Dual peak resonance and transmission spectrum characteristics in a coated long-period fiber grating

The dual peak resonance in a coated long-period fiber grating (LPFG) is presented and studied. By resolving the eigenvalue equation of the coated LPFG, the dual peak resonant wavelengths are determined based on a rigorous coupled mode theory. The relationships between the dual peak resonant wavelengths and the grating period for various cladding mode orders are studied. The results show that the dual resonant wavelengths of higher order cladding modes appear in a fiber within the commonly observed wavelength range. The smaller the grating period is, the higher the cladding mode order corresponding to the dual peak resonance is. Furthermore, the influence of the film optical parameters and the grating structural parameters on the wavelength interval between the dual resonant peak, the amplitude, and the bandwidth of the resonant peak is discussed. The obtained results have some similarity to those from the non-coated LPFG experiment by Shu (1999 Opt. Commun. 171, 65). In addition, the sensitivity of the coated LPFG sensor based on dual peak resonance is analyzed under the influences of the film optical parameters and the grating structural parameters. The resolution of the refractive index of this coated LPFG sensor is predicted to be 10−7, so it provides a theoretical foundation for the structural optimization of a high sensitivity dual peak resonance coated LPFG sensor.