Switchable optical filter

This invention relates to optical waveguide filters and, in particular, to a switchable optical filter particularly useful for gain equalization, for adding and dropping optical channels, and for optical cross-connection. A filter comprises a Mach-Zehnder interferometer (11) comprising a pair of waveguide arms (12,13) extending between a pair of couplers (14,15). The arms include a series of phase shifters (ϕ1...ϕN+1) in at least one arm separated by reflective filters (Λ1...ΛN) on both arms. By adjusting the phase shifters, the reflection by the filter of signals at wavelengths corresponding to the reflective filters can be adjusted independently.