Design of a gas sensor based on a sensitive film coated phase-shifted long-period fiber grating

A sensitive film coated long-period fiber grating with p phase shift introduced at the center is proposed for gas sensing. The sensitivity of the sensor is defined as the rate of change of the transmission at a specific wavelength with the film refractive index. Our analysis shows that it is most favorable to let the coupling strength K L of both parts of the long-period fiber grating be root 2 pi/4. The sensor has higher sensitivity than a sensor with coupling strength less than root 2 pi/4 and has wider monotonic range than a sensor with coupling strength larger than root 2 pi/4. The sensitivity of the sensor with coupling strength K L = root 2 pi/4 is qualitatively and numerically analyzed. The sensitivity can be improved by two orders of magnitude by selecting an optimum film thickness. The smaller the index modulation in the fiber core, the higher the sensitivity of the sensor. The sensitivity can be increased to 1.5 times when the index modulation is decreased to 2 times with an optimum film thickness being chosen.