Elimination of temperature cross-sensitivity in torsion and curvature measurements based on arc-induced long period fiber gratings

Abstract

We demonstrate a long period fiber grating (LPFG) with two prominent resonant dips for eliminating the temperature cross-sensitivity in torsion and curvature measurements. The versatile LPFG is fabricated on a conventional single mode fiber (SMF) using the arc discharge method. Due to the resonant coupling between the core mode and antisymmetric cladding modes, the transmission spectrum shows the polarization dependence, which is proved to be useful in improving the sensing performances. The different torsion sensitivities and similar temperature sensitivities for the resonant wavelengths of both dips indicate that the LPFG has the ability of temperature-independent torsion sensing. By optimizing the state of polarization (SOP), a torsion sensitivity of 0.104 nm/(rad/m) can be achieved with the discrimination of twist direction. Notably, the influence of temperature on the resonant contrast for the dip with a higher contrast can be negligible. Thus, its contrast variation is merely determined by curvature, meaning that the LPFG is intrinsically suitable for temperature-independent curvature sensing. The curvature sensitivity can be up to about 55 dB/m⁻¹ by adjusting the SOP. The demonstrations show that our fabricated LPFG can realize the elimination of cross-sensitivity induced by temperature in both torsion and curvature measurements. Additionally, the SMF-based LPFG has the advantages of low fabrication difficulty and cost, providing a simple and economic way for temperature-independent sensing.