Distributed optical fiber vibration sensing method and system based on two-section optical fiber

Abstract

The present application relates to the technical field of distributed optical fiber vibration sensing, in particular to a distributed optical fiber vibration sensing method and system based on double-section optical fiber, two backward Rayleigh scattering light signals are generated by setting double-section optical fiber at the same vibration position; each backward Rayleigh scattering light signal is converted into electric signal and transmitted through several radio frequency lines with different lengths, several converted signals with the same vibration information and different initial phases are obtained, the converted signals are sampled according to the sampling time interval, and the demodulation vibration signals are obtained; the phase demodulation is performed on the two demodulation vibration signals corresponding to the same backward Rayleigh scattering light signal, and the vibration phase is obtained, so that the vibration signal demodulation is accurately realized under high spatial resolution; the first vibration phase and the second vibration phase are obtained according to the two backward Rayleigh scattering light signals, the third vibration phase is obtained after overcoming the coherent fading, and the problem that the vibration phase cannot be continuously demodulated when the vibration signal appears fading is solved.

Description

Technical Field

[0001] This invention relates to distributed optical fiber vibration sensing technology, specifically to a distributed optical fiber vibration sensing method and system based on dual-segment optical fibers. Background Technology

[0002] In recent years, phase optical time domain reflectometers based on direct demodulation have attracted much attention from researchers as a direct detection type of distributed fiber optic vibration sensing system. They can quantitatively measure long-distance vibrations due to their high sensitivity in vibration detection, flexibility in implementation, resistance to electromagnetic interference, cost-effectiveness, and excellent spatiotemporal resolution. They have played an irreplaceable role in the safety and health monitoring of large structures such as pipeline intrusion detection, bridges and tunnels, and urban integrated pipe corridors.

[0003] In distributed fiber optic vibration sensing systems, vibration, as an external disturbance, affects the backscattered Rayleigh light signal transmitted in the fiber, potentially causing coherent fading and ultimately preventing continuous demodulation of the vibration signal. When traditional direct-probe distributed fiber optic vibration sensing systems sample, if the sampling time interval of the data acquisition module is large, the system must use a large pulse width to include the vibration information of adjacent sampling points, thus reducing the system's spatial resolution. The traditional direct-probe distributed fiber optic vibration sensing systems used by Fang et al. in their paper "Phase-sensitive optical time domain reflectometer based on phase-generated carrier algorithm" and Masoudi et al. in their paper "A distributed optical fibredynamic strain sensor based on phase-OTDR" require the use of an interferometer to extract the two vibration signals during demodulation. However, this method is sensitive to external noise, impractical, and costly.

[0004] Therefore, it is necessary to propose a distributed optical fiber vibration sensing method and system that can overcome coherent fading and achieve accurate vibration signal demodulation at high spatial resolution, thereby reducing the system's operating cost. Summary of the Invention

[0005] The purpose of this application is to propose a distributed optical fiber vibration sensing method and system based on dual-segment optical fibers to address the aforementioned technical problems.

[0006] In a first aspect, the present invention provides a distributed optical fiber vibration sensing method based on a dual-segment optical fiber, comprising the following steps:

[0007] The modulated optical signal is fed into a two-segment optical fiber at the same vibration position after passing through an optical fiber circulator, and two backscattered Rayleigh optical signals are generated.

[0008] The system receives two backscattered Rayleigh light signals and performs photoelectric conversion, outputting two converted signal groups. Each backscattered Rayleigh light signal is transmitted through several radio frequency lines of different lengths after photoelectric conversion, resulting in several converted signals with the same vibration information but different initial phases. These several converted signals constitute a converted signal group.

[0009] The two converted signal groups are sampled according to the sampling time interval to obtain two vibration signal groups to be demodulated. Each vibration signal group to be demodulated includes several vibration signals to be demodulated.

[0010] The two vibration signal groups to be demodulated are phase demodulated using a differential cross-multiplication algorithm to obtain the first vibration phase and the second vibration phase.

[0011] If the first vibration phase has phase distortion, the first vibration phase is compensated based on the second vibration phase to obtain the third vibration phase.

[0012] Preferably, if the first vibration phase has phase distortion, the first vibration phase is compensated based on the second vibration phase to obtain the third vibration phase, specifically including:

[0013] The first vibration phase and the second vibration phase are divided into N time intervals according to the given time interval;

[0014] Calculate the mean amplitude of the first vibration phase to obtain the first mean, and calculate the variance between the amplitude of the first vibration phase and the first mean in the j-th time interval, where j = 1, 2, ..., N, to obtain the first variance;

[0015] If the first variance is less than the given variance threshold, then the first vibration phase in the j-th time interval has phase distortion. The first vibration phase in the j-th time interval is compensated according to the amplitude and frequency of the second vibration phase in the j-th time interval to obtain the third vibration phase.

