Distortion-resistant distributed optical fiber sensing method and system based on array phase spectrum demodulation

Abstract

The invention belongs to the technical field of distributed optical fiber sensing, and relates to an anti-distortion distributed optical fiber sensing method and system based on array phase spectrum demodulation, an ultra-weak reflection optical fiber Bragg grating array is used as a sensing optical fiber, a series of reflection peaks with the signal-to-noise ratio higher than that of RBS are generated, a reflection peak sequence is formed, and the beat frequency signal spectrum amplitude is increased. A discrete beat frequency signal with frequency domain distortion output by an OFDR sensing system is converted to a frequency domain through Fourier transform of a symmetric window function to obtain a corresponding discrete frequency spectrum, and characteristic parameters such as amplitude and phase of a reflection peak sequence in the frequency spectrum are tracked and extracted; and then frequency correction factors of the reflection peak sequence in all measurement periods are calculated by using an amplitude interpolation correction method to obtain a corrected accurate array phase spectrum, and phase difference and strain calculation are performed to realize multi-point quasi-distributed strain demodulation for suppressing frequency domain distortion such as Doppler frequency shift and the like. The frequency resolution limitation caused by the fence effect is broken, and the fidelity of the frequency domain distortion signal is recovered.

Description

Technical Field

[0001] This invention relates to the fields of distributed optical fiber sensing and measurement technology, and in particular to an anti-distortion distributed optical fiber sensing method and system based on array phase spectrum demodulation. Background Technology

[0002] Distributed fiber optic sensing technology detects and senses disturbances in the external environment by detecting changes in the characteristic parameters of the transmitted light in the optical fiber. Due to its advantages such as resistance to electromagnetic interference, high positioning accuracy, wide coverage, and low cost, it can be widely used in many fields such as pipeline safety monitoring, earthquake and tsunami monitoring, and perimeter security.

[0003] Currently, mainstream distributed fiber optic sensing technologies include Optical Time Domain Reflectometer (OTDR) and Optical Frequency Domain Reflectometer (OFDR) distributed fiber optic sensing technologies. These technologies acquire Rayleigh backscattering (RBS) signals at various locations along the entire fiber and demodulate the amplitude, phase, and other parameter changes of the RBS, thereby inverting and calculating the spatiotemporal distribution of parameters such as strain and temperature at various locations along the entire fiber. Compared to traditional pulse measurement mechanisms, OFDR technology employs a continuous wave measurement mechanism with linear frequency modulation. Benefiting from the characteristics of de-skewed coherent reception and beat frequency domain demodulation, it can decouple the mutual constraints between measurement distance and spatial resolution in traditional pulse measurement mechanisms. It has the potential advantage of maintaining high spatial resolution and high measurement sensitivity while achieving long-distance sensing, making it widely used in aerospace, component health, and biomedicine. Its refined, distributed measurement capabilities are key technologies for achieving precise and transparent intelligent manufacturing.

[0004] Based on their working principle, OFDRs can generally be divided into two types: amplitude-based and phase-based, which respectively use spectral correlation and phase difference to demodulate the signal under test. Both methods rely on frequency domain analysis based on Fourier transform to process the signal within a single measurement cycle. According to the time-frequency analysis characteristics of Fourier transform, it requires that the parameter changes sensed by the optical fiber remain stable within a single measurement cycle. However, environmental disturbances or high-frequency, rapidly changing components in dynamic signals make it difficult for the optical fiber to remain stable within a single measurement cycle. This means that the amplitude, phase, and other parameters of the RBS signal at various points in the optical fiber will undergo certain dynamic changes within a single measurement cycle. This phenomenon manifests as Doppler frequency shift or phase modulation in the spectrum after Fourier transform. The resulting frequency domain distortion can cause spatial mismatch and large demodulation errors, or even irreversible demodulation failure. At the same time, although phase-based OFDRs have higher sensitivity than amplitude-based OFDRs, they directly demodulate through the phase spectrum of each frequency component, and the phase deviation caused by spatial mismatch has a more serious impact on their measurement performance.

[0005] In recent years, with the gradual improvement of core performance such as measurement distance of OFDR, especially in practical applications, signal impairment and demodulation failure caused by frequency domain distortion have become key factors restricting OFDR performance. Generally, overcoming signal impairment caused by frequency domain distortion requires accurate estimation and compensation to achieve spatial mismatch calibration, thereby restoring signal fidelity and promoting normal demodulation of the measured signal. However, RBS signals are generated by random fluctuations in the refractive index along the fiber, and their spectral signals are weak and exhibit a certain degree of disorder, making them difficult to directly distinguish through spectral characteristics. Simultaneously, the inherent picket-fence effect of the Fast Fourier Transform (FFT) limits frequency resolution, hindering the effective implementation of frequency shift estimation and correction. To address this, researchers have proposed methods such as piecewise cross-correlation, two-dimensional adaptive cross-correlation, pulse segmentation to reduce individual measurement periods, and frequency tracking combined with time-domain zero-padding for amplitude-type and phase-type OFDRs, respectively. However, these methods typically incur significant computational costs, thus losing their potential for fast demodulation. Summary of the Invention

