A distributed ultrasonic sensing method and system based on frequency-phase quadrature encoding

Abstract

The application discloses a kind of distributed ultrasonic sensing method and system based on frequency phase quadrature encoding, and the orthogonal frequency and phase encoding sequence is generated according to target system performance index;Backscattering signal of Rayleigh corresponding to each orthogonal encoding channel is collected, and the frequency and phase encoding sequence and the backscattering signal collected are used to carry out cross-correlation calculation, and the Rayleigh scattering signal of each encoding channel is separated out;IQ demodulation is carried out to the Rayleigh scattering signal of each encoding channel separated out, and in-phase and quadrature components are separated out, and the phase information of each orthogonal encoding channel is obtained;Phase response compensation processing is used to reconstruct the ultrasonic signal in each orthogonal encoding channel.The frequency response range, spatial resolution and signal-to-noise ratio of the vibration data processed by the method and system are greatly improved, and it is suitable for various complex monitoring environments, and the accuracy and positioning accuracy of the detection result of weak ultrasonic signal in the use of distributed optical fiber acoustic wave sensing system are improved.

Description

Technical Field

[0001] This application relates to the field of fiber optic sensing technology, and in particular to a distributed ultrasonic sensing method and system based on frequency-phase orthogonal coding. Background Technology

[0002] Structural health monitoring is a crucial means of ensuring the safety of major infrastructure such as aerospace, bridges, and pipelines. Ultrasonic testing is highly sensitive to minute defects, but traditional point sensors struggle to achieve continuous monitoring over large areas. Distributed fiber optic acoustic sensing (DAS) can transform an entire optical fiber into a continuous sensing unit, offering advantages such as resistance to electromagnetic interference and corrosion, providing an ideal platform for large-scale structural monitoring. To further enhance the response capability of DAS to high-frequency ultrasonic signals, it is urgent to increase the system's frequency response upper limit, thereby constructing a high-performance distributed ultrasonic sensing system to achieve highly sensitive identification and precise location of internal damage in large structures.

[0003] In distributed acoustic sensing systems, channel multiplexing technology is widely used to improve the upper limit of frequency response. This technology significantly increases the density of sound field information acquired per unit time by simultaneously transmitting signals of multiple wavelengths, frequencies, or pulse time slots in the same sensing fiber, thereby effectively improving the frequency response bandwidth of the system.

[0004] However, channel multiplexing technology has inherent drawbacks in practical applications: on the one hand, the multiple channels of frequency division multiplexing and time slot multiplexing are separated in the time or frequency domain, and demodulation requires strict differentiation of backscattered signals from different channels but close locations, which limits the optimal spatial resolution of the system; on the other hand, although code division multiplexing does not require additional frequency band resources, the multiple channels are only quasi-orthogonal, which easily causes crosstalk and nonlinear effects between channels. Conventional methods to avoid crosstalk will lead to a decrease in spatial resolution and signal-to-noise ratio. Therefore, the channel multiplexing technology in the present technology is difficult to meet the requirements of high-precision positioning.

[0005] Therefore, existing technologies need to be improved. Summary of the Invention

[0006] In view of the shortcomings of the prior art, the purpose of this invention is to provide users with a distributed ultrasonic sensing method and system based on frequency-phase orthogonal coding, overcoming the defects of low data spatial resolution and signal-to-noise ratio after processing by the distributed ultrasonic sensing method in the prior art.

[0007] The technical solution adopted by this invention to solve the technical problem is as follows: In a first aspect, the present invention provides a distributed ultrasonic sensing method based on frequency-phase orthogonal coding, wherein: Based on the target system performance indicators, the parameters of the orthogonal frequency and phase coding sequence are defined to obtain the initial coding sequence; wherein, the target system performance indicators include the total bandwidth threshold, target spatial resolution, and dynamic response range; the parameters of the orthogonal frequency and phase coding sequence include: code length, number of codes, bandwidth of each code element, frequency interval, phase range, and chirp rate range; The initial encoded sequence is optimized using the objective function of a sequential quadratic programming algorithm to obtain an optimized orthogonal frequency and phase encoded sequence; The backscattered Rayleigh signals corresponding to each orthogonal coding channel are collected, and the frequency and phase coding sequence is used to perform cross-correlation calculation with the collected backscattered Rayleigh signals to separate the Rayleigh scattering signal of each orthogonal coding channel. The Rayleigh scattering signals of each orthogonal coded channel were demodulated using IQ to separate the in-phase and quadrature components and obtain the phase information of each orthogonal coded channel. Phase response compensation processing was then used to reconstruct the ultrasonic signals in each orthogonal coded channel.

[0008] Optionally, the step of generating orthogonal frequency and phase coding sequences based on the target system performance indicators includes: Based on the target system performance indicators, the parameters of the orthogonal frequency and phase coding sequences are defined to obtain the initial coding sequence; wherein, the target system performance includes the total bandwidth threshold, target spatial resolution, and dynamic response range; the parameters of the coding sequence include: code length, number of codes, bandwidth of each code element, frequency interval, phase range, and chirp rate range; The initial coding sequence is optimized using the objective function of a sequential quadratic programming algorithm to obtain an optimized orthogonal frequency and phase coding sequence.

[0009] Optionally, the step of defining the parameters of the orthogonal frequency and phase coding sequences according to the target system performance indicators to obtain the initial coding sequence includes: Using a sequential quadratic programming algorithm, several sets of frequency-phase orthogonal coding sequences are created, and constraints are imposed on the parameters of each code point; The initial phase, frequency, and chirp rate of each code point are set to obtain the initial coded sequence.

