Ultra low noise fiber optic acoustic sensor for underwater applications

Abstract

Fiber optic sensor have many advantages over the conventional sensor technologies such as electromangnetic compatibility, light weight, exible geometric design, sensing signal at a remote point and no electrical signal at the signal detection point, easy sensor multiplexing,compatibility to fiber optic communication. Experimental studies have been available for underwater acoustic sensor with using fiber optics. In this thesis mandrel type interferometric fiber optic acoustic sensor design is analyzed in terms of noise performance characteristics and design parameters' contribution to noise performance is evaluated. Acoustic signals create phase change at the fiber optic acoustic probe and phase change is detected by using an interferometer. Experimental setup is used to emulate the design of the sensor and different components are used to evaluate the effect on the noise performance. Phase generated carrier method is evaluated for phase detection method. Finally noise measurement is done with a experimental setup emulating the design. Noise level is measured around -100 db re rad/VpHz and responsivity of the sensor(-140 dB re rad/VHzPa) is used to convert noise unit to pressure units and 40 dB re uPa=VpHz noise level in terms of pressure is obtained. This level of noise oor is close to sea sate 0 (SS0) noise level of the ocean. Sea state 0 is the state where there is no wind, wave or shipping traffic. This state is the lowest possible ambient noise available in the ocean and it is quite rare to find such state of the ocean. Therefore self noise of the designed fiber optic acoustic sensor is not the limiting factor for acoustic signal detection. Any signal over the ambient noise of the ocean can be detected using the designed fiber optic acoustic sensor.