Liquid Surface Tension and Refractive Index Sensor Based on a Side-Hole Fiber Bragg Grating

A novel side-hole fiber Bragg grating (SHFBG)-based sensor is proposed and realized surface tension, refractive index (RI), and temperature of liquid monitoring simultaneously. A segment of side-hole fiber (SHF) is spliced of a single mode fiber (SMF), and a 10 mm long fiber Bragg grating (FBG) is inscribed within the core of the open end of the SHF. Due to the capillary effect, the liquid level in the SHF changes as a function of the surface tension of the liquid, which can be measured by monitoring the liquid level in the hole. The reflectivity of the FBG is affected by the fraction length immersed in the liquid, while the Bragg wavelength is affected by the refractive index and the temperature of the liquid. The absolute shift and relative shift of the Bragg wavelength can be used to measure the temperature and the refractive index of the liquid, respectively. The sensor described in this paper can measure the surface tension and the refractive index easily taking advantage of the simple structure and bring, an additional benefit of the “internal thermometer” provided by the absolute value of the Bragg wavelength.