[0016] As a preferred option, the processing steps of the differential cross-multiplication algorithm specifically include:

[0017] Select any two vibration signals to be demodulated, denoted as I. rbs (d1, t) and I rbs (d1+Δl, t), where d1 and d1+Δl represent the distances between the two demodulated vibration signals and the vibration positions on the optical fiber, Δl represents the length difference between the two RF lines, and t represents time. For the two demodulated vibration signals I... rbs (d1, t) and Irbs (d1+Δl,t) is low-pass filtered to obtain two filtered vibration signals, which are represented as I1(d1,t) and I2(d1+Δl,t) respectively.

[0018] The two filtered vibration signals I1(d1,t) and I2(d1+Δl,t) are processed using the first-order differential cross-multiplication method to obtain the first intermediate signal I. DCM1 (d1, t), as shown in the following formula:

[0019]

[0020] Among them, the first intermediate signal I DCM1 (d1, t) includes the initial phase difference coefficient G and the vibration phase to be integrated, dθ. vibration (t);

[0021] The two filtered vibration signals I1(d1,t) and I2(d1+Δl,t) are processed using the second-order differential cross-multiplication method to obtain the second intermediate signal I. DCM2 (d1, t), as shown in the following formula:

[0022]

[0023] According to the first intermediate signal I DCM1 (d1, t) and the second intermediate signal I DCM2 (d1, t), the initial phase difference coefficient G is calculated;

[0024] The first intermediate signal I DCM1 Dividing (d1,t) by the initial phase difference coefficient G and integrating the result, we obtain the vibration phase, as shown in the following equation:

[0025]

[0026] Where C represents a constant, θ vibration (t) represents the vibration phase, written as:

[0027] θ vibration (t)=A vibration cos(2πf vibration t);

[0028] Among them, A vibration f represents the amplitude of the vibration phase. vibration This indicates the frequency of the vibration signal.

[0029] Preferably, the method further includes: amplifying and filtering the modulated optical signal before inputting it into an optical fiber circulator.

[0030] As a preferred embodiment, the method further includes amplifying and filtering the received two backscattered Rayleigh light signals before performing photoelectric conversion.

[0031] In a second aspect, the present invention provides a distributed optical fiber vibration sensing system based on a dual-segment optical fiber, used to implement the distributed optical fiber vibration sensing method based on a dual-segment optical fiber described in any implementation of the first aspect, including a signal generator, a narrow linewidth laser, an optical pulse modulator, an optical fiber circulator, a dual-segment optical fiber connected end to end, a photoelectric detection module, several radio frequency lines, a multi-channel data acquisition module, and a signal processing module.

[0032] A narrow-linewidth laser is connected to an optical pulse modulator. The narrow-linewidth laser is used to emit laser light and transmit the laser light to the optical pulse modulator.

[0033] The signal generator is connected to the optical pulse modulator to generate an electrical signal with adjustable pulse width and repetition frequency, and transmits the electrical signal to the optical pulse modulator. The signal generator is also connected to the multi-channel data acquisition module to transmit the frequency information of the electrical signal to the multi-channel data acquisition module.

[0034] An optical pulse modulator is used to modulate the laser emitted by a narrow-linewidth laser using an electrical signal output from a signal generator, thereby generating a modulated optical signal.

[0035] The fiber optic circulator is connected to a dual-segment fiber. The fiber optic cable is used to generate a backscattered Rayleigh light signal. The fiber optic circulator is used to input the modulated light signal into the dual-segment fiber which is in the same vibration position, and to output a backscattered Rayleigh light signal.

[0036] The photoelectric detection module is used to receive the backscattered Rayleigh light signal, perform photoelectric conversion on it, and output the converted signal.

[0037] Several radio frequency (RF) lines are connected at both ends to the output of the photoelectric detection module and the multi-channel data acquisition module, respectively. The RF lines are used to transmit the converted signal to the multi-channel data acquisition module.

[0038] The multi-channel data acquisition module is connected to the signal processing module. The multi-channel data acquisition module is used to synchronously acquire the converted signals output from several radio frequency lines according to the sampling time interval, and obtain several vibration signals to be demodulated.

[0039] The signal processing module is used to perform phase demodulation on several vibration signals to be demodulated, and to overcome coherent fading in the vibration phase obtained by phase demodulation.

[0040] Preferably, the lengths of the RF lines are different, and the length difference between any two RF lines is different.

[0041] Preferably, the length difference between any two RF lines is less than the distance the converted signal travels within the sampling time interval.

[0042] Preferably, the system also includes a first optical amplification and filtering module, which is connected to the optical pulse modulator and the fiber optic circulator, and is used to amplify and filter the modulated optical signal before outputting it.

[0043] Preferably, a second optical amplification and filtering module is also included. The second optical amplification and filtering module is connected to the fiber optic circulator and the photodetector module, and is used to amplify and filter the received backscattered Rayleigh light signal before transmitting it to the photodetector module.

