Objective Optical fiber sensors have been extensively used to measure the external physical properties, such as temperature, refractive index, pressure, and strain, because of their characteristics such as simple structure, compact size, low weight, electromagnetic immunity, and high sensitivity. Generally, the fiber optic temperature and strain sensors are manufactured using fiber Bragg gratings (FBG) and optical fiber interferometers. However, the existing optical fiber sensors cannot meet the requirements in some scenarios that require high measurement sensitivity. Therefore, in this study, we propose using the optical vernier effect to enhance the performances of the optical fiber sensors. Normally, the optical vernier effect is generated by cascading two interferometers, among which one is set as the sensing interferometer and the other is set as the reference interferometer. The current interferometers based on the optical vernier effect generally include two cascading interferometers exhibiting almost identical responses to external disturbance, decreasing the sensitivity of the system. A complex isolation structure is usually applied to the reference interferometer to increase the response difference of the two interferometers with respect to the external disturbance, increasing the complexity of the sensors. Therefore, it is considerably important to design a reference interferometer not sensitive to the changes in external environment and achieve high-sensitivity and high-accuracy sensing of temperature and strain.

Methods In this study, a reference interferometer based on the fiber Sagnac interferometer (FSI) is constructed by introducing an elliptical core polarization-maintaining fiber (ECPMF), which exhibits insensitivity to temperature, strain, bending, and torsion. The cascaded sensor is constructed by cascading a reference interferometer and a sensing interferometer based on polarization-mode interferometers (PMI). The optical vernier effect is obtained by controlling the free spectral range (FSR) of the two interferometers to be close but not equal. Thus, high-sensitivity and high-accuracy temperature and strain sensing is realized. The pigtail of the 3-dB coupler of the reference FSI is cut as short as possible to minimize its response to environmental disturbance. The length, major-axis radius, minor-axis radius, and cladding diameter of ECPMF (IVG PME-1300-125) are 10m, 3μm, 1μm, and 125μm, respectively. The PMI is considered to be the sensing interferometer, which is obtained by splicing a polarizer and a polarization-maintaining fiber (PMF) with the fast/slow axis at of an angle of 45°; the other end of the PMF is gold-coated. The length, core, and cladding diameters of the PMF (YOFC PM1017-A) are 0.94m, 6.5μm, and 125μm, respectively. Furthermore, theoretical analysis, numerical simulation, and experimental verification have been successively conducted based on the above design.

Results and Discussions The output spectra of single FSI, single PMI, and cascaded sensors are analyzed theoretically and simulated to verify that the vernier effect can improve the system sensitivity. Results show that the FSR of PMI and FSI are 2.87 and 3.15nm, respectively. When the birefringence parameter of the PMF is increased from 3.5 × 10 ^-4to 3.51 × 10 ^-4, the interference spectrum of single PMI exhibits a 1.12-nm red shift, whereas that of the cascaded sensor exhibits a 12.7-nm red shift. The magnification factor is 11.34. When no strain is applied to the fiber at room temperature, the FSR of PMI, FSI, and cascaded sensors are 2.83, 3.1, and 35nm, respectively. Based on theoretical analysis, the birefringence parameter and beat length of ECPMF are 6 × 10 ^-5and 25.8mm at 1550 nm, respectively, which are consistent with the product specifications. The magnification factor with respect to the vernier effect obtained via theoretical calculation is 11.48, which is consistent with the simulation results. FSI and FMI are placed on a heating plate with a precision of 0.01 ℃ to investigate the temperature sensing characteristics of FMI and the relative temperature insensitivity of FSI. In the experiment, the temperature measurement ranges from 30 ℃ to 37 ℃ with a step of 1 ℃; each temperature state is maintained for 20min. The temperature sensitivities of dip 1 and 2 are 1.40 and 1.38 nm/℃ for a single PMI, respectively, whereas those of dip 1 and 2 are 168.38 and 157.44 pm/℃ for a single FSI, resulting in a temperature sensitivity that is approximately 1/8 of the single PMI temperature sensitivity. Thus, FSI is insensitive to temperature. After being amplified by the vernier effect, the temperature sensitivity of the cascaded sensors becomes 15.56 nm/℃, which is 11.12 times greater than that of a single PMI and is consistent with the theoretical value of 11.34. FSI and PMI are fixed using an optical adhesive (NORLAND 81) on a microdisplacement platform (Thorlabs, KMTS25E/M) with an accuracy of 1 μm to investigate the strain sensing characteristics of PMI and the relative strain insensitivity of FSI, and the distance between the two platforms is 40cm. Controlled by software, one of the microdisplacement platforms move outward; further, a displacement step of 16μm and a corresponding PMI strain step of 40με are realized, and the strain measurement range is from 0 to 280με. The strain sensitivities of dip 1 and 2 are 13.04 and 12.90pm/με for a single PMI, whereas those of dip 1 and 2 are 2.52 and 2.50pm/με for a single FSI, resulting in a strain sensitivity that is approximately 1/5 of the single PMI strain sensitivity. Thus, FSI is insensitive to strain. After being amplified by the vernier effect, the strain sensitivity of the cascaded sensors becomes 15.56nm/℃, which is 11.81 times greater than that of a single PMI and is consistent with the theoretical value of 11.34. The deviation in values can be attributed to the manual reading of the spectrum.

Conclusions In this study, a highly sensitive optical fiber temperature and strain sensor is proposed by introducing ECPMF into the Sagnac ring as a reference interferometer. The sensor includes a reference interferometer FSI and a sensing interferometer PMI, and the lengths of PMF and ECPMF are reasonably designed to make the FSR of FSI and PMI close but not equal, resulting in a vernier effect. The experimental results show that the temperature and strain sensitivity of the cascaded sensor become 15.56 nm/℃ and 154.04 pm/με, respectively, which are 11.12 and 11.81 times greater than that associated with a single PMI, respectively, and consistent with the simulation results. Thus, the effectiveness and reliability of the proposed scheme are verified based on the experimental results. The obtained sensor exhibits high sensitivity, a simple structure, good stability, and a good application prospect in various fields such as aerospace and industrial production.