connections to star couplers 14 2 14 k 1 have also been omitted for simplicity for each of the star couplers 14 the inputs are so arranged that each receives an individual wavelength from the groups of different wavelengths produced by each of the input modules 10 and 12 each star coupler 14 operates in a manner well known per se so as to mix all the input signal trains and feed them to all of its outputs thus considering for example star coupler 14 1 a mixture of all of its inputs is applied to n outputs namely to each of the output modules 15 1 15 n 16 1 16 n 39 the nature of each of the output modules will now be described with reference to fig3 referring to fig3 c the structure of each optical output module 15 is shown the module consists of k inputs 22 1 22 k which are individually connected to one output of each of the star couplers 14 1 14 k each input 22 is connected to a receptor in the form of a tunable filter 23 1 23 k which is individually tunable in order to select individual wavelengths from the mixture of signals applied to each input 22 commercially available tunable filters are.