In recent years, modulation methods such as Super channel and Nyquist channel, whose frequency utilization efficiency is high, have been put into practical use. In an optical network in which a modulation method whose frequency utilization efficiency is high is used, interference may occur between light signals when wavelengths of the light signals are not appropriately adjusted. In this case, the quality of signals degrades and there is a risk that the bit error rate increases. Therefore, it is desired that the wavelengths of light signals (that is, the optical frequencies) are accurately adjusted in the optical network.
A wavelength selection switch that switches a path of a light signal for each wavelength in optical signal layer is introduced in nodes inside the optical network, so that a request for improving the wavelength accuracy is increased.
Related arts are disclosed in, for example, Japanese Laid-Open Patent Publication No. 62-159929, Japanese Laid-Open Patent Publication No. 09-162849, and Japanese Laid-Open Patent Publication No. 09-252283.
However, in the related arts, the wavelengths used in the optical network may not be accurately adjusted. It is possible to solve this problem by providing an accurate light source to each transmission device in the optical network. However, in this configuration, the cost to build the optical network increases.