Abstract: An optical signal modulation device comprises first and second optical waveguides (1, 2) fabricated on Z cut lithium niobate. The waveguides (1, 2) are connected at one end to an optical signal splitter (7) and at the other end to a Y coupler (11). The output of the Y coupler (11) is connected to a single mode optical fibre (13). A first pair of modulator electrodes (3, 4) are positioned across the waveguides (1, 2) to cause relative phase shifts between signals in the two optical waveguides (1, 2) in response to a first control signal V.sub.1. These relative phase shifts are caused in both TM mode and TE mode signals. The device also includes a second pair of bias electrodes (14, 15) arranged to cause a relative phase shift between signals of the TE mode only whereby the transfer characteristics of the two modes may be shifted relatively to one another in use in accordance with the second control function.