Receiver performance is calculated for two types of free-space atmospheric optical communications systems -a direct detection system using a p-i-n photodiode preceded by an optical amplifier, and an ideal coherent receiver operated in its shot-noise limited regime without an optical pre-amplifier. The signal format for both is amplitude shift keying (onoff modulation) with an assumed rectangular light pulse of duration T seconds. The more accurate Beckman probability density function (pdf) is used to describe the intensity fading rather than the more commonly used log-normal pdf which overestimates receiver performance. A coherent receiver using asynchronous detection is shown to outperform the direct detection system with an optical pre-amplifier gain of 30 dB by anywhere from 7 to about 10 dB depending on Rytov variance intensity fading levels. In addition, the effect of inner scale turbulence parameter, l 0 , on system bit error rate is demonstrated.

A design is presented which combines concepts of 2-D spatial light modulation with photorefractive coupling for a subsystem applicable to optical communications. Laser diodes are the transmitters of choice in laser spacecraft communications. However, the outputs from these devices are not directly usable in that massive bulky conventional optics are required to correct and direct a given beam. Furthermore, beam characteristics change with time and in some applications, due to channel effects, affecting SNRs and overall system efficiencies. Hence a means to not only replace heavy bulky optics with lightweight optical elements but to modify these optics in situ in real time is highly desirable.