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

•A single-frequency all-PM HDF oscillator operating at 2.09 μm with an output power of 57.9 mW has been realized for the first time. To the best of our knowledge, this achievement represents the highest reported single-frequency power for a fiber oscillator at this wavelength.•The rapid switching of longitudinal mode states through FBG temperature tuning, as well as the optical bistability of longitudinal modes induced by saturable absorption, were recorded and discussed for the first time in the HDF oscillator.•A high-efficiency, high-polarization extinction ratio 2.09 μm single-frequency laser with a power of 1 W, generated through a first-stage HDF amplifier, serves as an optimal seed source for narrow-linewidth pulsed Ho:YAG solid-state lasers. 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.