Liquid-Level Sensor Based on Reflective Mechanically Induced Long-Period Grating Using Double-Cladding Fiber

A reflection-type liquid-level sensor based on mechanically induced long-period fiber grating (MLPFG) is proposed and experimentally demonstrated. The sensor, acting as a Michelson interferometer, includes MLPFG with double-cladding fiber (DCF) whose end-face is coated with a silver film. DCF is used here in order to be more conducive for interference-spectrum generation. The liquid level can be measured by monitoring the shift of the interference fringes. By theoretical and experimental analysis of the interference characteristics, it is concluded that there are certain relationships between the interference-fringe spacing and the distance from the grating to the reflector, and that the fringe contrast ratio is influenced either by the stress or the position of the MLPFG. After the appropriate parameters are selected for the liquid-level sensor, the sensor sensitivity of 0.03136 nm/mm is obtained, which is considerably superior to that of conventional interferometric liquid-level sensors. In addition, the device has advantages, such as low cost and structural simplicity.