Bending sensor based on random coupling multi-core optical fiber and sensing method thereof

Abstract

The invention discloses a bending sensor based on a random coupling multi-core optical fiber and a sensing method thereof. The bending sensor realizes bending sensing based on the inter-core coupling characteristic of the random coupling multi-core optical fiber. The inter-core coupling frequency spectrum of the random coupling multi-core optical fiber shows higher sensitivity to the bending change of the optical fiber, and high-resolution curvature sensing can be realized by detecting the frequency spectrum information of the random coupling multi-core optical fiber. Besides, frequency and light intensity information in a plurality of fiber cores can be monitored at the same time by utilizing a plurality of channels and fan-in and fan-out devices of the randomly coupled multi-core optical fiber, so that bending direction identification is realized with a higher integration level. The coupling characteristic of the random coupling multi-core optical fiber is used for sensing, compared with an existing optical fiber interferometer and grating type devices, the sensor is simple in structure, a sensing system does not need complex equipment, and therefore the manufacturing and testing cost of the sensor can be reduced.

Description

Technical Field

[0001] The present application relates to the technical field of optical sensors, and in particular to a bending sensor based on randomly coupled multi-core optical fibers and a sensing method thereof. Background Art

[0002] Bending refers to the process of a rod-like structure changing from a straight line to a curved shape under the action of an external force. As sensors gradually develop towards miniaturization, high-precision bending sensors play a vital role in assessing the health status of precision structures, medical diagnosis, and improving the intelligent perception capabilities of robots. In order to meet the growing demand for precision structural bending sensing, the development of bending sensing technology with high sensitivity, high integration, and bending direction recognition capabilities and the development of corresponding sensor devices have become important research directions in the field of sensor detection.

[0003] Fiber optic sensors have significant advantages in the field of bending sensing due to their small size, resistance to electromagnetic interference and compact structure. Their flexibility and bendability enable them to be flexibly embedded in tiny structures without causing significant interference or damage. Common fiber optic bending sensors are usually based on fiber gratings, fiber interferometers or scattering effects, and achieve bending measurement through changes in optical path, refractive index or strain caused by fiber bending.

[0004] In the prior art, vector bending sensing is usually achieved by using a coupler to produce coupled interference in the light in the symmetrical fiber core. Alternatively, a Mach-Zehnder interferometer is constructed by using the core interference of strongly coupled multi-core optical fibers, and combined with fiber Bragg grating technology to achieve high-sensitivity bending sensing and temperature compensation.

[0005] However, in the curvature sensing test of precision structures, the changes in optical path, refractive index or strain directly caused by optical fiber microbending are relatively limited, and the curvature resolution based on spectral information or scattered signals is difficult to achieve high precision. In addition, in order to achieve bending direction recognition, it is usually necessary to design a complex optical fiber sensing structure or adopt precision processing technology. Existing sensors still have performance bottlenecks in high-precision vector bending sensing and cannot fully meet application requirements. Summary of the invention

[0006] The embodiments of the present application provide a bending sensor based on randomly coupled multi-core optical fiber and a sensing method thereof, thereby enabling high-resolution vector bending sensing.

[0007] In order to achieve the above object, the technical solution of the embodiment of the present invention is:

[0008] In the first aspect, an embodiment of the present invention provides a bending sensor method based on randomly coupled multi-core optical fiber, including: a randomly coupled multi-core optical fiber and a sensing test module; the randomly coupled multi-core optical fiber includes a common outer cladding and a plurality of cores arranged in the common outer cladding; the plurality of cores are equally spaced along the circumferential direction, the cores are transmitted in a single-mode form, and the optical waveguides are randomly coupled within the range surrounded by the cores; the sensing test module includes a broadband light source, an electro-optic modulator, a polarization controller, a coupler, a fan-out device, and a plurality of groups of photodetectors; the broadband light source is connected to the electro-optic modulator and the polarization controller through a single-mode optical fiber, and then connected to a core of the randomly coupled multi-core optical fiber through a coupler; one end of the fan-out device is connected to the randomly coupled multi-core optical fiber, and the other end is connected to a plurality of single-mode optical fibers, and each single-mode optical fiber is connected to a photodetector.

[0009] In some possible implementations, the length of the randomly coupled multi-core fiber exceeds the fiber coupling period, so that the light source enters from one core of the incident end of the randomly coupled multi-core fiber, is transmitted through the coupling length, and is coupled to all the cores of the output port.

[0010] In some possible implementations, the sensing test module is a transmission-type bending sensing test module; in the transmission-type bending sensing test module, the light emitted by the wide-spectrum light source passes through the electro-optic modulator and the polarization controller, and then enters a core of the randomly coupled multi-core optical fiber via a coupler. After random coupling transmission, the energy is gradually distributed to other cores, and the coupled optical signal is distributed to multiple groups of photodetectors for detection through a fan-out.

