OPTICAL SIGNAL AMPLIFIER

摘要

PROBLEM TO BE SOLVED: To reduce an unnecessary output voltage at an output which is generated because an unnecessary photocurrent flows to a photodetector for detecting signal light and is inputted to a differential amplifier circuit as a signal source. SOLUTION: A 2nd photodetector 11 is placed at a position not on the optical axis 2 of signal light, the 2nd photodetector 11 produces a photocurrent 12 such as a dark current and a photocurrent due to stray light 7 nearly equal in amount to that of an unnecessary photocurrent 4 from a 1st photodetector 3. The unnecessary photocurrent 4 from the 1st photodetector 3 and the unnecessary photocurrent 12 from the 2nd photodetector 11 are added while inverting the polarity of the currents to each other so as to cancel each other and the added current is given to and amplified by a differential amplifier circuit 5 as a signal source to be amplified. Thus, the unnecessary output voltage due to the unnecessary photocurrent 4 from the 1st photodetector 3 to detect the signal light can be canceled or reduced.