Multiparameter Sensor Based on the Long-Period Fiber Gratings Inscribed in a Pre-Twisted Polarization-Maintaining Fiber

We demonstrate the fabrication of pre-twisted long-period fiber gratings (PT-LPFGs) in polarization-maintaining fibers (PMFs) using a CO2 laser. This technique leverages pre-twisted fibers combined with laser exposure, eliminating the need for precise alignment along the PMF axis, a key limitation of conventional single-side CO2 laser methods. The PT-LPFGs with the periods of 420– $480~\mu $ m were successfully fabricated, achieving a maximum polarization-dependent loss (PDL) exceeding 20 dB at the resonance wavelength. The dual-dips resonance of the LP11 core mode was observed in the $1.0~\mu $ m wavelength band. Further chemical etching was applied to adjust the cladding mode resonance wavelength near the dispersion turning point (DTP). This adjustment enabled dual-dips resonance with LP14 and LP15 cladding modes along the PMF’s slow and fast axes, respectively, within the $2.0~\mu $ m wavelength band. The sensing performance of PT-LPFGs near the DTP was evaluated. Experimental results demonstrate that these gratings exhibit a high sensitivity of up to 14100 nm/RIU for the surrounding refractive index range of 1.448–1.457. Furthermore, the temperature and strain sensitivities were measured to be at 713 pm/°C and −49.4 pm/ $\mu \varepsilon $ , respectively. These findings highlight the potential of PT-LPFGs for high-sensitivity fiber sensors, PDL compensators, and other advanced optical applications.