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7 March 2014 Coherent combining of SHG converters through active phase control of the fundamental waves
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Coherent beam combining (CBC) by active phase control has proven to be an efficient and scalable way to increase the output power delivered by fiber laser systems. This study investigates the potential of this technique when applied to another type of laser sources: nonlinear optical frequency converters. Given that the efficiency of nonlinear conversion processes, here Second Harmonic Generation (SHG), relies on a phase-matching condition between the fundamental and the harmonic waves, an indirect control of the phase of an SHG beam through the fundamental wave is theoretically possible. This paper experimentally demonstrates such indirect phase control and its application to CBC of frequency converters. Two continuous-wave 1.55-μm fiber amplifiers are frequency doubled in PPLN crystals to generate 775-nm beams. These SH beams are coherently combined using frequency-tagging active phase control. A standard fibered electro-optic modulator (EOM) is used to control the phase of one of the 1.55-μm fiber amplifiers. This EOM provides both phase modulation for frequency-tagging and proper phase shifts to compensate for the phase fluctuations of the other 1.55-μm amplifier. Efficient coherent combining of these two 775-nm beams is successfully achieved: a λ/19.5 residual phase error is measured.
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A. Durécu, C. Aubert, G. Canat, J. Le Gouët, L. Lombard, and P. Bourdon "Coherent combining of SHG converters through active phase control of the fundamental waves", Proc. SPIE 8961, Fiber Lasers XI: Technology, Systems, and Applications, 89611Y (7 March 2014);

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