Experimental verification and theory for an eight-element multiple-aperture equal-gain coherent laser receiver for laser communications.

The detection and processing of laser communication signals are affected by the fading induced onto these signals by atmospheric turbulence. One method of reducing this fading is to use an array of detectors in which each of the detector outputs are added together coherently. We present experimental verification and theory of a 1.06 μm eight-element coherent receiver used to mitigate the effects of fading over a 1-km outdoor range. The carrier-to-noise ratio (CNR) was measured on a single channel and was then compared with the CNR obtained from the coherent sum of the eight channels. The increase of the mean CNR for the coherent sum as compared with a single aperture was observed proportional to the number of the apertures under different conditions of atmospheric turbulence. The measured mean CNR gain fitted the theoretical prediction well when the laser intensity fluctuations followed the gamma distribution.