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The use of an optical local oscillator for coherent detection with a photodiode can significantly reduce the responsivity of the detector because of saturation effects. Consequently, local-oscillator shot-noise-limited operation of the detector may not be possible. This effect is analyzed and formulations are developed for the optimum optical local-oscillator power level and the resultant maximum possible signal-to-noise ratio in terms of parameters derived from the photodiode current versus the optical power response curve. An effective heterodyne responsivity that can be used as a part of the specifications when one is procuring photodiodes for use in coherent detection systems is defined.
Holmes et al. (Mon,) studied this question.