Optical fiber device for detecting three-dimensional shape and shape reconstruction method

Abstract

According to the optical fiber device for detecting the three-dimensional shape, scattering points are inscribed on a cladding of a single-mode optical fiber, the number of projections of the scattering points on the end face of the single-mode optical fiber is at least two, the scattering points corresponding to each projection form a group, each group comprises at least two scattering points, and the scattering points in each group and the scattering points in other groups are arranged in a staggered mode. By detecting the energy change of each scattering point and calculating the curvature component of the optical fiber, high-precision measurement of the bending state of the optical fiber can be realized, and meanwhile, the two-dimensional bending state of a single point is reflected by utilizing the scattering points which are distributed in multiple directions in the radial direction, and the three-dimensional bending state along the optical fiber is obtained by combining the scattering points which are distributed in the axial direction of the optical fiber in a staggered manner. According to the invention, the measurement of the three-dimensional shape is completed only by using a single-channel sensing signal measured in one optical fiber at a time, so that the size of an optical fiber shape measurement device and the complexity of signal measurement are greatly reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical elements, and in particular relates to an optical fiber device for detecting three-dimensional shapes and a shape reconstruction method. Background Art

[0002] Soft robotics technology has rapidly gained popularity worldwide, and their superior performance in terms of compliance, flexibility, and robustness has been widely recognized. With the continuous expansion of applications, the core components of soft robotic systems—actuation, sensing, and control—are facing greater demands for integration and simplification. Among these core components, there is a growing demand for high-precision, high-resolution shape perception in robotic systems. Traditionally, robotic sensing systems have been primarily provided by electrical sensors. However, electrical sensors face significant challenges in reusability and large-scale integration, limiting their further application in the field of soft robotics.

[0003] Fiber-optic three-dimensional shape sensors are an emerging shape sensing solution in the field of flexible robotics. They can measure three-dimensional spatial information, such as the posture, orientation, trajectory, and position of an optical fiber or its attached object, in real time. Compared to traditional electrical sensors, fiber-optic sensors offer numerous advantages, including small size, flexibility, high precision, electromagnetic immunity, and in-situ monitoring capabilities. Existing shape sensing technologies are primarily based on fiber Bragg gratings (FBGs) and optical frequency domain reflectometry (OFDR). However, these two solutions rely on multi-channel fiber bundles or multi-core optical fibers, requiring complex system components and high multi-channel signal demodulation costs. Furthermore, they require a large sensor size (>1 mm), limiting sensor miniaturization and large-scale integration. Therefore, a new single-channel three-dimensional fiber-optic shape sensor device is urgently needed to achieve smaller, higher-resolution shape sensing capabilities. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide an optical fiber device for detecting three-dimensional shapes and a shape reconstruction method with reasonable design, small size and high precision.

[0005] The technical solution adopted to solve the above technical problems is: an optical fiber device for detecting three-dimensional shapes, wherein scattering points are engraved on the cladding of a single-mode optical fiber, the number of projections of the scattering points on the end face of the single-mode optical fiber is at least 2, and the scattering points corresponding to each projection are a group, each group has at least 2 scattering points, and each group of scattering points is axially offset from other groups of scattering points along the axial direction.

[0006] As a preferred technical solution, the scattering points are inscribed in a cladding region that is 10 μm away from the fiber core in the radial direction.

[0007] As a preferred technical solution, the scattering points are elliptical refractive index modulation points generated by femtosecond laser direct writing.

[0008] As a preferred technical solution, the scattering point has a long axis of 5 μm, a short axis of 1 μm, and a depth of 1 μm.

[0009] As a preferred technical solution, the distance L between two adjacent scattering points in the axial direction of the single-mode optical fiber is 1 μm to 1 cm.

[0010] As a preferred technical solution, the scattering points are in 2 groups and their projections on the end face of the single-mode optical fiber are distributed orthogonally; or the scattering points are in 4 groups and their projections on the end face of the single-mode optical fiber are distributed in a cross; or the scattering points are in 3 groups and their projections on the end face of the single-mode optical fiber are distributed in an equilateral triangle or an equilateral triangle; or the scattering points are in 6 groups and their projections on the end face of the single-mode optical fiber are distributed in a regular hexagon.

