NASA’s Langley Research Center has demonstrated a patent-pending method and apparatus for determining the position, in three dimensions, of any point on an optical fiber. The new method uses low-reflectance Fiber Bragg Grating (FBG) strain sensors in a multicore fiber to determine how any point along that fiber is positioned in space. The characteristics of optical fibers and the FBGs vary with curvature, and by sensing the relative change of FBGs in each of three or more fiber cores, the three-dimensional change in position can be determined. By using this method in monitoring applications where optical fibers can be deployed — such as in structures, medical devices, or robotics — precise deflection, end position, and location can be determined in near real time. This innovative position detection method offers 10 times greater positional accuracy than comparable optical techniques.
Multi-core optical fibers contain multiple light-guiding cores arranged symmetrically. Sensors, such as FBGs, are embedded into each of the cores (see figure, left). Such an arrangement allows for the measurement of strain in each core of the fiber at specific axial locations along the fiber. When a multi-core fiber is subjected to bending, the strain imposed in each core relative to one another is used to provide position information (see figure, right).
In the past, shape-sensing measurements using optical fibers estimated bending at sequential points along the fiber, and the resulting measurement had many discontinuities and errors. The combination of these errors resulted in a very poor indication of actual fiber position in three-dimensional space.
NASA’s patent-pending algorithms and apparatus incorporate not only fiber bending measurements, but fiber twisting measurements, to eliminate previous sources of error. The uniqueness of the algorithm is in how the curvature, bend direction, and twisting information of the fiber are all brought together to obtain a highly accurate 3D location and shape characterization. The new methods have been demonstrated to significantly improve the accuracy of multi-core fiber optic shape sensors.
This technology provides high spatial resolution for fibers up to 10 meters in length, and can be extended to other forms of cable. Potential uses include aerospace safety systems, medical applications, cabled remote vehicles, and space exploration.