Bend-Insensitive Simultaneous Measurement of Strain and Temperature based on Cascaded Long-Period Fiber Gratings Inscribed on a Polarization-Maintaining Photonic Crystal Fiber

Abstract

We report an optical fiber sensor capable of bend-insensitive simultaneous measurements of strain and temperature by using cascaded long-period fiber gratings (LPFGs) inscribed on a polarizationmaintaining photonic crystal fiber (PMPCF). The cascaded LPFGs written on the PMPCF have two different interference spectra for orthogonal input polarization states exciting the principal axes of the PMPCF. Each fringe spectrum is created by the interference between the uncoupled core mode and the cladding mode recoupled to the core mode at the second LPFG. Due to the birefringence of the PMPCF, the order of this cladding mode can be different depending on the input polarization. The lowest dip in each interference spectrum is designated as a sensor indicator dip (SID) A or B. These SIDs originating from cladding modes with dissimilar orders can have different strain or temperature sensitivities. The strain and the temperature responses of these SIDs were investigated in an applied strain range of 0 −2244 με and an ambient temperature range of 25 −95 °C, respectively. The strain sensitivities of the SIDs A and B were measured as approximately −0.93 and −1.41 pm/με (R² values: ~0.9984 and ~0.9949), and their temperature sensitivities as ~11.48 and ~15.14 pm/°C (R² values: ~0.9974 and ~0.9905), respectively. In particular, their bending responses were also explored over a wide curvature range of 0 − 7.984 m⁻¹. Themaximum bend-induced wavelength shift was ~0.06 nm. Owing to their linear and independent responses to strain and temperature and their insensitivity to bending, our LPFG device can be a strong candidate for a practical sensor head for simultaneous measurements of strain and temperature.