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Coherent combination is one of the most promising ways to realize high power laser output. A three- laser-beam coherent
combination system based on adaptive optics (AO) technique has been set up in our laboratory. In this system, three
1064nm laser beams are placed side-by-side and compressed by two reflective mirrors. An active segmented deformable
mirror (DM) is used to compensate the optical path difference (OPD) among three laser beams. The beams are
overlapped onto a 2900Hz CCD camera to form an interference pattern while the peak intensity of the interference
pattern is taken as the cost function to optimize by a stochastic parallel gradient descent (SPGD) algorithm. SPGD
algorithm is realized on a RT-Linux dual-core industrial computer. A series of experiments have been accomplished and
experimental results show that both static distorted aberrations in the beams and active distorted aberrations (which are
brought in by a hot iron and the frequency is about 5Hz) can be compensated successfully when the gain coefficients and
the perturbation amplitude of SPGD are chosed appropriately, thereby three beams can be well combined. For controlling
the phase of fiber lasers, the phase characteristics of beams passing through Yb-doped dual-clad fiber amplifier are
measured by means of investigating the interference pattern under different output power through experiments. The
frequency of phase fluctuation is evaluated through analyzing the fluctuation of power within a 90um aperture of
far-field focal spot. Experimental results show that the phase fluctuation frequencies of laser beam transmitted through
fiber amplifier are mainly in the range of 100~1500Hz. As a result, to control the phase fluctuation of beams passing
through fiber amplifier, the bandwidth of any potential phase control scheme must be greater than 1.5 kilohertz.
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