Sensitivity enhancing of transition mode long-period fiber grating as methane sensor using high refractive index polycarbonate/cryptophane A overlay deposition

A highly sensitivity long-period fiber grating (LPFG) methane sensor system is presented, which was working in transition mode with high refractive index overlay deposition of polycarbonate/cryptophane A. The transparent composition coating of polycarbonate/cryptophane A was used as high refractive index overlay deposition based on automatically dip-coating technique to tune the working point of the sensor in the transition region. Effects of overlay thickness and refractive index (RI) on the LPFG's resonant wavelength were studied in order to obtain the optimal overlay thickness. For the overlay thickness of 530 nm, the overlay RI of 1.5605 and the grating period of 520 μm, the maximal RI sensitivity of the designed LPFG sensor could reach 3.56 × 103 nm/RIU. The experimental results demonstrate that the new sensor has the features of high sensitivity (∼2.5 nm %−1) and low detection limit of 0.2% (v/v).