Abstract: An acousto-optic device which is incorporated into a fiber optic system to define, based upon device configuration, an element such as a phase modulator, optical tap, frequency shifter or mode coupler. The device comprises an acoustic transmission member having a groove therein which is secured in the Hertzian contact along a continuous portion of its surface with an optical waveguide so as to communicate acoustic signals from a transducer affixed on the member through the Hertzian contact area and into the optical waveguide. In one preferred embodiment the member comprises a capillary tube having a bore that is substantially the same diameter as the optical waveguide to which the device is to be affixed. A portion of the capillary tube is removed to expose the base, thereby defining a groove along the capillary substrate. In one preferred embodiment an acoustic transducer is deposited on the curved surface opposite the groove to focus acoustic signals from the transducer toward the groove.