A method was developed for concurrently measuring bending and twisting along an optical fiber using only the properties of light guided within the fiber. The method exploits polarization-dependent reflectivity from fiber Bragg gratings (FBGs) in a centrally located core (see figure). This polarization-based approach fundamentally separates the detection of fiber bending and fiber twist, and no twist bias must be applied during fiber draw.
Conventional strain sensing in cores near the fiber's periphery is used to measure bending, and this information is combined with measurements of the state of polarization of light launched into the central core to deduce twist. Because bending only moderately affects birefringence in the central core, measurable polarization effects due to twisting are practicably separable. By accurately measuring both bending and twisting deflections along its length, the overall shape of a fiber can be estimated.
The method requires no change to the existing FBG fiber manufacturing process, and allows for single-fiber end connection to instrumentation. Many areas of interest involve obscuring environments where conventional telemetry is impractical. Search and rescue in mines and collapsed structures, submarine deployments, and medical procedures via laparoscopy and catheterization would all benefit from location and attitude information best provided by shape-sensing fibers.