[0011] In some possible implementations, the sensing test module is a reflective bending sensing test module. In the reflective bending sensing test module, light emitted by a wide-spectrum light source passes through an electro-optic modulator and a polarization controller, and then enters a core of a randomly coupled multi-core optical fiber via a coupler. After random coupling transmission, the energy is gradually distributed to other cores, and through multi-core optical fiber end face reflection or Bragg grating reflection, the reflected optical signal is distributed to multiple groups of photodetectors for detection through a fan-out.

[0012] In a second aspect, an embodiment of the present invention provides a sensing method for a bending sensor based on a randomly coupled multi-core optical fiber, which is applied to the bending sensor of the first aspect, comprising: emitting an optical signal through a wide-spectrum light source, and modulating the optical signal through an electro-optical modulator and a polarization controller; inputting the modulated optical signal into one of the cores of the randomly coupled multi-core optical fiber through a coupler; wherein the modulated optical signal is transmitted in the randomly coupled multi-core optical fiber, random coupling occurs between the cores, and the coupled optical signal is distributed to multiple groups of photodetectors through a fan-out device; and the bending curvature and bending direction of the optical fiber are identified by analyzing the spectrum changes in different cores.

[0013] In some possible implementations, the bending curvature and bending direction of the optical fiber are identified by analyzing the spectral changes in different fiber cores, including: based on the power changes of the optical signals in different fiber cores, the spectral changes of the energy coupling envelope are calculated according to the spectral information of the power changes to determine the bending curvature of the optical fiber; based on the inhibitory effect of bending on the coupling between the fiber cores, the power changes of multiple fiber cores are monitored, and the bending direction of the optical fiber is determined by comparing the power change differences of different fiber cores.

[0014] In some possible implementations, based on the power variation of optical signals in different fiber cores and the spectrum information of the power variation, the spectrum variation of the energy coupling envelope is calculated to determine the bending curvature of the optical fiber; based on the inhibitory effect of bending on the coupling between fiber cores, the power variation of multiple fiber cores is monitored, and the bending direction of the optical fiber is determined by comparing the power variation difference of different fiber cores, including: for the light field coupling between parallel transmission waveguides, the coupled mode theory based on the perturbation approximation is analyzed, and the influence of the mode field of one fiber core on the transmission mode field of another fiber core is regarded as a perturbation term; for multiple fiber core structures, the power conversion efficiency is related to the difference in the coupling coefficient and the waveguide propagation constant, and the power conversion efficiency between different fiber cores is expressed as:

[0015]

[0016] in,

[0017]

[0018] Where η is the power conversion efficiency between fiber cores, n eff is the effective refractive index of the waveguide, λ is the operating wavelength, β represents the propagation constant, β2 is the propagation constant of core 2, β1 is the propagation constant of core 1, and κ 12 is the coupling coefficient from core 1 to core 2; when the effective refractive index of adjacent cores is slightly different, the power conversion efficiency η decreases significantly; fiber bending affects the coupling between cores, which is analyzed by introducing an equivalent refractive index model; a bent fiber is equivalent to a straight fiber with an equivalent refractive index distribution, expressed as:

[0019]

[0020] Where (r, θ) represents the local polar coordinates of a reference point in the fiber cross section, θ is the angle with the bending radial direction, R is the bending radius of the fiber, and n(r, θ) is the intrinsic refractive index. The center of one fiber core is the origin of the local polar coordinates, and the equivalent effective refractive index of the remaining fiber cores is n eqeff,2 It is expressed as:

[0021]

[0022] The equivalent propagation constant β of the remaining fiber cores eq,2 It is expressed as:

[0023]

[0024] Among them, n eff,2 is the effective refractive index of core 2, Λ 21 is the core spacing of different fiber cores, θ 21 is the angle between the line segment connecting different fiber cores and the bending radial direction; due to β eq,2 As variables, the coupled mode equation of randomly coupled multi-core fiber is further expressed as:

[0025]

[0026] in, is the local increment of the electric field amplitude envelope of core 2 as the propagation length changes, is the distance increment along the propagation direction, j is an imaginary unit, Φ1(z) is the phase function of core 1 changing with the propagation distance z, Φ2(z) is the phase function of core 2 changing with the propagation distance z, A1 is the slowly varying complex amplitude of the electric field in core 1, A2 is the slowly varying complex amplitude of the electric field in core 2, Φ(z) is the phase function changing with the propagation distance z, β eq (z') is the equivalent propagation constant at position z; according to the determined coupled mode equation, the inter-core power conversion of the randomly coupled multi-core optical fiber under the corresponding geometric parameters, wavelength and bending radius is calculated to determine the bending curvature and bending direction of the optical fiber.

