Receiver sensitivity

Receivers are integral part of a long distance fiber optic communication system.  A receiver includes photodetectors such as Avalanche photodiode, positive-intrinsic-negative semiconductor photodiode etc., demodulators and couplers. Receiver sensitivity is more for an optical receiver when it achieves the same performance with less optical power incident on it. The three important factors which influences receiver sensitivity are bit-error rate (BER), minimum received power and quantum limit of photodetection. Bit-error rate is defined as the probability of incorrect identification of a bit by the decision circuit of the receiver. Minimum received power is a cut-off value below which receiver operation ceases. Use of avalanche photodiode improves receiver sensitivity. But excess noise factor may degrade receiver sensitivity. Quantum limit of photodetection in almost all practical optical receivers is more than 20 dB or exceeds 1000 photons.