[0006] The purpose of this invention is to address the problems existing in existing OFDR-based distributed fiber optic sensing systems, such as spatial mismatch and large demodulation errors caused by frequency domain distortion due to Doppler frequency shift or phase modulation induced by environmental disturbances or high-frequency, rapidly changing components in dynamic signals, leading to even irreversible demodulation failures. This invention provides an anti-distortion distributed fiber optic sensing method and system based on array phase spectrum demodulation: An ultra-weak reflection fiber Bragg grating array is used as the sensing fiber to generate a high signal-to-noise ratio reflection peak sequence; a symmetrical window function is applied to transform the frequency-domain distorted discrete beat frequency signal output from the OFDR sensing system to the frequency domain using Fourier transform to obtain its corresponding discrete spectrum; the amplitude, phase, and other characteristic parameters of the reflection peak sequence in the spectrum are tracked and extracted; then, the frequency correction factor array of the reflection peak sequence in all measurement periods is calculated using the amplitude interpolation correction method to obtain the corrected and accurate array phase spectrum; finally, phase difference and strain calculations are performed to achieve multi-point quasi-distributed strain demodulation that suppresses frequency domain distortions such as Doppler frequency shift, breaking the frequency resolution limitation caused by the picket fence effect and restoring the fidelity of the frequency domain distorted signal.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides an anti-distortion distributed optical fiber sensing method based on array phase spectrum demodulation, comprising the following steps: S1. Acquire frequency-domain distorted discrete beat frequency signals; S2. Use a symmetrical window function with a window length of one measurement period of the OFDR system to continuously extract the frequency domain distorted discrete beat frequency signal and perform Fourier transform to obtain the spectrum corresponding to the extracted frequency domain distorted discrete beat frequency signal; S3. Track and identify the reflection peak sequence in the obtained spectrum, and extract the first... The measurement cycle, the first The maximum amplitude frequency of each reflection peak Maximum amplitude at and the second amplitude value of two frequency points with a frequency resolution adjacent to it. Combining the spectral centroid theorem for symmetric window functions, interpolation is used to calculate the first... The measurement cycle, the first Frequency correction factor between the frequency point corresponding to the maximum amplitude of each reflection peak and the true peak frequency The frequency correction factor array can be obtained by calculating the frequency correction factors of all reflection peaks in all measurement periods; S4. Based on the first The measurement cycle, the first The maximum amplitude frequency of each reflection peak Phase value at and the corresponding frequency correction factor The corresponding corrected phase value is calculated. The corrected array phase spectrum is composed of all the corrected phase values; S5. Perform differential calculations on the time and distance axes of the corrected array phase spectrum to obtain the corrected phase change value array, which is the corrected signal to suppress frequency domain distortion. The corrected demodulation strain can be further obtained through the phase strain relationship to achieve multi-point quasi-distributed demodulation.

[0008] As one possible implementation, the frequency-domain distortion discrete beat frequency signal is generated in the following way: S10. A frequency-modulated continuous wave with a preset sweep frequency range and a preset sweep frequency period is used as probe light to enter an unbalanced Mach-Zehnder interferometer for sensing; the fiber under test in the OFDR sensing system includes, in the direction of probe light propagation, a single-mode fiber wound on a piezoelectric transducer and an ultra-weak reflection fiber grating array, with one grating in the grating array pasted on a piezoelectric ceramic stack. S11. Apply a first signal to drive the piezoelectric transducer to induce a Doppler frequency shift in the single-mode fiber; apply a second signal to drive the piezoelectric ceramic stack to induce strain in the grating attached thereto; S12. Use a photodetector and oscilloscope to acquire the frequency-domain distorted discrete beat frequency signal after Rayleigh scattering and grating reflection through the fiber under test.

[0009] As one possible implementation, either a rectangular window function or a Hanning window function can be used to truncate the frequency-domain distorted discrete beat frequency signal.

[0010] As one possible implementation, when using a rectangular window function to truncate the frequency-domain distorted discrete beat frequency signal, the frequency correction factor is calculated as follows: in, Indicates the first The measurement cycle number Frequency correction factor corresponding to each reflection peak; Indicates the first The measurement cycle number The maximum amplitude of each reflection peak For its corresponding frequency point, For the first The measurement cycle number The second largest amplitude value among two adjacent frequency points of the largest amplitude of the reflection peak.

[0011] As one possible implementation, when using the Hanning window function to truncate the frequency-domain distorted discrete beat frequency signal, the frequency correction factor is calculated as follows: in, Indicates the first The measurement cycle number Frequency correction factor corresponding to each reflection peak; Indicates the first The measurement cycle number The maximum amplitude of each reflection peak For its corresponding frequency point, For the first The measurement cycle number The second largest amplitude value among two adjacent frequency points of the largest amplitude of the reflection peak.