[0027] One or more technical solutions provided in the embodiments of the present invention have at least the following technical effects or advantages:

[0028] In the embodiment of the present invention, different from the method of performing bending sensing through optical path difference of the traditional high-resolution fiber interferometer sensor, the present invention realizes bending sensing based on the core-to-core coupling characteristics of randomly coupled multi-core optical fiber. The core-to-core coupling spectrum of the randomly coupled multi-core optical fiber shows higher sensitivity to the bending change of the optical fiber. By detecting its spectrum information, high-resolution curvature sensing can be achieved. In addition, the present invention utilizes multiple channels and fan-in and fan-out devices of the randomly coupled multi-core optical fiber to simultaneously monitor the frequency and light intensity information in multiple cores, and realize bending direction identification with a higher degree of integration. In addition, the present invention utilizes the coupling characteristics of the randomly coupled multi-core optical fiber itself for sensing. Compared with the fiber interferometer and grating devices, the sensor structure is simple, and the sensing system does not require complex equipment, thereby reducing the cost of sensor manufacturing and testing. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present invention, the accompanying drawings required for use in the embodiments of the present invention will be briefly introduced below. Obviously, the accompanying drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying any creative work.

[0030] Figure 1 A schematic diagram of the structure of a bending sensor based on randomly coupled multi-core optical fiber provided for the implementation of the present invention;

[0031] Figure 2 A schematic diagram of a cross-sectional geometric structure of a randomly coupled multi-core optical fiber in an embodiment of the present invention;

[0032] Figure 3 Schematic diagram of the structure of another bending sensor based on randomly coupled multi-core optical fiber in an embodiment of the present invention;

[0033] Figure 4 It is a schematic diagram of an embodiment flow of a sensing method based on a bending sensor of a randomly coupled multi-core optical fiber in an embodiment of the present invention;

[0034] Figure 5 Schematic diagram of the power coupling curve between randomly coupled multi-core optical fibers in an embodiment of the present invention;

[0035] Figure 6 Schematic diagram of the variation of the coupling frequency between cores of a multi-core optical fiber with curvature in an embodiment of the present invention. DETAILED DESCRIPTION

[0036] The technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present application.

[0037] In the relevant description of this embodiment, the terms "including, containing, having" and the like are open terms and are generally understood to include but not be limited to; the term "at least one" is generally understood to mean one or more, where "plurality" refers to two or more; the term "at least one of the following" or similar expressions refers to any combination of these items, including any combination of single or plural items, for example, "at least one of a, b or c", or "at least one of a, b and c", can all represent: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, c can be single or multiple, respectively; the symbol "A / B" is used to describe the selection relationship of associated objects, generally indicating an "or" relationship before and after.

[0038] In the following description of the present embodiment, the terms used in the embodiments of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application. The singular forms "a" and "the" used in the embodiments of the present application and the appended claims are also intended to include plural forms, unless the context clearly indicates other meanings.

[0039] Those skilled in the art should understand that in the following description of the embodiments of the present application, the order of serial numbers does not mean the order of execution, some or all of the steps can be executed in parallel or sequentially, and the execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0040] Those skilled in the art will appreciate that the numerical ranges in the embodiments of the present application are to be construed as also specifically disclosing each intermediate value between the upper and lower limits of the scope. Each smaller range between the intermediate value in any stated value or stated range and any other stated value or intermediate value in the range is also included in the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded in the scope.

[0041] Unless otherwise specified, the technical / scientific terms used herein have the same meanings as those generally understood by those skilled in the art to which this application belongs. Although this application only describes preferred methods and materials, any methods and materials similar or equivalent to these may also be used in the implementation or testing of this application. All documents mentioned in this specification are incorporated by reference to disclose and describe methods and / or materials related to the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0042] In order to illustrate the technical solution of the present invention, specific embodiments are provided below for illustration.

[0043] Bending refers to the process of a rod-like structure changing from a straight line to a curved shape under the action of an external force. As sensors gradually develop towards miniaturization, high-precision bending sensors play a vital role in assessing the health status of precision structures, medical diagnosis, and improving the intelligent perception capabilities of robots. In order to meet the growing demand for precision structural bending sensing, the development of bending sensing technology with high sensitivity, high integration, and bending direction recognition capabilities and the development of corresponding sensor devices have become important research directions in the field of sensor detection.

[0044] Fiber optic sensors have significant advantages in the field of bending sensing due to their small size, resistance to electromagnetic interference and compact structure. Their flexibility and bendability enable them to be flexibly embedded in tiny structures without causing significant interference or damage. Common fiber optic bending sensors are usually based on fiber gratings, fiber interferometers or scattering effects, and achieve bending measurement through changes in optical path, refractive index or strain caused by fiber bending.

