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The general expressions of the autocorrelation function and the output SNR of a photodetector are derived for a coherent laser either amplitude of intensity modulated by a stationary Gaussian random modulating signal in the presence of background light. Two types of the modulationg signal are considered: the baseband and the bandpass modulating signals. The effects of the center frequency of the modulating signal, the modulation degree, the effective average quantum rate, the wave interaction noise, the shot noise, the modulation method, and the input CNR on the output SNRs are discussed. It is shown that the output SNRs increase as the modulation degree, the effective average quantum rate. and the input CNRs become large. The bandpass modulating signal is better than the baseband modulating signal when the effective average quantum rate and input CNRs are large. It is also shown that the intensity modulation is superior to the amplitude modulation only for the baseband modulating signal case with large effective average quantum rate and large input CNR. This paper gives results useful for the field of laser communication.

Output Signal-to-Noise Ratios for Amplitude and Intensity Modulated Lasers