[0012] As one possible implementation, the corrected phase value is calculated as follows: in, Indicates the first The measurement cycle number The maximum amplitude frequency of each reflection peak Phase value at; It is combined with its corresponding frequency correction factor The calculated correction phase.

[0013] As one possible implementation, performing differential calculations on the time and distance axes of the corrected array phase spectrum specifically includes: On the distance axis, the corrected phase value corresponding to each reflection peak in the reflection peak sequence is calculated using discrete difference. On the time axis, the corrected phase value of the reflection peak sequence for all measurement periods is subtracted from the corrected phase value of the first measurement period.

[0014] In a second aspect, the present invention provides an anti-distortion distributed optical fiber sensing system based on array phase spectrum demodulation, comprising: Frequency domain distortion discrete beat frequency signal acquisition module, used to acquire frequency domain distortion discrete beat frequency signals; The interception module continuously intercepts the frequency-domain distorted discrete beat frequency signal using a symmetrical window function with a window length equal to a single measurement cycle of the OFDR system. The transformation module performs a Fourier transform on the truncated frequency-domain distorted discrete beat frequency signal to obtain the spectrum corresponding to the truncated frequency-domain distorted discrete beat frequency signal. The correction factor array calculation module tracks and identifies the reflection peak sequence in the obtained spectrum and extracts the first... The measurement cycle, the first The maximum amplitude frequency of each reflection peak Maximum amplitude at and the second amplitude value of two frequency points with a frequency resolution adjacent to it. Combining the spectral centroid theorem for symmetric window functions, interpolation is used to calculate the first... The measurement cycle, the first Frequency correction factor between the frequency point corresponding to the maximum amplitude of each reflection peak and the true peak frequency The frequency correction factor array can be obtained by calculating the frequency correction factors of all reflection peaks in all measurement periods; The array phase spectrum calculation module is based on the first The measurement cycle, the first The maximum amplitude frequency of each reflection peak Phase value at and the corresponding frequency correction factor The corresponding corrected phase value is calculated. The corrected array phase spectrum is composed of all the corrected phase values; The demodulation module performs differential calculations on the time and distance axes of the corrected array phase spectrum to obtain the corrected phase change value array, which is the corrected signal to suppress frequency domain distortion. The corrected demodulation strain can be further obtained through the phase strain relationship to achieve multi-point quasi-distributed demodulation.

[0015] As one possible implementation, the frequency domain distortion discrete beat frequency signal acquisition module specifically includes: The probe light generating unit is used to generate a frequency-modulated continuous wave with a preset sweep frequency range and a preset sweep frequency period; The first coupler is used for beam splitting. Its input end is connected to the probe light generating unit, its first output end is connected to the circulator to form a sensing arm, and its second output end is connected to the second coupler to form a reference arm. The circulator is used to input the split probe light into the fiber under test, and to transmit the sensing light signal composed of the back Rayleigh scattering signal with frequency domain distortion and the grating reflection signal to the second coupler and the photodetector in sequence. The fiber under test, in the direction of the probe light propagation, includes a single-mode fiber wound on a piezoelectric transducer and an ultra-weak reflection fiber grating array, with one grating in the array attached to a piezoelectric ceramic stack. The first signal generator generates a high-frequency vibration signal and applies the high-frequency vibration signal to drive the piezoelectric transducer so that the single-mode fiber generates a Doppler frequency shift. The second signal generator produces a low-frequency vibration signal and applies the low-frequency vibration signal to drive the piezoelectric ceramic stack so that the grating attached to it is strained. The second coupler is used for beam combining. Its first input is connected to the circulator to receive the sensing signal, and its second input is connected to the first coupler to receive the reference signal. Its output is connected to the balanced photodetector. In addition, a balanced photodetector and an oscilloscope are used to convert optical signals into electrical signals and acquire frequency-domain distorted discrete beat frequency signals.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Compared with existing methods, the anti-distortion distributed optical fiber sensing method and system based on array phase spectrum demodulation proposed in this invention uses an ultra-weak reflection fiber Bragg grating array as the sensing fiber to generate a series of reflection peaks with a higher signal-to-noise ratio than RBS, forming a reflection peak sequence, increasing the spectral amplitude of the beat frequency signal, providing a basis for realizing reflection peak sequence tracking and suppressing frequency domain distortions such as Doppler frequency shift, and enabling multi-point quasi-distributed demodulation.

