Fiber-Bragg-grating WDM underwater acoustic sensor with directivity

An optical fiber sensor with two fiber Bragg gratings in serial connection which is multiplexed in an optical wavelength domain is constructed to sense an acoustic field in water. Both the gratings are inserted between optical isolators in order to stabilize the sensor by suppressing Fabry-Perot interference in fiber that is caused by small reflections from fiber end facet, connectors and an optical coupler. The two gratings in the sensor are confirmed to work independently and have little influence on each other. Comparison of the experimental data to the theoretical prediction shows that the developed sensor can detect the relative phase between two points in the acoustic field and thereby determine the direction of the acoustic source. Since a fiber Bragg grating can make an optical sensor of high sensitivity and wide dynamic range and can be easily multiplexed with an arrayed structure in either one or two dimension, it will then offer a convenient tool to measure distribution of amplitude and phase of an acoustic field in water.