Seeking the wavelength responsible for the photo-bleaching of Yb-doped fiber lasers

In radiation environments, the radiation induced attenuation (RIA) of the active fiber will induce severe performance degradation to the fiber laser system. One effective way to solve this problem is to bleach the active fiber with pumps at certain wavelengths, namely photo-bleaching. Experiments have shown that, output power of irradiated Yb-doped fiber lasers experiences remarkable recovery with 976 nm pump. However, under 976 nm pump, signals at both 976 nm and 1070 nm co-exist inside of the Yb-doped fiber. And, it can hardly tell which wavelength is responsible for the photo-bleaching process. Here, a one-hundred level Yb-doped fiber laser is irradiated with gamma-ray radiation. During the radiation process, significant output decline from 129 W at 0 Gy to 81 W at 100 Gy is witnessed. Then, self-bleaching test is conducted with 976 nm pump. After 2 h of bleaching, the output power restored to 111 W, corresponding to a recovery ratio of about 37.0%. To verify the specific wavelength responsible for the performance recovery, photo-bleaching characteristics of Yb-doped fiber lasers are investigated under different pump wavelengths including 915 nm, 976 nm, 1070 nm and 1550 nm. Experiments show that, laser signal at 1 μm waveband is the primary cause for the bleaching of Yb-doped fibers, while, the pump at 915 nm, 976 nm and 1550 nm can hardly bleach the irradiated Yb-doped fiber. The RIA recovery curves of Yb-doped fibers under different 1070 nm bleaching powers are measured. And, related evolution parameters are obtained through curve fitting. With these parameters, the RIA evolution of the Yb-doped fiber and the corresponding output power evolution of the Yb-doped fiber laser during the radiation and bleaching process are simulated. Comparisons show that, the numerical results are consistent with the experiments qualitatively, demonstrating the reliability of the model. This work should be instructive for the performance prediction of fiber laser systems under radiation and bleaching environments.