[0017] 2. The anti-distortion distributed fiber optic sensing method and system based on array phase spectrum demodulation proposed in this invention utilizes the linear phase-frequency relationship within the main lobe of the spectral spectrum of the sensing signal after being truncated by a symmetric window function and processed by FFT, and the spectral centroid theorem of adjacent frequency points, to provide a clear interpolation calculation formula. Without the need for time-domain zero-padding, only a limited number of arithmetic operations are added to break the frequency resolution limitation and reduce the computational load caused by spectrum refinement. While effectively suppressing large Doppler frequency shifts, it achieves high-precision phase extraction and correct strain demodulation, and has the potential for fast real-time demodulation.

[0018] 3. Experiments show that the anti-distortion distributed fiber optic sensing method based on array phase spectrum demodulation proposed in this invention can suppress Doppler frequency shift, restore signal fidelity, achieve correct strain demodulation, and greatly reduce demodulation time. Attached Figure Description

[0019] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this invention, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is a flowchart of the anti-distortion distributed optical fiber sensing method based on array phase spectrum demodulation proposed in an embodiment of the present invention; Figure 2 The figures show the amplitude and corresponding phase diagrams under different distortion conditions in the embodiments of the present invention; wherein (a) corresponds to (d), (b) corresponds to (e), and (c) corresponds to (f); Figure 3 This is a schematic diagram of the anti-distortion distributed optical fiber sensing system based on array phase spectrum demodulation proposed in an embodiment of the present invention; Figure 4 This is a comparison chart of the three-dimensional strain demodulation results of the traditional phase demodulation method and the amplitude interpolation method of this embodiment of the invention; Figure 5 This is a comparison chart showing the demodulation time required when demodulating the same data using the time-domain zero-padding method in this embodiment of the invention and the amplitude interpolation method in this scheme.

[0020] Figure Labels 1-Frequency domain distortion discrete beat frequency signal acquisition module, 10-Detector light generation unit, 11-First coupler, 12-Circulator, 13-Fiber optic cable under test, 14-First signal generator, 15-Second signal generator, 16-Second coupler, 17-Balanced photodetector, 18-Piezoelectric transducer, 19-Piezoelectric ceramic stack, 20-Oscilloscope; 2-Cutout module, 3-Transformation module, 4-Correction factor array calculation module, 5-Array phase spectrum calculation module, 6-Demodulation module. Detailed Implementation

[0021] To facilitate a clear description of the technical solutions in the embodiments of the present invention, the terms "first" and "second" are used to distinguish identical or similar items with essentially the same function and effect. For example, the first threshold and the second threshold are merely used to distinguish different thresholds and do not limit their order. Those skilled in the art will understand that the terms "first" and "second" do not limit the quantity or execution order, and that the terms "first" and "second" are not necessarily different.

[0022] It should be noted that in this invention, the terms "exemplary" or "for example" are used to indicate examples, illustrations, or descriptions. Any embodiment or design described as "exemplary" or "for example" in this invention should not be construed as being more preferred or advantageous than other embodiments or designs. Specifically, the use of terms such as "exemplary" or "for example" is intended to present the relevant concepts in a concrete manner.

[0023] In this invention, "at least one" refers to one or more, and "more than one" refers to two or more. "And / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A alone, A and B simultaneously, or B alone, where A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects are in an "or" relationship. "At least one" or similar expressions refer to any combination of these items, including any combination of singular or plural items. For example, "at least one of a, b, or c" can represent: a, b, c, a combination of a and b, a combination of a and c, a combination of b and c, or a, b, and c, where a, b, and c can be single or multiple.

[0024] The present invention aims to provide an anti-distortion distributed optical fiber sensing method and system based on array phase spectrum demodulation, in order to solve the problems of existing methods, such as the large amount of computational consumption and loss of the potential for fast demodulation caused by frequency domain distortion caused by Doppler frequency shift or phase modulation due to environmental disturbances or high-frequency, fast-changing components in dynamic signals, resulting in spatial mismatch and large demodulation errors, or even irreversible demodulation failure.

[0025] In a first aspect, embodiments of the present invention provide an anti-distortion distributed optical fiber sensing method based on array phase spectrum demodulation, see [link to previous section]. Figure 1 It includes the following steps: S1. Acquire frequency-domain distorted discrete beat frequency signals; As one possible implementation, the frequency-domain distortion discrete beat frequency signal is generated in the following way: S10. A frequency-modulated continuous wave with a preset sweep frequency range and a preset sweep frequency period is used as probe light to enter an unbalanced Mach-Zehnder interferometer for sensing; the fiber under test in the OFDR sensing system includes, in the direction of probe light propagation, a single-mode fiber wound on a piezoelectric transducer and an ultra-weak reflection fiber grating array, with one grating in the grating array pasted on a piezoelectric ceramic stack. S11. Apply a first signal to drive the piezoelectric transducer to induce a Doppler frequency shift in the single-mode fiber; apply a second signal to drive the piezoelectric ceramic stack to induce strain in the grating attached thereto; S12. Use a photodetector and oscilloscope to acquire the frequency-domain distorted discrete beat frequency signal after Rayleigh scattering and grating reflection through the fiber under test.

