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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 1μPa. It is consequently considered at the moment that the only practical method for measuring an acoustic field in water with an FBG is to make use of the intensity modulation of a laser light transmitting through or reflecting back from an FBG of which optical wavelength is at the slope of the transmission or reflection spectrum curve of the FBG.

Pressure and temperature dependence of fiber Bragg grating for acoustic sensing.