A Cascaded Eccentric Hole Fiber Sensor With Dumbbell-Shaped Structure for Large-Range Torsion Measurement

A cascaded eccentric hole fiber (EHF) sensor with a dumbbell-shaped structure for large-range torsion measurement is proposed and prepared. It is consisting of an EHF and two sections of single-mode fiber (SMF) which are made into spheres by electric discharge. The main sensing structure is the SMF sphere–EHF–SMF sphere. In the first sphere structure and EHF fusion point position, the appearance of mode-field mismatch and refractive index perturbation, the noncircular symmetric (NS) higher order mode (HM) is created, the effective refractive index (ERI) difference between the HM and the fundamental mode (FM) is modulated by the torsion effect during the transmission process, and then, the HM interferes with the FM at the second sphere structure. Experimental results show a maximum sensitivity of 0.759 nm/(rad/m), where the sensor undergoes a torsional change from 0° to ±720°. The blue shift occurs in the torsional range of
$0\pi $
to
$\pm 4\pi $
and
$\pm 8\pi $
to
$\pm 12\pi $
rad/m, and a red shift in the range of
$\pm 4\pi $
to
$\pm 8\pi $
and
$\pm 12\pi $
to
$\pm 16\pi $
rad/m. The periodic shift phenomenon enables the sensor torsion range to be monitored. In addition, the axial strain has a very small impact on the results.