Pressure Tolerant Fiber Optic Hydrophone

An interferometric hydrophone is disclosed that comprises a first mandrel defining an interior that is open to surrounding fluid. A sensing optical fiber is wound upon the first mandrel. A second mandrel is positioned in surrounding relationship with respect to the first mandrel. The first and second mandrels define a first chamber there between. A case encloses the first and second mandrels and first chamber; The cylindrical case and the second cylindrical mandrel define a second chamber there between, which is sealed and filled with gas or vacuum.