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.
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