[0045] In the prior art, vector bending sensing is usually achieved by using a coupler to produce coupled interference in the light in the symmetrical fiber core. Alternatively, a Mach-Zehnder interferometer is constructed by using the core interference of strongly coupled multi-core optical fibers, and combined with fiber Bragg grating technology to achieve high-sensitivity bending sensing and temperature compensation.

[0046] However, in the curvature sensing test of precision structures, the changes in optical path, refractive index or strain directly caused by optical fiber microbending are relatively limited, and the curvature resolution based on spectral information or scattered signals is difficult to achieve high precision. In addition, in order to achieve bending direction recognition, it is usually necessary to design a complex optical fiber sensing structure or adopt precision processing technology. Existing sensors still have performance bottlenecks in high-precision vector bending sensing and cannot fully meet application requirements.

[0047] Based on this, an embodiment of the present invention provides a bending sensor based on randomly coupled multi-core optical fiber and a sensing method thereof, which can achieve high-resolution vector bending sensing.

[0048] Figure 1 A schematic diagram of a bending sensor based on a randomly coupled multi-core optical fiber provided for the implementation of the present invention is shown in FIG. Figure 1 As shown, the bending sensor may include: a randomly coupled multi-core optical fiber 11 and a sensing test module 12;

[0049] The randomly coupled multi-core optical fiber 11 comprises: a common outer cladding and a plurality of optical cores arranged in the common outer cladding;

[0050] Among them, multiple fiber cores are distributed at equal intervals along the circumferential direction, the fiber cores transmit in a single-mode form, and the optical waveguides are randomly coupled within the range surrounded by the fiber cores;

[0051] The sensor test module 12 includes a broadband light source 121, an electro-optic modulator 122, a polarization controller 123, a coupler 124, a fan-out device 125, and a plurality of photodetectors 126;

[0052] The broadband light source 121 is connected to the electro-optic modulator 122 and the polarization controller 123 through a single-mode optical fiber, and then connected to a core of the randomly coupled multi-core optical fiber 11 through a coupler 124; one end of the fan-out 125 is connected to the randomly coupled multi-core optical fiber 11, and the other end is connected to multiple single-mode optical fibers, each of which is connected to a photodetector 126.

[0053] It is understandable that in order to achieve effective random coupling of optical signals between fiber cores, the number of the above-mentioned fiber cores needs to be at least two. In the embodiment of the present invention, in order to achieve omnidirectional vector bending sensing, the number of fiber cores of the above-mentioned randomly coupled multi-core optical fiber 11 is at least 3, ensuring the comprehensiveness and accuracy of the sensing function. The upper limit of the number of fiber cores can be based on the typical design of multi-core optical fiber for sensing and considerations in practical applications. Generally, 7 or less fiber cores can be used. This choice is mainly based on the size limitation of the common outer cladding and the optimization of optical fiber transmission performance.

[0054] In the specific layout of the optical fiber, multiple cores are equally spaced along the circumference, which can ensure uniform coupling of optical signals between the cores. The interval between two adjacent cores can be determined based on the needs of actual applications, or can also be determined based on the interval length used in commonly used multi-core optical fiber structures.

[0055] It can be understood that the core spacing is an important parameter that affects the coupling strength of multi-core optical fibers. In the randomly coupled multi-core optical fiber 11, the coupling strength between its cores is usually between that of strongly coupled and weakly coupled multi-core optical fibers. Since the inter-core coupling strength of weakly coupled multi-core optical fibers is too weak, an extremely long distance is required to achieve effective coupling, resulting in insufficient signal strength and the inability to support the need for curvature change monitoring. However, since the core spacing of strongly coupled multi-core optical fibers is too small, it is difficult to achieve the separation of multi-channel transmission signals through fan-in and fan-out devices, and it is not suitable for multi-channel sensing testing. Based on this, the randomly coupled multi-core optical fiber 11 in the embodiment of the present invention, based on the current conventional single-mode optical fiber parameters as a typical example, its core spacing range can be set to 15 to 25 μm, so as to simultaneously meet the technical requirements of higher inter-core coupling strength and channel separation.

[0056] For example, Figure 2 FIG. 1 is a schematic diagram of a cross-sectional geometric structure of a randomly coupled multi-core optical fiber 11 in an embodiment of the present invention. Figure 2 As shown, the randomly coupled multi-core optical fiber 11 includes four fiber cores distributed at equal intervals along the circumferential direction, which are represented by 1 to 4 respectively, and the four fiber cores are all located in a common outer cladding 111. Wherein, Λ represents the interval between two fiber cores.