[0044] Compared with the prior art, the present invention has the following beneficial effects:

[0045] (1) The distributed optical fiber vibration sensing method based on dual-segment optical fiber proposed in this invention samples the signals transmitted by several radio frequency lines of different lengths to obtain two vibration signals to be demodulated in space with the same vibration information but different initial phases. The demodulation results are obtained with high accuracy under high spatial resolution. The backscattered Rayleigh light signal generated by the dual-segment optical fiber at the same vibration position is analyzed to overcome coherent fading and realize continuous demodulation of vibration signal. Thus, an economical, practical and highly stable direct detection type distributed optical fiber vibration sensing system is realized.

[0046] (2) The distributed optical fiber vibration sensing system based on dual-segment optical fiber proposed in this invention connects several radio frequency lines of different lengths at the output end of the photoelectric detection module, and the length difference between any two radio frequency lines is different, so as to obtain multiple converted signals with the same vibration information but different initial phases, so as to obtain vibration signal demodulation results with high accuracy; further, the length difference between any two radio frequency lines is less than the distance that the converted signal is transmitted within the sampling time interval, so as to select two converted signals with a relatively close spatial interval corresponding to the backscattered Rayleigh light signal for sampling, thereby realizing vibration signal demodulation over a shorter distance in space and improving the spatial resolution of the system, avoiding the reduction of the spatial resolution of the system when the sampling time interval of the multi-channel data acquisition module is large.

[0047] (3) The distributed optical fiber vibration sensing system based on dual-segment optical fiber proposed in this invention lays dual-segment optical fiber at the vibration location and performs phase demodulation on the vibration signal to be demodulated corresponding to the dual-segment optical fiber to obtain two vibration phases. The vibration phase obtained after demodulation of the vibration signal to be demodulated with signal fading has phase distortion. Taking advantage of the fact that the initial phases of the vibration signals to be demodulated corresponding to the dual-segment optical fiber are different, the other vibration phase with phase distortion can be compensated according to one vibration phase, thus overcoming coherent fading and realizing continuous demodulation of vibration signal. It avoids the use of expensive devices and is an economical distributed optical fiber vibration sensing system to overcome coherent fading. Attached Figure Description

[0048] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0049] Figure 1 This is a schematic flowchart of a distributed optical fiber vibration sensing method based on a dual-segment optical fiber, according to an embodiment of this application.

[0050] Figure 2 This is a schematic diagram of the structure of a distributed optical fiber vibration sensing system based on a dual-segment optical fiber, as an embodiment of this application.

[0051] Figure 3 This is a schematic diagram of two groups of vibration signals to be demodulated, a first vibration phase, a second vibration phase, and a third vibration phase, according to an embodiment of this application. In the diagram, a represents the first vibration signal to be demodulated corresponding to the first optical fiber segment, b represents the first vibration signal to be demodulated corresponding to the second optical fiber segment, c represents the second vibration signal to be demodulated corresponding to the first optical fiber segment, d represents the second vibration signal to be demodulated corresponding to the second optical fiber segment, e represents the first vibration phase, f represents the second vibration phase, and g represents the third vibration phase.

[0052] Reference numerals in the attached diagram: 1. Signal generator; 2. Narrow linewidth laser; 3. Optical pulse modulator; 4. First optical amplification and filtering module; 5. Fiber optic circulator; 51. First port; 52. Second port; 53. Third port; 6. Optical fiber; 7. Second optical amplification and filtering module; 8. Photodetector module; 91. First radio frequency line; 92. Second radio frequency line; 10. Multi-channel data acquisition module; 11. Signal processing module. Detailed Implementation

[0053] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0054] Figure 1 An embodiment of this application illustrates a distributed optical fiber vibration sensing method based on a dual-segment optical fiber, comprising the following steps:

[0055] S1 inputs the modulated optical signal into a two-segment optical fiber at the same vibration position after passing through an optical fiber circulator, and generates two backscattered Rayleigh optical signals.

[0056] In a specific embodiment, the method further includes: amplifying and filtering the modulated optical signal before inputting it into an optical fiber circulator.

[0057] Specifically, in a distributed fiber optic vibration sensing system, the vibration location can be determined by utilizing the flight time of the modulated optical signal within the optical fiber. Because the modulated optical signal input into the fiber undergoes elastic collisions with the non-uniform medium within the fiber, a backscattered Rayleigh light signal is generated. The vibration signal influences the backscattered Rayleigh light signal, causing multipath interference within half the pulse width of the modulated optical signal, resulting in changes to the backscattered Rayleigh light signal. Therefore, the backscattered Rayleigh light signal carries vibration information from the vibration signal. By analyzing the backscattered Rayleigh light signal, the frequency and phase information of the vibration signal can be determined.

[0058] S2 receives two backscattered Rayleigh light signals and performs photoelectric conversion, outputting two converted signal groups. Each backscattered Rayleigh light signal is transmitted through several radio frequency lines of different lengths after photoelectric conversion, resulting in several converted signals with the same vibration information but different initial phases. These several converted signals constitute a converted signal group.

[0059] In a specific embodiment, the method further includes: amplifying and filtering the received two backscattered Rayleigh light signals before performing photoelectric conversion.

[0060] S3, sample the two converted signal groups according to the sampling time interval to obtain two vibration signal groups to be demodulated. Each vibration signal group to be demodulated includes several vibration signals to be demodulated.

