Half-mJ All-Fiber-Based Single-Frequency Nanosecond Pulsed Fiber Laser at 2- $\mu{\rm m}$

We report a high energy, high peak power, all-flber-based single-frequency ~15-ns pulsed laser source at ~1918.4 nm in master oscillator-power amplifier configuration. The pulsed fiber laser seed was achieved by directly modulating a continuous- wave single-frequency fiber laser using an electro-optic modulator driven by an arbitrary waveform generator to pre-shape the pulses before amplification to mitigate pulse steepening and dynamic gain saturation. One piece of large (25-μm) core, polarization-maintaining highly thulium-doped germanate glass fiber was used in the power amplifier stage to boost the energy and the peak power of the 15-ns pulses to >;0.5 mj and >;33 kW without any nonlinearities.