An optical interference filter (2) is provided by a substrate (1) in which is formed a plurality of parallel slots so as to define layers (slots and substrate laminae) of alternating refractive index. As shown, the filter comprises a central Fabry-Perot resonator slot cavity (3) between two mirror groups (4, 5) of alternating slots and substrate laminae, and is coupled to optical fibers (7) accommodated in V-grooves (6) on either side. For tuning, one mirror group (5) may be movable in the direction of arrow (A), or, where the central resonator is provided by substrate material, by altering its temperature. One or more further Fabry-Perot filters may be formed in the substrate and coupled to the filter (2). The invention encompasses non-resonating filters.