[0057] In some embodiments, the length of the randomly coupled multi-core optical fiber 11 exceeds the optical fiber coupling period, so that after the light source enters from a core at the incident end of the randomly coupled multi-core optical fiber 11, it can be coupled to all the cores of the output port after being transmitted through the coupling length.

[0058] It can be understood that the length of the randomly coupled multi-core optical fiber 11 exceeds the optical fiber coupling period. This design can ensure that when the light source enters from a specific core at the incident end of the randomly coupled multi-core optical fiber 11, the optical signal can be transmitted along the length direction of the optical fiber through a sufficiently long coupling region. Within this coupling length, the optical signal can be effectively randomly coupled between multiple cores, so that the optical signal is coupled to all cores.

[0059] Among them, the fiber coupling period determines the transmission distance required for optical signals to be significantly coupled between different fiber cores. This period is related to multiple factors, including but not limited to the spacing between the cores, the working wavelength used by the fiber, and the bending radius that the fiber may experience in a specific application scenario. Therefore, when designing randomly coupled multi-core optical fibers, these factors can be comprehensively considered to ensure that the length of the optical fiber can exceed its coupling period, thereby achieving sufficient coupling of optical signals between multiple cores.

[0060] In some embodiments, the sensor test module 12 is a transmissive bending sensor test module; the specific structure of the transmissive bending sensor test module can also be found in Figure 1 shown.

[0061] Specifically, in the transmission-type bending sensing test module, the light emitted by the broadband light source 121 passes through the electro-optic modulator 122 and the polarization controller 123, and then enters a core of the randomly coupled multi-core optical fiber 11 through the coupler 124. After random coupling transmission, the energy is gradually distributed to other cores. The coupled optical signal is distributed to multiple groups of photodetectors 126 for detection through the fan-out 125. Afterwards, the electrical signals collected by the multiple groups of photodetectors 126 are subjected to data processing. By analyzing the spectrum changes in different cores, the curvature and bending direction information of different cores can be identified, thereby realizing multi-core optical fiber bending sensing.

[0062] In some embodiments, the sensor test module 12 is a reflective bending sensor test module. The specific structure of the reflective bending sensor test module can be as follows: Figure 3 As shown, Figure 3 Schematic diagram of the structure of another bending sensor based on randomly coupled multi-core optical fiber in an embodiment of the present invention.

[0063] See also Figure 3As shown, in the reflective bending sensing test module, the light emitted by the wide-spectrum light source 121 passes through the electro-optic modulator 122 and the polarization controller 123, and then enters a core of the randomly coupled multi-core optical fiber 11 through the coupler 124. After random coupling transmission, the energy is gradually distributed to other cores. Then, through the multi-core optical fiber end face reflection or Bragg grating reflection, the reflected optical signal is distributed to multiple groups of photodetectors 126 for detection through the fan-out 126. Afterwards, the electrical signals collected by the multiple groups of photodetectors 126 are processed, and by analyzing the spectrum changes in different cores, the curvature and bending direction information of different cores can be identified, thereby realizing multi-core optical fiber bending sensing.

[0064] It can be understood that the signal path of the transmission test is unidirectional transmission, from the input end to the output end; while the signal path of the reflection test is bidirectional transmission, including the forward transmission and the reflected path. In practical applications, the transmission bend sensing test can be applied to scenarios where it is necessary to directly detect the signal changes at the output end of the optical fiber; the reflection bend sensing test can be applied to scenarios where it is necessary to use the reflected signal for measurement, such as when there is a reflective structure at the end of the optical fiber or when it is necessary to detect the bending condition of a specific position inside the optical fiber.

[0065] In the embodiment of the present invention, different from the method of performing bending sensing through optical path difference of the traditional high-resolution fiber interferometer sensor, bending sensing is realized based on the core-to-core coupling characteristics of the randomly coupled multi-core optical fiber. The core-to-core coupling spectrum of the randomly coupled multi-core optical fiber shows higher sensitivity to the bending change of the optical fiber. By detecting its spectrum information, high-resolution curvature sensing can be realized. In addition, the present invention utilizes multiple channels and fan-in and fan-out devices of the randomly coupled multi-core optical fiber to simultaneously monitor the frequency and light intensity information in multiple cores, and realize bending direction identification with a higher degree of integration. In addition, the present invention utilizes the coupling characteristics of the randomly coupled multi-core optical fiber itself for sensing. Compared with the fiber interferometer and grating devices, the sensor structure is simple, and the sensing system does not require complex equipment, thereby reducing the cost of sensor manufacturing and testing.

[0066] On the other hand, an embodiment of the present invention further provides a sensing method of a bending sensor based on a randomly coupled multi-core optical fiber, which can be applied to the bending sensor based on a randomly coupled multi-core optical fiber as described above.