[0010] Optionally, the step of optimizing the initial encoded sequence using the objective function of a sequence quadratic programming algorithm to obtain an optimized orthogonal frequency and phase encoded sequence includes: Construct a sequence quadratic programming problem and calculate the correlation function and objective function of the encoding; the objective function includes one or more of the following: calculating the autocorrelation sidelobe peak, the cross-correlation sidelobe peak, and the synthetic correlation sidelobe peak. The correlation function for the encoding is: ; in, Let be the i-th frequency and phase encoded sequence, and τ be the cross-correlation delay. Represents complex conjugation; when i=k, it is autocorrelation, and when i≠k, it is cross-correlation. By using linear approximation of the objective function, the search direction and step size are solved and optimized to obtain a frequency and phase encoded sequence that meets the preset convergence conditions.

[0011] Optionally, the optical fiber under test used in the distributed ultrasonic sensing system based on frequency-phase orthogonal coding applied by the method is a scattering-enhanced optical fiber, and the objective function is to calculate the sidelobe peak value and the synthetic sidelobe intensity at the location of the scattering enhancement point.

[0012] Optionally, the step of acquiring the backscattered Rayleigh signals corresponding to each orthogonal coding channel includes: The generated frequency and phase encoded sequence is injected into an arbitrary waveform generator, and the arbitrary waveform generator is used to generate an encoded pulse modulated electrical signal. Furthermore, the laser continuously emitted by the laser is modulated with coded pulses using an IQ electro-optic modulator. After the modulated coded optical pulses are transmitted to the sensing fiber, a backscattered Rayleigh signal is generated in the sensing fiber.

[0013] Optionally, the step of separating the Rayleigh scattering signal of each coded channel by performing cross-correlation calculation between the coded sequence and the acquired backscattered Rayleigh signal includes: Obtain the complex signal obtained by cross-correlation calculation of the frequency and phase encoded sequence and the backscattered Rayleigh signal; Based on a preset time window truncation method, the backscattered Rayleigh signal of each coded channel is separated from the complex signal.

[0014] Optionally, the step of performing IQ demodulation on the Rayleigh scattering signals of each separated orthogonal coded channel to separate the in-phase and quadrature components and obtain the phase information of each orthogonal coded channel includes: The backscattered Rayleigh signal interferes with the local oscillator light, and is then split into X and Y polarized 1 / Q components by a 90° optical mixer. A balanced photodetector converts the light into an electrical signal, and data is acquired using an oscilloscope or data acquisition card.

[0015] Optionally, the step of reconstructing the ultrasonic signals in each orthogonal coded channel by employing cross-correlation decoding and phase response compensation processing includes: Modal decomposition is used to separate the high-frequency modes of vibration from the low-frequency noise. Low-frequency vibration signals were used to calibrate the strain-phase response of each orthogonal coding channel. The calibrated phases are spliced ​​together to reconstruct the ultrasonic signals in each orthogonal coded channel.

[0016] Secondly, the present invention provides a distributed ultrasonic sensing system based on frequency-phase orthogonal coding, comprising: a laser, an electro-optic modulator, an optical fiber amplifier, a waveform generator, a photodetector, a data acquisition and processing module, and a sensing optical fiber connected together. The data acquisition and processing module is used to generate orthogonal frequency and phase coding sequences based on the target system performance indicators; acquire the backscattered Rayleigh signals corresponding to each orthogonal coding channel, and perform cross-correlation calculations using the orthogonal frequency and phase coding sequences and the acquired backscattered Rayleigh signals to separate the Rayleigh scattering signals of each coding channel; perform IQ demodulation on the separated Rayleigh scattering signals of each orthogonal coding channel to separate the in-phase and quadrature components and obtain the phase information of each orthogonal coding channel; and use phase response compensation processing to reconstruct the ultrasonic signals in each orthogonal coding channel.

[0017] This invention discloses a distributed ultrasonic sensing method and system based on frequency-phase orthogonal coding. By introducing orthogonal coding modulation and chirped pulse compression techniques into the backscattered Rayleigh signal acquired by the distributed fiber optic ultrasonic sensing system, high-resolution demodulation and localization of ultrasonic vibration signals are achieved. The spatial resolution and signal-to-noise ratio of the vibration data processed by this method are significantly improved, making it suitable for various complex monitoring environments. This method is of great significance for improving the accuracy and localization precision of weak ultrasonic signal detection results in the use of distributed fiber optic sensing acoustic wave systems. Attached Figure Description

[0018] Figure 1 This is a flowchart illustrating the steps of the distributed ultrasonic sensing method based on frequency-phase orthogonal coding in an embodiment of the present invention. Figure 2 This is a schematic diagram illustrating the implementation principle of the distributed ultrasonic sensing method based on frequency-phase orthogonal coding in an embodiment of the present invention. Figure 3 This is a flowchart illustrating the steps for generating frequency and phase encoded sequences in the method of this embodiment of the invention. Figure 4 This is a schematic diagram of the relevant side lobes in an embodiment of the present invention; Figure 5 This is a schematic diagram illustrating the principle of the Rayleigh scattering phase demodulation algorithm in the method of this embodiment of the invention; Figure 6 This is a modulation and demodulation flowchart for a specific application embodiment of the method of the present invention. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0020] Rayleigh scattering distributed fiber optic sensing systems primarily utilize the backscattering effect of laser light propagating in optical fibers. This system constructs a distributed strain and acoustic wave monitoring network based on elastic scattering. Its core principle is that when a laser pulse propagates in the fiber, it undergoes elastic scattering with the microscopic inhomogeneities of the fiber medium's refractive index, generating characteristic backscattering. Specifically, when incident photons interact with static defects such as density fluctuations and doping distributions in the fiber, backscattered light with the same frequency as the incident light—that is, Rayleigh scattered light—is produced. Phase-sensitive Rayleigh scattering signals exhibit linear response characteristics to strain and acoustic wave disturbances. The geometric deformation and refractive index changes in the fiber caused by strain and acoustic waves directly modulate the phase and intensity of the scattered light. Simultaneously, the system can effectively extract the location, frequency, and amplitude information of the disturbance through phase demodulation or interferometric detection techniques, achieving highly sensitive distributed measurement. These characteristics have led to the widespread application of Rayleigh scattering in distributed fiber optic strain, acoustic wave, and vibration sensing systems.

