A detector or a modulator for converting between optical and radio frequency signals comprising an optical guide (11 to 14) for propagating two optical signal components having frequencies that differ by an amount corresponding to a radio frequency and a microstrip radio signal guide (15, 16) for propagating a radio signal at the radio frequency, the microstrip radio signal guide being in travelling-wave coupling with an interaction one (14) of the optical guide comprising material in which interaction between the optical signal components and the radio signal occur. The microstrip radio signal guide element (15, 16) comprises an electrically conductive strip (15) juxtaposed with and extending along the interaction zone (14) on one side thereof and an electrically conductive ground plane (16) juxtaposed with and extending along the interaction zone (14) on an opposite side thereof. Transition radio signal guide elements (20,21) in the form of Vivaldi antennas extending transversely to the microstrip (15) connect the microstrip radio signal guide element (15,16) with a rectangular section wave-guide (18) and a matched load. Each of the transition radio signal guide elements (20, 21) comprises an opening of progressively varying width formed in the electrically conductive ground plane of the microstrip guide (16). Radio frequency signal resonators (24 to 27) extend the ends of the microstrip (15) and the small ends of the Vivaldi antennas.