Patent ID: 7433600

Claim:
A system comprising: a closed loop provided by an optical fiber; and a plurality of optical node devices arranged along said closed loop; each of said plurality of optical node device comprising: a tunable wavelength selecting element adapted to input WDM signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths, said tunable wavelength selecting element having a function of dropping at least one optical signal from said WDM signal light and a function of adding at least one optical signal to at least one unassigned wavelength channel of said WDM signal light; and a wavelength selecting filter optically connected to said tunable wavelength selecting element for removing noise present in any bands other than a signal band of each optical signal passing through said tunable wavelength selecting element; said wavelength selecting filter comprising: an optical demultiplexer having an input port for inputting WDM signal light output from said tunable wavelength selecting element and N output ports for respectively outputting said N optical signals separated from said WDM signal light, and an optical multiplexer having N input ports for respectively inputting N optical signals output from said demultiplexer, and an output port for outputting WDM signal light obtained by wavelength division multiplexing said N optical signals input to said N input ports; wherein said transmission band of each of said optical demultiplexer and said optical multiplexer per wavelength channel is wider than the band of each wavelength channel of said WDM signal light; wherein transmission bands per wavelength channel of said optical demultiplexer and multiplexer have central wavelengths shifted from the central wavelength of each wavelength channel of said WDM signal light toward shorter wavelength and longer wavelength, respectively, or transmission bands per wavelength channel of said optical demultiplexer and multiplexer have central wavelengths shifted from the central wavelength of each wavelength channel of said WDM signal light toward longer wavelength and shorter wavelength, respectively.