A layer (14) of optically active material overlaps the mode field of a single-mode optical waveguide (10) and has regions (16, 17) to which electric fields (E1, E2) can be applied by way of respective electrodes (18, 19) so as to vary the refractive index of the layer (14) in those regions. The regions (16, 17) are spaced apart longitudinally of the waveguide (10), and the fields (E1, E2) are so arranged that they extend at different angles to an interface (13) between the waveguide (10) and the layer (14) and act sequentially upon different polarisation components of radiation propagating along the waveguide (10). In one arrangement, the fields (E1, E2) are orthogonal to one another. In an alternative arrangement, a third field (E3) can be applied to a further longitudinally-spaced region of the layer (14) and the three fields (E1, E2 and E3) are arranged at (120°) to one another.