A wireless-optical interface device for converting a received wireless signal to a corresponding optical signal for transferring digital information from the wireless domain to the optical domain. The interface device includes an oscillator (e.g. a negative differential resistance oscillator such as a resonant tunnelling diode) capable of synchronisation with the wireless signal, and an optical output device (e.g. semiconductor laser) controllable by an output of the oscillator to provide the corresponding optical signal. The oscillator and the optical output device may be integrated on the same semiconductor chip. Also disclosed is a corresponding wireless-optical interface device for converting a received optical signal to a corresponding wireless signal for transferring digital information from the optical domain to the wireless domain.