PURPOSE: To provide an optical feedback type photodetector which has high S/N ratio and a wide dynamic range and can respond at a high speed by suppressing noise.
CONSTITUTION: A semiconductor laser 120 externally emits a light modulated by an optical intensity in a frequency fo based on the modulation drive signal from a drive signal output circuit 110, emits an element 130 to be measured via an optical system 140, and feedbacks to input the light reflected on the element 130 to be measured to the laser 120 via the system 140. The laser 120 having an optical amplifying function varies the optical intensity of the light according to the return light. A photoconductive photoreceiver 160 applied with the modulation voltage signal of the frequency fo from a voltage applying unit 150 receives the emitted light of the laser 120 in which the intensity is varied according to the return light, and synchronously detects it. The modulation current signal flowing to the photoreceiver 160 is converted into a voltage signal by a current-voltage converter 170, etc., and its low frequency component is extracted and output.
SHIRAKAWA KOICHI
TAKESHIMA AKIRA
