Beam-combinable high-power fiber laser sources for directed energy applications

We discuss methods and lessons learned during the integration and testing of five individual kW-class fiber amplifiers seeded with pseudorandom phase-modulated light, using a 1x5 Diffractive Optical Element (DOE). Each amplifier is capable of producing approximately 1.2 kW of near diffraction-limited output power ( M2 <1.1). Low power samples from each amplifier are used for active polarization control. Phase control of each amplifier was accomplished using a low power combined beam sample and AFRL's Locking of Optical Coherent via Single-detector Electronic-frequency Tagging (LOCSET) control system. Approximately 5 kW of signal output was achieved with a combined efficiency of 82%. Losses in the system arise from DOE efficiency limitations, Amplified Spontaneous Emission (ASE), polarization errors, uncorrelated wavefront errors, optical path length mismatches, and beam misalignments. We discuss the impact of recent amplifier developments and how these developments impact this beam combining method.