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 from satellite to earth (ground or airplane) occasionally involves clouds over part of the optical channel. Most of the energy of optical pulses propagating through thin clouds passes through the clouds. The propagating energy is concentrated around the center of the beam. The distribution of the energy relative to the center of the beam is not uniform. Using the received energy in an efficient way reduces the transmitter power needed for given bit error rate. The advantages of low transmitter power are less radiation exposure and greater immunity to eavesdropping. To use the received energy efficiently, a mathematical model of spatial widening of the optical beam is derived using Monte Carlo simulation. The simulation is carried out at three different wavelengths in the visible and the near IR. Important aspects of this work include the fact that (1) using shorter wavelengths such as 0.532 micrometers results in least spatial widening and maximal received power, and is thus preferable for optical communication, and (2) the mathematical model derived is a basis for adaptive communication with less transmitted energy consumption.