The optical fiber interferometer is a very important optical device, which has been widely used in the fields of physics, chemistry, medicine, and biological environment monitoring. By changing the substrate material of the inserted optical fiber, the performance of the Sagnac optical fiber sensor has been effectively improved. However, limited by the inherent characteristics of optical fiber (such as photoelastic effect and photothermal effect), the sensitivity improvement of optical fiber sensors based on Sagnac interference is hindered. Therefore, improving the sensitivity of the Sagnac sensor is of important research significance. The Sagnac sensors with metal-filled photonic crystal fiber (PCF) can obtain higher sensitivity. However, manufacturing metal-filled PCF requires more stringent technology and costs. The manufacturing of functional liquid-filled PCF is simpler than that of metal-filled PCF. At present, most optical fiber sensors are used for single-element detection, which greatly limits the application of optical fiber sensors. In order to realize two-parameter detection in complex environments, a two-parameter optical fiber sensor based on the Sagnac interference principle is designed to detect temperature and strain.

The polarization-maintaining PCF (PM-PCF) model selected in the experiment is LMA-PM-10. The PM-PCF's core diameter is about 9.9 μm. The diameter of the air hole cladding is about 54.3 μm. The diameter of PM-PCF's cladding is about 235 μm. The PM-PCF has a strain-sensitive material that is the strain-applying part of the PM-PCF. Therefore, strain detection can be effectively realized through the shift of the Sagnac spectrum. Moreover, the material of the fiber itself and the strain-applying part have a good photothermal effect and thermal expansion effect. Therefore, the PM-PCF is also extremely sensitive to ambient temperature. The sensor can be used for temperature detection because the change in the outside temperature will cause an obvious shift in the interference spectrum. The temperature transformation can be calculated by the movement of the interference spectrum. In addition, by using the nitrogen pressurization device, the ethanol solution is filled into the air hole of PM-PCF. By extending the filling time, each air hole of the optical fiber is filled with ethanol. The filling of temperature-sensitive materials can greatly improve the temperature sensitivity of optical fiber sensors, which is the reason for filling ethanol in the PM-PCF. Experiments have proved that the sensing performance of the sensor has been improved.

First of all, the strain sensitivity of the sensor is tested. Before connecting the optical path, the PM-PCF is welded into the Sagnac ring. The clamp is used to fix the optical fiber in the strain test device. The strain is gradually increased according to the principle of screw micrometer. The strain sensitivity achieves 35.35 pm/μ? in the strain range of 0-900 μ?. During repeated measurements, the sensor shows excellent hysteresis. Then, the temperature sensitivity of the sensor is detected. The whole sensor is placed in the temperature control box for temperature detection. The sensor achieves a temperature-sensing sensitivity of -1.72 nm/℃ within the temperature range of 26-50 ℃ when the PM-PCF is not filled with ethanol. The PM-PCF is placed in a closed air chamber, and each air hole is filled with ethanol by a nitrogen pressurization device. After the temperature detection, the sensor temperature sensitivity reaches -2.66 nm/℃, which is 1.55 times that of the raw PM-PCF. This phenomenon effectively proves the importance of filling ethanol. During repeated temperature detection, the sensor shows excellent hysteresis. The Sagnac interferometric sensor for temperature and strain detection has outstanding stability.

In this paper, an optical fiber sensor based on the Sagnac interference principle is reported, which is used to detect temperature and strain in the environment. In the experiment, the PM-PCF is selected as the sensing unit. First, the PM-PCF without ethanol is fused into the Sagnac interference loop. The sensor relies on the photothermal effect and photoelastic effect of PM-PCF substrate material to achieve a temperature sensing sensitivity of -1.72 nm/℃ within the temperature range of 26-50 ℃ and achieve a strain sensitivity of 35.35 pm/μ? in the strain range of 0-900 μ?, respectively. The sensing performance of the Sagnac interferometer can be enhanced by using the external field's tuning effect of functional materials. In this way, ethanol is filled into the air hole of PM-PCF cladding by a nitrogen pressurization device. The temperature sensitivity is -2.66 nm/℃, which is 1.55 times that of the raw PM-PCF. The Sagnac interferometric sensor for temperature and strain detection has a simple structure and excellent hysteresis, which can be used to improve the sensing sensitivity.