CN113701656B - Single-channel circulating series connection type multi-core optical fiber shape sensor - Google Patents
Single-channel circulating series connection type multi-core optical fiber shape sensor Download PDFInfo
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Abstract
The invention provides a single-channel circulating series connection type multi-core optical fiber shape sensor, which is characterized in that: the optical fiber comprises a seven-core optical fiber, a seven-core optical fiber Fan-in device and a 45-degree cone round table at one end of the seven-core optical fiber; the seven-core optical fiber is provided with a middle core and three pairs of peripheral fiber cores which are distributed in a hexagon shape; the 45-degree cone frustum can reflect light beams transmitted by the peripheral fiber cores, so that the light beams enter the symmetrical fiber cores after being reflected twice and are transmitted reversely; and sequentially connecting each fiber core of the seven-core optical fiber in series through the seven-core optical fiber Fan-in device and the 45-degree cone frustum at the fiber end, thereby realizing the function of mapping the seven fiber cores into a one-dimensional topological light path. The invention can be used for three-dimensional shape sensing, has the characteristics of simple structure, single-end freedom and single-channel use, and can be widely used for shape monitoring of medical instruments, spacecraft wings and other equipment.
Description
Technical Field
The invention relates to a single-channel circulating series-connection type multi-core optical fiber shape sensor which can be used for monitoring the health of an intelligent structure, can also be used for detecting the shape change of a skin structure of a robot or an airplane wing in real time, and belongs to the technical field of distributed optical fiber deformation sensing.
Background
Optical fiber deformation sensing is a distributed sensing technology, which detects information such as bending and torsion of an optical fiber by using a backscattering signal generated by local strain of the optical fiber, and then processes the information to reconstruct the spatial deformation of the optical fiber. The technology has wide application value in the fields of medical treatment, energy, national defense, aerospace, structural safety monitoring, other intelligent structures and the like.
The optical fiber shape sensing may be classified into a single-mode optical fiber beam combining scheme and a multi-core optical fiber scheme according to the kind of optical fiber used. The single-mode optical fiber beam combination is formed by combining and packaging a plurality of common single-mode optical fibers, has certain complexity and difficulty in the aspects of packaging and calibration of the shape sensor, and has higher shape resolution due to larger fiber core spacing. The multi-core optical fiber shape sensor utilizes a multi-core optical fiber to replace a single-mode optical fiber bundle, so that the shape sensor has the advantages of more compact volume, better sensor consistency and more convenient packaging and use.
The patent CN110243301a provides a dynamic BOTDA-based core-by-core scanning type multi-core fiber shape sensor, which uses an optical switch to realize core-by-core scanning detection of a multi-core fiber, so as to obtain brillouin scattering light signals of each fiber core, and then demodulates the brillouin scattering light signals to obtain the shape change of the fiber. In the application of the sensor, two multi-channel optical switches and two multi-core optical fiber fan-out devices need to be adopted, and two ends of a multi-core optical fiber need to be connected with a BOTDA instrument, so that the use of the multi-core optical fiber shape sensor is limited. For example, in medical applications of multicore fiber shape sensors, the sensor-binding catheter needs to be inserted into the body, which requires that a section of the multicore fiber remains free of connections.
The invention discloses a multi-core circulating series-connection type optical fiber shape sensor based on dynamic BOTDA (Brillouin optical time domain reflectometer). According to the invention, a plurality of fiber cores of a multi-core optical fiber are connected into a single channel in a channel series connection mode of fan-in fan-out devices at two ends of the multi-core optical fiber, so that two optical switches are omitted, the sensor structure is simplified, but two ends of the multi-core optical fiber are also required to be connected to a BOTDA instrument, free ends do not exist, and the application range of shape sensing is limited.
Patent CN110243302B proposes a reflective multi-core circulating tandem fiber shape sensor. The invention realizes the serial connection of each fiber core of the multi-core optical fiber by adopting a plurality of circulators and the plane reflecting mirror at the fiber end, and realizes the purpose of single-end freedom. However, the sensor includes a plurality of circulators, which not only complicates the sensor structure, but also increases transmission loss.
Disclosure of Invention
The invention aims to provide a single-channel circulating series connection type multi-core optical fiber shape sensor which is simple and compact in structure and free at one end.
