# Liquid Crystal-Embedded Hollow Core Fiber Temperature Sensor in Fiber Ring Laser
## 1 Introduction  
In recent years, optical fiber sensors have attracted much attention in aerospace engineering [1], manufacturing industries [2], and biological sensing regions [3]. Due to their specific merits of immunity to electromagnetic interference, miniaturization, high sensitivity, and flexibility [4]. Various kinds of sensors have been designed to measure temperature [5], electric field [6], biomolecule [7], curvature [8], etc. Among these optical fiber sensors, a variety of temperature measurement methods, such as Fabry-Perot interferometer [9], photonic crystal fiber (PCF) [10], tapered fiber [11], multimode interference (MMI) [12], and fiber Bragg grating (FBG) [13], have been extensively studied. Among them, hollow fiber has been widely concerned [14; 15; 16; 17; 18]. The thermal conductivity of the hollow cladding is different from that of the solid cladding, and it is easy to inject the thermosensitive liquid through the capillary effect, with low cost and high sensitivity. [PERSON] et al. reported a compact MZI based on hollow fiber [19], which is used as a splitter and combiner to form a Mach-Zehnder interferometer for temperature detection. It achieves 30.92 pm/\({}^{\circ}\)C. [PERSON] et al. reported a diaphragm free ultra-high temperature FPI sensor based on hollow fiber [20], which achieved a detection sensitivity of 15.2 pm/\({}^{\circ}\)C at 1000 \({}^{\circ}\)C. Besides, [PERSON] et al. proposed a temperature sensor with a sensitivity of 31.8 pm/\({}^{\circ}\)C based on hollow photonic band gap fiber with mode interference [21]. However, a common problem is that the sensor is limited by the large bandwidth of broadband light source and low signal-to-noise ratio.  
Recently, FRL system has been successfully developed as a temperature sensor. Compared with the traditional optical fiber temperature sensor using super continuous wide-band light source, it has the advantages of narrow line width and high signal-to-noise ratio. [PERSON] et al. proposed a fiber Mach-Zehnder interferometer sensor based on Er doped fiber peanut (EDFP) structure without using extra filter. The temperature sensitivity of the EDFP is 0.158 nm/\({}^{\circ}\)C [22]. Besides, [PERSON] et al. designed a temperature sensor based on single-mode-no-core-hollow-core-no-core-single-mode interferometer to measure the temperature variation. The temperature sensitivity up to 8.53 pm/\({}^{\circ}\)C [23]. A 2 \(\upmu\)m tapered seven-core fiber structure was used for temperature measurement in fiber ring laser [24]. The sensitivity is up to \(-4\) pm/\({}^{\circ}\)C and the range is 35 \({}^{\circ}\)C to 70 \({}^{\circ}\)C. However, due to the limitation of the thermal expansion coefficient of the fiber, the detection sensitivity of these structures has not reached \(-\)nm/\({}^{\circ}\)C magnitude.  
In this paper, we propose and experimentally demonstrate a highly sensitivity temperature sensor based on liquid crystal filled HCF in FRL system. The device consists of a hollow core fiber (HCF) filled with LC-E7 welded between two core-less fibers. The pores at the welding site collapse to form MZI. When the external temperature changes, the refractive index (RI) of the liquid crystal changes due to the thermo-optic effect, and the corresponding resonant wavelength also changes accordingly. In the FRL system, the output resonance spectrum of the device is demodulated by MZI, and the temperature tuning ability of the device is verified. The resonance wavelength is offset with temperature, and the maximum sensitivity is 1.318 nm/\({}^{\circ}\)C. By changing the HCF size more modes can be excited and higher sensitivity can be obtained. The device is easy to manufacture because only simple filling methods and processes are used. In addition, the liquid crystal and optical fiber used in the device are both economical and efficient. These advantages make the device competitive in practical applications such as high sensitivity temperature sensors, high efficiency biomolecule culture, and environmental temperature monitoring.