FREE-SPACE OPTICAL COMMUNICATION - ANALYSIS OF SPATIAL WIDENING OF OPTICAL PULSES FOR PROPAGATION THROUGH CLOUDS

As part of a communication channel, clouds cause spatial widening and attenuation of optical pulse power. Free-space optical communication f rom satellite to earth (ground or airplane) occasionally involves clou ds over part of the optical channel. Most of the energy of optical pul ses propagating through thin clouds passes through the clouds. The pro pagating energy is concentrated around the center of the beam. The dis tribution of the energy relative to the center of the beam is not unif orm. Using the received energy in an efficient way reduces the transmi tter power needed for given bit error rate. The advantages of low tran smitter power are less radiation exposure and greater immunity to eave sdropping. To use the received energy efficiently, a mathematical mode l of spatial widening of the optical beam is derived using Monte Carlo simulation. The simulation is carried out at three different waveleng ths in the visible and the near IR. Important aspects of this work inc lude the fact that (1) using shorter wavelengths such as 0.532 mu m re sults in least spatial widening and maximal received power, and is thu s preferable for optical communication, and (2) the mathematical model derived is a basis for adaptive communication with less transmitted e nergy consumption.