Parameters Optimization Of Pi-Phase-Shifted Long-Period Fiber Grating For Gas Sensing

The gas-sensitive film coated -phase-shifted long-period fiber grating (LPFG), consisting of two same long-period fiber gratings cascaded by a regular fiber with length of half of the grating period, is proposed for gas sensing. The sensitivity of the sensor is defined by the change rate of the transmission at specific wavelength with respect to the film refractive index. The qualitative and numerical analyses show that to get a high sensitivity, the optimal coupling strength of each long-period fiber grating is root 2 pi/4. With this coupling strength, the longer the grating is, the higher sensitivity the sensor has. By choosing appropriate grating period, a high sensitivity for the film with a certain refractive index and thickness can be attained.