Patent ID: 7877023

Claim:
A light detector for detecting loss of an optical signal in an optical communication system, the light detector comprising: a photoelectric conversion circuit which converts an input optical signal into an electrical signal; an amplifier which amplifies the electrical signal from the photoelectric conversion circuit to produce an amplified signal; a threshold adjustment circuit which outputs a threshold value based on which of respective signal information is present in the input optical signal; and an optical signal loss detection circuit which compares electrical amplitude of the amplified signal from the amplifier to the threshold value from the threshold adjustment circuit, and which outputs results of comparing the amplified signal to the threshold value, wherein the threshold adjustment circuit comprises a memory storing a plurality of threshold values uniquely determined in correspondence to the respective signal information that may be in input optical signals, a computation circuit which reads from the memory the threshold value corresponding to the signal information in the input optical signal, and which outputs the threshold value read as a digital value, a digital-to-analog converter which converts the digital value output from the computation circuit into an analog value, and which outputs the analog value to the optical signal loss detection circuit, and a plurality of switches which select a corresponding threshold value, according to which signal information is in the input optical signal, from a plurality of threshold values set in advance, and which output the threshold value selected to the optical signal loss detection circuit.