Optical heterodyne detection of an atmospherically distorted signal wave front

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Abstract
The effect of atmospheric distortion of an optical wave front on the performance of an optical heterodyne detection system is examined theoretically. With the distortion measured by the phase and log-amplitude structure functions, an exact expression is derived relating the distortion to the detector signal-to-noise ratio. It is shown that because of distortion there is a limit to the achievable signal-to-noise ratio no matter bow large the detector collection aperture is. The minimum aperture diameter which will (nearly) achieve this limit, called the efficiency saturation dimension r0, is obtained in terms of the phase and log-amplitude structure fractions. Using the current theory for propagation in a turbulent medium, an expression for r0is obtained for a receiver in the atmosphere looking at a source above the atmosphere. The expression is in terms of the wavelength, the receiver altitude, and the zenith angle of the receiver line-of-sight.