[0026] S2. Use a symmetrical window function with a window length of one measurement period of the OFDR system to continuously extract the frequency domain distorted discrete beat frequency signal and perform Fourier transform to obtain the spectrum corresponding to the extracted frequency domain distorted discrete beat frequency signal; As one possible implementation, either a rectangular window function or a Hanning window function can be used to truncate the frequency-domain distorted discrete beat frequency signal.

[0027] S3. Track and identify the reflection peak sequence in the obtained spectrum, and extract the first... The measurement cycle, the first The maximum amplitude frequency of each reflection peak Maximum amplitude at and the second amplitude value of two frequency points with a frequency resolution adjacent to it. Combining the spectral centroid theorem for symmetric window functions, interpolation is used to calculate the first... The measurement cycle, the first Frequency correction factor between the frequency point corresponding to the maximum amplitude of each reflection peak and the true peak frequency The frequency correction factor array can be obtained by calculating the frequency correction factors of all reflection peaks in all measurement periods; See Figure 2 As one possible implementation, when using a rectangular window function to truncate the frequency-domain distorted discrete beat frequency signal, the frequency correction factor is calculated as follows: in, Indicates the first The measurement cycle number Frequency correction factor corresponding to each reflection peak; Indicates the first The measurement cycle number The maximum amplitude of each reflection peak For its corresponding frequency point, For the first The measurement cycle number The second largest amplitude value among two adjacent frequency points of the largest amplitude of the reflection peak.

[0028] See Figure 2 As one possible implementation, when using the Hanning window function to truncate the frequency-domain distorted discrete beat frequency signal, the frequency correction factor is calculated as follows: in, Indicates the first The measurement cycle number Frequency correction factor corresponding to each reflection peak; Indicates the first The measurement cycle number The maximum amplitude of each reflection peak For its corresponding frequency point, For the first The measurement cycle number The second largest amplitude value among two adjacent frequency points of the largest amplitude of the reflection peak.

[0029] S4. Based on the first The measurement cycle, the first The maximum amplitude frequency of each reflection peak Phase value at and the corresponding frequency correction factor The corresponding corrected phase value is calculated. The corrected array phase spectrum is composed of all the corrected phase values; As one possible implementation, the corrected phase value is calculated as follows: in, Indicates the first The measurement cycle number The maximum amplitude frequency of each reflection peak Phase value at; It is combined with its corresponding frequency correction factor The calculated correction phase.

[0030] S5. Perform differential calculations on the time and distance axes of the corrected array phase spectrum to obtain the corrected phase change value array, which is the corrected signal to suppress frequency domain distortion. The corrected demodulation strain can be further obtained through the phase strain relationship to achieve multi-point quasi-distributed demodulation.

[0031] As one possible implementation, performing differential calculations on the time and distance axes of the corrected array phase spectrum specifically includes: On the distance axis, the corrected phase value corresponding to each reflection peak in the reflection peak sequence is calculated using discrete difference. On the time axis, the corrected phase value of the reflection peak sequence for all measurement periods is subtracted from the corrected phase value of the first measurement period.

[0032] Compared to existing methods, the anti-distortion distributed fiber sensing method based on array phase spectrum demodulation proposed in this invention uses an ultra-weak reflection fiber Bragg grating array as the sensing fiber, generating a series of reflection peaks with a higher signal-to-noise ratio than RBS, forming a reflection peak sequence, increasing the spectral amplitude of the beat frequency signal, providing a basis for realizing reflection peak sequence tracking and suppressing frequency domain distortions such as Doppler frequency shift, and enabling multi-point quasi-distributed demodulation.

[0033] Secondly, embodiments of the present invention provide an anti-distortion distributed optical fiber sensing system based on array phase spectrum demodulation, see [link to relevant documentation]. Figure 3 ,include: Frequency domain distortion discrete beat frequency signal acquisition module 1 is used to acquire frequency domain distortion discrete beat frequency signals; As one possible implementation, the frequency domain distortion discrete beat frequency signal acquisition module 1 specifically includes: The probe light generating unit 10 is used to generate a frequency-modulated continuous wave with a preset sweep frequency range and a preset sweep frequency period; The first coupler 11 is used for beam splitting. Its input end is connected to the probe light generating unit 10, and its first output end is connected to the circulator to form a sensing arm. Its second output end is connected to the second coupler to form a reference arm. The circulator 12 is used to input the split probe light into the fiber under test, and to transmit the sensing light signal composed of the frequency domain distortion discrete beat frequency signal of back Rayleigh scattering and grating reflection to the second coupler and the photodetector in sequence. The fiber under test 13 includes, in the direction of the probe light propagation, a single-mode fiber wound on a piezoelectric transducer 18 and an ultra-weak reflection fiber grating array, with one grating in the grating array attached to a piezoelectric ceramic stack 19. As one possible implementation, the length of the single-mode fiber is 60m; the grating spacing is 2.5cm, and the grating length is 1.2m.