[0061] S4. The two vibration signal groups to be demodulated are phase demodulated using the differential cross-multiplication algorithm to obtain the first vibration phase and the second vibration phase.

[0062] In a specific embodiment, the processing procedure of the differential cross-multiplication algorithm specifically includes:

[0063] Select any two vibration signals to be demodulated, denoted as I. rbs (d1, t) and I rbs (d1+Δl, t), where d1 and d1+Δl represent the distances between the two demodulated vibration signals and the vibration positions on the optical fiber, Δl represents the length difference between the two RF lines, and t represents time. For the two demodulated vibration signals I... rbs (d1, t) and I rbs (d1+Δl,t) is low-pass filtered to obtain two filtered vibration signals, which are represented as I1(d1,t) and I2(d1+Δl,t) respectively.

[0064] The two filtered vibration signals I1(d1,t) and I2(d1+Δl,t) are processed using the first-order differential cross-multiplication method to obtain the first intermediate signal I. DCM1 (d1, t), as shown in the following formula:

[0065]

[0066] Among them, the first intermediate signal I DCM1 (d1, t) includes the initial phase difference coefficient G and the vibration phase to be integrated, dθ. vibration (t);

[0067] The two filtered vibration signals I1(d1,t) and I2(d1+Δl,t) are processed using the second-order differential cross-multiplication method to obtain the second intermediate signal I. DCM2 (d1, t), as shown in the following formula:

[0068]

[0069] According to the first intermediate signal I DCM1 (d1, t) and the second intermediate signal I DCM2 (d1,t), the initial phase difference coefficient G is calculated;

[0070] The first intermediate signal I DCM1 Dividing (d1,t) by the initial phase difference coefficient G and integrating the result, we obtain the vibration phase, as shown in the following equation:

[0071]

[0072] Where C represents a constant, θ vibration (t) represents the vibration phase, written as:

[0073] θ vibration (t)=A vibration cos(2πfvibration t);

[0074] Among them, A vibration f represents the amplitude of the vibration phase. vibration This indicates the frequency of the vibration signal.

[0075] S5. If the first vibration phase has phase distortion, the first vibration phase is compensated according to the second vibration phase to obtain the third vibration phase.

[0076] In a specific embodiment, if the first vibration phase has phase distortion, the first vibration phase is compensated based on the second vibration phase to obtain the third vibration phase, specifically including:

[0077] The first vibration phase and the second vibration phase are divided into N time intervals according to the given time interval;

[0078] Calculate the mean amplitude of the first vibration phase to obtain the first mean, and calculate the variance between the amplitude of the first vibration phase and the first mean in the j-th time interval, where j = 1, 2, ..., N, to obtain the first variance;

[0079] If the first variance is less than the given variance threshold, then the first vibration phase in the j-th time interval has phase distortion. The first vibration phase in the j-th time interval is compensated according to the amplitude and frequency of the second vibration phase in the j-th time interval to obtain the third vibration phase.

[0080] Specifically, when the vibration signal to be demodulated corresponding to an optical fiber at a vibration location experiences coherent fading due to phase offset point drift, the differential cross-multiplication (DCM) algorithm cannot achieve continuous demodulation of the vibration signal, resulting in phase distortion in the demodulated vibration phase. In the embodiments of this application, by utilizing the characteristic that the initial phases of the vibration signals to be demodulated corresponding to two optical fiber segments at the same vibration location are different, the vibration signals to be demodulated corresponding to the two optical fiber segments are demodulated to obtain two vibration phases. By performing variance analysis on the amplitude of one vibration phase, if the variance of the vibration phase amplitude within a certain time interval is greater than a variance threshold, then the vibration phase within that time interval has phase distortion. The vibration phase with phase distortion can be compensated based on the other vibration phase within the same time interval, overcoming coherent fading and achieving continuous demodulation of the vibration signal. This solves the problem that vibration signals with coherent fading cannot be continuously demodulated.

[0081] The steps S1-S5 above do not necessarily represent the order of the steps, but are represented by step symbols. The order of the steps can be adjusted.

[0082] Further reference Figure 2As an implementation of the above method, this application provides an embodiment of a distributed optical fiber vibration sensing system based on a dual-segment optical fiber. This system embodiment is similar to... Figure 1 The method embodiment shown includes a signal generator 1, a narrow linewidth laser 2, an optical pulse modulator 3, an optical fiber circulator 5, a two-segment optical fiber connected end to end 6, a photoelectric detection module 8, several radio frequency lines, a multi-channel data acquisition module 10, and a signal processing module 11.

[0083] Narrow linewidth laser 2 is connected to optical pulse modulator 3. Narrow linewidth laser 2 is used to emit laser light and transmit the laser light to optical pulse modulator 3.

[0084] The signal generator 1 is connected to the optical pulse modulator 3 to generate an electrical signal with adjustable pulse width and repetition frequency, and transmit the electrical signal to the optical pulse modulator 3. The signal generator 1 is also connected to the multi-channel data acquisition module 10 to transmit the frequency information of the electrical signal to the multi-channel data acquisition module 10.

