Abstract

Photoconductors can be used as optical heterodyne detectors and they have the advantage of lower cost and larger size. Much literature is available for the perfect case where the circuit connected to the detector has no effect on the response of the detector. For direct detection when the signal is very small these results may be satisfactory. However, application of an optical local oscillator in the milliwatt range to the photoconductor modifies its conductance sufficiently to have a considerable effect on both the responsivity of the detector and its interaction with the signal processing circuitry. Penin et al. [1] recognized that there was a local oscillator induced interaction with the circuit and derived a formulation for the noise factor of a photoconductive detector as a function of local oscillator power. Unfortunately, they did not recognize that the equivalent source term is also modified by the local oscillator; and consequently, their result is not correct. In this paper, the signal to noise ratio (S/N) is derived for a photoconductive detector as a function of local oscillator power and includes both effects. It will be found that there is a finite level of local oscillator power for which the S/N is a maximum.

© 1987 Optical Society of America