[0021] The DAS (Distributed optical fiber Acoustic Sensing) system has the following advantages in terms of applicability: (1) The optical fiber has both sensing and transmission functions, adopts an all-optical passive design, and the sensing optical fiber is inherently safe. It has the characteristics of anti-electromagnetic interference, corrosion resistance, and high voltage resistance, and is especially suitable for continuous acoustic / vibration monitoring in environments with strong electromagnetic interference, flammable and explosive or harsh environments such as oil and gas pipelines, high-voltage transmission lines, tunnels, and border lines. (2) A single sensing optical fiber can be equivalent to an array of hundreds of thousands or even millions of vibration sensors, realizing distributed sound field reconstruction at meter-level or even sub-meter-level spatial resolution. The system is based on φ-OTDR (Phase-sensitive Optical Time-Domain Reflectometer) or similar coherent detection technology, which can simultaneously acquire multi-dimensional information such as intensity, frequency, phase and time difference of arrival of vibration events, and support accurate classification and location of events. (3) Optical fibers are lightweight, small in diameter, and highly adaptable to bending, making them easy to lay along complex structures such as pipelines, fences, and rail transit lines. They can also be embedded in structures or integrated into cables. With high dynamic range optical transceivers, a single system can monitor distances of tens to hundreds of kilometers, achieving passive, long-distance, continuous distributed acoustic wave monitoring. As a new generation of intelligent sensing technology, DAS has been widely used in fields such as oil and gas pipeline safety early warning, perimeter security, foreign object intrusion monitoring in rail transit, seismic wave detection, and submarine cable health monitoring, providing an efficient solution for all-weather dynamic sensing of industrial IoT and infrastructure.

[0022] Specifically, the core technology of distributed ultrasonic sensing systems in structural health monitoring lies in integrating ultrasonic guided wave technology with distributed sensor networks. Leveraging the long propagation distance and high sensitivity to defects of guided waves in waveguide structures such as plates and pipes, a sensor array deployed on the structural surface enables continuous dynamic scanning of large areas. In typical scenarios such as steel plate crack monitoring, the system excites and receives ultrasonic guided wave signals propagating along the structure. When fatigue cracks or corrosion defects appear in the steel plate, the propagation path, amplitude, and phase of the guided waves change significantly. The distributed sensor network can simultaneously acquire multi-point response data, and combined with algorithms, reconstruct high-resolution damage probability images from the differences in multi-source signals, thereby achieving precise location of cracks and quantitative assessment of their propagation trends.

[0023] In distributed acoustic wave sensing systems, channel multiplexing technology significantly increases the density of acoustic field information acquired per unit time by simultaneously transmitting signals of multiple wavelengths, multiple frequencies, or multiple pulse time slots in the same sensing fiber. This effectively improves the frequency response bandwidth of the system, making it possible to achieve distributed, large dynamic range acoustic wave monitoring without increasing the number of fiber cores.

[0024] To overcome the limitations of existing DAS (Distributed Amplifier) ​​technologies, which rely on channel multiplexing for limited improvement in spatial resolution and where increasing the number of multiplexed channels further degrades the minimum resolvable distance between adjacent sensing units, thus failing to meet the increasingly demanding high-precision positioning requirements for ultrasonic structural health detection in industrial monitoring, this invention provides a distributed ultrasonic sensing method and system based on frequency-phase orthogonal coding. This method generates a coding sequence that satisfies frequency and phase orthogonality, jointly modulates the frequency and phase of the probe light, and employs phase response compensation processing at the receiver to achieve high spatial resolution multi-channel parallel multiplexing and high-fidelity ultrasonic signal reconstruction. This method improves the spatial resolution and signal-to-noise ratio of vibration data, is adaptable to various complex monitoring environments, and significantly enhances the accuracy and positioning precision of weak ultrasonic signal detection results in DAS systems.

[0025] The following description, in conjunction with the accompanying drawings, provides a more detailed account of a distributed ultrasonic sensing method and system based on frequency-phase orthogonal coding disclosed in this invention.

[0026] In a first aspect, the present invention provides a distributed ultrasonic sensing method based on frequency-phase orthogonal coding, such as... Figure 1 As shown, it includes: Step S1: Define the parameters of the orthogonal frequency and phase coding sequence according to the target system performance indicators to obtain the initial coding sequence; wherein, the target system performance includes the total bandwidth threshold, target spatial resolution and dynamic response range; the parameters of the orthogonal frequency and phase coding sequence include: code length of the coding sequence, number of codes, bandwidth of each code element, frequency interval, phase range and chirp rate range.

[0027] This step includes the generation and optimization of frequency and phase coding sequences to maintain sufficiently high orthogonality between channels. Specifically, the steps for generating frequency and phase coding sequences based on the target system performance indicators include: Step S11: Define the parameters of the frequency and phase coding sequence according to the target system performance indicators to obtain the initial coding sequence; wherein, the target system performance includes the total bandwidth threshold, target spatial resolution and dynamic response range; the parameters of the coding sequence include: code length, number of codes, bandwidth of each code element, frequency interval, phase range and chirp rate range.