[0067] Figure 4 FIG. 1 is a flow chart of a sensing method based on a bending sensor of a randomly coupled multi-core optical fiber according to an embodiment of the present invention. Figure 4 As shown, the method may include:

[0068] S401, emitting an optical signal through a broadband light source, and modulating the optical signal through an electro-optic modulator and a polarization controller;

[0069] S402 inputs the modulated optical signal into a core of a randomly coupled multi-core optical fiber through a coupler; wherein the modulated optical signal is transmitted in the randomly coupled multi-core optical fiber, random coupling occurs between the cores, and the coupled optical signal is distributed to multiple groups of photodetectors through a fan-out device;

[0070] S403, identifying the bending curvature and bending direction of the optical fiber by analyzing the frequency spectrum changes in different fiber cores.

[0071] In some embodiments, the above S403 may specifically include:

[0072] S4031, based on the power variation of the optical signals in different fiber cores and the spectrum information of the power variation, calculating the spectrum variation of the energy coupling envelope, and determining the bending curvature of the optical fiber;

[0073] S4032, based on the inhibitory effect of bending on the coupling between fiber cores, monitor the power changes of multiple fiber cores, and determine the bending direction of the optical fiber by comparing the power change differences of different fiber cores.

[0074] Specifically, the identification of the optical fiber bending curvature and bending direction through the above S4031 to S4032 specifically includes the following contents:

[0075] For the optical field coupling between parallel transmission waveguides, the coupled mode theory based on perturbation approximation is analyzed, and the influence of the mode field of one core on the transmission mode field of another core is regarded as a perturbation term; for multiple core structures, the power conversion efficiency is related to the difference in coupling coefficient and waveguide propagation constant. Taking the two-core optical fiber structure as an example (only the two-core structure is used as an example in the embodiment of the present invention, and other multi-core optical fiber structures are the same, which will not be repeated here), the power conversion efficiency between core 1 and core 2 can be expressed by the following formula:

[0076]

[0077] in,

[0078]

[0079] Where η is the power conversion efficiency between fiber cores, n eff is the effective refractive index of the waveguide, λ is the operating wavelength, β represents the propagation constant, β2 is the propagation constant of core 2, β1 is the propagation constant of core 1, and κ 12 is the coupling coefficient from core 1 to core 2; when the effective refractive index of adjacent cores is slightly different, the power conversion efficiency η decreases significantly;

[0080] The influence of fiber bending on the coupling between fiber cores is analyzed by introducing an equivalent refractive index model; a bent fiber is equivalent to a straight fiber with an equivalent refractive index distribution, which can be expressed as:

[0081]

[0082] Among them, n eq (r, θ, R) is the equivalent refractive index of the optical fiber in the polar coordinate system, r is the polar diameter, (r, θ) represents the local polar coordinates of a reference point in the optical fiber cross section, θ is the angle with the bending radial direction, R is the bending radius of the optical fiber, and n(r, θ) is the intrinsic refractive index; if the center of core 1 is taken as the origin of the local polar coordinates, the equivalent effective refractive index of core 2 n eqeff,2 It is expressed as:

[0083]

[0084] The equivalent propagation constant β of core 2 eq,2 It is expressed as:

[0085]

[0086] Among them, n eff,2 is the effective refractive index of core 2, Λ 21 is the core spacing between core 1 and core 2, θ 21 is the angle between the line segment connecting core 1 and core 2 and the bending radial direction; due to β eq,2 As variables, the coupled mode equation of randomly coupled multi-core fiber is further expressed as:

[0087]

[0088] Where A represents the slowly varying complex amplitude of the electric field, z represents the axial length of the multi-core optical fiber, is the local increment of the electric field amplitude envelope of core 2 as the propagation length changes, is the distance increment along the propagation direction, j is an imaginary unit, Φ1(z) is the phase function of core 1 changing with the propagation distance z, Φ2(z) is the phase function of core 2 changing with the propagation distance z, A1 is the slowly varying complex amplitude of the electric field in core 1, A2 is the slowly varying complex amplitude of the electric field in core 2, Φ(z) is the phase function changing with the propagation distance z, β eq (z') is the equivalent propagation constant at position z.

[0089] Based on the above theory, the inter-core power conversion of randomly coupled multi-core optical fibers under different corresponding geometric parameters, wavelengths and bending radii can be calculated, thereby determining the bending curvature and bending direction of the optical fiber.

