Experimental verification and theory of CNR gain for an eight-element multiple-aperture coherent laser receiver

摘要

Abstract: The detection and processing of laser communicationsignals are drastically affected by the fading inducedonto these signals by atmospheric turbulence. Onemethod of reducing this fading is to use an array ofdetectors in which each of the detector outputs areadded together coherently. This requires measuring thephase difference between each of the receivers andco-phasing each of the detector outputs. This paperpresents experimental verification at the InnovativeScience and Technology Experimentation Facility over anoutdoor range of a 1.06 micron eight element coherentreceiver used to mitigate the effects of fading. Thesystem is composed of a 60 mw Nd:Yag laser used as thetransmitter and a 27 MHz AO modulator used to frequencyshift the transmitted beam. The receiver is composed ofeight 1 cm lenses launching the eight received opticalsignals into eight signal mode optical fibers. Phasecompensation between each of the eight receivers isaccomplished using single mode fibers wrapped aroundPZT cylinders that are controlled by phase compensatingelectronics. The carrier-to-noise (CNR) ratio wasmeasured on a single channel and was then compared withthe CNR obtained from the coherent sum of the eightchannels. The improvement of the CNR for the coherentsum as compared to a single channel was then comparedagainst theoretical predictions. !12