An optical device comprises an optical fibre directional coupler having at least a first optical fibre (10) optionally coupled at a coupling region (30) to a second optical fibre (20) such that light can propagate in the coupling region in at least two possible electromagnetic transmission modes; and means (40, 50, 60) for transferring energy between the modes by a spatially periodic perturbation (e.g. a flexural acoustic vibration or an electric field generated by a pair of interdigital electrodes) of at least a part of the coupling region.