[0085] The optical pulse modulator 3 is used to modulate the laser emitted by the narrow linewidth laser 2 using the electrical signal output by the signal generator 1, thereby generating a modulated optical signal.

[0086] The fiber optic circulator 5 is connected to the dual-segment fiber 6. The fiber optic 6 is used to generate a backscattered Rayleigh light signal. The fiber optic circulator 5 is used to input the modulated light signal into the dual-segment fiber 6 which is in the same vibration position, and to output a backscattered Rayleigh light signal.

[0087] The photoelectric detection module 8 is used to receive the backscattered Rayleigh light signal, perform photoelectric conversion on it, and output the converted signal.

[0088] Several radio frequency lines are connected at both ends to the output end of the photoelectric detection module 8 and the multi-channel data acquisition module 10, respectively. The radio frequency lines are used to transmit the converted signal to the multi-channel data acquisition module 10.

[0089] The multi-channel data acquisition module 10 is connected to the signal processing module 11. The multi-channel data acquisition module 10 is used to synchronously acquire the converted signals output from several radio frequency lines according to the sampling time interval, and obtain several vibration signals to be demodulated.

[0090] The signal processing module 11 is used to perform phase demodulation on several vibration signals to be demodulated, and to overcome coherent fading in the vibration phase obtained by phase demodulation.

[0091] Specifically, the signal generator 1 is connected to the multi-channel data acquisition module 10, and transmits the frequency information of the electrical signal to the multi-channel data acquisition module 10 for demodulation.

[0092] In a specific embodiment, the lengths of the radio frequency lines are different, and the length difference between any two radio frequency lines is different.

[0093] Specifically, by setting up several radio frequency (RF) lines of different lengths and connecting them to the multi-channel data acquisition module 10, vibration signals to be demodulated that are spatially close, have the same vibration information, but different initial phases are acquired. This facilitates obtaining highly accurate vibration signal demodulation results, enabling frequency detection and phase demodulation of vibration signals. This eliminates the need for an interferometer in the distributed fiber optic vibration sensing system, thereby reducing external noise interference and lowering system operating costs. Furthermore, the different lengths of any two RF lines allow for demodulation of vibration signals over shorter distances in space, improving the system's spatial resolution. This solves the problems of large sampling time intervals in the multi-channel data acquisition module 10 affecting the system's spatial resolution, high operating costs of traditional direct-detection distributed fiber optic vibration sensing systems, and sensitivity to external noise.

[0094] In a specific embodiment, the length difference between any two RF lines is less than the distance the converted signal travels within the sampling time interval.

[0095] Specifically, if the length difference between any two RF lines is less than the distance the converted signal travels within the sampling time interval, the sampled vibration signal to be demodulated can be protected from interference from other vibration sources, and the two sampled vibration signals to be demodulated can be guaranteed to have the same vibration information, thereby obtaining a highly accurate vibration demodulation result. When the sampling time interval of the multi-channel data acquisition module 10 in a traditional direct-detection distributed fiber optic vibration sensing system is large, the system must use a large pulse width to include the vibration information of adjacent sampling points, thus reducing the spatial resolution of the system. In the embodiments of this application, without increasing the cost of the multi-channel data acquisition module 10, by selecting any two RF lines to transmit the converted signal for sampling, two vibration signals to be demodulated that are spatially close and have the same vibration information are obtained, so as to achieve accurate vibration signal demodulation at high spatial resolution when the sampling time interval of the multi-channel data acquisition module 10 is small. Therefore, the distributed fiber optic vibration sensing system based on dual-segment fiber proposed in this application can improve the spatial resolution of the system.

[0096] In a specific embodiment, it also includes a first optical amplification and filtering module 4, which is connected to the optical pulse modulator 3 and the fiber optic circulator 5, and is used to amplify and filter the modulated optical signal before outputting it.

[0097] In a specific embodiment, a second optical amplification and filtering module 7 is also included. The second optical amplification and filtering module 7 is connected to the fiber optic circulator 5 and the photoelectric detection module 8, and is used to amplify and filter the received backscattered Rayleigh light signal before transmitting it to the photoelectric detection module 8.

[0098] The following is about the adoption Figure 2 The specific process of implementing the distributed fiber optic vibration sensing method based on dual-segment fiber optics is further explained, and the vibration signal to be demodulated and the phase demodulation results acquired during the process are as follows: Figure 3 As shown.

[0099] refer to Figure 2 Under the premise of ensuring a sufficiently high signal-to-noise ratio, the control signal generator 1 outputs an electrical signal with adjustable pulse width and repetition frequency, and injects it into the input of the optical pulse modulator 3. The laser output from the narrow linewidth laser 2 is transmitted to the optical pulse modulator 3. The optical pulse modulator 3 uses the electrical signal output from the signal generator 1 to modulate the laser emitted by the narrow linewidth laser 2, generating a modulated optical signal. In the embodiment of this application, the fiber optic circulator 5 is a three-port fiber optic circulator. The three clockwise ports of the fiber optic circulator 5 are the first port 51, the second port 52, and the third port 53. The first optical amplification and filtering module 4 is connected to the first port 51 of the fiber optic circulator 5, and the optical fiber 6 is connected to the second port 52 of the fiber optic circulator 5. After the modulated optical signal is amplified and filtered by the first optical amplification and filtering module 4, it is injected into the two-segment optical fiber 6 at the same vibration position through the fiber optic circulator 5, thereby generating two backscattered Rayleigh light signals. The two backscattered Rayleigh light signals return through the third port 53 of the fiber optic circulator 5.