[0028] In detail, the step of defining the parameters of the encoding sequence based on the target system performance indicators to obtain the initial encoding sequence includes: Step S111: Using a nonlinear sequence programming algorithm, create several sets of frequency-phase orthogonal coding sequences and impose constraints on the parameters of each code point.

[0029] The sequence quadratic programming algorithm is used when the code points in the sequence are non-linear during sequence planning.

[0030] Step S112: Set the initial phase, frequency and chirp rate for each code bit to obtain the initial coding sequence.

[0031] The generation of frequency and phase coding sequences includes defining the parameters of the frequency and phase coding sequences and presetting the defined parameters to obtain the initial coding sequence. The code length, number of codes, bandwidth per symbol, frequency interval, phase range, and chirp rate range of the coding sequence need to be preset according to system specifications. Total bandwidth threshold B th The theoretical spatial resolution is determined to be W≈c / (2n·Bth), where c is the speed of light and n is the effective refractive index of the fiber. The choice of free variables such as code length and number of codes affects the computational cost of the coding optimization algorithm. Too many variables will lead to excessively long optimization time and make it easier to get trapped in local optima, while too few variables will make it difficult to effectively suppress crosstalk. Generally, the values ​​are defined based on the expected spatial resolution and dynamic response range of the system.

[0032] Step S2: Optimize the initial coding sequence using the objective function of the sequence quadratic programming algorithm to obtain the optimized frequency and phase coding sequence.

[0033] The optimization of frequency and phase coding sequences involves constructing a quadratic programming problem and using a nonlinear sequence quadratic programming algorithm to optimize the peak values ​​of autocorrelation sidelobes, cross-correlation sidelobes, synthetic correlation sidelobes, and Doppler tolerance of each code.

[0034] In detail, the step of optimizing the initial encoded sequence using the objective function of the sequence quadratic programming algorithm to obtain the optimized frequency and phase encoded sequence includes: Step S21: Construct a sequence quadratic programming problem and calculate the correlation function and objective function of the encoding; the objective function includes one or more of the following: calculating the autocorrelation sidelobe peak, the cross-correlation sidelobe peak, and the synthetic correlation sidelobe peak.

[0035] The correlation function for the encoding is: ; in, Let be the i-th frequency and phase encoded sequence, and τ be the cross-correlation delay. It represents complex conjugate; when i=k, it is autocorrelation, and when i≠k, it is cross-correlation.

[0036] like Figure 3The diagram illustrates autocorrelation sidelobes, cross-correlation sidelobes, and composite correlation sidelobes. An autocorrelation sidelobe is defined as the correlation value at a non-zero shift (peak value at the main lobe position) obtained by performing a correlation operation between a signal and its shifted copy. A cross-correlation sidelobe is the correlation value at all shifts when performing a correlation operation between two different codes (or different waveforms from the same code set). The main lobe is usually not cross-correlated with the main lobe, except that the meaning of zero shift is different. A composite correlation sidelobe is defined as follows: for M mutually orthogonal codes, the composite correlation sidelobe of the i-th code is the sum of the autocorrelation sidelobe of the i-th code and the cross-correlation sidelobes of the i-th code and the remaining M-1 codes.

[0037] This step focuses on the objective function of the sequential quadratic programming algorithm. The objective function determines the final optimization effect of the algorithm, and its selection is related to the fiber under test. Typically, the synthetic sidelobe volume of orthogonal coding is larger than the autocorrelation main lobe volume, meaning the main lobe signal is completely submerged in crosstalk. This necessitates optimization of both the main lobe energy and the synthetic sidelobe energy. In one implementation, the fiber under test used in the distributed ultrasonic sensing system based on frequency-phase orthogonal coding is a scattering-enhanced fiber. The objective function calculates the sidelobe peak value and synthetic sidelobe intensity at the scattering enhancement point location. Since using a scattering-enhanced fiber reduces the number of effective scattering points, the synthetic sidelobes only need to satisfy partial orthogonality. Therefore, when setting the objective function, sidelobes at some locations can be ignored, and only the sidelobe peak value and synthetic sidelobe intensity at the scattering enhancement point are calculated. This reduces computational burden and improves the local orthogonality of the coding.

[0038] Step S22: Using the objective function linear approximation, solve for the search direction and step size, and optimize to obtain the frequency and phase encoding sequence that meets the preset convergence conditions.

[0039] like Figure 4 As shown, Taylor expansion is used to linearly approximate the objective function, and the search direction and step size are determined. Finally, it is determined whether the gradient and step size satisfy the convergence condition.

[0040] The frequency-phase orthogonal coding technique proposed in this step utilizes frequency and phase coding sequences to jointly modulate the frequency and phase of the probe light, thereby achieving multi-channel multiplexing and high-precision demodulation. This method, by designing a coding set that simultaneously satisfies frequency and phase orthogonality, enables the parallel transmission of multiple independent probe signals in the same sensing fiber, effectively improving the system's acoustic frequency response bandwidth and spatial resolution, and significantly suppressing crosstalk and phase noise between channels.

