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Coherent beam combining of lasers: toward wavelength versatility and long-range operation compliance
Fortunately, nonlinear conversion processes rely on a phase-matching condition, correlating not only the wave-vectors of the coupled waves, but also their phases. It is therefore possible to control the phase indirectly, using more standard phase modulators.
Feasibility of this technique was previously demonstrated for second harmonic generators (SHG). Controlling the phase of the fundamental wave, excellent harmonic wave combining efficiency was achieved in both cases of phase matching and quasi phase matching, with lower than λ/30 residual phase error.
In this paper, coherent combining of difference frequency generators (DFG) is experimentally tested. Even if DFG is more challenging than SHG as it implies handling three waves instead of two, phase control of the sole 1-μm pump waves is sufficient to combine the 3.4-μm waves generated.
The mid-infrared DFG crystals are pumped and signal-seeded with standard all-fiber sources at 1 μm and 1.5 μm respectively. Phase control is performed with an electro-optic phase modulator which is a standard all-fiber component operating at 1 μm.
CBC of mid-infrared DFG modules is a first step towards combining continuous wave OPOs.
To reach long range within a short acquisition time, coherent wind Lidars require high power (~kW), narrow linewidth (few MHz) pulsed laser sources with nearly TF limited pulse duration (~1μs). Eyesafe, all-fiber laser sources based on MOPFA (master oscillator, power fiber amplifier) architecture offer many advantages over bulk sources such as low sensitivity to vibrations, efficiency and versatility. However, narrow linewidth pulsed fiber lasers and amplifiers are usually limited by nonlinear effects such as stimulated Brillouin scattering (SBS) to 300W with commercial fibers. We investigated various solutions to push this limit further. For example, a source based on a new fiber composition yielded a peak power of 1120W for 650ns pulse duration with excellent beam quality. Based on these innovative solutions we built a Lidar with a record range of 16km in 0.1s averaging time.
In this proceeding, we present some recent results obtained with our wind Lidars based on these high power sources with record ranges. EDR measurements using the developed algorithm based on structure function calculation are presented, as well as its validation with simulations and measurements campaign results.
For the sake of demonstration, this new technique is experimentally applied twice for continuous wave second-harmonic-generator (SHG) combination: i) combining 2 SHG of 1.55-μm erbium-doped fiber amplifiers in PPLN crystals generating 775-nm beams; ii) combining 2 SHG of 1.064-μm 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 λ/20 and λ/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 fundamen tal 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.
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