A tunable mixer comprising: a solid state device formed of a (non-linear) material having a second or higher order electrical susceptibility, and configured to receive an input light signal having a principle modal frequency; and a grating, provided as a series of a plurality of grating elements, arranged to provide distributed feedback within the device such that the mixer is electrically controllable: to add, relative to the principle modal frequency, a sideband mode to the input light signal at any selected one of at least two respective sideband modal frequencies; and to output the resulting signal.