220 microJ Monolithic Single-Frequency Q-switched Fiber Laser at 2 micrometers by 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 1920nm 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 micrometers highly Tm-doped germanate fiber was used in the power amplifier stage. For 80 ns pulses with 20 kHz repetition rate, we achieved 220 microJ pulse energy, which corresponds to a peak power of 2.75kW with transform-limited linewidth.