[0011] As a preferred technical solution, there are at least two groups of scattering points, and their projections on the end face of the single-mode optical fiber are randomly distributed.

[0012] A shape reconstruction method for an optical fiber device for detecting a three-dimensional shape comprises the following steps: Step 1. Record the peak energy of the reflected signal of the scattering point array in the straight state of the single-mode optical fiber and set it as the reference value. Subtract the reference value from the peak energy during the bending deformation process to get the energy change. ΔI i , i is the i-th scattering point; Step 2. Obtain the corresponding curvature component based on the energy change of each scattering point , c is the curvature sensitivity coefficient; Step 3. For the scattering points arranged in multiple radial directions, the scattering points in each direction provide the curvature component in that direction. The curvature components in different directions are fused through geometric relationships to obtain the total curvature. and bending angle ; Step 4. Total curvature and bending angle Perform function interpolation to obtain the curvature function and angle function, and differentiate the angle function to obtain the torsion function; Step 5. Substitute the curvature function and torsion function into the Frenet geometry framework and numerically solve to obtain the tangent vector function; Step 6. Integrate the tangent vector function along the arc length to obtain the coordinates of each point in space, and then perform curve fitting to obtain a smooth curve in space, thereby reconstructing the three-dimensional shape.

[0013] The beneficial effects of the present invention are as follows: The present invention accurately calculates the curvature component of the optical fiber by detecting the energy change at each scattering point, thereby achieving high-precision measurement of the bending state of the optical fiber and meeting the requirements for shape perception in fields such as precision measurement and minimally invasive medical surgery.

[0014] The present invention utilizes scattering points arranged in multiple radial directions to fully reflect the two-dimensional bending state of a single point. By combining scattering points that are offset along the optical fiber axis, the three-dimensional bending state along the optical fiber is obtained. This three-dimensional shape reconstruction capability enables the present invention to be applied to the monitoring of complex shapes, such as the position and shape monitoring of medical catheters within the body.

[0015] The present invention is insensitive to temperature and strain changes, exhibiting excellent temperature and strain stability, and thus strong resistance to environmental interference. It can achieve millimeter-level shape sensing through a single optical fiber, accurately capturing minute shape changes along the fiber's length, achieving high-precision shape monitoring with high spatial resolution.

[0016] The present invention does not require spectral encoding, breaks through the limitation of Fiber Bragg Grating (FBG) scheme on spectral resources, arranges more sensing points in the same optical fiber, and thus realizes more intensive shape monitoring.

[0017] The present invention only requires a single fiber optic sensing channel, breaking through the optical frequency domain reflectometry (OFDR) solution's demand for multi-channel sensing and demodulation resources. A single optical fiber has a smaller sensor size and can be used for shape measurement in smaller spaces. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 Schematic diagram of the structure of an optical fiber device for detecting three-dimensional shapes according to embodiment 1 of the present invention.

[0019] Figure 2 Schematic diagram of the projection of the scattering point on the end face of the single-mode optical fiber in Example 2 of the present invention.

[0020] Figure 3 Schematic diagram of the projection of the scattering point on the end face of the single-mode optical fiber in Example 3 of the present invention.

[0021] Figure 4 Schematic diagram of the projection of the scattering point on the end face of the single-mode optical fiber in Example 4 of the present invention.

[0022] Figure 5 Schematic diagram of the projection of the scattering point on the end face of the single-mode optical fiber in Example 5 of the present invention.

[0023] Figure 6 3 is a backscattered signal profile diagram of a scattering point of the optical fiber sensor of Example 1.

[0024] Figure 71 is a graph showing the intensity variation of six adjacent backscattered signals of the optical fiber sensor of Example 1 at different optical fiber bending angles.

[0025] Figure 8 3 is a graph showing the changing trend of the bending angle-intensity sensitivity of the backscattered signals of two adjacent scattering points of the optical fiber sensor in Example 1.

[0026] Figure 9 Graphs showing curvature, angle, and torsion measured at 36 scattering points when the optical fiber sensor of Example 1 is deformed into arcs of different radii.

[0027] Figure 10 These are the reconstructed curve shapes when the optical fiber sensor of Example 1 is deformed into arcs of different radii.

