6 kW-Level Linearly Polarized Near-Diffraction-Limited Monolithic Fiber Laser with Linewidth Being 0.43nm

A high power narrow-linewidth, all-fiber polarization-maintaining (PM) amplifier has been demonstrated. A lasing power of 5870W has been delivered in master oscillator power amplifier architecture with cascaded white noise source (WNS) phase modulation and bidirectional pumping schemes. The maximal power was limited by the onset of Stimulated Brillouin scattering. At the maximum power operation, the amplifier exhibited a 3dB spectral linewidth of 0.43nm with beam quality be M² < 1.3 and polarization extinction ratio (PER) being 16.3dB. To the best of our knowledge, this represents the highest spectral brightness and PER achieved by PM fiber laser systems around 6kW-level operation.