Conventionally used optical transmission systems transmit and receive optical signals among optical transmission devices having optical cross-connects (OXCs) by using the wavelength division multiplexing (WDM) method. In an optical transmission system of the WDM method, optical signals from each route are cross-connected in optical switches that make up the OXCs. Representative configurations of an optical transmission system of the WDM method include the broadcast and selector (BS) type in which input WDM signals are demultiplexed by a splitter and optical signals to be output are multiplexed by a wavelength selective switch (WSS), and the route and selector (RS) type in which input WDM signals are demultiplexed by a WSS and signals to be output are multiplexed by a WSS. In the BS type, in order to monitor input WDM signals by using an optical channel monitor (OCM), optical transmission devices have a 2×1 optical switch mounted at the stage prior to the OCM, and switch between monitored ports and monitor the wavelengths of input WDM signals. In contrast to this, in the RS type, in addition to monitoring the wavelengths of the input WDM signals, optical transmission devices also monitor optical signals that are multiplexed by a WSS and optical signals that are demultiplexed by a WSS. An RS type that uses WSSs for demultiplexing has less signal loss compared to a BS type that uses splitters for demultiplexing, and is advantageous for realizing a colorless, directionless, and contentionless (CDC) function.
In recent years, configurations in which the BS type and the RS type coexist within one optical transmission system have also been proposed. In an optical transmission system in which the BS type and the RS type coexist, optical transmission devices use a multi-input optical switch at the stage prior to an OCM in order to monitor optical signals from each port including also optical signals that are output. Thus, optical transmission devices are able to monitor the optical signals of each channel while individually selecting optical signals.
Japanese Laid-open Patent Publication No. 2010-245993 is an example of related art.