Beam combinable, kilowatt, all-fiber amplifier based on phase-modulated laser gain competition.

We report power scaling results of a highly efficient narrow-linewidth monolithic Yb-doped fiber amplifier seeded with two signals, operating at 1038 and 1064 nm. With the appropriate seed power ratio applied, this technique was shown to suppress stimulated Brillouin scattering in conjunction with phase modulation, while generating the output power in predominantly the longer wavelength signal. Notably, the integration of laser gain competition with pseudo-random bit sequence phase modulation, set at a clock rate of 2.5 GHz and utilizing an optimized pattern to match the shortened effective nonlinear length, yielded 1 kW of output power. The beam quality was measured to be near the diffraction limit with no sign of transverse mode instability. Furthermore, the coherent beam combination performance of the amplifier provided a 90% combining efficiency with no indication of spectral broadening when compared to the single-tone case. Overall, the power scaling results represent a significant reduction in spectral linewidth compared to that of commercially available narrow-linewidth Yb-doped fiber amplifiers.