[0034] The first signal generator 14 generates a high-frequency vibration signal and applies the high-frequency vibration signal to drive the piezoelectric transducer 18 so that the single-mode optical fiber generates a Doppler frequency shift. The second signal generator 15 generates a low-frequency vibration signal and applies the low-frequency vibration signal to drive the piezoelectric ceramic stack 19 so that the grating attached thereon is strained. The second coupler 16 is used for beam combining. Its first input terminal is connected to the circulator to receive the sensing signal, its second input terminal is connected to the first coupler to receive the reference signal, and its output terminal is connected to the balanced photodetector. In addition, a balanced photodetector 17 and an oscilloscope 20 are used to convert optical signals into electrical signals and acquire frequency-domain distorted discrete beat frequency signals.

[0035] Module 2 uses a symmetrical window function with a window length equal to a single measurement cycle of the OFDR system to continuously extract frequency-domain distorted discrete beat frequency signals; Transformation module 3 performs Fourier transform on the truncated frequency domain distorted discrete beat frequency signal to obtain the spectrum corresponding to the truncated frequency domain distorted discrete beat frequency signal. Correction factor array calculation module 4 tracks and identifies the reflection peak sequence in the obtained spectrum, and extracts the first... The measurement cycle, the first The maximum amplitude frequency of each reflection peak Maximum amplitude at and the second amplitude value of two frequency points with a frequency resolution adjacent to it. Combining the spectral centroid theorem for symmetric window functions, interpolation is used to calculate the first... The measurement cycle, the first Frequency correction factor between the frequency point corresponding to the maximum amplitude of each reflection peak and the true peak frequency The frequency correction factor array can be obtained by calculating the frequency correction factors of all reflection peaks in all measurement periods; Array phase spectrum calculation module 5, based on the first The measurement cycle, the first The maximum amplitude frequency of each reflection peak Phase value at and the corresponding frequency correction factor The corresponding corrected phase value is calculated. The corrected array phase spectrum is composed of all the corrected phase values; And demodulation module 6 performs differential calculations on the time axis and distance axis of the corrected array phase spectrum to obtain the corrected phase change value array, which is the corrected signal to suppress frequency domain distortion. The corrected demodulation strain can be further obtained through the phase strain relationship to achieve multi-point quasi-distributed demodulation.

[0036] The proposed method and system for anti-distortion distributed optical fiber sensing based on array phase spectrum demodulation utilizes the linear phase-frequency relationship within the main lobe of the spectral spectrum of the sensing signal after truncating with a symmetric window function and performing FFT processing, as well as the spectral centroid theorem for adjacent frequency points, to provide a clear interpolation calculation formula. Without the need for time-domain zero-padding, it only requires a limited number of arithmetic operations to overcome the limitations of frequency resolution and reduce the computational load caused by spectrum refinement. While effectively suppressing large Doppler frequency shifts, it achieves high-precision phase extraction and correct strain demodulation, and has the potential for fast real-time demodulation.

[0037] See Figure 3 To more clearly illustrate the sensing and demodulation principles of the anti-distortion distributed fiber optic sensing method and system based on array phase spectrum demodulation proposed in this invention, a linear sweep frequency signal generated by an arbitrary waveform generator drives a phase modulator to modulate the output of a fiber laser with a wavelength of 1554.4 nm. A frequency-modulated continuous wave with a sweep frequency range of 12.5 GHz and a sweep frequency period of 1 ms, obtained through a programmable optical filter, is then used as probe light to enter an OFDR interferometer for sensing. A balanced photodetector 17 receives the coherent beat frequency signal. The fiber under test in the OFDR includes a 60 m long single-mode fiber wound on a piezoelectric transducer 18 and an ultra-weak reflection fiber grating array with a grating spacing of 2.5 cm and a length of 1.2 m, one of which is attached to a piezoelectric ceramic stack 19. A high-frequency vibration signal is applied by a first signal generator 14 to drive the piezoelectric transducer 18, causing a Doppler frequency shift in the single-mode fiber segment. A low-frequency vibration signal is applied by a second signal generator 15 to drive the piezoelectric ceramic stack 19, causing strain on the grating. FFT calculations are performed on the frequency-domain distorted discrete beat frequency signals acquired by the balanced photodetector 17 and oscilloscope 20. The maximum amplitude and adjacent second-largest amplitude of the reflection peak sequence in the FFT spectrum are tracked and identified in all measurement periods. The frequency correction factor array and the corrected array phase spectrum are calculated. Then, the corrected phase change value array and the corrected demodulated strain are calculated differentially to achieve high-fidelity demodulation. The demodulation results of the traditional phase demodulation method are as follows: Figure 4 As shown in (a), frequency domain distortion phenomena such as Doppler frequency shift cause signal demodulation failure, resulting in the inability to obtain the correct strain and producing a large error; the demodulation result of this invention is as follows. Figure 4 As shown in (b), the amplitude interpolation correction method proposed in this invention can suppress Doppler frequency shift, restore signal fidelity, and achieve correct strain demodulation.

