Structure optimization of coated LPFG based on dual-peak resonance

Abstract

A dual-peak resonance in coated long-period fiber grating (LPFG) is presented in this paper. By resolving the characteristic equation of the coated LPFG, the dual peak resonant wavelengths are determined based on rigorous coupled mode theory. The relationships between the dual peak resonant wavelengths and grating period and the mode ordinal are studied. The results show that the dual resonance appears in higher cladding mode, and the higher the cladding mode ordinal is, the smaller the related grating period required to couple with core mode is. Further, the influence of film optical parameters (the film thickness h3 and the refractive index n3) on the intervals of dual resonant wavelengths, as well as the attenuation peak of transmission spectra, are analyzed whether the optical dispersion is under consideration or not. The transmittance spectra show the dual resonant peaks shift in the opposite direction with the variation of the film refractive index. This can be used to construct a refractive index sensor, in which the films sensitive to the surrounding gases are coated on the cladding of the fiber grating region, and the intervals of dual peak resonant wavelengths change with the film refractive index. By using optimization method, the optimal film optical parameters and the grating structure parameters are obtained. Data simulation shows that the resolution of the refractive index of the films is predicted to be more than 10^-7. The theoretic analysis provides straightforward foundation for the actual highly sensitive film sensors.