The purpose of the invention is realized as follows:
a single-channel circulating series connection type multi-core optical fiber shape sensor comprises a seven-core optical fiber, a seven-core optical fiber Fan-in device and a 45-degree cone round table at one end of the seven-core optical fiber; the seven-core optical fiber is provided with a middle core and three pairs of peripheral fiber cores which are distributed in a hexagon shape; the cone frustum with the angle of 45 degrees can reflect light beams transmitted by the peripheral fiber cores, so that the light beams enter the symmetrical fiber cores after being reflected twice and are transmitted reversely; and sequentially connecting each fiber core of the seven-core optical fiber in series through the seven-core optical fiber Fan-in device and the 45-degree cone frustum at the fiber end, thereby realizing the function of mapping the seven fiber cores into a one-dimensional topological light path.
In order to reduce the reflection coupling loss of the 45-degree cone circular truncated cone, a light beam is efficiently coupled from a peripheral fiber core to a symmetrical peripheral fiber core of the light beam, and the 45-degree cone circular truncated cone is an optimized arc-shaped reflection cone circular truncated cone. Preferably, the rotational symmetry plane of the arc cone frustum meets the equation of a paraboloid, the surface in the shape can reflect and focus the light beam, and the diffraction diffusion effect of the light beam in the transmission process after reflection is counteracted, so that the coupling efficiency is improved.
Preferably, in order to adapt to different application environments of the sensor, such as a liquid environment, the surface of the 45-degree cone frustum is coated with a reflective film to ensure that the light beam can be subjected to total reflection rather than refraction.
The shape sensor acquires one-dimensional data information, and the one-dimensional data information is subjected to segmented mapping and extraction to obtain deformation information corresponding to each fiber core of the seven-core optical fiber.
The data information of six peripheral fiber cores and the data information of the middle fiber core of the seven-core optical fiber are subjected to difference, so that the influence of temperature distribution change on the demodulation of the shape of the optical fiber can be eliminated.
The data information of the three pairs of peripheral fiber cores can be divided into two independent groups of forward transmission and reverse transmission, the two groups of data can be verified mutually, and the shape of the optical fiber is demodulated after averaging, so that the shape demodulation precision is improved.
Except the fiber end of the seven-core optical fiber at the position of the 45-degree cone frustum, other shape sensing positions can be prepared into a spiral structure, so that six peripheral fiber cores are spirally distributed around the circumference of the middle core.
The demodulation equipment of the shape sensor is OFDR,
BOTDR or BOTDA.
Compared with the prior art, the invention has at least the following advantages:
(1) Compared with other multi-core optical fiber shape sensors, the multi-core optical fiber shape sensor has the characteristic of single-channel transmission, and redundant optical switches or a plurality of light sources are not needed to scan each fiber core channel respectively.
(2) The invention uses the reflecting structure of the multicore fiber end to complete the purpose of fiber core serial connection, and has simple structure. More importantly, it enables single-ended freedom, which makes possible the application of fiber optic shape sensors in interventional medicine.
Drawings
Fig. 1 is a view showing an end face structure of a seven-core optical fiber used in the present invention.
Fig. 2 (a) is a three-dimensional diagram of a 45-degree cone frustum reflection structure of a seven-core optical fiber end, and fig. 2 (b) is a three-dimensional diagram of an arc-shaped cone frustum obtained by optimization on the basis of (a).
FIG. 3 is a method of making an arcuate pyramidal reflective frustum.
Fig. 4 (a) is an axial cross section of a cone frustum structure with a base angle of 45 degrees, and fig. 4 (b) is an axial cross section of an arc-shaped cone reflection structure.
Fig. 5 (a) and (b) are graphs of simulation results of the directions of light beams at the fiber ends before and after the cone frustum at the fiber end of the multi-core optical fiber is optimized in an arc shape.
Fig. 6 is a diagram of a system of the present invention as a three-dimensional shape sensor.
Fig. 7 is a schematic diagram of the case where one-dimensional data sequence segments are mapped to each core. The optical fiber comprises (a) one-dimensional data information acquired by a BOTDR system, (b) deformation information of each fiber core obtained by mapping, and (c) a seven-core optical fiber.
Fig. 8 is a schematic diagram of the working principle of the bending sensor of the invention: (a) The cross-sectional view is schematically shown (plane N-N' is the neutral plane of the fiber bend, 1 is the azimuth angle of the core 1 with respect to the y-axis, and the distances of the core from the center of the cladding are r, theta b Is the angle between the bending direction of the optical fiber and the y-axis); (b) fiber bend schematic.
Fig. 9 is a schematic diagram of the working principle of the torsion sensor of the present invention: (a) a schematic diagram of a sensing principle; (b) twisting the multi-core fiber schematic.
Detailed Description
The invention is further illustrated below with reference to specific examples.
Firstly, a 45-degree cone frustum reflection structure at the fiber end of the sensor is introduced.
