Sensing Characteristics of Collapsed Long Period Fiber Gratings in Tri-Hole Fiber

We demonstrated a novel long-period fiber grating (LPFG) written in a tri-hole fiber (THF). The LPFG was fabricated by periodically collapsed micro holes utilizing CO ₂ laser irradiation integrated with pressure-assisted technology. The mode coupling between the fundamental mode and higher mode of the proposed grating is mainly caused by geometric deformation along the fiber axis. Compared with conventional LPFGs, such a grating has strong refractive index modulation due to periodic collapsed structure. We have established an approximate hypothesis to simulate the transmission spectrum of the grating. The measured spectrum is basically consistent with the simulated transmission spectrum at the resonant peak position. We investigated the sensing characteristics of the proposed LPFG, including axial strain, bending, torsion, and temperature. The sensitivity of bending, temperature and torsion are −9.56 nm/m ⁻¹ , 162.8 pm/(rad/m) in clockwise and −346.9 pm/(rad/m) in counterclockwise and 86.8 pm/°C, respectively. The experimental data shows that the fabricated LPFG has potential applications in the field of external high-sensitivity strain sensing with a sensitivity of −26 pm/μϵ.