Coherent beam combining of two W-level fiber amplifiers in turbulence atmospheric environment based on stochastic parallel gradient descent algorithm

We demonstrate coherent beam combining of two W-level fiber amplifiers based on stochastic parallel gradient descent (SPGD) algorithm at 8 m distance in a simulated turbulence atmospheric environment. A photodetector was used to get the coherent optical intensity of the main-lobe from a pinhole, based on the intensity and the SPGD algorithm, the phase controlling was performed by the digital signal processor. In order to simulate the atmospheric environment, turbulence was induced in the free space of the light path by fans and air-conditions in the lab. Experimental result shows that the system performs well for long-time both with and without the simulating turbulence in close-loop, combining efficiency as high as 84.25% with turbulence and 84.85% without turbulence were realized. Visibility increased from near zero in open-loop to 0.432 with turbulence and 0.505 without turbulence in close-loop and the residual phase error is controlled to be less than lambda/18. The probability of energy encircled in the main-lobe to be more than 70% of its ideal value was increased from 18.66 to 93.71% without turbulence and from 14.80 to 92.49% with turbulence when the system evolves from open-loop to close-loop.