Patent ID: 7751715

Claim:
An optical transmission system which is provided with a master unit and a remote unit connected with each other via an optical fiber, and interactively transmits an optical signal modulated by a radio signal to be used for a radio area communication in a mobile communication system between the master unit and the remote unit, wherein the master unit includes an optical transmitter which transmits the optical signal to the optical fiber at a fixed averaged optical output level, the remote unit includes an optical receiver which light-receives the optical signal incoming via the optical fiber, and the optical receiver includes: an light receiving element which converts the optical signal into an electric signal to reproduce the radio signal; a variable gain amplifier which amplifies the reproduced radio signal at a gain corresponding to a value of a given adjustment voltage; a gain control circuit which is supplied power for a defined control period to be driven, controls the adjustment voltage so as to stabilize an averaged level of the reproduced radio signal to a defined value, and feed-forward-controls a gain of the variable gain amplifier; a shut off unit which shuts off supply of power to the gain control circuit when the control period ends; and a keeping unit which keeps a value of an adjustment voltage applied to the variable gain amplifier at the time when the supply of the power to the gain control circuit is shut off.