A vector vibration sensing method and system based on a seven-core optical fiber

Abstract

The present invention provides a vector vibration sensing method and system based on a seven-core optical fiber, comprising the following steps: obtaining a laser; transmitting the laser into the centrifugal and central cores of the seven-core optical fiber to serve as signal light and reference light respectively; applying a vibration signal to the seven-core optical fiber to obtain the phase difference between the signal light and the reference light; and respectively realizing the calculation of vibration direction and vibration amplitude information based on the phase difference and its relative relationship. The system of the present invention can realize the calculation of multi-dimensional vibration vector information including vibration frequency, direction, amplitude, etc.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber sensing, and particularly relates to a vector vibration sensing method and system based on a seven-core optical fiber. Background Art

[0002] Optical fiber is not only an excellent transmission channel but also a special sensitive element that can be used to sense and obtain changes in external physical quantities. Relying on its characteristics such as being passive, having low loss, being light in weight, small in size, corrosion-resistant, and immune to electromagnetic interference, the application scenarios of optical fiber sensing technology can extend to many places where human and electrical power are inaccessible. Compared with the solutions of traditional electronic sensing technology, it has special advantages. Therefore, significant achievements have been made in the sensing and measurement of various physical quantities such as temperature, vibration, and strain, demonstrating the great development potential of this technical method. In particular, due to its excellent property of being able to work in harsh environments, optical fiber sensing technology plays an irreplaceable and crucial role in fields such as safety production, national defense construction, and precision manufacturing.

[0003] Among them, optical fiber vibration sensing can effectively extract the laws of changes in optical fiber length, effective refractive index, etc. over time by measuring parameters such as the intensity, wavelength, frequency, or phase of the optical signal transmitted therein, and thereby sense and obtain information such as the frequency and amplitude of external vibration signals. Optical fiber vibration sensors have advantages such as a wide monitoring range, high sensitivity, and a simple system structure, and thus are widely used in many fields, such as optical fiber vibration intrusion detection systems, mechanical structure health monitoring systems, and seismic wave detection systems. Along with the maturity of technology comes higher requirements. Taking border intrusion monitoring as an example, the analysis of the received optical signal is no longer limited to the relatively single information acquisition of real-time monitoring of the vibration of the optical fiber caused by potential intrusion. Further, it is required to determine intrusion events, which requires more diverse information acquisition and more accurate data analysis.

[0004] However, existing optical fiber vibration sensing technologies mainly focus on the measurement, perception, and analysis of vibration frequency and amplitude, and the information they interpret is relatively monotonous. Facing the increasingly demanding measurement performance requirements and the continuously developing diverse application scenarios, the future development of optical fiber sensing technology faces many challenges. The effective excavation and accurate interpretation of richer multi-dimensional information perception and solution are receiving attention, and it is crucial to effectively mine and accurately interpret multi-vibration vector information including vibration frequency, direction, amplitude, etc. Summary of the Invention

[0005] To solve the above technical problems, the present invention proposes a vector vibration sensing method and system based on a seven-core optical fiber, and the vibration direction and amplitude information can be solved by obtaining the magnitude of the phase difference between each centrifugal core and the central core and their relative relationship.

[0006] On the one hand, to achieve the above object, the present invention provides a vector vibration sensing method based on a seven-core optical fiber, comprising the following steps:

[0007] Obtain a laser;

[0008] Transmit the laser to the seven-core optical fiber to serve as the signal light and the reference light respectively;

[0009] Apply a vibration signal to the seven-core optical fiber to obtain the phase difference between the signal light and the reference light;

[0010] Based on the phase difference and its relative relationship, respectively implement the calculation of the vibration direction and vibration amplitude information.

[0011] Optionally, when applying a vibration signal to the seven-core optical fiber, the radial deformation amount δL of the centrifugal core i simultaneously reflects the multi-component vibration vector information including the vibration direction and vibration amplitude, and δL i has the following expression:

[0012] δL i =d i ·α = -d0cos(θ b -θ i )·α

[0013] wherein, d0 is the core pitch of the seven-core optical fiber, d i is the distance from core i to the neutral plane, θ b is the angle between the x-axis and the vibration direction, θ i is the angle between the x-axis and core i, and α is the central angle of the bent optical fiber, and the vibration amplitude has a linear relationship with α.

[0014] Optionally, the expression of the phase difference between the signal light and the reference light is:

[0015]

[0016] wherein, i = 2, 3,..., 7 is the core number of the seven-core optical fiber, k is the wave number of the input light, n is the effective refractive index of the optical fiber, C2 is a constant related to the optical fiber material, and δL i is the radial deformation amount of the centrifugal core i.

