Patent ID: 6873758

Claim:
An add-drop filter for filtering wavelength division multiplex information bearing radiation, said information bearing radiation comprising a plurality of communication channel radiation components, each component being associated with a respective communication channel and having a wavelength within a first range of radiation wavelengths and at least one supervisory radiation component associated with supervisory traffic and having a wavelength within a second different range of radiation wavelengths, the filter comprising: a) a main input for receiving the input information bearing radiation at the filter; b) a main output for outputting output information bearing radiation from the filter; c) at least one subsidiary input and at least one subsidiary output; d) primary filtering means through which the main input is coupled to the main output; e) secondary filtering means through which the primary filtering means is coupled to said at least one subsidiary input and subsidiary output; f) the primary filtering means being operative for extracting at least one preferred channel radiation component and said at least one supervisory radiation component from the input information bearing radiation and coupling said at least one preferred component and said at least one supervisory component to the secondary filtering means, and for receiving from the secondary filtering means combined radiation comprising at least one substitute channel radiation component corresponding to said at least one preferred radiation component and at least one substitute supervisory radiation component corresponding to said at least one supervisory radiation component and outputting the combined radiation together with the input information bearing radiation transmitted through the primary filtering means from the main input at the main output; and g) the secondary filtering means being operative for isolating from the radiation coupled to the secondary filtering means from the primary filtering means said at least one preferred and supervisory radiation components and coupling the latter to corresponding subsidiary outputs, and combining the substitute radiation components received at the secondary filtering means to generate the combined radiation.