Optical communication systems perform wavelength-division multiplexing (WDM) transmission in which a plurality of optical signals is transmitted with a plurality of optical channels having different wavelengths.  There is an optical communication system for switching between a currently used line and a backup line using the Optical Unidirectional Path Switched Ring (OUPSR) method to increase the reliability thereof. 
Switching between a currently used line and a backup line is performed when an optical receiving terminal included in an optical communication system cannot receive an optical signal owing to the failure of an optical channel.  Accordingly, in this case, the optical receiving terminal may receive light from an optical transmission path and detect the fault of the optical signal.  In a standard such as SONET, it is required to perform switching between a currently used line and a backup line within 50 ms of an optical signal fault in an optical channel. 
On the other hand, it is difficult to actually detect an optical signal fault.  In the case of optical communication using a transmission path formed of an optical fiber, in order to increase a transmission distance, each of an optical transmission terminal, an optical relay terminal, and an optical receiving terminal included in an optical communication system includes an optical amplifier for directly amplifying incident light.  The optical amplifier generates amplified spontaneous emission (ASE) light when amplifying a received optical signal.  Accordingly, since light received by the optical receiving terminal includes not only signal light but also ASE light, it cannot be determined whether received light is ASE light or signal light only by detecting light transmitted from an optical transmission path. 
Japanese Laid-open Patent Publication No.  2001-244900 discusses a technique for switching between a currently used line and a backup line.