[0017] Optionally, the method for calculating the vibration direction based on the phase difference includes:

[0018] Based on the positive / negative and relative magnitude of the phase difference, obtain the quadrant where the vibration direction is located;

[0019] Construct a correlation function between the phase difference and the vibration direction;

[0020] Based on the quadrant where the vibration direction is located and the correlation function, obtain the angle between the vibration direction and the x-axis, and realize the calculation of the vibration direction.

[0021] Optionally, the correlation function expression between the phase difference and the vibration direction is:

[0022]

[0023] where j = 2, 3,..., 7 (j ≠ i, i + 3), θ i is the angle between the x-axis and the core i, θ j is the angle between the x-axis and the core j, θ b is the angle between the x-axis and the vibration direction.

[0024] Optionally, solve the correlation function between the phase difference and the vibration direction, perform vibration direction calculation, and obtain the calculation results of the angles θ b between the vibration direction and the x-axis for all 12 groups;

[0025] Perform a linear fit with a slope of 0 on the calculation results of the angles θ b between the 12 groups of vibration directions and the x-axis to obtain the calculation results of the uniquely determined vibration direction angle θ b with improved accuracy.

[0026] Optionally, the method for calculating the vibration amplitude based on the phase difference includes:

[0027] Construct the correlation function between the phase difference and the vibration amplitude;

[0028] Based on the correlation function between the phase difference and the vibration amplitude, perform an inverse mapping of the phase difference to the corresponding core direction, obtain 6 groups of inverse mapping values, and perform a linear fit on the 6 groups of solved inverse mapping values to obtain the inverse mapping values with improved accuracy;

[0029] Through vibration experiments with multiple different vibration amplitudes, obtain the linear coefficient between the inverse mapping value and the actually applied vibration amplitude;

[0030] Based on the linear coefficient, obtain the one-to-one correspondence between the vibration amplitude and the inverse mapping value, and realize the calculation of the vibration amplitude.

[0031] Optionally, the correlation function expression between the phase difference and the vibration amplitude is:

[0032]

[0033] where A vibis the vibration amplitude, γ is a constant, L0 is the initial length of the seven-core optical fiber, α is the central angle of the bent optical fiber, d0 is the core pitch of the seven-core optical fiber, the left side of the equal sign is the corresponding inverse mapping value of the phase difference, and the rightmost side of the equal sign is the relationship between the inverse mapping value and the vibration amplitude to be measured;

[0034] Performing linear fitting on the six groups of inverse mapping values obtained by solving the correlation function between the phase difference and the vibration amplitude can improve the calculation accuracy of the vibration amplitude.

[0035] On the other hand, to achieve the above object, the present invention provides a vector vibration sensing system based on a seven-core optical fiber, including:

[0036] a laser, a polarization maintaining coupler, a fan-in module, a fan-out module, a seven-core optical fiber, a first polarization tracker, an i-th polarization tracker, a phase modulator, a loop filter, and a balanced photodetector;

[0037] The laser emits laser light, which successively passes through the polarization maintaining coupler and the fan-in module and is input into the seven-core optical fiber. At the same time, the seven-core optical fiber is subjected to vibration, and the laser light after being affected by the vibration is output after passing through the fan-out module. Among them, the first output passes through the first polarization tracker, and the second to seventh outputs respectively pass through the i-th polarization tracker and the phase modulator. The laser light passing through the first polarization tracker and the laser light passing through the i-th polarization tracker and the phase modulator are coupled by the polarization maintaining coupler and then output through the balanced photodetector; the signal output by the balanced photodetector is processed by the loop filter and fed back to the phase modulator as its driving signal.

[0038] Compared with the prior art, the present invention has the following advantages and technical effects:

[0039] The present invention uses a seven-core optical fiber as a sensitive element. When it is subjected to vibration and thus generates a bending deformation along the vibration direction, according to the relevant theory of material mechanics, the middle core of the seven-core optical fiber is located within the bending neutral plane and will not be affected by the bending, while the centrifugal cores will be subjected to corresponding compression or tension according to their different geometric distribution positions, so that the light transmitted therein generates a phase difference relative to the light transmitted through the middle core. The magnitude and relative relationship of the phase differences of each core have a fixed relationship with the vibration direction and amplitude. Therefore, by obtaining the magnitude and relative relationship of the phase differences of each core, the calculation of multi-dimensional vibration vector information including vibration frequency, direction, amplitude, etc. can be realized. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The drawings constituting a part of this application are used to provide a further understanding of this application. The schematic embodiments of this application and their descriptions are used to explain this application and do not constitute an improper limitation to this application. In the drawings:

