Demodulation of low-level broad-band optical signals with semiconductors

A simple approach to the problem of detection of optical signals with semiconductors leads to a general understanding of the performance. It shows that the low impedance level of broad-band circuits prevents high efficiency conversion of the incident power. Because of this, the envelope detector has a signal-to-noise ratio (SNR) which is determined by the noise of the following amplifier. As in radio communication, addition of a coherent optical signal from a local oscillator can increase SNR; however, for optical signals the limiting value of the noise figure is determined by the shot noise of the conversion current. Comparison with the limiting SNR expected from a photo-emission detector reveals no important distinction between them for frequency conversion. The unit which uses material with the higher quantum conversion efficiency will have the possibility of better SNR. For envelope detection, the photo emitter can be far superior to the photoconductive detector. The two cases of a photovoltaic and a photoelectromagnetic solid-state device are analyzed and general expressions as well as numerical examples given for the design parameters. The devices are comparable both in dimensions and performance.