Atmospheric modulation noise in an optical heterodyne receiver

Abstract

Atmospheric modulation noise in an optical heterodyne receiver, as measured by <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\sigma^{2}_{M}</tex> the normalized variance of the signal power, and by <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\langle(\Delta f)^{2}\rangle</tex> , the mean-square frequency spread of what would otherwise be a monochromatic signal, are evaluated in this paper. The analysis is based on the statistics of optical propagation in a randomly inhomogeneous medium and particularly on results for the wave-structure function. It is shown that to avoid a large signal-power variance, the receiver-collector diameter should be no larger than r <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> (r <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> is the diameter associated with saturation of receiver performance as measured by the average signal-to-noise ratio.) It is found that the rms frequency spread due to atmospheric effects is small enough that, even under worst conditions, it will not seriously affect the velocity resolution of an optical Doppler radar.