Optimum local oscillator levels for coherent detection using photoconductors

Use of an optical local oscillator with a photoconductor can modify its conductance sufficiently to have a considerable effect on both the effective responsivity of the detector and its interaction with the signal processing circuitry. These effects are analyzed and formulations for the signal-to-noise ratio (SNR) are derived for both constant voltage bias and constant current bias. It was found that the SNR peaks at a finite optimum local oscillator power for the voltage bias case. For the current bias case, there is a saturationlike effect rather than a true maximum when operating the photoconductor in the linear region and a true maximum when operating in the quadratic region.