An optical interference modulator comprises a main input, a main output, an optical splitter connected to the main input, first and second MMI couplers, each with a first primary-end access port connected to the splitter; a second primary-end access port connected to the main output; a first secondary-end access port connected to a respective primary waveguide; and a second secondary-end access port connected to a respective secondary waveguide. A light reflector is arranged to reflect light incident from said primary and secondary waveguides back into the same respective waveguide such that light travelling through the respective waveguide from the respective secondary-end access port, after reflection, travels back to the same secondary-end access port. For the MMI couplers, at least one of the respective primary and secondary waveguides comprises a respective light phase modulating device arranged to modulate the phase of light travelling along the corresponding waveguide in either direction.