An apparatus (32) includes an electronic circuit (76, 80, 84), an electro-acoustic transducer (60) and a coupler (64). The electronic circuit is configured to receive data to be transmitted over an optical cable (24), and to convert the data into a modulating signal. The electro-acoustic transducer is configured to convert the modulating signal into an acoustic wave. The coupler is mechanically coupled to a section of the optical cable, and is configured to apply to the section a longitudinal strain that varies responsively to the acoustic wave, so as to modulate the data onto an optical carrier traversing the optical cable.