[0041] In detail, combined Figure 4As shown, in the first step, M sets of frequency and phase coding sequences (code length N) are applied to a nonlinear sequence planning algorithm, and constraints are imposed on the bandwidth B0, frequency interval Δf, phase range, and chirp rate range k of each code point, and the total bandwidth threshold B is also constrained. th The second step is to set the initial phase, frequency, and chirp rate for each symbol. The initial expression for the frequency and phase coding sequence of the m-th frequency-phase sequence is then: ; Among them, T n f n κ n and φ n These represent the length, starting frequency, starting chirp rate, and starting phase of the nth symbol, respectively. In the third step, a quadratic programming problem is constructed to calculate the correlation function and objective function of the codes. The correlation function between codes is: ; Where τ is the cross-correlation delay, This represents complex conjugate. When i=k, it is autocorrelation; when i≠k, it is cross-correlation.

[0042] In autocorrelation, when τ=0, the correlation function X i,k (0) represents the peak value of the autocorrelation main lobe. The full width at half maximum (FW) of the main lobe is limited by the total bandwidth threshold: W≈c / 2nB th Where c is the speed of light and n is the effective refractive index of the optical fiber. Figure 3 Taking an example to illustrate sidelobes and the objective function, all cross-correlation functions and autocorrelation functions after removing the main lobe range are considered sidelobes. The objective function consists of three parts: the peak value of the autocorrelation sidelobes, the peak value of the cross-correlation sidelobes, and the composite correlation sidelobes. The peak values ​​of the autocorrelation and cross-correlation sidelobes are normalized by comparing them to the peak value of the autocorrelation main lobe, while the composite correlation sidelobes are normalized by comparing them to the volume of the autocorrelation main lobe. In the fourth step, Taylor expansion is used to linearly approximate the objective function, and the search direction and step size are determined. Finally, it is determined whether the gradient and step size satisfy the convergence condition.

[0043] In this step, frequency and phase coding sequences are generated based on system performance indicators. These sequences, combined with orthogonal phase coding multiplexing, enable channel multiplexing at the same frequency, further enhancing coding flexibility. Furthermore, flexible adjustment of multiple parameters, such as coding frequency and chirp rate, results in optimized frequency and phase coding sequences with lower peak sidelobes and smaller synthetic sidelobe volumes, effectively suppressing sensing. Moreover, unlike traditional coding schemes, frequency coding ensures that the final spatial resolution depends on the total coding bandwidth, rather than being limited by the symbol width.

[0044] Step S3: Collect the backscattered Rayleigh signals corresponding to each orthogonal coding channel, and perform cross-correlation calculation between the frequency and phase coding sequence and the collected backscattered Rayleigh signals to separate the Rayleigh scattering signal of each coding channel.

[0045] In this step, the optimized encoding information is loaded into the probe light pulse sequence using an arbitrary waveform generator, and then transmitted to the fiber under test via an fiber amplifier and circulator. Subsequently, multiplexed Rayleigh backscattered signals are acquired.

[0046] In detail, the step of acquiring the backscattered Rayleigh signals corresponding to each orthogonal coding channel includes: Step S31: Inject the generated frequency and phase encoded sequence into an arbitrary waveform generator, and use the arbitrary waveform generator to generate an encoded pulse modulated electrical signal.

[0047] Step S32: The laser continuously emitted by the laser is modulated with coded pulses using an IQ electro-optic modulator. After the modulated coded light pulses are transmitted to the sensing fiber, a backscattered Rayleigh signal is generated in the sensing fiber.

[0048] In detail, the step of separating the Rayleigh scattering signal of each coded channel by performing cross-correlation calculations using the frequency and phase coded sequence and the acquired backscattered Rayleigh signal includes: Step S33: Obtain the complex signal after cross-correlation calculation between the frequency and phase encoded sequence and the backscattered Rayleigh signal.

[0049] The cross-correlation calculation in this step is a mathematical tool for measuring the similarity and relative time delay between two signals. It involves sliding one signal relative to the other and calculating the degree of matching between the two signals at each translation position, usually through an inner product, to find the optimal alignment and similarity between them.

[0050] In this step, the complex signal after the inner product is obtained by calculating the cross-correlation between the signal corresponding to each code point in the frequency and phase coding sequence and the backscattered Rayleigh signal.

[0051] Step S34: Based on the preset time window interception method, separate the backscattered Rayleigh signal of each coded channel from the complex signal.

[0052] In this step, the complex signal in the target fiber segment is intercepted through a pre-set time window, and then the backscattered Rayleigh signal of each coded channel is separated from the intercepted complex signal.

[0053] Step S4: IQ demodulation is performed on the Rayleigh scattering signals of each separated coding channel to separate the in-phase and quadrature components, obtain the phase information of each quadrature coding channel, and phase response compensation processing is used to reconstruct the ultrasonic signals in each quadrature coding channel.

[0054] After the multiplexed Rayleigh backscattered signals are extracted separately using cross-correlation decoding, the all-fiber phase corresponding to each channel is obtained using the IQ demodulation algorithm.

[0055] In detail, the step of performing IQ demodulation on the Rayleigh scattering signals of each separated coding channel to separate the in-phase and quadrature components and obtain the phase information of each quadrature coding channel includes: The backscattered Rayleigh signal interferes with the local oscillator light and beats at the same frequency. It is then split into X and Y polarized 1 / Q components by a 90° optical mixer. The light is converted into an electrical signal using a balanced photodetector, and the data is acquired using an oscilloscope or data acquisition card.

[0056] In one implementation, the arctangent algorithm is used to demodulate the phase information of each orthogonal coded channel. The arctangent algorithm is a method that uses the arctangent function to calculate an angle or phase, which can demodulate phase information from the in-phase and quadrature components of a complex signal.