[0090] For example, Figure 5 Schematic diagram of the power coupling curve between the cores of a randomly coupled multi-core optical fiber in an embodiment of the present invention. Figure 5The horizontal axis is the core spacing length (Length), in centimeters (cm), and the vertical axis is the normalized power (NormalizedPower). Figure 5 As shown, for a core spacing of 20 μm, a common outer cladding diameter of 125 μm, and an undeformed two-core optical fiber, at a wavelength of 1550 nm, when energy is injected from core 1, coupling will occur to core 2, and the coupling period is about 12 cm.

[0091] Among them, the propagation constant difference information caused by bending carried by the power coupling curve between cores in the randomly coupled multi-core optical fiber can be modulated by the electro-optic modulator to modulate the RF signal, and finally detected by the photodetector. By analyzing the frequency response, the change of the coupling period in the randomly coupled multi-core optical fiber can be converted into the shift of the passband center frequency in the electronic domain. The linear dependence of this frequency shift on the curvature can be seen in Figure 6 shown. Figure 6 Schematic diagram of the variation of the coupling frequency between the cores of a multi-core optical fiber according to the curvature in an embodiment of the present invention. Figure 6 The power coupling curves of multi-core optical fibers with a core spacing of 23 μm under different curvatures are obtained through simulation calculation, and the corresponding frequency information is obtained through fast Fourier transform (FFT). The relationship between frequency and curvature is obtained by linear fitting of the simulated data. Figure 6 In the figure, the horizontal axis represents the curvature, in m-1; the vertical axis represents the frequency, in GHz. The fitting result of frequency F and curvature C is F=21.26×C+2.9, where the slope of the fitting curve represents the curvature sensitivity of the sensor, and the linearity of the linear fitting result is R 2 is 0.99, and its curvature sensitivity is about 21.26GHz / m -1 In addition, according to the multi-core fiber coupling power theory, the difference in propagation constants of different cores in the bending state will inhibit the coupling efficiency between the cores. Therefore, the bending direction of the optical fiber can be identified based on the power changes in multiple groups of cores.

[0092] In the embodiment of the present invention, different from the method of performing bending sensing through optical path difference of the traditional high-resolution fiber interferometer sensor, the present invention realizes bending sensing based on the core-to-core coupling characteristics of randomly coupled multi-core optical fiber. The core-to-core coupling spectrum of the randomly coupled multi-core optical fiber shows higher sensitivity to the bending change of the optical fiber. By detecting its spectrum information, high-resolution curvature sensing can be achieved. In addition, the present invention utilizes multiple channels and fan-in and fan-out devices of the randomly coupled multi-core optical fiber to simultaneously monitor the frequency and light intensity information in multiple cores, and realize bending direction identification with a higher degree of integration. In addition, the present invention utilizes the coupling characteristics of the randomly coupled multi-core optical fiber itself for sensing. Compared with the fiber interferometer and grating devices, the sensor structure is simple, and the sensing system does not require complex equipment, thereby reducing the cost of sensor manufacturing and testing.

[0093] The various embodiments in this specification are described in a progressive manner, and the same or similar parts between the various embodiments can be referenced to each other. Each embodiment focuses on the differences from other embodiments.

[0094] The above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit the present application. Although the present application has been described in detail with reference to the aforementioned embodiments, a person of ordinary skill in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some or all of the technical features thereof may be replaced by equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the present application.

Claims

1. A bending sensor based on randomly coupled multi-core optical fiber, characterized in that: include: Randomly coupled multi-core optical fiber and sensor test module; The randomly coupled multi-core optical fiber comprises a common outer cladding and a plurality of optical cores arranged in the common outer cladding; The plurality of fiber cores are distributed at equal intervals along the circumferential direction, the fiber cores are transmitted in a single-mode form, and the optical waveguides are randomly coupled within the range surrounded by the fiber cores; The sensor test module includes a broadband light source, an electro-optic modulator, a polarization controller, a coupler, a fan-out device, and a plurality of groups of photodetectors; The broadband light source is connected to the electro-optic modulator and the polarization controller through a single-mode optical fiber, and then connected to one core of the randomly coupled multi-core optical fiber through the coupler; one end of the fan-out is connected to the randomly coupled multi-core optical fiber, and the other end is connected to multiple single-mode optical fibers, and each single-mode optical fiber is connected to a photodetector.

2. The bending sensor according to claim 1, characterized in that: The length of the randomly coupled multi-core optical fiber exceeds the optical fiber coupling period, so that after the light source enters from one core of the incident end of the randomly coupled multi-core optical fiber, it is transmitted through the coupling length and coupled to all the cores of the output port.

3. The bending sensor according to claim 2, characterized in that: The sensing test module is a transmission-type bending sensing test module; in the transmission-type bending sensing test module, the light emitted by the wide-spectrum light source passes through the electro-optic modulator and the polarization controller, and then enters one core of the randomly coupled multi-core optical fiber via the coupler. After random coupling transmission, the energy is gradually distributed to other cores, and the coupled optical signal is distributed to multiple groups of photodetectors for detection through the fan-out.

