Heterodyne experiments have been performed in the middle infrared region of the electromagnetic spectrum using the CO2 laser as a radiation source. Theoretically optimum operation has been achieved at kHz heterodyne frequencies using photoconductive Ge:Cu detectors operated at 4o K, and at kHz and MHz frequencies using Pb1-Snx.Se photovoltaic detectors at 77°K. In accordance with the theory, the minimum detectable power observed is a factor of 2/n greater than the theoretically perfect quantum counter, hvΔf. The coefficient 2/n varies from 5 to 25 for the detectors investigated in this study. A comparison is made between photoconductive and photodiode detectors for heterodyne use in the infrared, and it is concluded that both are useful. Heterodyne detection at 10.6 μn is expected to be useful for communications applications, infrared radar, and heterodyne spectroscopy. It has particular significance because of the high radiation power available from the CO2 laser, and because of the 8 to 14 μm atmospheric window. Copyright © 1968 by The Institute of Electrical and Electronics Engineers, Inc.