[0028] Figure 11 1 is a graph of the curve shape reconstructed by the optical fiber sensor of Example 1 at different temperatures.

[0029] Figure 12 1 is a graph showing the relationship between the error in the shape of the curve reconstructed by the optical fiber sensor of Example 1 and the temperature change.

[0030] Figure 13 3 is a graph of the curve shape reconstructed under different strains of the optical fiber sensor of Example 1.

[0031] Figure 14 3 is a graph showing the relationship between the error in the curve shape reconstructed by the optical fiber sensor of Example 1 and the strain change. DETAILED DESCRIPTION

[0032] The present invention will be further described in detail below with reference to the accompanying drawings and examples, but the present invention is not limited to the following embodiments. Example 1

[0033] exist Figure 1 In the optical fiber device for detecting three-dimensional shapes of this embodiment, the elliptical refractive index modulation points generated by femtosecond laser direct writing on the cladding of a single-mode optical fiber are scattering points. The number of projections of the scattering points on the end face of the single-mode optical fiber is 2, and each projection corresponds to a group of scattering points, with each group consisting of 36 scattering points. The projections of the two groups of scattering points on the end face of the single-mode optical fiber are orthogonally distributed and axially offset. The two groups of scattering points are distributed in the cladding region at a radial distance of 10 μm from the fiber core, and the axial offset L is 2.5 mm. The scattering points are elliptical points with a major axis of 5 μm, a minor axis of 1 μm, and a depth of 1 μm.

[0034] When a single-mode fiber bends, the optical field intensity within the fiber shifts, concentrating the energy in a specific direction. Each scattering point experiences a significant increase in light signal scattering. The intensity of the scattered signal at this cladding scattering point is related to the optical field intensity at that location. Therefore, the intensity of the scattered signal at that point reflects the one-dimensional bending state of the fiber at that point. By arranging scattering points in multiple radial directions, the two-dimensional bending state of a single point can be fully characterized. By combining the misalignment of the scattering points along the fiber's axial dimension, the three-dimensional bending state along the fiber can be determined.

[0035] The sensor of this embodiment uses single-channel optical frequency domain reflectometry to measure the backscattered signal in a single-mode optical fiber and extract the scattering peak value of the cladding scattering point, thereby achieving sensor signal detection. Optical frequency domain reflectometry is a well-known and mature optical fiber detection method. The main technical specifications of the optical frequency domain reflectometry selected in this embodiment are: spatial resolution greater than the axial offset distance of the scattering point, so as to distinguish the scattering peak value of each scattering point; and detection length greater than the length of the prepared cladding scatterer fiber.

[0036] The shape reconstruction method of an optical fiber device for detecting a three-dimensional shape of this embodiment includes the following steps: Step 1. Record the peak energy of the reflected signal of the scattering point array in the straight state of the single-mode optical fiber and set it as the reference value. Subtract the reference value from the peak energy during the bending deformation process to get the energy change. ΔI i , i is the i-th scattering point; Step 2. Obtain the corresponding curvature component based on the energy change of each scattering point , , c is the curvature sensitivity coefficient; Step 3. For the scattering points arranged in multiple radial directions, the scattering points in each direction provide the curvature component in that direction. The curvature components in different directions are fused through geometric relationships to obtain the total curvature. and bending angle ; Where, is the curvature component of the i-th scattering point in the X direction, is the curvature component of the i-th scattering point in the Y direction; this X / Y coordinate system can be optionally referenced to the measurement plane. Maintaining consistency within a single optical fiber ensures the correct shape of the curve, and there is no strict requirement. Step 4. Total curvature and bending angle Perform function interpolation to obtain the curvature function and angle function, and differentiate the angle function to obtain the torsion function; Step 5. Substitute the curvature function and torsion function into the Frenet geometry framework and numerically solve them to obtain the tangent vector function; Step 6. Integrate the tangent vector function along the arc length to obtain the coordinates of each point in space, and then perform curve fitting to obtain a smooth curve in space, thereby reconstructing the three-dimensional shape. Example 2

[0037] like Figure 2 In this embodiment of the optical fiber device for detecting three-dimensional shapes, four groups of scattering points, each with at least two, are inscribed on the cladding of a single-mode optical fiber. The projections of the four groups of scattering points on the end face of the single-mode optical fiber are arranged in a cross pattern. Each group of scattering points is staggered from the other groups. The scattering points are inscribed within a cladding region 10 μm radially from the fiber core. The distance L between two adjacent scattering points in the axial direction is 20 μm to 100 μm. The scattering points are inscribed within a cladding region 10 μm radially from the fiber core. The scattering points are elliptical, with a major axis of 5 μm, a minor axis of 1 μm, and a depth of 1 μm.

