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11 March 2016 Multi-kW coherent combining of fiber lasers seeded with pseudo random phase modulated light
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We report efficient coherent beam combining of five kilowatt-class fiber amplifiers with a diffractive optical element (DOE). Based on a master oscillator power amplifier (MOPA) configuration, the amplifiers were seeded with pseudo random phase modulated light. Each non-polarization maintaining fiber amplifier was optically path length matched and provides approximately 1.2 kW of near diffraction-limited output power (measured M2<1.1). Consequently, a low power sample of each laser was utilized for active linear polarization control. A low power sample of the combined beam after the DOE provided an error signal for active phase locking which was performed via Locking of Optical Coherence by Single-Detector Electronic-Frequency Tagging (LOCSET). After phase stabilization, the beams were coherently combined via the 1x5 DOE. A total combined output power of 4.9 kW was achieved with 82% combining efficiency and excellent beam quality (M2<1.1). The intrinsic DOE splitter loss was 5%. Similarly, losses due in part to non-ideal polarization, ASE content, uncorrelated wavefront errors, and misalignment errors contributed to the efficiency reduction.
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Angel Flores, Thomas Ehrehreich, Roger Holten, Brian Anderson, and Iyad Dajani "Multi-kW coherent combining of fiber lasers seeded with pseudo random phase modulated light", Proc. SPIE 9728, Fiber Lasers XIII: Technology, Systems, and Applications, 97281Y (11 March 2016);

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