A seven-core fiber as shown in fig. 1 was selected, having a central core and six side cores in a regular hexagonal distribution. In order to reduce the crosstalk of the core member, a low-refractive-index spacer layer may be added to the core.
As shown in fig. 2, a rotationally symmetric cone-shaped reflecting circular truncated cone is provided at the fiber end of the seven-core optical fiber. Wherein fig. 2 (a) is a cone-shaped reflecting circular table with a base angle of 45 degrees, and fig. 2 (b) is an arc-shaped cone-shaped reflecting circular table optimized on the basis of (a).
The manufacturing method of the arc-shaped cone reflection truncated cone is shown in fig. 3, and is specifically described as follows:
step 1: and a section of coreless optical fiber 2 is welded at the fiber end of the seven-core optical fiber 1 and is cut flatly. The core of the seven-core optical fiber 1 is at a distance d =42 μm from the center, and the length of the remaining coreless fiber 2 is about 60 μm.
And 2, step: the coreless optical fiber at the fiber end of the seven-core optical fiber 1 is ground into a cone round table 3 with a bottom angle of 45 degrees.
And 3, step 3: and (4) placing the 45-degree cone round table 3 in the step (3) under an electrode for high-temperature polishing and shaping to form an arc cone round table 4.
And 4, step 4: and plating a reflecting film 5 on the paraboloid of the arc cone frustum, wherein a gold film is selected to be plated.
And 5: and connecting the other end of the seven-core optical fiber 1 with a fan-in fan-out device of the seven-core optical fiber, selecting a pair of side cores as an input end and an output end respectively, evaluating the quality of the fiber end arc-shaped cone reflecting structure, and preparing to obtain the arc-shaped optimized cone fiber end full reflector.
As shown in fig. 4 (a), the axial section of the cone frustum structure with a base angle of 45 ° is shown, and the dotted line is the direction of three light rays output by the fiber core b after being reflected twice on the cone frustum. Because the light rays can generate diffraction effect after leaving the constraint of the fiber core b, the light beams are transmitted in a divergent mode, and therefore the three light rays cannot be collected by the fiber core a after being reflected twice, and the coupling connection loss of the fiber cores a and b is large.
In order to counteract the diffraction and diffusion effect of the light beam after leaving the fiber core, the optical fiber after the fiber core b is emitted needs to be focused. This requires the reflective surface to be optimized to be an arc-shaped reflective surface. As shown in fig. 4 (b), in order to realize efficient coupling of the light beam into the fiber core a after the second reflection, the focusing point of the first reflection needs to be optimally set on the axis of the optical fiber, and symmetric reciprocal coupling concatenation of the two fiber cores can be realized.
The design criteria are mainly two-fold:
(1) The parabolic surface is chosen as the shape of the reflecting surface function because off-axis parabolic surfaces have the property of focusing a parallel beam of light to a point.
(2) In order to make the beam reflected at 45 deg., it is necessary that the parabola (generatrix) satisfies that the angle between the tangent at point a and the horizontal is 45 deg., i.e., the slope at point a is-1.
In conclusion, the generatrix of the rotationally symmetrical paraboloid of the arc-shaped cone frustum structure meets the parabola equation:
wherein r is the radial direction of the optical fiber, z is the axial direction of the optical fiber, d is the distance from the peripheral fiber core of the multi-core optical fiber to the center, and the coordinate of the intersection point of the peripheral fiber core center and the arc is A (42,0).
Fig. 5 (a) and (b) are graphs of simulation results of the directions of light beams at the fiber ends before and after the cone frustum at the fiber end of the multi-core optical fiber is optimized in an arc shape. It can be seen that before optimization, the light beam output by the b fiber core cannot be completely coupled into the a fiber core after the light beam is reflected twice by the frustum structure. After optimization, the light beam is output from the fiber core b, is focused on the axis of the optical fiber after being reflected for the first time, is then scattered, is symmetrically reflected for the second time, and is completely coupled into the fiber core a. Therefore, the optimization effect is obvious.
Secondly, taking BOTDR-based three-dimensional dynamic shape sensing as an example, the structure and the working principle of the invention as a three-dimensional shape sensor are introduced.
Fig. 6 shows a system diagram of the present invention as a three-dimensional shape sensor. One end of the seven-core optical fiber 1 is connected with a fan-in fan-out device 6 of the seven-core optical fiber, and the other end is provided with an arc optimized cone round table reflection structure 4. Fan-in fan-out device 6 corresponds the input of fibre core an and the output connection of BOTDR7, then connects in series the circulation in proper order of fan-in fan-out device 6 remaining six inputs, combines the reflection of the cone round platform reflection configuration of the arc optimization of fine end, and the transmission path of the pulse of the BOTDR7 output that finally obtains in seven core optic fibre is: a → a ' → b ' → b → c → c ' → d ' → d → e → e ' → f ' → f → g → g '.
