An optical apparatus includes a waveguide-based frequency-selective structure with one or more input ports and N1 output ports where N1 is 2 or more. If input signals in a pair of input signals are separated in frequency by an integer multiple of the structure's free spectral range, and one of the pair is routed onto one output port, the other in the pair is also routed onto that port. The apparatus also includes: N2 waveguides, where N2