In particular, starting from the results obtained in , new experiments have been carried out using a cylindrical geometry with two different coating: Damival and UR5041, showing that it is not the shape but the transversal dimension and the material characteristics of the coating that influence the sensitivity.2.?Operating PrincipleThe relation between the normalized Bragg wavelength shift ����B/��B and a spatial uniform sound pressure P(t) = p?sin(��St) around the FBG (where p and ��S are the amplitude and angular frequency of the sound pressure, respectively), is given by :����B��B(t)=[?(1?2��)E+n22(1?2��)E(2p12+p11)]P(t)(1)where n = 1.465, E = 70 GPa, �� = 0.17 and p11 = 0.121 and p12 = 0.270 are the effective refractive index of the guided mode, the Young’s modulus, Poisson ratio and the elasto-optic coefficients of the optical fiber, respectively.
Thus, the spectral response of the FBG moves without changing its shape at the same frequency of the applied acoustic pressure. For a GE-doped FBG at 1550 nm, ����B/��P was measured as -3��10-3 nm/MPa over a pressure range of 70 MPa . This means that with interrogation units able to perform wavelength shift measurements with a resolution of 10-4 pm in the investigated acoustic frequency range, an acoustic pressure limit of detection of hundreds of Pa can be obtained.When optical fibers are coated with a plastic material, they exhibit some order of magnitude increase in their pressure sensitivity [7-8].
In fact, according to the Hocker analysis , if the FBG is coated with a thick layer of polymer, the normalized wavelength Carfilzomib pressure sensitivity is given by:����B��B(t)=[?1+n22[p12?��(p11+p12)]](1?2��coa)EcoaP(t)(2)where Ecoa and ��coa are the Young’s modulus and the Poisson ratio of the coating, respectively.It can be seen from Equation 2 that for coatings with small Young’s modulus compared with the fiber one, the wavelength pressure sensitivity of the FBG can be increased significantly. The experimental demonstration of the pressure gain sensitivity was proven in the static case , but the concept can be extended in the case of acoustic fields if the coating dimensions are small compared with the acoustic wavelength.In addition, the extension of results is valid if the acoustic damping within the overlay is low in order to not affect the dynamic strain amplitude within the coating itself. Also, the acoustic impedance, related to the coating thickness and elastic modulus, should be very close to that of the water, in order to minimize the acoustic reflection at the water-coating interface.3.