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 CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> 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 CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser methods. The PT-LPFGs with the periods of 420 to 480 μm were successfully fabricated, achieving a maximum polarization-dependent loss (PDL) exceeding 20 dB at the resonance wavelength. The dual-dips resonance of the LP<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> core mode was observed in the 1.0 μm 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 LP<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sub> and LP<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sub> cladding modes along the PMF’s slow and fast axes, respectively, within the 2.0 μm 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 to 1.457. Furthermore, the temperature and strain sensitivities were measured to be at 713 pm/°C and -49.4 pm/με, respectively. These findings highlight the potential of PT-LPFGs for high-sensitivity fiber sensors, PDL compensators, and other advanced optical applications.