Resonant Hydrophones Based on Coated Fiber Bragg Gratings For Underwater Monitoring

In this chapter, we report on recent numerical and experimental results obtained with Fiber Bragg Grating (FBG) hydrophones for underwater acoustic detection. The optical hydrophones under investigation consist of FBGs coated with ring-shaped polymers of different size. Via full-wave numerical simulations, we study the complex opto-acousto-mechanical interaction among an incident acoustic wave traveling in water, the optical fiber surrounded by the ring shaped coating, and the FBG inscribed the fiber. Our results fully characterize the opto-acoustic response of the optical hydrophone, and highlight the key role played by the coating in enhancing significantly its sensitivity, over the whole acoustic wave range of interest (up to 30 kHz) by comparison with a standard uncoated configuration. The coating material was selected and designed in order to provide mechanical amplification, via choice of its acoustic-mechanical properties. Our underwater acoustic measurements reveal the resonant behavior of these optical hydrophones, as well as its dependence on the coating size. By comparison with uncoated FBGs, responsivity enhancements were found, demonstrating the effectiveness of polymeric coating in tailoring the acoustic response of FBG based hydrophones.