Plastic Coated Fiber Bragg Gratings As High Sensitivity Hydrophones

The authors reports on the use of fiber Bragg grating for underwater acoustic sensing. The Bragg wavelength shifts as a consequence of the strain induced in the fiber when a sound pressure is applied. In order to improve the behavior of the FBG-based hydrophone in terms of sensitivity and bandwidth, FBGs have been coated with proper materials, characterized by different elastic modulus (much lower than the fiber one), dimensions and shapes. The minimum detectable acoustic pressure has been found to be of the order of ~10 Pa in the investigated frequency range, with excellent performances in terms of linear response and wide dynamic range. The experimental analysis also reveals that, by a proper design of the coating features, sensor bandwidth and sensitivity can be tailored for specific applications. Thanks to the simplicity of its operating principle, geometry and interrogation scheme, these hydrophones can provide an alternative to the current state of the art in piezoelectric hydrophone technology.