The coupling of optical power between a fibre, side polished close to the core, and a planar multimode waveguide overlay is highly wavelength selective. Vacuum deposited films and polished lithium niobate have been used in the role of the overlay and a wavelength resolution of 3nm (with subnanometre feasibility) has been demonstrated with losses of less than 0.2dB. Electro-optic tunability in the region of a few nanometres was also observed for the lithium niobate devices.