Rapid Decay of Type-II Femtosecond Laser Inscribed Gratings Within $Q$ -switched Yb $^{3+}$ -Doped Fib

Fiber lasers that utilize femtosecond (fs) laser inscribed type-II fiber Bragg gratings within the active medium are known to produce stable output powers in excess of 100 W during continuous-wave operation. However, here we show that the gratings undergo rapid annealing when employed as high reflectors in pulsed Q-switched fiber lasers operating at average power levels of less than 0.5 W. We demonstrate that a combination of a high population inversion and a high optical field seems to drive this annealing process and point out similarities to photodarkening.