Development of an erbium-doped fibre laser as a deep-sea hydrophone

The present work deals with the development of a prototype of a hydrophone for deep-sea acoustic detection. The base sensitive element is a single mode fibre laser, realized within an optically pumped erbium-doped fibre. It was obtained by inducing an index modulation within the fibre core by illuminating it with ultra-violet coherent light through a phase-mask grating; in this way a couple of Bragg reflectors delimiting an optical cavity were realized. The emission wavelength depends on the cavity length and on the Bragg gratings' optical characteristics. The environmental conditions, in terms of temperature and static and dynamic pressure, modify these geometrical factors and optical parameters, inducing a wavelength shift of the optical signal. Fibre laser sensors were characterized both optically and acoustically within a closed tub in the laboratory, using a Mach–Zender interferometer and an electronic lock-in system allowing the transformation of the wavelength shift into amplitude variations, in order to greatly increase the sensitivity. The high sensitivity makes these sensors very suitable for a wide range of deep-sea acoustic applications, including geological surveys, marine mammal surveys and overall as acoustic sensors in the high energy cosmic neutrino underwater telescopes.