Methods and apparatus to determine a twist parameter and/or a bend angle associated with a multi-core fiber
A multi-core fiber includes multiple optical cores, and for each different core of a set of different cores of the multiple optical cores, a total change in optical length is detected. The total change in optical length represents an accumulation of all changes in optical length for multiple segments of that different core up to a point on the multi-core fiber. A difference is determined between the total changes in optical length for cores of the set of different cores. A twist parameter and/or a bend angle associated with the multi-core fiber at the point on the multi-core fiber is/are determined based on the difference.
1. A shape-sensing method for a multi-core optical fiber, the method comprising:
for each core of multiple cores of the optical fiber, continuously tracking a phase difference accumulated along the optical fiber, the phase difference being between phases of reflected light signals in a measurement state of the optical fiber and in a reference state of the optical fiber;
based on the continuously tracked phase differences for the multiple cores, determining a shape of the optical fiber.
2. The method of claim 1 , further comprising:
measuring the phase difference using optical frequency domain reflectometry (OFDR).
3. The method of claim 1 , wherein an accuracy of the tracked phase difference is maintained over an entire length of the optical fiber.
4. The method of claim 1 , wherein continuously tracking the phase difference comprises aligning the reflected light signals between the measurement state and the reference state to account for a shift in apparent location of segments of the optical fiber between the measurement state and the reference state.
5. The method of claim 1 , wherein continuously tracking the phase difference comprises:
measuring a change in the phase difference at each segment along the optical fiber; and
unwrapping the measured change in the phase difference.
6. The method of claim 1 , wherein determining the shape of the optical fiber comprises tracking a pointing vector along the optical fiber.
7. The method of claim 6 , wherein tracking the pointing vector comprises computing two orthogonal differential strain signals from derivatives of the phase differences tracked for two of the multiple cores.
8. The method of claim 7 , wherein tracking the pointing vector further comprises:
using a discrete representation of the orthogonal differential strain signals to compute rotation matrices for segments along the optical fiber; and
computing local pointing vectors from the rotation matrices.
9. The method of claim 8 , wherein determining the shape of the optical fiber further comprises determining a position and direction at a point along the optical fiber by summing all local pointing vectors up to that point.
10. The method of claim 7 , wherein the differential orthogonal strain signals are based in part on wobble and twist signals measured along the optical fiber.
11. A shape-sensing system for a multi-core optical fiber, the system comprising:
a plurality of acquisition interferometers and associated data acquisition networks to measure reflected light signals of a plurality of cores of the optical fiber; and
a system controller and data processor to process the measured reflected light signals to:
continuously track, for each core of the plurality of cores, a phase difference accumulated along the optical fiber, the phase difference being between phases of the measured reflected light signals and reference reflected light signals; and
based on the continuously tracked phase differences for the plurality of cores, determine a shape of the optical fiber.
12. The shape-sensing system of claim 11 , further comprising a tunable laser, wherein the system controller and data processor further initiate sweeps of the tunable laser over a defined wavelength range to interrogate the plurality of cores of the optical fiber.
13. The shape-sensing system of claim 11 , wherein the system controller and data processor aligns the measured reflected light signals with the reference reflected light signal to account for a shift in apparent location of segments of the optical fiber between a measurement state and a reference state.
14. The shape-sensing system of claim 11 , wherein the system controller and data processor is to determine the shape of the optical fiber by tracking a pointing vector along the fiber.
15. The shape-sensing system of claim 14 , wherein the system controller and data processor is to track the pointing vector by:
computing two orthogonal differential strain signals from derivatives of the phase differences tracked for two of the plurality of cores;
using a discrete representation of the orthogonal differential strain signals to compute rotation matrices for segments along the optical fiber; and
computing local pointing vectors from the rotation matrices.
16. The shape-sensing system of claim 15 , wherein the system controller and data processor is to determine the shape of the optical fiber by determining positions and directions at points along the optical fiber by summing all local pointing vectors up to those points.
17. The shape-sensing system claim 11 , wherein the phase difference is caused by at least one of distortions within a continuous Bragg grating along the core or distortions within a Rayleigh scatter pattern.
18. A non-transitory computer-readable storage medium storing program instructions for processing reflected light signals measured for multiple cores of a multi-core optical fiber in a measurement state and a reference state, the program instructions, when executed by a computer, causing the computer to perform operations comprising:
for each core of multiple cores of the optical fiber, continuously tracking a phase difference accumulated along the optical fiber, the phase difference being between phases of the reflected light signals in the measurement state of the optical fiber and in the reference state; and
based on the continuously tracked phase differences for the multiple cores, determining a shape of the optical fiber.
19. The non-transitory computer-readable storage medium of claim 18 , wherein continuously tracking the phase difference comprises aligning the reflected light signals between the measurement state and the reference state to account for a shift in apparent location of segments of the optical fiber between the measurement state and the reference state.
20. The non-transitory computer-readable storage medium of claim 18 , wherein continuously tracking the phase difference comprises:
measuring a change in the phase difference at each segment along the optical fiber; and
unwrapping the measured change in the phase difference.