Dual-dip long-period fiber gratings for directional bending measurement

A high-sensitivity bending sensor based on dual-dip long period fiber gratings (LPFGs) in a single-mode fiber is proposed. The length of the gratings is very short only 7.2 mm. The asymmetric perturbation in fiber is generated during the fabrication, which makes the dual-dip LPFGs show different bending sensing characteristics at opposite directions. Meanwhile, the highest bending sensitivity of -35.91 nm/m- 1 and -20.74 dB/m-1 can be obtained at bending directions of 0 degrees and 180 degrees, respectively. Furthermore, the torsion and temperature sensing characteristics are investigated experimentally. The torsion sensitivities follow sine function showing potential for directional sensing. The temperature sensitivities are 41.9 p.m./degrees C and 45.1 p.m./degrees C are close, which can eliminate the cross-sensitivity to temperature.