[0057] In this step, mode decomposition is used to separate and compensate for phase bias, suppressing system common-mode noise and low-frequency drift. Simultaneously, low-frequency vibration signals are used for calibration to compensate for strain-phase response differences between different coded channels, ensuring the continuity and amplitude-phase consistency of the broadband vibration signal. Subsequently, the calibrated phases of each channel are stitched together according to the actual pulse transmission sequence. Using OTDR principles, the time delay information corresponding to each location is obtained, completing the distributed vibration signal localization output. The phase data of each channel, arranged in channel order and distance order, jointly construct a two-dimensional phase field image.

[0058] In detail, the step of employing phase response compensation processing to reconstruct the ultrasonic signals in each orthogonal coded channel includes: Step S41: Use the modal decomposition method to perform modal separation and distinguish between high-frequency modes and low-frequency noise of vibration.

[0059] To address the inherent phase offset differences between different channels, a mode decomposition method is used for mode separation, distinguishing between high-frequency vibration modes and low-frequency noise, and removing low-frequency drift of the system.

[0060] Step S42: Use low-frequency vibration signals to calibrate the strain-phase response of each orthogonal coding channel.

[0061] While eliminating low-frequency drift in the system, low-frequency vibration signals are used to calibrate the strain-phase response of each channel to ensure the continuity and amplitude-phase consistency of the broadband ultrasonic signal.

[0062] Step S43: The calibrated phases are spliced ​​together to reconstruct the ultrasonic signals in each orthogonal coded channel.

[0063] The calibrated phases are stitched together according to the actual detection pulse emission sequence to restore the ultrasonic waveform with high spatial resolution.

[0064] like Figure 5 As shown, unlike existing technologies that ignore the inherent phase offset differences between different channels or only use simple subtraction for compensation, this invention employs mode decomposition (MDD) to separate and compensate for the phase offset, and uses a reference signal to calibrate the response differences of each channel. Specifically, for the inherent phase offset differences between different coding channels, mode decomposition is used to decompose the phase signal into different frequency components, identify and separate common-mode noise and low-frequency drift components, and achieve accurate offset compensation. Simultaneously, low-frequency vibration signals are used to calibrate the strain-phase response differences of each coding channel, and the inconsistencies in the responses between different channels are compensated and spliced ​​based on the calibration results. This method effectively suppresses system common-mode noise and low-frequency drift, ensures the continuity, amplitude, and phase consistency of the broadband ultrasonic signal across the entire frequency band, and eliminates the need for additional probe pulses, reducing system complexity.

[0065] Combination Figure 6 The diagram shown illustrates the steps of the method in a specific application embodiment of the present invention. The method first uses a sequential quadratic programming algorithm to generate frequency and phase encoded sequences, which are then injected into an arbitrary waveform generator to drive an electro-optic modulator for pulse modulation. Backscattered Rayleigh signals from each orthogonal encoded channel are acquired after the injection of the frequency and phase encoded sequences. After acquiring the backscattered Rayleigh signals corresponding to each orthogonal encoded channel, cross-correlation calculations are performed between the orthogonal encoding and Rayleigh scattering signals to separate the individual Rayleigh scattering signal for each encoded channel. Cross-correlation decoding is then used to eliminate crosstalk between different encoded channels. Finally, a computer is used to perform IQ demodulation on the decoded Rayleigh scattering signals to separate the in-phase and quadrature components, thereby obtaining the phase information of each channel.

[0066] Furthermore, after separating the Rayleigh backscattered complex signals corresponding to each coded channel through cross-correlation decoding, the CLEAN algorithm combined with the least squares method is used to estimate the Rayleigh backscattered complex signals to reduce sidelobe interference.

[0067] Specifically, the steps for estimating the Rayleigh backscattered complex signal using the CLEAN algorithm combined with the least squares method include: The Rayleigh scattering complex signal obtained from cross-correlation decoding is considered as the result of the convolutional superposition of the ideal scattering point response and the encoded correlated sidelobe response. Before demodulation, the scattering enhancement points are manually located using the high-precision Φ-OTDR method to obtain the prior information of the scattering point positions. Combined with the scattering point positions, the complex amplitude of the scattering points is estimated using the coherent CLEAN algorithm and the least squares method to minimize the sum of squares of the residual signal. At the same time, the correlation function of the encoded sequence is used as the point spread function to eliminate the sidelobes corresponding to the identified scattering points until the energy of the residual signal is lower than a preset threshold or a preset number of iterations is reached.

[0068] This process can effectively suppress the interference of spurious scattering points introduced by autocorrelation sidelobes and crosscorrelation sidelobes, weaken the phase distortion caused by crosstalk, and thus obtain a purer complex amplitude distribution of scattering points. This provides high-fidelity in-phase and quadrature component inputs for subsequent IQ demodulation, and ultimately improves the spatial resolution and phase accuracy of ultrasound signal reconstruction.

[0069] It is conceivable that the above method can also be directly used to estimate the phase and complex amplitude of the scattering point.

[0070] Compared to the traditional coherent CLEAN algorithm for estimating the target point's position and amplitude, in this demodulation process, the target point is the scattering point, and its position is used as known information. The algorithm's goal is to accurately estimate the complex amplitude and phase. Furthermore, the point spread function in the above steps can be either an autocorrelation function or a cross-correlation function.

[0071] In one implementation, taking the channel corresponding to the first code as an example, the Rayleigh backscattered complex signal acquired by the acquisition module is cross-correlated with the first code and decoded. The first peak of the decoded Rayleigh backscattered complex signal is then obtained as the first scattering point. The ideal scattering point is modeled as a delta function. Using the known location information of the scattering point, the convolution between the scattering point and the point spread function is calculated, denoted as the estimated "Rayleigh backscattering - first code". The "Rayleigh backscattering - first code" complex signal is subtracted from the decoded Rayleigh backscattered complex signal to calculate the residual. The estimated phase and complex amplitude of the "Rayleigh backscattering - first code" are iteratively optimized using the least squares method to minimize the residual, thereby accurately estimating the complex amplitude and phase of the scattering point.