[0100] Each backscattered Rayleigh beam returns through the third port 53 of the fiber optic circulator 5, then is amplified and filtered by the second optical amplification and filtering module 7, and finally input to the photodetector module 8 for photoelectric conversion, yielding a converted signal. This converted signal is then transmitted through several radio frequency lines of different lengths, resulting in several converted signals with the same vibration information but different initial phases. The distance to the vibration position on fiber 6 is d. i The vibration signal to be demodulated at point I can be represented as I rbs (d i ,t), as shown in the following formula:

[0101]

[0102] Where i = 1, 2, ..., M, i represents the i-th distance value d i The sequence number, M represents the number of sequence numbers; t represents time; D(d i,t) represents the distance d between the backscattered Rayleigh light signal and the vibration position on fiber 6. i The DC component of the vibration signal to be demodulated at the corresponding location; A(d i ,t) represents the distance d between the backscattered Rayleigh light signal and the vibration position on fiber 6. i The AC component of the vibration signal to be demodulated at the corresponding location; The distance d represents the distance between the backscattered Rayleigh light signal and the vibration position on fiber 6. i The initial phase sum of the vibration signal to be demodulated at the corresponding location, and varies with the external environment in the range [0, 2π]; A vibration The amplitude of the vibration phase is proportional to the amplitude of the vibration signal; f vibration This represents the frequency of the vibration signal. In the vibration signal I to be demodulated... rbs (d i In the case of t), since the intensity of the backscattered Rayleigh light signal has a nonlinear relationship with the amplitude of the vibration signal, the vibration signal is demodulated by using a phase demodulation algorithm.

[0103] Two RF lines of different lengths are randomly selected, and their converted signals are input to the multi-channel data acquisition module 10 for synchronous signal acquisition according to the sampling time interval. This yields two vibration signals with the same vibration information but different initial phases, which are to be demodulated. The signal generator 1 is connected to the multi-channel data acquisition module 10 and transmits the trigger signal and clock synchronization signal to the module, ensuring synchronous operation of the signal generator 1 and the multi-channel data acquisition module 10. This allows for correct sampling and processing of the converted signals for subsequent demodulation. The converted signals output from the two RF lines, after sampling, yield the first and second vibration signals to be demodulated, as shown in the following formula:

[0104]

[0105] Because the backscattered Rayleigh light signal interferes with the modulated light signal at half the pulse width, the scattering medium at each location in fiber 6 has a different size and is affected by different environmental changes. Furthermore, the backscattered Rayleigh light signal and the converted signal have different transmission speeds in fiber 6 and the radio frequency line. Therefore, the initial phase of the first demodulated vibration signal is represented as... The initial phase of the second vibration signal to be demodulated is expressed as follows:

[0106] Two vibration signals with identical vibration information but different initial phases are transmitted to the signal processing module 11. A differential cross-multiplication algorithm is used for phase demodulation to obtain the vibration phase. Based on the vibration phase, the vibration frequency and phase information at the vibration location can be obtained. Specifically, a low-pass filter is used on the first and second vibration signals to eliminate DC noise, coherent noise, electrical noise introduced by the photoelectric detection module 8, and quantization noise introduced by the multi-channel data acquisition module 10, resulting in the first filtered vibration signal I1(d1, t) and the second filtered vibration signal I2(d1+Δl, t), respectively expressed as:

[0107]

[0108] The first filtered vibration signal I1(d1,t) and the second filtered vibration signal I2(d1+Δl,t) are processed using the first-order differential cross-multiplication method to obtain the first intermediate signal I. DCM1 (d1, t), as shown in the following formula:

[0109]

[0110] The two filtered vibration signals I1(d1,t) and I2(d1+Δl,t) are processed using the second-order differential cross-multiplication method to obtain the second intermediate signal I. DCM2 (d1, t), as shown in the following formula:

[0111]

[0112] According to the first intermediate signal I DCM1 (d1, t) and the second intermediate signal I DCM2 (d1, t), the initial phase difference coefficient G is calculated using the following formula:

[0113]

[0114] The first intermediate signal I DCM1 Dividing (d1, t) by the initial phase difference coefficient G and integrating the result, we obtain the vibration phase, as shown in the following equation:

[0115]

[0116] Vibration phase θ vibration (t) Writing:

[0117] θ vibration (t)=A vibration cos(2πf vibration t);

