Abstract
Optical receivers are either operated in the photon-counting mode or in the photoelectric-current mode. An ideal photon-counting detector measures the number of photons received within the counting time. Its performance is governed by the photon statistics. For Poisson statistics, the signal-to-noise ratio (SNR) equals the mean number of photons per counting time, SNR=n¯. For a detector with quantum efficiency η, SNR =ηn¯. The performance of detectors that measure the photoelectric current is limited by a combination of photon statistics and circuit noise. To eliminate circuit noise, devices with internal gain, such as the photomultiplier tube and the avalanche photodetector (APD) are used. Unfortunately, the gain mechanism itself introduces noise. Photon-counting receivers discriminate against gain noise by using a nonlinear filtering circuit that counts the electric pulse associated with each photoevent as a single count, regardless of its height. Thus they eliminate both circuit noise and gain noise and, ideally, achieve SNR =ηn¯.
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