A description of the nonlinear response of a HgCdTe photoconductor on the incident optical radiation is derived from a two-level rate equation for the photoexcited carriers. The model accounts for a limited number of electrons available for photoexcitation, and the possibility of photostimulated relaxation of an already excited photoelectron. The detector response as well as the level of incident optical power where saturation occurs is directly related to the physical constants of the detector material. The saturation of the detector is the same phenomenon as is known for the saturable absorbers—the detector material becomes transparent when the incident optical power exceeds the saturation power. Even though the description given here is applied to the HgCdTe detector, the general model can be applied to other detector materials as well. The parameters for the model are fitted to a thermoelectrically cooled HgCdTe detector and afterwards employed to predict the detector noise performance in a heterodyne setup. Unlike for liquid-nitrogen-cooled detectors, shot-noise-limited detection cannot be obtained with use of only thermoelectrical cooling (226 K) of the detector. However, the detector performance can be optimized to be able to perform Doppler measurements from aerosol backscatter by use of the model presented to optimize the applied optical power in the reference wave.
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