[0118] The two backscattered Rayleigh light signals are processed according to the above steps to obtain the first vibration phase and the second vibration phase. If there is signal fading in the first vibration phase, the first vibration phase is compensated according to the second vibration phase to overcome coherent fading and obtain the third vibration phase. Specifically, in the embodiment of this application, the linewidth of the narrow linewidth laser 2 is 3kHz, the optical pulse modulator 3 is an acousto-optic modulator, and two radio frequency lines of different lengths are connected to the output end of the photoelectric detection module 8. The first radio frequency line 91 is 0.9m long, the second radio frequency line 92 is 0.8m long, the total length of the optical fiber 6 is 2.2km, and the vibration position is located at 1.24km on the optical fiber 6. The vibration source is implemented by driving a PVC pipe with a horn. In view of the situation where there is coherent fading in the vibration signal to be demodulated due to the phase offset point drift caused by the change of the external ambient temperature, a 30cm long optical fiber 6 with a certain distance between its two ends is used as a sensing unit to overcome coherent fading. The final demodulation result is as follows. Figure 3 As shown in (g), where Phase represents the amplitude of the vibration phase in radians (rad) and Time represents the duration in seconds, this indicates that the distributed optical fiber vibration sensing method and system based on dual-segment optical fibers proposed in this application can effectively solve the problems of discontinuous demodulation of vibration signals and phase distortion in the demodulated vibration phase. In the embodiments of this application, a single-mode optical fiber of the same length as the first segment is attached parallel to the same vibration location as the second segment of optical fiber. A sinusoidal signal with a voltage of 16Vpp and a frequency of 10Hz is selected as the vibration source. The backscattered Rayleigh light signals corresponding to the first and second segments of optical fibers are obtained from the first RF line 91 and the second RF line 92, respectively, and sampled to obtain the first demodulated vibration signal, the second demodulated vibration signal, the first demodulated vibration signal, and the second demodulated vibration signal. The vibration duration of the above four demodulated vibration signals is 10s. The two demodulated vibration signals corresponding to the first segment of optical fiber are as follows: Figure 3 (a) Figure 3 As shown in (c), where Voltage represents the amplitude of the vibration signal to be demodulated, and its unit is V. The two vibration signals to be demodulated corresponding to the second fiber segment are as follows: Figure 3 (b) Figure 3 As shown in (d), the two vibration signals to be demodulated corresponding to the first fiber segment and the two vibration signals to be demodulated corresponding to the second fiber segment are subjected to phase demodulation using the aforementioned differential cross-multiplication algorithm, resulting in two vibration phases, namely the first vibration phase and the second vibration phase, as shown in (d). Figure 3 (e) Figure 3 As shown in (f). In Figure 3In the diagram, the area within the red box represents the intensity fading point of the vibration signal to be demodulated. At this point, the vibration phase reconstructed using a phase demodulation algorithm will exhibit phase distortion. Therefore, for... Figure 3 (e) Figure 3 (f) For vibration phase points where phase restoration fails, compensation is performed based on the frequency and amplitude of the vibration signals within the same time interval of the two vibration phases. This ensures continuous phase restoration and continuous demodulation of the vibration signals. The third vibration phase obtained after compensation is as follows: Figure 3 As shown in (g).

[0119] This invention utilizes the characteristic that the initial phases of the vibration signals to be demodulated corresponding to the two fiber segments are different, thereby avoiding signal fading of the vibration signals to be demodulated corresponding to the two fiber segments within the same time interval. The phase of one vibration signal is used to compensate for the phase distortion of the other vibration signal, thus overcoming coherent fading and achieving continuous demodulation of the vibration signal. Furthermore, by acquiring the vibration signals to be demodulated transmitted through several radio frequency lines, accurate demodulation of the vibration signal is achieved at high spatial resolution.

[0120] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method of distributed optical fiber vibration sensing based on two- segment optical fiber, characterized in that, The method comprises the following steps: The modulated light signal is input into two sections of optical fiber in the same vibration position through an optical fiber circulator, and two backward Rayleigh scattering light signals are generated; The two backward Rayleigh scattering light signals are received and photoelectrically converted, and two converted signal groups are output, wherein each of the two backward Rayleigh scattering light signals is transmitted through a plurality of radio frequency lines with different lengths after photoelectric conversion, a plurality of converted signals with the same vibration information and different initial phases are obtained, and the plurality of converted signals constitute a converted signal group; Two converted signal groups are sampled according to sampling time intervals, and two groups of demodulation vibration signals are obtained, wherein each of the two groups of demodulation vibration signals comprises a plurality of demodulation vibration signals; The two groups of demodulation vibration signals are phase demodulated by using a differential cross-multiplication algorithm, and a first vibration phase and a second vibration phase are obtained, and the processing process of the differential cross-multiplication algorithm comprises the following steps: Selecting any two paths of the to-be-demodulated vibration signals, respectively denoted as and , wherein, and respectively represent the distance between the two paths of the to-be-demodulated vibration signals and the vibration position on the optical fiber, represents the length difference of the two radio frequency lines, t represents time, and the two paths of the to-be-demodulated vibration signals and are low-pass filtered to obtain two paths of filtered vibration signals, respectively denoted as and ; two said filtered vibration signals and a first intermediate signal is obtained by processing with a first order differential cross multiplication method as shown in the following equation: ； wherein the first intermediate signal comprising an initial phase difference coefficient G and a vibration phase to be integrated ; two said filtered vibration signals and a second intermediate signal is obtained by processing with a second-order differential cross-multiplication method as shown in the following formula: ； According to the first intermediate signal and the second intermediate signal , a initial phase difference coefficient G is calculated. dividing the first intermediate signal by the initial phase difference coefficient G and integrating, to obtain the vibration phase as follows: ； where C represents a constant, denotes the vibration phase, written as: ； wherein denotes the amplitude of the vibration phase, denotes the frequency of the vibration signal; If the first vibration phase has phase distortion, the first vibration phase is compensated according to the second vibration phase, and a third vibration phase is obtained.

