An underwater acoustic sensor using an optical fiber Bragg grating (FBG) in the reflection mode is examined to characterize the sensor under various conditions. For the fabrication of the sensor, one end of the FBG fiber pigtail is terminated by a low reflection facet such as an APC connector or an angled cut fiber and the output signal is extracted by an optical circulator of low back reflection. The sensor demonstrated as good performance of fundamental characteristics as a FBG acoustic sensor in the transmission mode. Considering many of the basic properties such as sensitivity, linearity, frequency response and compactness, an acoustic sensor using a FBG is as good as or better than a conventional PZT acoustic sensor.
Dependence of fiber-Bragg-grating (FBG) characteristics on static pressure around an FBG is analyzed in conjunction with the operation of an FBG as an underwater acoustic sensor. As the pressure increases from 0 to 69 MPa, the reflection and transmission spectra of an FBG shifts to a shorter wavelength while keeping the spectral shape unchanged. The rate of shift of the Bragg reflection wavelength is about -0.006 nm/MPa and therefore the pressure more than 0.17 MPa can be detected with an instrument available today. However, the sensitivity is too small to detect sound pressure of our interest that is less than 140 dB re
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.
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