Abstract: A method for compensating a center wavelength of optical transmission of a planar lightwave circuit includes heating the planar lightwave circuit from a first set temperature to a second set temperature. The first set temperature is at least approximately a room temperature and at most approximately 500° C. The second set temperature is at least approximately 500° C. and at most approximately 900° C. The planar lightwave circuit is maintained at the second set temperature for a predetermined retention time. The planar lightwave circuit is cooled from the second set temperature to a third set temperature. The third set temperature is at least approximately a room temperature and at most approximately 500° C.

Abstract: A 2-input/2-output directional coupler is arranged as an element coupler/splitter for a first stage, and 1-input/2-output coupler/splitters are used as element coupler/splitters for a second stage and subsequent stages. Output ports of these element coupler/splitters are concatenated with input ports of other element coupler/splitters to form a 2-input/multi-output coupler/splitter. An optical signal can be inputted through one input port of the first-stage 2-input/2-output directional coupler, and an optical signal for optical line monitoring can be inputted through the other input port. It is unnecessary, therefore, to use an optical wavelength division multiplexer/demultiplexer or other optical part which conventionally is connected at the time of optical line monitoring.