220 μJ monolithic single-frequency Q-switched fiber laser at 2 μmby using highly Tm-doped germanate fibers

We report a unique all fiber-based single-frequency Q-switched laser in a monolithic master oscillator power amplifier configuration at $\sim 1920\mathrm{nm}$ by using highly Tm-doped germanate fibers for the first time. The actively Q-switched fiber laser seed was achieved by using a piezo to press the fiber in the fiber Bragg grating cavity and modulate the fiber birefringence, enabling Q-switching with pulse width and repetition rate tunability. A single-mode polarization maintaining large core $25\mathrm{\mu m}$ highly Tm-doped germanate fiber was used in the power amplifier stage. For $80\mathrm{ns}$ pulses with $20\mathrm{kHz}$ repetition rate, we achieved $220\mathrm{\mu J}$ pulse energy, which corresponds to a peak power of $2.75\mathrm{kW}$ with transform-limited linewidth.