Characteristics of film sensor based on tilted long-period fiber gratings

In order to design a high-sensitive fiber-grating-based film sensor, the coupling features and sensing characteristics to the film refractive index of tilted long-period fiber gratings (TLPFG) coated with high-refractive-index sensitive films were studied, based on the coupled-mode theory. TLPFGs can couple the core mode to higher-order(azimuthal number l1) cladding modes whose coupling coefficients increased with the tilt angle and were much less than that of the first-order cladding mode for 80. It is confirmed that the tilt angle plays a role in determining the spectral transmittance rather than the resonant wavelength of the cladding-mode coupling. The response of spectral transmittance related to the 1-order and 5-number cladding mode of TLPFG to the variation of film refractive index were investigated under 80. The dependence of sensitivity of coated TLPFGs on the grating structural parameters(L,,) and film optical parameters(h3,n3) were analyzed. Result shows the sensitivity is greater than that of the non-tilted LPFGs by one order and the resolution of film refractive index is available to be 10-9.