A Highly sensitive LPFG torsion sensor with step-index cladding modulation

Based on the current research on long-period fiber gratings (LPFGs), we propose a stepped LPFG torsion sensor designed to enhance the torsional sensitivity of existing sensors. The sensor combines CO2 laser polishing and pre-twisting techniques to increase the refractive index difference between neighboring notches. This method is effective in enhancing the introduction of elliptical birefringence to improve the torsional sensing sensitivity. Through comparative experiments, the effect of different pre-twist angles $\beta$ on sensor sensitivity was studied. When the applied pre-twist force was greater, the residual twist force in the optical fiber was greater, and the corresponding twist sensitivity also increased. The experimental results demonstrate a torsional sensitivity of up to 0.45 nm/(rad/m) in the range of 0 to -18.48 rad/m. However, due to non-uniform modulation of elliptical birefringence, the sensitivity decreases to 0.362 nm/(rad/m) for the range of 0–18.48 rad/m. Furthermore, the temperature characteristics of the sensor were investigated, demonstrating a sensitivity of 65 pm/ °C within the measurement range of 20 °C to 110 °C. The results demonstrate that the proposed sensor offers a novel solution for enhancing torsional sensing performance, with significant application value in engineering measurement fields.