ON THE MUTUAL COHERENCE FUNCTION OF AN OPTICAL WAVE IN A TURBULENT ATMOSPHERE
RAND CORP SANTA MONICA CA
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Part of an overall study of phenomena that affect the performance of optical or infrared reconnaissance and guidance equipment, the present work shows that the most commonly used expression for the mutual coherence function for an optical wave propagating in a turbulent atmosphere is, in general, incorrect. This expression is based on an unphysical extrapolation of the Kolomogorov spectrum. In modifying the spectrum, however, the corrections to the coherence function, the implied resolution, spreading of a finite beam, and the signal-to-noise ratio using heterodyne detection, can be considerable. Approximate expressions for the coherence function, valid over three distinct propagation distance regimes, are derived. The field is shown to have a greater transverse coherence length than that predicted by other researchers, the difference being more pronounced at shorter ranges. Thus, under many conditions, the performance degradation of devices that depend on the above properties is less than previously reported.