Patent ID: 8351790

Claim:
An optical transmission system comprising: first, second, and third terminal stations each transmitting and receiving a WDM light formed by combining a plurality of lights having different wavelengths; a main transmission path that bi-directionally couples the first terminal station with the second terminal station; and an optical add/drop multiplexer disposed on the main transmission path, the optical add/drop multiplexer branching light having a certain wavelength from a WDM light input from the main transmission path on the side of the first terminal station and outputting the branched light to a branch transmission path coupled with the third terminal station, the optical add/drop multiplexer combining a light input from the branch transmission path with lights included in the WDM light other than the light having a certain wavelength, and outputting the combined light to the main transmission path on the side of the second terminal station, the optical add/drop multiplexer branching light having a certain wavelength from a WDM light input from the main transmission path on the side of the second terminal station and outputting the branched light to the branch transmission path, and the optical add/drop multiplexer combining a light input from the branch transmission path with lights included in the WDM light other than the light having the certain wavelength and outputting the combined light to the main transmission path on the side of the first terminal station; the optical add/drop multiplexer comprising: an input determination unit and an optical path switching unit, wherein the input determination unit detects the power of the light input from the main transmission path on the side of the first terminal station, the power of the light input from the main transmission path on the side of the second terminal station, and the power of the light input from the branch transmission path and determines whether each of the input lights is lost or not, and wherein, if the input determination unit determines that any one of the input lights is lost, the optical path switching unit forms a loop-back route by decoupling an optical path through which an output light output to the transmission path corresponding to the input light is propagated from the transmission path and by coupling the optical path to the opposing optical path to which the input light is provided from the transmission path.