The laser light intensity modulated by the FBG subjected to the sound pressure is detected by a fast photodiode.Indeed, in the transmission mode and by working on the edge of the grating spectrum, the transmitted optical power Pt is directly related to the sound pressure according to the following expression:Pt(t)=Pi[T0+?T?��0?��0?pP(t)](3)where Pi is the incident optical power, T0 is the transmission value at FWHM, ?T/?��0 and ?��0/?p represent the edge slope of the grating spectral response and the wavelength sensitivity to the pressure, respectively. Yet, in order to achieve the maximum sensitivity and dynamic range, the laser wavelength should be set in correspondence of the full width at half maximum (FWHM), on either the longer or shorter wavelength side of the spectrum curve.
It is seen from Equation 3 that the ac component of the transmitted light power is proportional to the sound pressure experienced by the FBG. Thus, the detection of the light with a photodiode provides an electrical output directly proportional to the acoustic field in the water. From the resulting temporal waveform, the amplitude, the frequency and the phase of the field can easily be measured after a proper calibration procedure.The experimental set-up is shown in Figure 1. Field trials have been carried out in a professional tank at the Whitehead Alenia Sistemi Subacquei’s laboratory. The tank size was 11 �� 5 m, and its depth was 7 m. The sensor’s symmetry axis was orthogonally to the direction of propagation of the acoustic wave. A piezoelectric hydrophone has been used as reference.
A computer-controlled scanning stage that allows independent translations in X, Y and Z directions and rotations about the vertical axis is used to place the optical fiber hydrophone.Figure 1.Lateral view of the experimental set-up.4.?Results and DiscussionIn this section, we report a series of measurements carried out to evaluate the sensitivity, the linearity and the resolution of the FBG-based hydrophone coated with materials of different geometrical and acousto-mechanical properties and to compare Anacetrapib the obtained performances with a reference PZT hydrophone.The first comparison is between two hydrophones with the FBGs (one characterized by a central wavelength of 1,554.20 nm and a FWHM of 0.5 nm and the other by a central wavelength of 1,547.6 nm and a FWHM of 0.45 nm) embedded in a material of cylindrical geometry with diameter of 4 mm and length of 25 mm exhibiting an elastic modulus of ~100 MPa and in a material (the Damival 13650) of spherical geometry with diameter of 4.4 cm (exhibiting elastic modulus lower than 100 Mpa and acoustic impedance that matches that of water), respectively (see Figure 2).Figure 2.