Strain-Insensitive Simultaneous Measurements of Torsion and Temperature Using Long-Period Fiber Grating Inscribed on Twisted Double-Clad Fiber

Here, we report an optical fiber sensor capable of performing strain-insensitive simultaneous measurements of torsion and temperature using a long-period fiber grating (LPFG) inscribed on twisted double-clad fiber (DCF) with a CO2 laser. The LPFG written on twisted DCF, referred to as twisted double-clad LPFG (TDC-LPFG), exhibited multiple resonance dips with different cladding-mode orders, and two of them were selected as sensor indicators for the simultaneous measurement of torsion and temperature. The measured torsion sensitivities of the sensor indicators were approximately - 2.167\times 10^{{2}} and - 4.667\times 10^{{2}} pm/(rad/m) in a twist angle range of −180° to 180°, and their temperature sensitivities were measured as \sim 6.580\times 10^{{1}} and \sim 7.189\times 10^{{1}} pm/°C in a temperature range of 25 ^{\circ }\text{C}-100\,\,^{\circ }\text{C} . Their linear and independent responses to torsion and temperature made possible the discrimination of the two measurands. Moreover, the strain sensitivity of the TDC-LPFG was measured and found to be less than 2.750\times 10^{{1}} pm/ \mu \varepsilon in magnitude, over four times less than that of a standard LPFG. This strain insensitiveness contributes strongly to diminishing strain-induced measurement uncertainties associated with torsion and temperature. Finally, we validated the simultaneous measurement capability of 25 applied points of torsion and temperature variations. The average torsion and temperature standard deviations were \sim 3.44\times 10^{-{2}} rad/m and \sim 3.01\times 10^{-{1}}\,\,^{\circ }\text{C} , respectively.