Directional Bend Sensing With a CO $_{2}$ -Laser-Inscribed Long Period Grating in a Photonic Crystal Fiber

In this paper, we present a directional bend sensor based on a long period grating (LPG) formed by introducing periodic grooves along one side of a photonic crystal fiber (PCF) with a focused CO₂ laser beam. A bend sensitivity of 2.26 nm/m^-1 within a range of - 5 ~ + 5 m^-1 is experimentally demonstrated. Numerical simulation suggests that the directional response is the result of asymmetric cladding geometry resulted from collapse and/or deformation of air holes and asymmetric material-index modulation caused by one-sided illumination of the CO₂ laser beam. The sensitivity could be further enhanced by increasing the area of the air-silica photonic crystal cladding and optimizing the size of individual air holes. The easy fabrication process and good linear response of the proposed sensor make it a suitable candidate for structural shape sensing in harsh environments.