A Mach-Zehnder interferometer modulator for modulating a beam of laser light includes a pair of separate waveguides through which the laser light is passed after splitting in a splitting zone and after which the light is recombined in a merge zone. The waveguides are formed in a semiconductor material with one of the electrodes of each pair being formed in a doped layer while the other electrode, the top electrode, is a surface metalisation. The doped layer is trenched so that adjacent electrodes in the doped layer are electrically isolated from one another so that one of the electrodes in the doped layer can be connected with a different electrical polarity to the other electrode in the doped layer thereby permitting the connection of the pairs of electrodes in parallel anti-phase mode.