4. The bending sensor according to claim 2, characterized in that: The sensing test module is a reflective bending sensing test module. In the reflective bending sensing test module, the light emitted by the wide-spectrum light source passes through the electro-optic modulator and the polarization controller, and then enters one core of the randomly coupled multi-core optical fiber via the coupler. After random coupling transmission, the energy is gradually distributed to other cores, and through multi-core optical fiber end face reflection or Bragg grating reflection, the reflected optical signal is distributed to multiple groups of photodetectors for detection through the fan-out.

5. A sensing method of a bending sensor based on a randomly coupled multi-core optical fiber, applied to the bending sensor according to any one of claims 1 to 4, characterized in that: include: An optical signal is emitted through a broadband light source, and the optical signal is modulated through an electro-optic modulator and a polarization controller; The modulated optical signal is input into a core of a randomly coupled multi-core optical fiber through a coupler; wherein the modulated optical signal is transmitted in the randomly coupled multi-core optical fiber, random coupling occurs between the cores, and the coupled optical signal is distributed to multiple groups of photodetectors through a fan-out device; By analyzing the spectrum changes in different fiber cores, the bending curvature and bending direction of the optical fiber can be identified.

6. The method according to claim 5, characterized in that The method of identifying the bending curvature and bending direction of the optical fiber by analyzing the spectrum changes in different fiber cores includes: Based on the power variation of the optical signal in different fiber cores and the spectrum information of the power variation, the spectrum variation of the energy coupling envelope is calculated to determine the bending curvature of the optical fiber; Based on the inhibitory effect of bending on the coupling between fiber cores, the power changes of multiple fiber cores are monitored, and the bending direction of the optical fiber is determined by comparing the power change differences of different fiber cores.

7. The method according to claim 6, characterized in that The method comprises: calculating the spectrum change of the energy coupling envelope based on the power change of the optical signal in different fiber cores and the spectrum information of the power change to determine the bending curvature of the optical fiber; monitoring the power change of multiple fiber cores based on the inhibitory effect of bending on the coupling between fiber cores, and determining the bending direction of the optical fiber by comparing the power change difference of different fiber cores, including: For the optical field coupling between parallel transmission waveguides, the coupled mode theory based on perturbation approximation is analyzed, and the influence of the mode field of one core on the transmission mode field of another core is regarded as a perturbation term; for multiple core structures, the power conversion efficiency is related to the difference in coupling coefficient and waveguide propagation constant, and the power conversion efficiency between different cores is expressed as: in, Where η is the power conversion efficiency between fiber cores, n eff is the effective refractive index of the waveguide, λ is the operating wavelength, β represents the propagation constant, β2 is the propagation constant of core 2, β1 is the propagation constant of core 1, and κ 12 is the coupling coefficient from core 1 to core 2; when the effective refractive index of adjacent cores is slightly different, the power conversion efficiency η decreases significantly; The influence of fiber bending on the coupling between fiber cores is analyzed by introducing the equivalent refractive index model; a bent fiber is equivalent to a straight fiber with an equivalent refractive index distribution, which is expressed as: Where (r, θ) represents the local polar coordinates of a reference point in the fiber cross section, θ is the angle with the bending radial direction, R is the bending radius of the fiber, and n(r, θ) is the intrinsic refractive index. The center of one fiber core is the origin of the local polar coordinates, and the equivalent effective refractive index of the remaining fiber cores is n eqeff，2 It is expressed as: The equivalent propagation constant β of the remaining fiber cores eq，2 It is expressed as: Among them, n eff，2 is the effective refractive index of core 2, Λ 21 is the core spacing of different fiber cores, θ 21 is the angle between the line segment connecting different fiber cores and the bending radial direction; due to β eq，2 As variables, the coupled mode equation of randomly coupled multi-core fiber is further expressed as: in, is the local increment of the electric field amplitude envelope of core 2 as the propagation length changes, is the distance increment along the propagation direction, j is an imaginary unit, Φ1(z) is the phase function of core 1 changing with the propagation distance z, Φ2(z) is the phase function of core 2 changing with the propagation distance z, A1 is the slowly varying complex amplitude of the electric field in core 1, A2 is the slowly varying complex amplitude of the electric field in core 2, Φ(z) is the phase function changing with the propagation distance z, β eq(z′) is the equivalent propagation constant at position z; According to the determined coupled mode equation, the inter-core power conversion of the randomly coupled multi-core optical fiber under the corresponding geometric parameters, wavelength and bending radius is calculated, so as to determine the bending curvature and bending direction of the optical fiber.

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