High-performance fiber ring laser torsion sensor based on polarization-maintaining photonic crystal fibers

Torsion information is important for rotating systems, industrial monitoring, transportation engineering, and medical equipment. Optical fiber torsion sensors have significant advantages, such as immune to electromagnetic interference, small size, and light weight. Sagnac loop interferometer (SI) torsion sensors have attracted much attention due to their compact structure, high sensitivity, excellent stability, and low cost. However, their nonlinear response limits the measurement range, while the wide full width at half maximum and low signal-to-noise ratio (SNR) reduce the resolution of torsion sensors. To solve these problems, a fiber ring laser torsion sensor (FRLTS) based on homemade polarization-maintaining photonic crystal fiber (PM-PCF) is proposed in this work. The torsion sensor introduces a PM-PCF based SI into the erbium-doped fiber ring cavity as a filter and torsion sensor device. The interference spectrum of SI is derived by the transmission matrix method and simulated, and then the sensing principle of the sensor is obtained. Subsequently, the experimental system is set up to study the lasing output characteristics and torsion response of the FRLTS. By taking advantage of the narrow linewidth and high signal-to-noise ratio (SNR) of fiber ring lasers, a high-resolution fiber torsion sensor is successfully obtained. The experimental results show that the maximum linear torsion measurement range of the sensor can be extended to 480° (from –260° to 220°), the maximum torsion sensitivity is 0.032 nm/(°), and the resolution is as high as 0.681°. Furthermore, in a temperature range from 20 ℃ to 95 ℃, the temperature-induced wavelength variation is only 4×10<sup>–3</sup> nm/℃, corresponding to a torsion angle measurement error of 0.16(°)/℃. Compared with existing reports, its temperature stability is increased by 37.5 times, while the temperature-induced error in angle measurements is reduced by 9.375 times. The proposed FRLTS not only successfully achieves high-resolution and wide-range torsion sensing, but also effectively suppresses cross-sensitivity caused by temperature. Therefore, the torsion sensor has significant potential applications in fields such as aerospace and robotics where precise measurement of minute torsion angle is required in special environments.