Coherent combining of fiber-laser-pumped frequency converters using all-fiber electro-optic modulator for active phase control

Coherent beam combining (CBC) by active phase control could be useful for power scaling fiber-laser-pumped optical frequency converters like OPOs. However, a phase modulator operating at the frequency-converted wavelength is needed, which is non standard component. Fortunately, nonlinear conversion processes rely on a phase-matching condition correlating, not only the wavevectors of the coupled waves, but also their phases. This paper demonstrates that, using this phase correlation for indirect control of the phase, coherent combining of optical frequency converters is feasible using standard all-fibered electro-optic modulators. For the sake of demonstration, this new technique is experimentally applied twice for continuous wave second-harmonicgenerator (SHG) combination: i) combining 2 SHG of 1.55-mu m erbium-doped fiber amplifiers in PPLN crystals generating 775-nm beams; ii) combining 2 SHG of 1.064-mu m ytterbium-doped fiber amplifiers in LBO crystals generating 532-nm beams. Excellent CBC efficiency is achieved on the harmonic waves in both these experiments, with lambda/20 and lambda/30 residual phase error respectively. In the second experiment, I/Q phase detection is added on fundamental and harmonic waves to measure their phase variations simultaneously. These measurements confirm the theoretical expectations and formulae of correlation between the phases of the fundamental and harmonic waves. Unexpectedly, in both experiments, when harmonic waves are phase-locked, a residual phase difference remains between the fundamental waves. Measurements of the spectrum of these residual phase differences locate them above 50 Hz, revealing that they most probably originate in fast-varying optical path differences induced by turbulence and acoustic-waves on the experimental breadboard.