[0072] Furthermore, to compensate for the inherent phase offset differences between channels, mode decomposition is employed to suppress common-mode noise. Simultaneously, due to inconsistencies in phase response across different frequencies or coding channels—such as the cumulative effects of modulator nonlinearity and fiber dispersion—phase response compensation and splicing are necessary to ensure the continuity and amplitude-phase consistency of the broadband vibration signal. Finally, based on the principles of OTDR (Optical Time-Domain Reflectometer), precise positioning along the fiber is achieved through pulse echo delay, ultimately demodulating the intensity, frequency, and phase information of the distributed vibration / acoustic signal to meet the high sensitivity, large dynamic range, and multi-channel multiplexing demodulation requirements of the DAS system.

[0073] Secondly, the present invention provides a distributed ultrasonic sensing system based on frequency-phase orthogonal coding, such as... Figure 2 As shown, it includes: a connected laser, an electro-optic modulator, an optical fiber amplifier, a waveform generator, a photodetector, a data acquisition and processing module, and a sensing optical fiber.

[0074] The data acquisition and processing module is used to generate orthogonal frequency and phase coding sequences based on the target system performance indicators; acquire the backscattered Rayleigh signals corresponding to each orthogonal coding channel, and perform cross-correlation calculations using the orthogonal frequency and phase coding sequences and the acquired backscattered Rayleigh signals to separate the Rayleigh scattering signals of each coding channel; perform IQ demodulation on the separated Rayleigh scattering signals of each coding channel to separate the in-phase and quadrature components and obtain the phase information of each orthogonal coding channel; and use phase response compensation processing to reconstruct the ultrasonic signals in each orthogonal coding channel.

[0075] The distributed ultrasonic sensing system based on frequency-phase orthogonal coding provided in this embodiment mainly consists of a narrow-linewidth laser, an electro-optic modulator, an optical fiber amplifier, a waveform generator, a balanced photodetector, a data acquisition and processing module, a sensing optical fiber, and an auxiliary clock synchronization and triggering module. In specific implementation, the synchronization trigger signal output from the data acquisition and processing module first controls the electro-optic modulator. Then, the continuous light output from the narrow-linewidth laser is modulated into pulsed light with adjustable repetition frequency and pulse width. After being amplified by the optical fiber amplifier, it enters the sensing optical fiber through a circulator. Subsequently, the pulsed light excites backscattered Rayleigh light in the fiber, and this scattered light carries phase change information caused by external vibrations. The backscattered light is then interfered and mixed with another local oscillator light in an optical mixer after passing through the circulator, and then enters the balanced photodetector to be converted into an analog electrical signal. The data acquisition and processing module then performs high-speed analog-to-digital conversion, followed by phase demodulation and positioning algorithm processing, ultimately extracting the vibration signal distribution along each position of the optical fiber.

[0076] This invention discloses a distributed ultrasonic sensing method and system based on frequency-phase orthogonal coding. By introducing orthogonal coding modulation and chirped pulse compression techniques into the backscattered signal acquired by the distributed fiber optic ultrasonic sensing system, high-resolution demodulation and localization of ultrasonic vibration signals are achieved. The spatial resolution and signal-to-noise ratio of the vibration data processed by this method are significantly improved, making it suitable for various complex monitoring environments. This method is of great significance for improving the accuracy and localization precision of weak ultrasonic signal detection results in the use of DAS systems.

[0077] The embodiments described above are merely examples of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application.

Claims

1. A distributed ultrasonic sensing method based on frequency-phase orthogonal coding, characterized in that, include: Based on the target system performance indicators, parameters of the orthogonal frequency and phase coding sequences are defined to obtain the initial coding sequence; wherein, the target system performance indicators include the total bandwidth threshold, target spatial resolution, and dynamic response range; the parameters of the orthogonal frequency and phase coding sequences include: code length, number of codes, bandwidth of each code element, frequency interval, phase range, and chirp rate range; The step of defining the number of orthogonal frequency and phase coding sequences based on the target system performance indicators to obtain the initial coding sequence includes: Using a sequential quadratic programming algorithm, several sets of frequency-phase orthogonal coding sequences are created, and constraints are imposed on the parameters of each code point; Set the initial phase, frequency, and chirp rate for each code point to obtain the initial coded sequence; The initial encoded sequence is optimized using the objective function of a sequential quadratic programming algorithm to obtain an optimized orthogonal frequency and phase encoded sequence; The backscattered Rayleigh signals corresponding to each orthogonal coding channel are collected, and the frequency and phase coding sequence is used to perform cross-correlation calculation with the collected backscattered Rayleigh signals to separate the Rayleigh scattering signal of each orthogonal coding channel. The Rayleigh scattering signals of each orthogonal coded channel are demodulated by IQ to separate the in-phase and quadrature components, obtain the phase information of each orthogonal coded channel, and use phase response compensation processing to reconstruct the ultrasonic signals in each orthogonal coded channel.