2. The dual- section fiber-based distributed optical fiber vibration sensing method of claim 1, wherein, If the first vibration phase has phase distortion, the first vibration phase is compensated according to the second vibration phase, and a third vibration phase is obtained. The first vibration phase and the second vibration phase are divided into N time intervals according to a given time interval; The amplitude mean value of the first vibration phase is calculated to obtain a first mean value, and the variance of the amplitude of the first vibration phase in the jth time interval and the first mean value is calculated, wherein j=1, 2, …, N, and a first variance is obtained; If the first variance is less than a given variance threshold, the first vibration phase in the jth time interval has phase distortion, and the first vibration phase in the jth time interval is compensated according to the amplitude and frequency of the second vibration phase in the jth time interval, and a third vibration phase is obtained.

3. The dual- section fiber-based distributed optical fiber vibration sensing method of claim 1, wherein, Further comprising: The modulated light signal is input into two sections of optical fiber in the same vibration position through an optical fiber circulator, and two backward Rayleigh scattering light signals are generated; 4. The dual- section fiber-based distributed optical fiber vibration sensing method of claim 1, wherein, Further comprising: The received two backward Rayleigh scattering light signals are amplified and filtered, and then photoelectrically converted.

5. A distributed optical fiber vibration sensing system based on two-section optical fiber for implementing the method of distributed optical fiber vibration sensing based on two-section optical fiber according to any one of claims 1-4, characterized in that, The signal generator, the narrow linewidth laser, the optical pulse modulator, the optical fiber circulator, the two sections of optical fiber connected end to end, the photoelectric detection module, the plurality of radio frequency lines, the multi-channel data acquisition module and the signal processing module are included. The narrow linewidth laser is connected with the optical pulse modulator, and is used to emit laser and transmit the laser to the optical pulse modulator. The signal generator is connected with the optical pulse modulator, is used to generate an electrical signal with adjustable pulse width and repetition frequency, and transmit the electrical signal to the optical pulse modulator, and is connected with the multi-channel data acquisition module, and is used to transmit frequency information of the electrical signal to the multi-channel data acquisition module. The optical pulse modulator is used to modulate the laser emitted by the narrow linewidth laser by using the electrical signal output by the signal generator, and generate a modulated light signal. The fiber circulator is connected to the two segments of the fiber, which are used to generate a backscattered Rayleigh light signal. The fiber circulator is used to input the modulated light signal into the two segments of the fiber that are in the same vibration position, and to output the backscattered Rayleigh light signal. The photoelectric detection module is used to receive the backscattered Rayleigh light signal, perform photoelectric conversion on it, and output the converted signal. The two ends of the plurality of radio frequency lines are respectively connected to the output end of the photoelectric detection module and the multi-channel data acquisition module, and the radio frequency lines are used to transmit the converted signal to the multi-channel data acquisition module; The multi-channel data acquisition module is connected to the signal processing module. The multi-channel data acquisition module is used to synchronously acquire the converted signals output from several radio frequency lines according to the sampling time interval, and obtain several channels of vibration signals to be demodulated. The signal processing module is used to perform phase demodulation on several channels of the vibration signals to be demodulated, and to overcome coherent fading in the vibration phase obtained by phase demodulation.

6. A two- section optical fibre based distributed optical fibre vibration sensing system according to claim 5, characterised in that, The lengths of the radio frequency lines are different, and the length difference between any two radio frequency lines is different.

7. A two- section optical fibre based distributed optical fibre vibration sensing system according to claim 6, characterised in that, The length difference between any two of the radio frequency lines is less than the distance the converted signal travels within the sampling time interval.

8. The dual- section fiber-based distributed optical fiber vibration sensing system of claim 5, wherein, It also includes a first optical amplification and filtering module, which is connected to the optical pulse modulator and the optical fiber circulator, and is used to amplify and filter the modulated optical signal before outputting it.

9. The dual- section fiber-based distributed optical fiber vibration sensing system of claim 5, wherein, It also includes a second optical amplification and filtering module, which is connected to the fiber optic circulator and the photodetector module, and is used to amplify and filter the received backscattered Rayleigh light signal before transmitting it to the photodetector module.

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