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

Abstract

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 <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -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.