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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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