Homodyne detection of infrared radiation from a moving diffuse target

Experiments have been performed in which the radiation from a CO2laser was coherently detected after being scattered from a moving diffuse reflector. This is generally the configuration of an infrared laser radar. The power-spectral-density of the heterodyne signal was measured and its width was shown to agree with the calculated value based on a theoretical model for the process. Expressions are obtained for the ratio of heterodyne signal bandwidth to heterodyne frequency for the cases of focused radiation, unfocused radiation, and for a typical radar configuration. In most cases, the heterodyne signal is found to possess a narrow-band character. The probability density of the signal envelope was also measured for a known scatterer (providing Gaussian scattered radiation) and was found to be Rayleigh distributed, as expected. The power-spectral-density and envelope probability distribution provide information about a scattering medium or target which cannot be obtained from average-value measurements of the heterodyne signal-to-noise ratio. This information is useful for communications applications, infrared radar, and heterodyne spectroscopy experiments. Finally, a simple and direct method of obtaining information about the statistics of an incident radiation field (which does not involve photocounting) is discussed.