Structure design of refractive index sensor based on LPFG with double-layer coatings

Based on the coupled-mode theory, the mode field distribution and mode transition of LP modes in an LPFG with double-layer coatings are studied theoretically. As a refractive index sensor, this LPFG structure is designed for high sensitivity by selecting a suitable thickness of the first coating. Date simulation indicates that the effective index of the LP cladding mode increases in steps with the first coating thickness, thus LP mode is guided to the overlay and the mode transition takes place, where the LP modes can be easily affected by the ambient. Thereby, the coated LPFG sensor should be designed to operate at the transition region. Further, for two types of LPFG with double-layer coatings, the transmittance spectrum, sensitivity and measurable dynamic range is analyzed. The sensitivity of the LPFG sensor is available to 10³ with suitable overlay thickness.