Abstract

The signal to noise ratio expected theoretically in coherent, infra-red, heterodyne systems has been well documented for many years. In practice however it is usually accepted as a general rule that up to an order of magnitude degradation of experiment with theory will be found. At the last Aspen conference on coherent laser radar systems in 1980 the closest agreement claimed by any worker in the field was a loss factor of ~ 4 (discussion session). Just such a loss factor was reported in an early paper by Brandewie and Davis¹ with a cw system and in more recent calibrations by several workers²,³. It has been suggested that these discrepancies, between predicted and observed signal to noise ratios, might be due at least in part to assumption of over optimistic values for the quantum efficiency of the detector, together with various optical mismatching and beam aberrations which could reduce heterodyning efficiency. An additional problem, which also renders difficult the comparison between the results of different groups, is the lack of reproducible, well calibrated scattering targets. We have recently had available surfaces of cut polystyrene and flame sprayed aluminium for which the scattering characteristics have been well established. Accordingly we have conducted a detailed examination of all the parameters entering the evaluation of signal to noise ratio with a two-fold aim: firstly to determine whether a discrepancy did exist that could be explained by factors not presently taken account of in the theoretical treatment, and secondly to provide an absolute calibration of our CO₂ laser heterodyne system so that the measured signal to noise ratios could be translated into absolute backscattering strengths.

© 1983 Optical Society of America