Design optimization of an intensity-interrogated long-period fibre grating film sensor operating near the phase-matching turning point

The intensity interrogation of a sensitive-film-coated long-period fibre grating (LPFG) operating near the phase-matching turning point (PMTP) is fully investigated. The coated LPFG is designed to operate near the PMTP and a little off resonance by properly selecting the film and grating parameters; the variation of the film refractive index induced by the interaction between the sensitive film and the surrounding medium leads to the change of coupling strength with a fixed resonant wavelength. Based on the coupled-mode theory and a four-layer model, the phase-matching property and the influence of film optical parameters on the turning points are investigated. Furthermore, the determination of the range of a little off resonance is discussed. By analysing the dependence of sensitivity on the film parameters (film refractive index n(3) and thickness h(3)) and the grating structural parameters (grating period A, length L and average index modulation sigma), the optimum parameters for high sensitivity are obtained. The simulation results show that the resolution of film refractive index is available in the order of 10(-7) which is comparable to that of the wavelength-interrogated LPFG sensor based on dual-peak resonance, but an expensive optical spectrum analyser can be excluded from the measuring system.