In this paper, a theoretical comparison is made of the mean carrier-to-noise ratio (CNR) for a coherent (heterodyne detection) equal gain (EG) optical array receiver system with that predicted by a conventional single-aperture monolithic coherent detector system. Our analysis shows that the mean CNR for an EG array receiver system improves significantly over that of a single aperture system. Experimental data taken from a recent outdoor experiment over a range up to 1 km between target and transceiver are also presented and compared with the theory for a particular eight-element EG system developed at the University of Central Florida. Optical signals received by the EG receiver array are launched into eight single mode optical fibers. Phase compensation between the individual receivers is accomplished by wrapping the fiber around PZT cylinders that are controlled by phase compensating electronics.
The detection and processing of laser communication signals are drastically 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. This requires measuring the phase difference between each of the receivers and co-phasing each of the detector outputs. This paper presents experimental verification at the Innovative Science and Technology Experimentation Facility over an outdoor range of a 1.06 micron eight element coherent receiver used to mitigate the effects of fading. The system is composed of a 60 mw Nd:Yag laser used as the transmitter and a 27 MHz AO modulator used to frequency shift the transmitted beam. The receiver is composed of eight 1 cm lenses launching the eight received optical signals into eight signal mode optical fibers. Phase compensation between each of the eight receivers is accomplished using single mode fibers wrapped around PZT cylinders that are controlled by phase compensating electronics. The carrier-to-noise (CNR) ratio was measured on a single channel and was then compared with the CNR obtained from the coherent sum of the eight channels. The improvement of the CNR for the coherent sum as compared to a single channel was then compared against theoretical predictions.