High power narrow-linewidth linearly polarized nanosecond all-fiber amplifier with near-diffraction-limited beam quality

We present a monolithic high power narrow-linewidth linearly polarized nanosecond all-fiber laser in a master oscillator power amplifier configuration. The pulsed seed is generated by modulating a single-frequency continuous wave laser at 1064 nm using an electro-optic intensity modulator. In order to suppress stimulated Brillouin scattering (SBS), the pulse width is set to be similar to 4 ns. When the repetition rate is set to be 20 MHz or 10 MHz, SBS has been observed in the main amplifier. The corresponding SBS thresholds are similar to 100 W and 120 W respectively. At the repetition rate of 5 MHz, the pulse width is compressed to similar to 3 ns and an average power of 136 W is obtained, corresponding to peak power of 8.5 kW. Further power scaling is limited by stimulated Raman scattering. At maximum output power, the beam quality (M-2 factor) is similar to 1.1 and the polarization extinction ratio is >15.8 dB.