Twist-Direction-Discriminable Torsion Sensor Using Long-Period Fiber Grating Inscribed on Polarization-Maintaining Photonic Crystal Fiber

Here we propose and experimentally demonstrate an optical fiber torsion sensor with twist direction discrimination capability by incorporating a long-period fiber grating (LPFG) inscribed on polarization-maintaining photonic crystal fiber (PMPCF) by CO₂ laser pulses. An LPFG formed on PMPCF has an input-polarization-dependent resonance dip, and two separated resonance dips, designated as the shorter wavelength-dip (SWD) and longer wavelength-dip (LWD), are obtained with respect to orthogonal input polarization. The torsion response of the sensor head was investigated in a wide twist angle range of -220° to 220° for the two resonance dips. Highly linear torsion responses of the SWD and LWD could be obtained with adjusted R² values of ~0.9960 and ~0.9967, respectively, in a twist angle range from -160° to 160°, when their transmittance levels measured in a logarithmic scale were converted into a linear scale. The torsion sensitivities of the SWD and LWD were measured as -5.067 and 4.830 %/rad in this twist angle range of high linearity, respectively. In particular, its temperature response was also examined in a temperature range from 35 to 85 °C. With pre-torsion applied to the sensor head at twist angles of 220° and -220°, the temperature sensitivities of the SWD and LWD were measured as ~3.64 and ~9.75 pm/°C in terms of the dip wavelength and -2.87 × 10^-3 and 5.45 × 10^-5 dB/°C in terms of the dip transmittance, respectively. The experimental results corroborate that our sensor can perform virtually temperature-insensitive torsion measurement with twist direction discrimination capability and good linearity in a wide measurement range.