2.09 μm single-frequency all-polarization-maintaining Ho-doped fiber laser based on dynamic Bragg grating frequency selection

We demonstrated a 2.09 μm single-frequency all-polarization-maintaining (all-PM) Ho-doped fiber (HDF) laser pumped by a 1940 nm Tm-doped fiber (TDF) laser. The oscillator employed a ring cavity scheme to enhance the net gain at the wavelength, with two circulators utilized to guarantee unidirectional laser operation and eliminate the spatial hole burning (SHB) effect in the gain fiber. An unpumped HDF was used as a fiber saturable absorber (FSA) to construct a powerful dynamic Bragg grating (DBG) for frequency selection. By optimizing the output coupling ratio, FSA length and FBG temperature, a maximum single-frequency output power of 57.9 mW was obtained with a center wavelength of 2089.73 nm. Furthermore, we recorded and discussed the rapid switching of longitudinal mode states through FBG temperature tuning, as well as the optical bistability of longitudinal modes induced by saturable absorption in the oscillator. Finally, a power level of 1 W with a polarization extinction ratio of 17 dB was successfully achieved by a first-stage HDF amplifier.