[0038] The shape reconstruction method of the optical fiber device for detecting the three-dimensional shape of this embodiment is the same as that of the first embodiment. Example 3

[0039] like Figure 3 In this embodiment of the optical fiber device for detecting three-dimensional shapes, three groups of scattering points are inscribed on the cladding of a single-mode optical fiber, each group having at least two scattering points. The projections of the three groups of scattering points on the end face of the single-mode optical fiber form a regular triangle or regular triangle distribution. Each group of scattering points is staggered from the other groups of scattering points. The distance L between two adjacent scattering points in the axial direction is 20μm-200μm. The scattering points are inscribed within a cladding region 10μm radially from the fiber core. The scattering points are elliptical, with a major axis of 5μm, a minor axis of 1μm, and a depth of 1μm.

[0040] The shape reconstruction method of the optical fiber device for detecting the three-dimensional shape of this embodiment is the same as that of the first embodiment. Example 4

[0041] like Figure 4 In this embodiment of the optical fiber device for detecting three-dimensional shapes, six groups of scattering points are inscribed on the cladding of a single-mode optical fiber, each group containing at least two scattering points. The projections of the six groups of scattering points on the end face of the single-mode optical fiber form a regular hexagonal distribution. Each group of scattering points is staggered with respect to the other groups, and the distance between two adjacent scattering points in the axial direction is 1 cm. The scattering points are inscribed within a cladding region 10 μm radially from the fiber core. The scattering points are elliptical, with a major axis of 5 μm, a minor axis of 1 μm, and a depth of 1 μm.

[0042] The shape reconstruction method of the optical fiber device for detecting the three-dimensional shape of this embodiment is the same as that of the first embodiment. Example 5

[0043] like Figure 5 In this embodiment of the optical fiber device for detecting three-dimensional shapes, seven groups of scattering points are inscribed on the cladding of a single-mode optical fiber, each group containing at least two scattering points. The projections of the seven scattering points on the end face of the single-mode optical fiber are randomly distributed. Each group of scattering points is staggered with respect to the other groups. The distance L between two adjacent scattering points in the axial direction is 1 cm. The scattering points are inscribed within a cladding region 10 μm radially from the fiber core. The scattering points are elliptical, with a major axis of 5 μm, a minor axis of 1 μm, and a depth of 1 μm.

[0044] The shape reconstruction method of the optical fiber device for detecting the three-dimensional shape of this embodiment is the same as that of the first embodiment.

[0045] experiment In order to verify the beneficial effects of the present invention, the inventors used the optical fiber device for detecting three-dimensional shapes of Example 1 to perform the following measurements: Measuring the backscattered signal of the scattering point, such as Figure 6 , the scattering signal enhancement amplitude of each scattering point is greater than 30dB, and the amplitude difference between each scatterer is about 5.7dB.

[0046] Measured at curvature C=20m -1 When the optical fiber is rotated 540°, the intensity of the six adjacent cladding scattering points changes, such as Figure 7 , the intensity of each scattering point shows a trigonometric function change, and the peak change trend of each mutually perpendicular scatterer differs by 90°.

[0047] The peak intensity at different curvature values was measured. Figure 8 As the curvature increases, the amplitude of the reflected intensity also increases accordingly, further confirming the high sensitivity of the cladding scattering points to changes in curvature. At different bending angles, the intensity changes of the scattering points show a certain regularity, proving that the cladding scatterer has the ability to recognize curvature and angle.