2. The distributed ultrasonic sensing method based on frequency-phase orthogonal coding according to claim 1, characterized in that, The step of optimizing the initial encoded sequence using the objective function of the sequence quadratic programming algorithm to obtain the optimized orthogonal frequency and phase encoded sequence includes: Construct a sequence quadratic programming problem and calculate the correlation function and objective function of the encoding; the objective function includes one or more of the following: calculating the autocorrelation sidelobe peak, the cross-correlation sidelobe peak, and the synthetic correlation sidelobe peak. The correlation function for the encoding is: ； in, Let be the i-th frequency and phase encoded sequence, and τ be the cross-correlation delay. Represents complex conjugation; when i=k, it is autocorrelation, and when i≠k, it is cross-correlation. By using linear approximation of the objective function, the search direction and step size are solved and optimized to obtain a frequency and phase encoded sequence that meets the preset convergence conditions.

3. The distributed ultrasonic sensing method based on frequency-phase orthogonal coding according to claim 1, characterized in that, The optical fiber under test used in the distributed ultrasonic sensing system based on frequency-phase orthogonal coding applied by the method is a scattering-enhanced optical fiber.

4. The distributed ultrasonic sensing method based on frequency-phase orthogonal coding according to claim 1, characterized in that, The objective function is to calculate the sidelobe peak value and the synthetic sidelobe intensity at the scattering enhancement point location.

5. The distributed ultrasonic sensing method based on frequency-phase orthogonal coding according to claim 1, characterized in that, The steps for acquiring the backscattered Rayleigh signals corresponding to each orthogonal coding channel include: An IQ electro-optic modulator is used to modulate the continuously emitted laser light into encoded pulses. The modulated encoded light pulses are then transmitted to a sensing fiber, where they excite backscattering Rayleigh signals.

6. The distributed ultrasonic sensing method based on frequency-phase orthogonal coding according to claim 1, characterized in that, Before the step of acquiring the backscattered Rayleigh signals corresponding to each orthogonal coding channel, the following steps are included: The generated frequency and phase encoded sequence is injected into an arbitrary waveform generator, and the arbitrary waveform generator is used to generate an encoded pulse modulated electrical signal.

7. The distributed ultrasonic sensing method based on frequency-phase orthogonal coding according to claim 2, characterized in that, The steps for separating the Rayleigh scattering signal of each orthogonal coded channel by performing cross-correlation calculations using the frequency and phase coded sequences and the acquired backscattered Rayleigh signals include: Obtain the complex signal obtained by cross-correlation calculation of the frequency and phase encoded sequence and the backscattered Rayleigh signal; Based on a preset time window interception method, the backscattered Rayleigh signal of each orthogonal coded channel is separated from the complex signal.

8. The distributed ultrasonic sensing method based on frequency-phase orthogonal coding according to claim 1, characterized in that, The steps of performing IQ demodulation on the Rayleigh scattering signals of each separated orthogonal coded channel to separate the in-phase and quadrature components and obtain the phase information of each orthogonal coded channel include: The backscattered Rayleigh signal interferes with the local oscillator light and beats at the same frequency. It is then split into X and Y polarized 1 / Q components by a 90° optical mixer. The light is converted into an electrical signal using a balanced photodetector, and the data is acquired using an oscilloscope or data acquisition card.

9. The distributed ultrasonic sensing method based on frequency-phase orthogonal coding according to claim 1, characterized in that, The step of using phase response compensation processing to reconstruct the ultrasonic signals in each orthogonal coded channel includes: Modal decomposition is used to separate the high-frequency modes of vibration from the low-frequency noise. Low-frequency vibration signals were used to calibrate the strain-phase response of each orthogonal coding channel. The calibrated phases are spliced ​​together to reconstruct the ultrasonic signals in each orthogonal coded channel.

10. A distributed ultrasonic sensing system based on frequency-phase orthogonal coding, characterized in that, include: Connected laser, electro-optic modulator, fiber amplifier, waveform generator, photodetector, data acquisition and processing module, and sensing fiber; The data acquisition and processing module is used to generate orthogonal frequency and phase coding sequences according to the target system performance indicators; acquire the backscattered Rayleigh signals corresponding to each orthogonal coding channel; and perform cross-correlation calculation between the orthogonal frequency and phase coding sequences and the acquired backscattered Rayleigh signals to separate the Rayleigh scattering signals of each coding channel. The Rayleigh scattering signals of each orthogonal coded channel were demodulated using IQ to separate the in-phase and quadrature components and obtain the phase information of each orthogonal coded channel. Phase response compensation processing was then used to reconstruct the ultrasonic signals in each orthogonal coded channel. The step of generating orthogonal frequency and phase coding sequences based on the target system performance indicators includes: Based on the target system performance indicators, parameters of the orthogonal frequency and phase coding sequences are defined to obtain the initial coding sequence; wherein, the target system performance indicators include the total bandwidth threshold, target spatial resolution, and dynamic response range; the parameters of the orthogonal frequency and phase coding sequences include: code length, number of codes, bandwidth of each code element, frequency interval, phase range, and chirp rate range; The step of defining the number of orthogonal frequency and phase coding sequences based on the target system performance indicators to obtain the initial coding sequence includes: Using a sequential quadratic programming algorithm, several sets of frequency-phase orthogonal coding sequences are created, and constraints are imposed on the parameters of each code point; Set the initial phase, frequency, and chirp rate for each code point to obtain the initial coded sequence; The initial encoded sequence is optimized using the objective function of a sequential quadratic programming algorithm to obtain an optimized orthogonal frequency and phase encoded sequence; The step of defining the number of orthogonal frequency and phase coding sequences based on the target system performance indicators to obtain the initial coding sequence includes: Using a sequential quadratic programming algorithm, several sets of frequency-phase orthogonal coding sequences are created, and constraints are imposed on the parameters of each code point; The initial phase, frequency, and chirp rate of each code point are set to obtain the initial coded sequence.

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