Methods and apparatus to determine a shape of an optical fiber sensor
To sense the shape of a multicore optical fiber sensor, light reflected in a center and two or more helixed outer cores of the optical fiber sensor is measured, and phases associated with strain in the center and helixed outer cores is tracked along the length of the fiber sensor. Further, a wobble signal indicative of a variation in the spin rate of the fiber sensor is determined. Based on the tracked phases and the wobble signal, the fiber shape is computed.
1. A shape-sensing method for an optical fiber sensor, the optical fiber sensor comprising a center core and two or more helixed outer cores, the method comprising:
measuring light reflected in the center core and the two or more helixed outer cores of the optical fiber sensor to track phases associated with strain in the center core and the two or more helixed outer cores along a length of the optical fiber sensor;
determining a wobble signal indicative of a variation in spin rate of the two or more helixed outer cores along the length of the optical fiber sensor; and
computing a shape of the optical fiber sensor based at least in part on the tracked phases and the determined wobble signal.
2. The method of claim 1 , further comprising determining a twist signal indicative of an extrinsic twist applied to the optical fiber sensor.
3. The method of claim 2 , wherein determining the twist signal comprises:
subtracting the phase associated with strain in the center core from an average of the phases associated with strain in the two or more helixed outer cores.
4. The method of claim 1 , wherein computing the shape of the optical fiber based at least in part on the tracked phases and the determined wobble signal comprises:
determining rotational positions of the two or more helixed outer cores based on the wobble signal; and
computing the shape of the optical fiber sensor based at least in part on the tracked phases and the rotational positions.
5. The method of claim 1 , further comprising tracking the phases by:
aligning a reference scan of the optical fiber sensor in a known shape against a measurement scan of the optical fiber sensor to maintain coherence; and
comparing the reference scan with the measurement scan.
6. The method of claim 1 , wherein computing the shape of the optical fiber sensor comprises:
determining first and second orthogonal differential strain signals based at least in part on the tracked phases and at least one signal selected from the group consisting of: a twist signal and the wobble signal;
creating a rotation matrix comprising bend angles determined based on the first and second orthogonal differential strain signals;
using the rotation matrix to iteratively compute pointing vectors along the length of the optical fiber sensor; and
computing the shape of the optical fiber sensor based on sums of the pointing vectors.
7. The method of claim 1 , wherein determining the wobble signal comprises:
measuring, as a function of distance along the length of the optical fiber sensor, a shift of a phase in at least one core of the two or more helixed outer cores relative to an expected phase while the optical fiber sensor is placed in a configuration causing a continuous bend in a single plane.
8. A system for measuring a shape of an optical fiber sensor that comprises a center core and two or more helixed outer cores, the system comprising:
interferometers configured to measure light reflected in the center core and the two or more helixed outer cores of the optical fiber sensor; and
a system controller and data processor configured to:
process the light to track phases associated with strain in the center core and the two or more helixed outer cores along a length of the optical fiber sensor;
determine a wobble signal indicative of a variation in spin rate of the two or more helixed outer cores along the length of the optical fiber sensor; and
compute a shape of the optical fiber sensor based at least in part on the tracked phases and the determined wobble signal.
9. The system of claim 8 , wherein the system controller and data processor is further configured to determine a twist signal indicative of an extrinsic twist applied to the optical fiber sensor.
10. The system of claim 9 , wherein determining the twist signal comprises:
subtracting the phase associated with strain in the center core from an average of the phases associated with strain in the two or more helixed outer cores.
11. The system of claim 8 , wherein the system controller and data processor is configured to compute the shape of the optical fiber sensor based at least in part on the tracked phases and the determined wobble signal by:
determining rotational positions of the two or more helixed outer cores based at least in part on the wobble signal; and
computing the shape of the optical fiber sensor based at least in part on the tracked phases and the determined rotational positions.
12. The system of claim 8 , wherein processing the light to track the phases comprises:
comparing a reference scan of the optical fiber sensor in a known shape against a measurement scan of the optical fiber sensor.
13. The system of claim 12 , wherein comparing the reference scan of the optical fiber sensor in the known shape against the measurement scan of the optical fiber sensor comprises:
using Rayleigh scatter signals in the reference and measurement scans.
14. The system of claim 12 , wherein processing the light to track the phases comprises:
aligning the reference and measurement scans to maintain coherence.
15. The system of claim 8 , wherein determining the wobble signal comprises:
measuring, as a function of distance along the length of the optical fiber sensor, a shift of a phase in at least one core of the two or more helixed outer cores relative to an expected phase while the optical fiber sensor is placed in a configuration causing a continuous bend in a single plane.
16. The system of claim 8 , wherein each core of the center core and the two or more helixed outer cores comprises gratings continuously along the length of the optical fiber sensor.
17. The system of claim 8 , wherein computing the shape of the optical fiber sensor comprises:
determining first and second orthogonal differential strain signals based at least in part on the tracked phases and at least one signal selected from the group consisting of: a twist signal and the wobble signal;
creating a rotation matrix comprising bend angles determined based on the first and second orthogonal differential strain signals;
using the rotation matrix to iteratively compute pointing vectors along the length of the optical fiber sensor; and
computing the shape of the optical fiber sensor based on sums of the pointing vectors.
18. The system of claim 8 , wherein:
the interferometers are configured to measure the light at two orthogonal polarization states; and
the system controller and data processor is configured to, when processing the light to track the phases, correct the tracked phase for birefringence based on measurements at the two orthogonal polarization states.
19. The system of claim 8 , wherein the system controller and data processor is configured to process the light to track the phases by: tracking the phases continuously along an entirety of the length of the optical fiber sensor.
20. A computer-readable storage medium storing processor-executable instructions for determining a shape of an optical fiber sensor by processing tracked phases, the optical fiber sensor comprising a center core and two or more helixed outer cores, and the tracked phases being associated with strains in the center core and the two or more helixed outer cores, wherein the instructions, when executed by one or more processors, cause the one or more processors to perform operations comprising:
determining a wobble signal indicative of a variation in spin rate of the two or more helixed outer cores along a length of the optical fiber sensor; and
computing a shape of the optical fiber sensor based at least in part on the tracked phases and the determined wobble signal.