[0048] The inventors used the shape reconstruction method for optical fiber devices for detecting three-dimensional shapes in Example 1 to verify the shape reconstruction of arc shapes with different radii. The arc radii were 6 cm, 7 cm, 8 cm, 9 cm, 10 cm, 11 cm, and 12 cm, and the curvature, angle, and torsion functions calculated by measuring the energy change values were as follows: Figure 9 As shown. The reconstructed arc shape is as follows Figure 10 The final curve has an error of less than 8% at the end and a relative error of less than 6% for the entire curve.

[0049] The inventors also verified the impact of environmental changes, namely temperature and strain changes, on the reconstruction effect of the curve shape, and the results are as follows: In the temperature range of 20℃~60℃, the curve shapes reconstructed at different temperatures are as follows: Figure 11 As shown in Figure 2, the effect of temperature change on the sensor signal is small. The experimental results show that the effect of temperature change on the shape reconstruction error is less than 0.052% / ℃, indicating that the present invention has good temperature stability. Figure 12 shown.

[0050] In the strain range of 0 to 600µε, the reconstructed curve shapes under different strains are as follows: Figure 13 As shown in Figure 2, the strain change has little effect on the sensing signal. The experimental results show that the effect of strain change on the shape reconstruction error is less than 0.0074% / µε, indicating that the present invention has good strain stability. Figure 14 shown.

Claims

1. An optical fiber device for detecting three-dimensional shapes, characterized in that: Scattering points are engraved on the cladding of the single-mode optical fiber. The number of projections of the scattering points on the end face of the single-mode optical fiber is at least 2, and each projection corresponds to a group of scattering points. Each group of scattering points has at least 2 scattering points, and each group of scattering points is axially offset from other groups of scattering points along the axial direction.

2. The optical fiber device for detecting three-dimensional shapes according to claim 1, characterized in that: The scattering point is inscribed in the cladding region which is 10 μm away from the core in the radial direction.

3. The optical fiber device for detecting three-dimensional shapes according to claim 1, characterized in that: The scattering points are elliptical refractive index modulation points generated by femtosecond laser direct writing.

4. The optical fiber device for detecting three-dimensional shapes according to claim 3, characterized in that: The scattering point has a major axis of 5 μm, a minor axis of 1 μm, and a depth of 1 μm.

5. The optical fiber device for detecting three-dimensional shapes according to claim 1, characterized in that: The distance L between two adjacent scattering points in the axial direction of the single-mode optical fiber is 1 μm to 1 cm.

6. The optical fiber device for detecting three-dimensional shapes according to claim 1, characterized in that: The scattering points are in two groups and their projections on the end face of the single-mode optical fiber are orthogonally distributed; or the scattering points are in four groups and their projections on the end face of the single-mode optical fiber are cross-distributed; or the scattering points are in three groups and their projections on the end face of the single-mode optical fiber are distributed in an equilateral triangle or an equilateral triangle; or the scattering points are in six groups and their projections on the end face of the single-mode optical fiber are distributed in a regular hexagon.

7. The optical fiber device for detecting three-dimensional shapes according to claim 1, characterized in that: There are at least two groups of scattering points, and their projections on the end face of the single-mode optical fiber are randomly distributed.

8. The shape reconstruction method of an optical fiber device for detecting a three-dimensional shape according to any one of claims 1 to 7, characterized in that: The following steps are involved: Step 1. Record the peak energy of the reflected signal of the scattering point array in the straight state of the single-mode optical fiber and set it as the reference value. Subtract the reference value from the peak energy during the bending deformation process to get the energy change. ΔI i , i is the i-th scattering point; Step 2. Obtain the corresponding curvature component based on the energy change of each scattering point , , c is the curvature sensitivity coefficient; Step 3. For the scattering points arranged in multiple radial directions, the scattering points in each direction provide the curvature component in that direction. The curvature components in different directions are fused through geometric relationships to obtain the total curvature. and bending angle ; Step 4. Total curvature and bending angle Perform function interpolation to obtain the curvature function and angle function, and differentiate the angle function to obtain the torsion function; Step 5. Substitute the curvature function and torsion function into the Frenet geometry framework and numerically solve them to obtain the tangent vector function; Step 6. Integrate the tangent vector function along the arc length to obtain the coordinates of each point in space, and then perform curve fitting to obtain a smooth curve in space, thereby reconstructing the three-dimensional shape.

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