High-Sensitive Liquid Level Sensor With Temperature Compensation Based on Cascaded LPFG and FBG Structure

This article proposes an intensity-modulated liquid level sensor (LLS) based on cascaded long-period fiber grating (CLBG) and fiber Bragg grating (FBG) structure with temperature compensation. Its unique structure converts the wavelength shift of the LPFG transmission peak caused by the liquid level change into a variable in the intensity of the FBG reflection peak. By inducing the combined action of LPFG and FBG, a new coupling peak is generated in the reflection spectrum for temperature compensation, reducing the interference of environmental temperature on liquid level measurement. A rigorous theoretical model was established, simulations were conducted, and experiments were performed to verify the results. In the experiment, the intensity variation of the FBG peak and the wavelength variation of the LPFG-FBG interaction peak in the reflection spectrum were used to measure the two parameters. The results show that when the liquid level changes within 15 mm, the intensity sensitivity of the corresponding FBG peak is –0.488 dB/mm. The wavelength of the LPFG-FBG interaction peak remains essentially unchanged. Within the temperature range of 30 ° C–60 ° C, the intensity sensitivity of the corresponding FBG peak is 0.0374 dB/°C, and the wavelength sensitivity of the LPFG-FBG interaction peak is 8.78 pm/°C. Using a $2\times 2$ matrix, simultaneous measurement of liquid level and temperature can be achieved, realizing temperature compensation. This sensor features high sensitivity, simple structure, high linearity, and low detection cost, promising a good application prospect in the field of liquid level sensing.