[0038] See Figure 5 The figure shows a comparison of the demodulation time required when using the existing time-domain zero-padding method and the amplitude interpolation method of this scheme to demodulate the same data. As can be seen from the figure, this scheme greatly reduces the demodulation time and improves the computational efficiency.

[0039] Although the invention has been described herein in conjunction with various embodiments, those skilled in the art will understand and implement other variations of the disclosed embodiments by reviewing the accompanying drawings, disclosure, and other materials. In this specification, the word "comprising" does not exclude other components or steps, and "a" or "an" does not exclude multiple components. A single processor or other unit can implement several functions listed in the specification. While certain measures are described in different embodiments, this does not mean that these measures cannot be combined to produce good results.

[0040] Although the invention has been described in conjunction with specific features and embodiments, it is obvious that various modifications and combinations can be made therein without departing from the spirit and scope of the invention. Accordingly, this specification and drawings are merely illustrative of the invention and are considered to cover any and all modifications, variations, combinations, or equivalents within the scope of the invention. Clearly, those skilled in the art can make various alterations and modifications to the invention without departing from its spirit and scope. Thus, if such modifications and modifications fall within the scope of the invention and its equivalents, the invention is also intended to include such modifications and modifications.

Claims

1. A distortion-resistant distributed fiber optic sensing method based on array phase spectrum demodulation, characterized in that, Includes the following steps: S1. Acquire frequency-domain distorted discrete beat frequency signals; S2. Use a symmetrical window function with a window length of one measurement period of the OFDR system to continuously extract the frequency domain distortion discrete beat frequency signal and perform Fourier transform to obtain the spectrum corresponding to the extracted frequency domain distortion discrete beat frequency signal; S3. Track and identify the reflection peak sequence in the obtained spectrum, and extract the first... The measurement cycle, the first The maximum amplitude frequency of each reflection peak Maximum amplitude at and the second amplitude value of two frequency points with a frequency resolution adjacent to it. Combining the spectral centroid theorem for symmetric window functions, interpolation is used to calculate the first... The measurement cycle, the first Frequency correction factor between the frequency point corresponding to the maximum amplitude of each reflection peak and the true peak frequency The frequency correction factor array can be obtained by calculating the frequency correction factors of all reflection peaks in all measurement periods; S4. Based on the first The measurement cycle, the first The maximum amplitude frequency of each reflection peak Phase value at and the corresponding frequency correction factor The corresponding corrected phase value is calculated. The corrected array phase spectrum is composed of all the corrected phase values; S5. Perform differential calculations on the time and distance axes of the corrected array phase spectrum to obtain the corrected phase change value array, which is the corrected signal to suppress frequency domain distortion. The corrected demodulation strain can be further obtained through the phase strain relationship to achieve multi-point quasi-distributed demodulation.

2. The anti-distortion distributed optical fiber sensing method based on array phase spectrum demodulation according to claim 1, characterized in that, Frequency-domain distorted discrete beat frequency signals are generated in the following way: S10. A frequency-modulated continuous wave with a preset sweep frequency range and a preset sweep frequency period is used as probe light to enter an unbalanced Mach-Zehnder interferometer for sensing; the fiber under test in the OFDR sensing system includes, in the direction of probe light propagation, a single-mode fiber wound on a piezoelectric transducer and an ultra-weak reflection fiber grating array, with one grating in the grating array pasted on a piezoelectric ceramic stack. S11. Apply a first signal to drive the piezoelectric transducer to induce a Doppler frequency shift in the single-mode fiber; apply a second signal to drive the piezoelectric ceramic stack to induce strain in the grating attached thereto; S12. Use a photodetector and oscilloscope to acquire the frequency-domain distorted discrete beat frequency signal after Rayleigh scattering and grating reflection through the fiber under test.

3. The anti-distortion distributed optical fiber sensing method based on array phase spectrum demodulation according to claim 1, characterized in that, Use either a rectangular window function or a Hanning window function to extract the frequency-domain distorted discrete beat frequency signal.

4. The anti-distortion distributed optical fiber sensing method based on array phase spectrum demodulation according to claim 3, characterized in that, When using a rectangular window function to extract a frequency-domain distorted discrete beat frequency signal, the frequency correction factor is calculated as follows: in, Indicates the first The measurement cycle number Frequency correction factor corresponding to each reflection peak; Indicates the first The measurement cycle number The maximum amplitude of each reflection peak For its corresponding frequency point, For the first The measurement cycle number The second largest amplitude value among two adjacent frequency points of the largest amplitude of the reflection peak.

