Polarization-insensitive Simultaneous Measurement of Torsion and Temperature Using Long-period Fiber Grating Written on Polarization-maintaining Photonic Crystal Fiber

Abstract

Here, we report a novel optical fiber grating sensor capable of simultaneous measurement of torsion and temperature by utilizing a long-period fiber grating (LPFG) written on a polarization-maintaining photonic crystal fiber (PMPCF) and a Faraday rotator mirror (FRM) concatenated in series with the LPFG. The LPFG written on the PMPCF, referred to as a PMPC-LPFG, which is connected in series with the FRM, exhibits a polarization-independent wavelength-dependent loss spectrum in its reflection spectrum, which has two attenuation dips (ADs) designated as AD 1 and AD 2. When these two ADs have different cladding-mode orders, they have different torsion responses as well as temperature responses. For the two ADs with resonance wavelengths of λ1 = ~1508.02 nm and λ2 = ~1533.48 nm, torsion and temperature responses were investigated in a twist angle range from −70 to 70° (step: 10°) and an ambient temperature range from 25 to 95 °C (step: 10 °C), respectively. The torsion sensitivities of AD 1 and AD 2 were measured as ~157.0 and ~272.7 pm/rad at room temperature, and their temperature sensitivities were measured as ~5.8 and ~8.2 pm/°C, respectively. Owing to their linear and independent responses to torsion and temperature, torsion and temperature changes applied to the PMPC-LPFG can be simultaneously evaluated from the measured wavelength shifts of AD 1 and AD 2 using their predetermined torsion and temperature sensitivities.