# Liquid Crystal-Embedded Hollow Core Fiber Temperature Sensor in Fiber Ring Laser
## 3 Results  
Firstly, the temperature response of the structure in the supercontinuum light source is studied. Temperatures range from 20 \({}^{\circ}\)C to 30 \({}^{\circ}\)C. Figure 6a shows the resonant wavelength excursions corresponding to different temperatures. As shown in Figure 6a, with the increase of temperature, the transmission spectrum moves to the short-wave direction. There is a good linear relationship between temperature and wavelength, as shown in Figure 6b. The temperature sensitivity of the structure from 20 \({}^{\circ}\)C to 30 \({}^{\circ}\)C.HCF is \(-1.041\) nm/\({}^{\circ}\)C, which shows that the blue shift of the spectrum occurs with the increase of temperature, which is consistent with Equation (2). In addition, it should be noted that in the experimental results, each data point is the mean of at least five independent measurements, and the error bar represents the standard deviation of the mean. The error line is small and the experiment repeatability is good.  
The measurement range is from 20 \({}^{\circ}\)C to 30\({}^{\circ}\)C according to Step 2 \({}^{\circ}\)C. The emission spectra of the fiber ring cavity at different temperatures are shown in Figure 7a,b. With the increase of temperature, the blue shift of the spectrum occurs, which is consistent with the Equation (2). Its sensitivity is shown in Figure 7b. The calculated laser spectral sensitivity is 1.181 nm/\({}^{\circ}\)C. In addition, the correlation coefficient in Figure 7b is 0.996, indicating a  
Figure 5: The schematic configuration of the proposed FRL temperature sensor system.  
Figure 6: Temperature response of the LC filled HCF structure in ASE source. (**a**) Transmission spectra of temperature from 20 \({}^{\circ}\)C to 30 \({}^{\circ}\)C; (**b**) relationship between the resonant wavelength response and the temperature.  
seen from Figure 9, the maximum wavelength shift and power fluctuation of the laser line were 0.132 nm and 0.32 dB, respectively. Without affecting the measurement results, the vibration of SNHNS structure in the FRL system can introduce the power change, and the power change can be mitigated by good encapsulation. This is another problem worth exploring. This problem can be solved with a compact and robust package to protect fragile uncoated fiber optic sensors, such as external hollow capillaries or embedded grooves.  
Since other liquid such as silicon oil has a similar isotropic refractive index variation with temperature. Replacing liquid crystal with silicone oil is an effective method for temperature measurement [26; 27; 28; 29]. However, limited by the material properties of silicone oil itself, it has the following inevitable disadvantages: poor lubricity; the high solubility of gas is difficult to achieve gas sealing device; certain hygroscopicity; and after adding water, performance decline. Besides, the surface tension is small and easy to leak. Liquid crystals are still one of the best choices for temperature measurement.  
Besides, surface treatments of the inner solid surface to induce planar alignment is an effective way to fixed liquid crystal orientation [30]. However, surface treatment inside optical fibers often require polishing or drilling. This greatly reduces the stability of the system. In addition, the exposed fiber is easily affected by dust in the air, which reduces the consistency of orientation. At the same time, due to the complex surface treatment process, the cost is high. Therefore, we do not choose to preprocess the fiber surface in this work.