Atmosphere Propagation and Communication Channel Model for Laser Wavelengths

The advent of the laser has generated much interest in its use for communication. Consideration is given in this paper to obtaining a communication channel model of the atmosphere for laser frequencies. For this purpose a fairly exhaustive, tutorial, and partly critical review of a major part of the extensive literature pertaining to theoretical and experimental results on the propagation of laser signals in the atmosphere is given. Some of the basic characteristics of the channel for Which detailed results are given are 1) channel time spread (or, equivalently, channel coherence bandwidth) due to multipathing and dispersion, 2) channel gain, 3) channel doppler spread and coherence time, 4) channel spatial coherence and amplitude covariance functions. Consideration is given to both point receivers and area receivers, heterodyne and direct detection receivers, nonplanar (i.e., spherical and beam) wave propagation, and infinite plane-wave propagation. The results are given for clear weather conditions. Brief attention is given to additive noise. In reviewing the extensive literature dealing with the propagation of optical signals through a turbulent medium, an attempt is made to put this varied, diverse, and in some instances controversial material into proper perspective; to note where theoretical results derived for one set of conditions were applied to experimental results obtained for another set of conditions; to correct some invalid conclusions drawn from some theoretical results as far as signal degradation effects and parameter variations; and to indicate where out-of-date vertical profile models were used with updated models being given, and where possible the new theoretical predictions based on these models provided. The paper should, as a result, be of help to those familiar with the literature as Well as those who are not. For those not acquainted with the literature, in addition to providing a fairly in-depth knowledge of the characteristics of propagati- on through a turbulent medium and an indication of the status of our understanding of this subject, the paper should provide a stepping stone for obtaining a more thorough understanding of the subject and for doing work in the field.