Patent ID: 7551857

Claim:
An optical transmission system comprising: a first terminal transmitting a wavelength division multiplex signal light amplified by a post-amplifier to a first transmission path; a second transmission path opposite to the first transmission path transmitting a signal light from a second terminal; and a plurality of repeaters provided in the first and the second transmission paths, having optical amplifiers amplifying the signal light from the first transmission path; wherein, when a fault in the first transmission path occurs between a first repeater located on an up-stream side and a second repeater located on a down-stream side and the second repeater detects this fault, the second repeater transmits fault information to the second transmission path; the repeaters located on the up-stream side from the first repeater for the first transmission path stop an amplifying operation of the signal light based on the fault information and transmit a stop request to the second transmission path; the first terminal stops transmission of the signal light on the basis of the stop request, controls the post-amplifier so that an intensity of an amplified spontaneous emission (ASE) light output by the post-amplifier can be detected by an input monitor of an optical amplifier located at a next stage and controls an intensity of an output light within a range satisfying a safety criterion, and transmits the ASE light to the first transmission path; and a repeater located on the up-stream side of the first transmission path from the first repeater controls, on the basis of the ASE light, an optical amplifier so that an intensity of the ASE light output by the optical amplifier can be detected with an input monitor of an optical amplifier located in a next stage and controls an intensity of an output light within a range for satisfying a safety criterion.