[0041] Figure 1Schematic flow chart of a vector vibration sensing method based on a seven-core optical fiber according to an embodiment of the present invention;

[0042] Figure 2 Side view of the seven-core optical fiber bent due to vibration according to an embodiment of the present invention;

[0043] Figure 3 Cross-sectional view of the seven-core optical fiber bent due to vibration according to an embodiment of the present invention;

[0044] Figure 4 Schematic diagram of a vibration vector sensing calculation model based on a seven-core optical fiber according to an embodiment of the present invention;

[0045] Figure 5 Schematic structural diagram of a vector vibration sensing system based on a seven-core optical fiber according to an embodiment of the present invention. Detailed implementation manners

[0046] It should be noted that, without conflict, the embodiments in the present application and the features in the embodiments may be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0047] It should be noted that the steps shown in the flowchart of the accompanying drawings may be executed in a computer system such as a set of computer-executable instructions, and although the logical order is shown in the flowchart, in some cases, the steps shown or described may be executed in a different order than here.

[0048] [[ID=2,7]]Example 1

[0049] As Figure 1 shown, the present invention provides a vector vibration sensing method based on a seven-core optical fiber, including the following steps: obtaining a laser; transmitting the laser to the seven-core optical fiber, respectively as a signal light and a reference light; applying a vibration signal to the seven-core optical fiber to obtain a phase difference between the signal light and the reference light; and respectively realizing the calculation of vibration direction and vibration amplitude information based on the phase difference and its relative relationship.

[0050] In this embodiment, in combination with Figure 2 and Figure 3 , the laser emitted by the laser is respectively input into a seven-core optical fiber with a regular hexagonal distribution of the fiber cores. When the seven-core optical fiber is vibrated and undergoes corresponding instantaneous bending, according to the relevant principles of material mechanics, the central fiber core, that is, core 1, is located in the bending neutral plane perpendicular to the bending direction and will not produce corresponding strain response; the centrifugal fiber cores, core i (i = 2, 3,..., 7) distributed on both sides of the neutral plane will be subjected to corresponding tensile or compressive effects according to their position distributions and produce radial length changes. The radial deformation amount δL i of the centrifugal fiber core simultaneously reflects the multi-element vibration vector information including the vibration direction and vibration amplitude:

[0051] δL i = d i ·α = -d0cos(θ b - θ i )·α (1)

[0052] where d0 is the core pitch of the seven-core optical fiber, d i is the distance from the core i to the neutral plane, θ b is the angle between the x-axis and the vibration direction, θ i is the angle between the x-axis and the core i, and α is the central angle of the bent optical fiber. This formula indicates that the farther the core position is from the neutral plane, the greater the radial length change. Moreover, for core pairs that are symmetrically distributed on different sides of the neutral plane but centrosymmetric, their δL i are equal in magnitude and opposite in phase. And the central angle α of the bent optical fiber implies the vibration amplitude information. The relationship between the vibration amplitude A vib and α is:

[0053]

[0054] where D is the displacement of the optical fiber caused by vibration, γ is a linear constant related to the vibration application method, and L0 is the initial length of this section of the seven-core optical fiber. It has been verified that within the optical fiber vibration measurement range, α is sufficiently small, so the vibration amplitude A vib has a linear relationship with α.

[0055] In actual measurement, six centrifugal cores are simultaneously used as signal light transmission channels, and the central core is used as a shared reference light transmission channel to construct six Mach-Zehnder interference structures with a common reference. The radial deformation of these centrifugal cores will introduce a phase difference between the signal light and the reference light, which is i linearly proportional to δL:

[0056]

[0057] where k is the wave number of the input light, n is the effective refractive index of the optical fiber, and C2 is a constant related to the optical fiber material. Therefore, by measuring , the vibration vector information can be solved, and a vibration vector model can be constructed, as shown in Figure 4 .

