pre-radiated fiber laser in this state could be confirmed [36,37]. When the output power of a fiber laser exceeds a certain threshold power, the mode instability effect occurs when the output laser's mode changed from a stable fundamental mode to an unstable mode. The energy between different modes is dynamically coupled at a millisecond scale, resulting in sudden degradation of the directionality and beam quality of the output beam. The time domain characteristics of typical mode instability effects exhibit periodicity at a millisecond scale, and the frequency domain exhibits a coupling frequency on the kHz level, consistent with the results in Figure 4.