532 nm pump induced photo-darkening inhibition and photo-bleaching in high power Yb-doped fiber amplifiers.

A novel method for mitigating photo-darkening and the effective photo-bleaching phenomenon by 532 nm cladding pump in Yb-doped fiber were herein reported. Compared with the pristine fiber, beyond 30% of photo-darkening induced excess loss was suppressed by 532 nm pretreatment. Moreover, the excess loss in the photo-darkened fiber was completely bleached with 532 nm pump. Additionally, the bleached fiber exhibited better photo-darkening resistance. Therefore, for high power application, a 20/400 gamma irradiated fiber was bleached in situ by 532 nm pump and the laser properties were explored. The output power restored to 421W accounting for 82% of the pristine fiber, with the mode instability threshold rising to over 2.6 times and the efficiency increasing from 37% to 63%. The results indicate 532 nm pump has bright prospects for the stable operation of high power fiber lasers.