Abstract: A hydrophone has a mandrel with a shell and a cylindrical cavity inwardly adjacent to the shell. A passage provides fluid communication between the cylindrical cavity and an exterior environment surrounding the mandrel. The hydrophone further has an optical fiber having an optical sensing section that is at least partially wound on the mandrel. The optical sensing section has an optical characteristic that varies as a function of a radial dimension of the mandrel. The mandrel has a core of solid material having a bulk modulus lower than 0.1 GPa. The cylindrical cavity is between the core and the shell.