In such a connection, the brillouin scattering signals of the respective cores connected in series are as shown in fig. 7 (a). The one-dimensional signal can be spread out according to different cores, as shown in fig. 7 (b). The developed signals can be divided into two groups which are verified mutually according to the transmission direction, wherein the two groups are respectively a fiber core, c fiber core, e fiber core and g fiber core, and the two groups are respectively b fiber core, d fiber core, f fiber core and g fiber core. Both sets of data can be used for three-dimensional shape recovery demodulation.
One group of cores (a, c, e, g) was chosen to illustrate the mechanism for shape sensing:
the fiber-optic distributed measurement using the BOTDR is achieved by modulating the incident light into pulses. The position of the optical fibre at each point along the line may be determined by the propagation time of the pulsed light in the optical fibre, with the amount of change in the brillouin shift Δ ν at each point along the line B Determined by the stress to which the fiber is subjected at that point and the ambient temperature:
Δv B ＝C ε ·Δε+C T ·ΔT (2)
in the formula: c ε Is a Brillouin frequency shift strain coefficient, C T The temperature coefficient of Brillouin frequency shift is shown, wherein delta epsilon is the stress variation and delta T is the temperature variation. When the temperature change is not considered, the formula (2) can be simplified as follows:
under pure bending conditions, for a circular section spring beam, the following relationship exists between axial strain and curvature:
in equation (4), ε is the value of the axial surface linear strain experienced by the sensing location based on the BOTDR fiber shape sensor, ρ is the radius of curvature of the sensing location of the sensor, C is the corresponding curvature, and D is the distance from the sensor to the neutral plane. Given D, C, the strain of the sensing fiber can be determined. As can be seen from equations (3) and (4), the change Δ v of strain and brillouin frequency shift B Is proportional, so the curvature C is proportional to Δ v B Is in direct proportion. Thus, by monitoring the amount of change Δ v in the Brillouin frequency shift B Can obtain the change of the curvature C of the optical fiberThe situation is.
As shown in fig. 8, the four core optical fibers selected were mainly composed of a central core g located at the center of the cladding and three cores (a, c, e) arranged in the form of a regular triangle. When the fiber is bent along the NN' axis with a radius of curvature ρ, the distance of the core i from the neutral plane can be obtained from the geometrical relationship in FIG. 8 (a):
D i ＝r i sin(θ b -2π/3-θ i ) (5)
by substituting formula (5) into formula (4) and formula (3), the change Δ v of Brillouin frequency shift in the core i can be obtained B Relation to curvature radius ρ:
in a practical BOTDR bend sensing system, the change amount Deltav of Brillouin frequency shift B /v B Can be obtained through experimental data, so that only three unknowns rho and theta are in the formula (6) b And theta i (Here, depending on the core arrangement, θ 1 、θ 2 And theta 3 There is a fixed positional relationship), so the three unknowns can be solved by simultaneously establishing an equation (formula (6)) corresponding to the three cores, and the local form change data of the optical fiber can be obtained according to the local bending radius and the bending direction of the optical fiber, and the three-dimensional deformation of the whole optical fiber can be reconstructed by means of the form change data.
In the data difference operation process, the ambient temperatures of the four fiber cores can be considered to be approximately the same because the diameter of the seven-core optical fiber is very small. After the difference operation between the three peripheral fiber cores (a, c and e) and the middle core g, the strain of each fiber core along the axial direction of the optical fiber is automatically eliminated, and the influence caused by the change of the environmental temperature is also automatically eliminated. The obtained information of pure bending and pure torsion of the seven-core optical fiber improves the stability and reliability of the three-dimensional deformation optical fiber sensor.