[0058] Specifically, on the one hand, the ratio of any two except for the core pairs with centrosymmetric distribution is a function that is uniquely related to the vibration direction θ b :

[0059]

[0060] where j = 2, 3, …, 7 (j ≠ i, i + 3), θi,j is the azimuth angle of the fiber cores i and j. Solving the equation above can obtain θ b . However, it should be noted that in order to eliminate the ambiguity problem in the solution caused by the non-monotonicity of the cosine function within the range of 2π, it is necessary to predict the quadrant where the vibration direction is located. Combining Equation 1 and the phase difference with δL i , it can be known that the farther the fiber core position is from the neutral plane, the value will be larger, and theoretically, the values will be equal in magnitude and opposite in phase in pairs. Then, simply comparing the relative magnitudes and polarities of the 6 measured can infer that the vibration direction is between the fiber cores where the maximum value and the median value appear and points to the side of the fiber core with a positive b . On this basis, θ

[0061] On the other hand, when the vibration direction is determined, for any specific fiber core, the magnitude of b will be uniquely determined by the vibration amplitude. Correspondingly, according to the geometric relationship between θ i and θ , perform an inverse mapping of

[0062]

[0063] As can be seen from the above formula, theoretically, performing an inverse mapping of vib to the directions of each fiber core, its magnitude is a linear function of α and no longer depends on the geometric distribution position of the fiber core. That is to say, when α is determined, or in other words, when the vibration amplitude A vib is determined, the results of the inverse mapping of each fiber core are equal, and when their ends are connected, they should form a regular hexagon. The larger the vibration amplitude, the larger the radius of this hexagon. However, due to the influence of measurement errors resulting in unequal inverse mappings, the solution error can be reduced by means of linear fitting, so as to obtain a vibration amplitude A

[0064] Therefore, in the vibration vector model as shown in Figure 4 , we obtain the angle θ b between the vibration direction and the x-axis, and a regular hexagon whose size intuitively reflects the magnitude of the vibration amplitude A vib

[0065] The specific steps include:

[0066] ​Step 1: Measure the phase difference of each fiber core relative to the light transmitted in the neutral core during vibration through experiments

[0067] Step 2: Based on the measured phase difference Make a rough judgment on the vibration direction according to the positive / negative and relative magnitude of the phase difference, and determine the quadrant where the vibration direction is located (the vibration direction is always between the two fiber cores with the largest phase difference and points to the side of the fiber core with a positive phase difference);

[0068] Step 3: Solve the equation shown in Formula 4 to calculate the vibration direction and obtain the calculation results of the included angle θ between all 12 groups of vibration directions and the x-axis b ;

[0069] Step 4: Perform a linear fitting with a slope of 0 on the calculation results of 12 groups of θ b to obtain the calculation results of θ with improved accuracy b ;

[0070] Step 5: Perform inverse mapping on 6 groups of according to Formula 5;

[0071] Step 6: Perform a linear fitting with a slope of 0 on the calculation results of 6 groups of inverse mapping to obtain the inverse mapping values with improved accuracy;

[0072] Step 7: Through vibration experiments with multiple groups of different vibration amplitudes, obtain the linear coefficient between the inverse mapping value and the actually applied vibration amplitude, and determine the one-to-one correspondence between the vibration amplitude A vib and the inverse mapping value.

[0073] Step 8: According to the calculation results of Step 3 and Step 7, construct a vibration vector model as shown in Figure 4 , where the arrow points to the vibration direction, and the size of the hexagon intuitively shows the size of the vibration amplitude.

[0074] Example 2

[0075] As shown in Figure 5 , the present invention also provides a vector vibration sensing system based on a seven-core optical fiber (only one of the 6 groups of Mach-Zehnder structures is shown in detail to avoid redundancy, and the other 5 groups have the same structure), including:

[0076] The vibration vector sensing system based on a seven-core optical fiber is composed of a laser (Laser), a 50:50 polarization-maintaining coupler (PMC), a fan-in / out module (Fan-in / out), a seven-core optical fiber (7-core fiber), a first polarization tracker (PT1), a i-th polarization tracker (PTi), a phase modulator (PM), a balanced photodetector (BPD), and a loop filter (LF) connected according to the relationship shown in the above figure.

[0077] The laser emits laser light which successively passes through the polarization-maintaining coupler and the fan-in module and is input into the seven-core optical fiber. At the same time, the seven-core optical fiber is subjected to vibration. The laser light after passing through the vibration then passes through the fan-out module and is output. Among them, the first output, that is, the output of core 1, passes through the first polarization tracker. The second to seventh outputs, that is, the output of core i, respectively pass through the i-th polarization tracker and the phase modulator. The laser light after passing through the first polarization tracker and the laser light passing through the i-th polarization tracker and the phase modulator are coupled by the polarization-maintaining coupler and then output through the balanced photodetector. The purpose of this loop filter is to filter the output signal to generate a feedback signal and feedback it to the phase modulator.

