Multiparameter sensor based on long-period grating in few-mode ring-core fiber for vector curvature and torsion measurement

With the rapid development of optical fiber sensor in physical measurement, it is of practical significant to realize multi -parameter measurement. In this work, we proposed and demonstrated a multiparameter sensor for curvature, torsion and temperature. The sensor is formed from CO 2 laser -inscribed long -period grating (LPG) in fewmode ring -core fiber (RCF), which can convert fundamental mode to LP 11 mode. One -side inscription enhances asymmetric index modulation in fiber, resulting in directionally sensitive to bend by intensity interrogation. The curvature sensitivities in various bending directions perform linearity in curvature range of 5 - 9.466 m -1 . Curvature sensitivities are related to the bending directions. The intensity sensitivity achieves - 0.584 dB/m -1 , 0.879 dB/m -1 , and - 0.735 dB/m -1 , respectively. And torsion sensitivity from -6.98 rad/m to 6.98 rad/m displays linearity with -0.554 dB/(rad/m). The measured influence of temperature can be negligible. The proposed RCF-LPG could not only be used as high -efficiency mode converter in mode division multiplexing systems but also be potential in few -mode fiber based multiparameter sensing fields.