Patent ID: 7142785

Claim:
An apparatus comprising: a wavelength selection filter including first, second, third and fourth ports, the first port receiving a wavelength division multiplexed (WDM) light including a plurality of lights at different wavelengths multiplexed together, the second port receiving an insertion light, and, in accordance with a surface acoustic wave generated in the wavelength selection filter via a selection signal applied to the wavelength selection filter, the wavelength selection filter branches a respective light from the received WDM light at a wavelength corresponding to a frequency of the surface acoustic wave, outputs the branched light from the third port and outputs the insertion light and the WDM light having said respective light branched therefrom from the fourth port; and an optical coupler, arranged in front of the first port, branching a portion of the WDM light; and a spectrum monitor monitoring a spectrum of the branched portion and a spectrum of light output from the fourth port, and monitoring whether the wavelength selection filter is operating properly in accordance with the monitored spectrums.