[0078] Essentially, this system constructs six mutually-referenced Mach-Zehnder interference structures with the middle core (core 1) of the seven-core optical fiber as the reference optical transmission channel and the other off-center cores as the signal optical transmission channels. When vibration is applied to a certain point on the seven-core optical fiber, the phase difference of each path of signal light relative to the reference light can be completed by means of the linear coherent phase demodulation technology based on the optical phase-locked loop. extraction. When the phase-locked loop is locked, the frequency of the system output signal is the vibration frequency; according to the steps described in the specific implementation manner of the method, the solutions of θ b and A vib are calculated, and the sensing of the vibration vector including the vibration direction and amplitude can be realized.

[0079] Thus, the vibration vector sensing based on the seven-core optical fiber is completed / realized.

[0080] The above is only a preferred specific implementation manner of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed in the present application should be covered by the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A vector vibration sensing method based on a seven-core optical fiber, characterized in that, It includes the following steps: Obtain laser light; Transmit the laser light to a seven-core optical fiber, which are used as signal light and reference light respectively; Apply a vibration signal to the seven-core optical fiber to obtain the phase difference between the signal light and the reference light; Based on the phase difference and its relative relationship, respectively implement the calculation of the vibration direction and vibration amplitude information; When a vibration signal is applied to the seven-core optical fiber, the radial deformation amount δL of the centrifugal core i simultaneously reflects the multi-dimensional vibration vector information including the vibration direction and the vibration amplitude, and δL i has the following expression: δL i = d i ·α = -d0cos(θ b - θ i )·α Among them, d0 is the core pitch of the seven-core optical fiber, d i is the distance from the core i to the neutral plane, θ b is the angle between the x-axis and the vibration direction, θ i is the angle between the x-axis and the core i, α is the central angle of the bent optical fiber, and the vibration amplitude has a linear relationship with α; The expression of the phase difference between the signal light and the reference light is: where k is the wave number of the input light, n is the effective refractive index of the optical fiber, C2 is a constant related to the optical fiber material, and δL i is the radial deformation of the centrifugal core; the phase difference between each signal light and the reference light is proportional to the radial deformation generated by the vibration of the transmission core of the signal light; The method for implementing the calculation of the vibration direction based on the phase difference includes: Based on the positive or negative and relative magnitude of the phase difference, obtain the quadrant where the vibration direction is located; Construct the correlation function between the phase difference and the vibration direction; Based on the quadrant where the vibration direction is located and the correlation function, obtain the angle between the vibration direction and the x-axis to implement the calculation of the vibration direction; The expression of the correlation function between the phase difference and the vibration direction is: where j = 2, 3,..., 7 (j ≠ i, i + 3), θ i is the angle between the x-axis and the core i, θ j is the angle between the x-axis and the core j, θ b is the angle between the x-axis and the vibration direction; Solve the correlation function between the phase difference and the vibration direction, perform vibration direction calculation, and obtain the included angles θ between all 12 groups of vibration directions and the x-axis b of the calculation result; For the included angle θ between the vibration direction and the x-axis of 12 groups b perform a linear fitting with a slope of 0 on the calculation results to obtain a uniquely determined vibration direction angle θ b with improved accuracy; The method for implementing the calculation of the vibration amplitude based on the phase difference includes: Construct the correlation function between the phase difference and the vibration amplitude; Based on the correlation function between the phase difference and the vibration amplitude, perform inverse mapping of the phase difference to the corresponding core direction to obtain 6 groups of inverse mapping values, and perform linear fitting on the solved 6 groups of inverse mapping values to obtain the inverse mapping values with improved accuracy; Through vibration experiments with multiple groups of different vibration amplitudes, obtain the linear coefficient between the inverse mapping value and the actually applied vibration amplitude; Based on the linear coefficient, obtain the one-to-one correspondence between the vibration amplitude and the inverse mapping value to implement the calculation of the vibration amplitude; The expression of the correlation function between the phase difference and the vibration amplitude is: where A vib is the vibration amplitude, γ is a constant related to the vibration application method, L0 is the initial length of the seven-core optical fiber, α is the central angle of the bent optical fiber, and d0 is the core pitch of the seven-core optical fiber; the left side of the equal sign is the corresponding inverse mapping value of the phase difference, and the rightmost side of the equal sign is the relationship between the inverse mapping value and the vibration amplitude to be measured; Performing linear fitting on the 6 groups of inverse mapping values solved from the correlation function between the phase difference and the vibration amplitude can improve the calculation accuracy of the vibration amplitude.

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