A saturable absorber based on bismuth-doped germanosilicate fiber for a 1.93 μm, mode-locked fiber laser

We demonstrate the use of an ion-doped fiber type saturable absorber for mode-locking of a fiber laser for the first time, to the best of the authors' knowledge. More specifically, it is experimentally demonstrated that an unpumped segment of bismuth doped germanosilicate fiber can readily be used as a mode-locker operating at 2 μm wavelengths. Incorporating a bismuth-doped germanosilicate fiber as a saturable absorber in a thulium/holmium codoped fiber ring cavity, stable mode-locked pulses with a temporal width of ∼3.94 ns were easily achieved at a wavelength of 1927.6 nm with a repetition rate of ∼6.806 MHz. A series of output characterizations were conducted to clarify whether the outputs are Q-switched or mode-locked pulses.