The mechanism of the invention for torsion sensing is as follows:
FIG. 9 (a) shows the pitchIs L p The spiral core fiber with the spiral core at the distance r from the center of the fiber generates theta under the action of external torsion t The torsion angle of (c). It can be seen from the figure that the length of the helical core changes from L to L ε Therefore, according to the geometrical relationship in the figure, the axial strain epsilon of the spiral fiber core and the torsion angle theta on the unit pitch can be obtained t The relationship between:
the Brillouin frequency shift quantity and the torsion angle theta on the spiral fiber core can be obtained by substituting the formula (7) into the formula (3) t The relationship of (1):
as can be seen from equation (8), the main factor affecting the sensitivity of BOTDR-based multicore fiber torsion sensing is the ratio L of the pitch to the distance from the core to the center of the fiber p And/r. In the helical multicore fibers shown in fig. 9 (a) and 9 (b), since the distances from the three helical cores to the center of the fiber are all equal, only the fiber twist pitch L is considered here p The effect on the sensitivity of the twist sensing, whereas the central core is not sensitive to twist and only serves to compensate for temperature or longitudinal stretching of the fiber. For a helical core fiber with good coaxiality, the pitches L of the three helical cores are used p Same, and therefore the amount of change in brillouin frequency shift Δ v across the three cores B /v B The response to fiber twist is uniform, that is, the amount of change in the brillouin shift in the three cores caused by the fiber twist is the same. For untwisted multicore fibers, the pitch L of the core p It can be seen as infinity, where the sensitivity of the fiber to twist sensing approaches zero (see equation (8)). However, once a twisted fiber is used, the sensitivity of the fiber to twist sensing will increase rapidly and the core pitch L p The smaller, the higher the sensitivity. Therefore, the present invention can employ a spiral seven-core fiber to enhance the detection of spatial torsional strainCapability. Of course, the core pitch L takes into account factors such as core bend loss p It cannot be too small, and generally needs to be more than millimeter. As can be seen from FIG. 9 (b), the amount of change Δ v in Brillouin frequency shift obtained along the optical fiber is used B /v B The strain amount of each position along the optical fiber can be obtained, so that a plurality of optical fiber local form parameters are obtained, and the three-dimensional deformation of the whole optical fiber can be reconstructed by using the obtained data of the plurality of optical fiber local form changes.
In fact, the seven-core optical fiber shape sensor provided by the invention can form two groups of forward and reverse shape sensing signals by means of a reflecting cone frustum structure at the fiber end, and can perform shape verification and improve the shape reduction precision in a mode of averaging the two groups of signals.
In the specification and drawings, there have been disclosed typical embodiments of the invention. The invention is not limited to these exemplary embodiments. Specific terms are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth.
Claims (8)
1. A single-channel circulating series connection type multi-core optical fiber shape sensor is characterized in that: the optical fiber comprises a seven-core optical fiber, a seven-core optical fiber Fan-in device and a 45-degree cone round table at one end of the seven-core optical fiber; the seven-core optical fiber is provided with a middle core and three pairs of peripheral fiber cores which are distributed in a hexagon shape; the cone frustum with the angle of 45 degrees can reflect light beams transmitted by the peripheral fiber cores, so that the light beams enter the symmetrical fiber cores after being reflected twice and are transmitted reversely; and sequentially connecting each fiber core of the seven-core optical fiber in series through the seven-core optical fiber Fan-in device and the 45-degree cone frustum at the fiber end, thereby realizing the function of mapping the seven fiber cores into a one-dimensional topological light path.
2. The single-channel circulating tandem multi-core optical fiber shape sensor of claim 1, wherein: the 45-degree cone round table is an optimized arc-shaped reflecting cone round table.
3. A single-channel circulating tandem type multi-core optical fiber shape sensor according to any one of claims 1 to 2, wherein: the surface of the 45-degree cone round table is plated with a reflecting film.
4. The single-channel circulating tandem type multi-core optical fiber shape sensor as claimed in claim 1, wherein: the shape sensor acquires one-dimensional data information, and the one-dimensional data information is subjected to segmented mapping and extraction to obtain deformation information corresponding to each fiber core of the seven-core optical fiber.
5. The single-channel circulating tandem type multi-core optical fiber shape sensor according to claim 1 or 4, wherein: the data information of six peripheral fiber cores and the data information of the middle fiber core of the seven-core optical fiber are subjected to difference, so that the influence of temperature distribution change on the demodulation of the shape of the optical fiber can be eliminated.
6. The single-channel circulating tandem type multi-core optical fiber shape sensor according to claim 1 or 4, wherein: the data information of the three pairs of peripheral fiber cores is divided into two independent groups of forward transmission and reverse transmission, the two groups of data are mutually verified, and the shape of the optical fiber is demodulated after averaging, so that the shape demodulation precision is improved.
7. The single-channel circulating tandem multi-core optical fiber shape sensor of claim 1, wherein: except for the fiber end of the seven-core optical fiber at the position of the 45-degree cone frustum, the other shape sensing positions are prepared into a spiral structure, so that six peripheral fiber cores are spirally distributed around the circumference of the middle core.
8. A single-channel circulating tandem type multi-core optical fiber shape sensor according to claim 1, wherein: the demodulation equipment of the shape sensor is OFDR, BOTDR or BOTDA.
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