Discrimination of Photon- and Dark-Initiated Signals in Multiple Gain Stage APD Photoreceivers

We demonstrate the ability of linear mode single carrier multiplication (SCM) avalanche photodiode (APD)-based optical receivers to discriminate single-photon-initiated avalanche events from dark-current-initiated events. Because of their random spatial origin in discrete regions of the depletion region, in the SCM APD the dark-generated carriers multiply differently than the photon-generated carriers. This causes different count distributions and necessitates different statistical descriptions of the signal contributions from photon- and dark-originating impulse responses. To include dark carriers in the performance models of the SCM APD, we considered the influence of the spatial origin of the ionization chains on a receiver's noise performance over the times the optical pulse is integrated by the receiver's decision circuits. We compare instantaneous (time-resolved) numeric and pseudo-DC analytical models to measured SCM APD data. It is shown that it is necessary to consider both the distribution of spatial origin and the instantaneous properties of the ionization chains to describe statistically an SCM APD receiver. The ability of SCM APD receivers to discriminate single photon events from single dark events is demonstrated, and the effective gain and excess noise contributions of the light- and dark-initiated avalanche events and their influence on receiver sensitivity and signal-to-noise characteristics are shown.