5. The anti-distortion distributed optical fiber sensing method based on array phase spectrum demodulation according to claim 3, characterized in that, When using the Hanning window function to extract a frequency-domain distorted discrete beat frequency signal, the frequency correction factor is calculated as follows: in, Indicates the first The measurement cycle number Frequency correction factor corresponding to each reflection peak; Indicates the first The measurement cycle number The maximum amplitude of each reflection peak For its corresponding frequency point, For the first The measurement cycle number The second largest amplitude value among two adjacent frequency points of the largest amplitude of the reflection peak.

6. The anti-distortion distributed optical fiber sensing method based on array phase spectrum demodulation according to claim 1, characterized in that, The corrected phase value is calculated as follows: in, Indicates the first The measurement cycle number The maximum amplitude frequency of each reflection peak Phase value at; It is combined with its corresponding frequency correction factor The calculated correction phase.

7. The anti-distortion distributed optical fiber sensing method based on array phase spectrum demodulation according to claim 1, characterized in that, The differential calculation of the corrected array phase spectrum on the time and distance axes specifically includes: On the distance axis, the corrected phase value corresponding to each reflection peak in the reflection peak sequence is calculated using discrete difference. On the time axis, the corrected phase value of the reflection peak sequence for all measurement periods is subtracted from the corrected phase value of the first measurement period.

8. A distortion-resistant distributed fiber optic sensing system based on array phase spectrum demodulation, characterized in that, include: Frequency domain distortion discrete beat frequency signal acquisition module, used to acquire frequency domain distortion discrete beat frequency signals; The interception module continuously intercepts the frequency-domain distorted discrete beat frequency signal using a symmetrical window function with a window length equal to a single measurement cycle of the OFDR system. The transformation module performs a Fourier transform on the truncated frequency-domain distorted discrete beat frequency signal to obtain the spectrum corresponding to the truncated frequency-domain distorted discrete beat frequency signal. The correction factor array calculation module tracks and identifies the reflection peak sequence in the obtained spectrum and extracts the first... The measurement cycle, the first The maximum amplitude frequency of each reflection peak Maximum amplitude at and the second amplitude value of two frequency points with a frequency resolution adjacent to it. Combining the spectral centroid theorem for symmetric window functions, interpolation is used to calculate the first... The measurement cycle, the first Frequency correction factor between the frequency point corresponding to the maximum amplitude of each reflection peak and the true peak frequency The frequency correction factor array can be obtained by calculating the frequency correction factors of all reflection peaks in all measurement periods; The array phase spectrum calculation module is based on the first The measurement cycle, the first The maximum amplitude frequency of each reflection peak Phase value at and the corresponding frequency correction factor The corresponding corrected phase value is calculated. The corrected array phase spectrum is composed of all the corrected phase values; The demodulation module performs differential calculations on the time and distance axes of the corrected array phase spectrum to obtain the corrected phase change value array, which is the corrected signal to suppress frequency domain distortion. The corrected demodulation strain can be further obtained through the phase strain relationship to achieve multi-point quasi-distributed demodulation.

9. The anti-distortion distributed optical fiber sensing system based on array phase spectrum demodulation according to claim 8, characterized in that, The frequency domain distortion discrete beat frequency signal acquisition module specifically includes: The probe light generating unit is used to generate a frequency-modulated continuous wave with a preset sweep frequency range and a preset sweep frequency period; The first coupler is used for beam splitting. Its input end is connected to the probe light generating unit, its first output end is connected to the circulator to form a sensing arm, and its second output end is connected to the second coupler to form a reference arm. The circulator is used to input the split probe light into the fiber under test, and to transmit the sensing light signal composed of the back Rayleigh scattering signal with frequency domain distortion and the grating reflection signal to the second coupler and the photodetector in sequence. The fiber under test, in the direction of the probe light propagation, includes a single-mode fiber wound on a piezoelectric transducer and an ultra-weak reflection fiber grating array, with one grating in the array attached to a piezoelectric ceramic stack. The first signal generator generates a high-frequency vibration signal and applies the high-frequency vibration signal to drive the piezoelectric transducer so that the single-mode fiber generates a Doppler frequency shift. The second signal generator produces a low-frequency vibration signal and applies the low-frequency vibration signal to drive the piezoelectric ceramic stack so that the grating attached to it is strained. The second coupler is used for beam combining. Its first input is connected to the circulator to receive the sensing signal, and its second input is connected to the first coupler to receive the reference signal. Its output is connected to the balanced photodetector. In addition, a balanced photodetector and an oscilloscope are used to convert optical signals into electrical signals and acquire frequency-domain distorted discrete beat frequency signals.

10. The distortion-resistant distributed optical fiber sensing system based on array phase spectrum demodulation according to claim 9, characterized in that, The length of the single-mode fiber is 60m; the grating spacing is 2.5cm, and the grating length is 1.2m.

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