Multi-kilowatt diffractive coherent combining of pseudorandom-modulated fiber amplifiers

We report efficient coherent beam combining of five kW-class fiber amplifiers seeded with pseudorandom phase-modulated light, using a 1 x 5 diffractive optical element (DOE). Each fiber amplifier channel was path length matched, actively polarized, and provided approximately 1.2 kW of near diffraction-limited output power (M-2 < 1.1). A low-power sample of the combined beam after the DOE provided an error signal for active phase stabilization. After phase stabilization, the beams were coherently combined via the DOE. Notably, a total output power of similar to 5 kW was achieved with 82% combining efficiency and excellent beam quality (M-2 < 1.1). The intrinsic DOE splitter loss was 5%. Additional losses due in part to nonideal polarization, amplified spontaneous emission content, uncorrelated wavefront errors, and fractional beam misalignments contributed to the efficiency reduction. Overall, multi-kW beam combining of pseudorandom-modulated fiber amplifiers was demonstrated for the first time. (C) The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License.