The hydrophone has an interferometer structure incorporating a measuring arm in which is provided a very long monomodal optical fibre immersed in the interaction medium in which is propagated the acoustic wave. A phase displacement is induced on the optical wave by the elastooptical effect and said wave is propagated in the fibre by the acoustic wave which creates an acoustic pressure field in the medium. A reference arm establishes a reference optical path and the phase displacement linked with the acoustic wave is detected by interferometry between the two optical waves emerging from the two arms. The optical connections between the source, the optical fibre immersed in the interaction medium, the reference arm and the detector being provided in guided optics in a compact structure.