Coherent combination of multiple laser amplifier is an important technique for high power and high beam quality laser. Laser amplifier uses master oscillator power amplifier (MOPA) configuration for narrow bandwidth and high beam quality laser. Then active phase control is used to make multiple laser amplifier phase synchronization for coherent spot in the far field. The center spot is N times brighter than the one of incoherent combination. At present, researchers have used this method to achieve 1.56kW coherent combination of nine fiber lasers and 105kW of seven slab lasers. The phase noise of the laser amplifier is an important factor affecting the coherent combination. There are two key technologies in the coherent combination. The one is fill factor of multiple laser spatial distribution, which determines main lobe energy of coherent spot. And the other is the phase noise of the laser amplifier, which decides stability of the coherent combination. The phase noise of laser amplifier is caused by many factors, mainly thermal disturbance and mechanical vibration. Due to the complexity of the phase noise generation, the research on phase noise is generally concentrated in the qualitative analysis. The phase noise is generally considered the time phase noise. It is phase dithering from variation refractive index by external disturbance. A variety of factors, such as temperature changes, mechanical vibrations, the pump power, affect phase noise. We establish an externally imposed discrete disturbance source on fiber laser amplifier and then analyze above-mentioned factors independently and quantitatively by the method. The experiments demonstrate space phase noise, dithering of beam direction, is simultaneously induced by either mechanical vibration or thermal disturbance. This experiment is significant for understanding mechanism of the phase noise and suppression of phase noise.
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