Anti-distortion distributed optical fiber sensing method and system based on array phase spectrum demodulation

Abstract

The application 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. A series of reflection peaks with a signal-to-noise ratio RBS higher than that of a reflection peak are generated by using a super-weak reflection fiber Bragg grating array as a sensing optical fiber, a reflection peak sequence is formed, and the spectral amplitude of a beat frequency signal is increased. The discrete beat frequency signal with frequency domain distortion output by an OFDR sensing system is converted to the frequency domain by Fourier transform with a symmetric window function to obtain the corresponding discrete frequency spectrum, the amplitude, phase and other characteristic parameters of the reflection peak sequence in the frequency spectrum are tracked and extracted, the frequency correction factor of the reflection peak sequence at all measurement periods is calculated by using an amplitude interpolation correction method, the corrected accurate array phase spectrum is obtained, and phase difference and strain calculation are performed, so that multi-point quasi-distributed strain demodulation for suppressing frequency domain distortions such as Doppler frequency shift is realized, the frequency resolution limitation caused by the fence effect is broken, and the fidelity of the frequency domain distortion signal is restored.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of distributed optical fiber sensing technology and measurement technology, and particularly relates to an anti-distortion distributed optical fiber sensing method and system based on array phase spectrum demodulation. BACKGROUND

[0002] Distributed optical fiber sensing technology realizes the detection and perception of external environmental disturbance by detecting the change of characteristic parameters of transmitted light in the optical fiber. Due to its advantages of anti-electromagnetic interference, high positioning accuracy, wide coverage, low cost, etc., it can be widely applied to pipeline safety monitoring, earthquake and tsunami monitoring, perimeter security and many other fields.

[0003] At present, the mainstream distributed optical fiber sensing technology includes optical time domain reflectometer (OTDR) and optical frequency domain reflectometer (OFDR) distributed optical fiber sensing technology. By obtaining the Rayleigh backscattering (RBS) signals at each position on the whole optical fiber, and demodulating the amplitude, phase and other parameter changes of the RBS, the spatial and temporal distribution of the strain, temperature and other measured parameters at each position on the whole optical fiber can be calculated. Compared with the traditional pulse measurement mechanism, the OFDR technology adopts a linear frequency modulation continuous wave measurement mechanism. Benefiting from the characteristics of optical dechirp coherent reception and beat frequency domain demodulation, it can decouple the mutual restriction between measurement distance and spatial resolution in the traditional pulse measurement mechanism, has the potential advantages of realizing long distance sensing while maintaining high spatial resolution and high measurement sensitivity, and has attracted widespread attention in the fields of aerospace, component health, biomedical, etc. The fine and distributed measurement capability provided by it is a key technology to realize the precision and transparency of intelligent manufacturing.

[0004] According to its working principle, OFDR can be generally divided into amplitude type and phase type, which respectively uses spectrum correlation and phase difference to realize the demodulation of the signal to be measured. Both methods rely on frequency domain analysis based on Fourier transform to process the signal in a single measurement period. According to the time-frequency analysis characteristics of Fourier transform, it requires that the changes of the parameters perceived by the optical fiber remain stable in a single measurement period. However, environmental disturbances or high-frequency and fast-changing components in dynamic signals make it difficult for the optical fiber to remain stable in a single measurement period. This means that the amplitude, phase and other parameters of the RBS signal at different positions of the optical fiber will experience certain dynamic changes in a single measurement period. This phenomenon is manifested as Doppler shift or phase modulation in the frequency spectrum after Fourier transform, and the distortion in the frequency domain caused thereby will cause spatial mismatch and large demodulation error, and even irreversible demodulation failure. At the same time, compared with the amplitude type, the phase type OFDR has higher sensitivity, but it directly demodulates through the phase spectrum of each frequency component, and the phase deviation caused by spatial mismatch has a more serious impact on the measurement performance.

[0005] In recent years, with the gradual improvement of the core performance of OFDR measurement distance, especially in practical application scenarios, the signal damage and demodulation failure caused by frequency domain distortion have become the key factors restricting the performance of OFDR. Generally, to overcome the signal damage caused by frequency domain distortion, it is necessary to accurately estimate and compensate it to realize the calibration of spatial mismatch, thereby restoring the signal fidelity and promoting the normal demodulation of the signal to be measured. However, the RBS signal is generated by the random fluctuation of the refractive index along the optical fiber, and its frequency spectrum signal is weak and presents a certain disorder, which is difficult to directly distinguish through spectral characteristics. At the same time, the inherent grating effect of fast Fourier transform (FFT) limits the frequency resolution, hindering the effective implementation of frequency shift estimation and correction. In this regard, researchers have proposed methods such as segmented cross-correlation, two-dimensional adaptive cross-correlation and pulse segmentation to reduce the single measurement period, frequency tracking combined with time domain zero padding, etc. for amplitude type and phase type OFDR respectively. However, these methods usually cause a large amount of calculation consumption, thereby losing the potential of fast demodulation. SUMMARY

[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:

[0008] 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:

[0009] S1. Acquire frequency-domain distorted discrete beat frequency signals;

[0010] 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;

[0011] 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;

[0012] S4. Based on the first The measurement cycle, the first The maximum amplitude frequency of each reflection peak Phase value of the phase spectrum of the array and the corresponding frequency correction factor , the corresponding correction phase value is calculated The corrected array phase spectrum is composed of all the correction phase values;

[0013] S5. The corrected array phase spectrum is differentially calculated on the time axis and the distance axis to obtain a correction phase change value array, which is a correction signal for suppressing frequency domain distortion, and the corrected demodulation strain can be further obtained through the phase strain relationship to realize quasi-distributed demodulation of multiple points.

[0014] As a possible implementation, the frequency domain distortion discrete beat frequency signal is generated by the following method:

[0015] S10. The light frequency modulation continuous wave with a preset sweep frequency range and a preset sweep frequency period is used as the probe light to enter the unbalanced Mach-Zehnder interferometer for sensing; the measured optical fiber in the OFDR sensing system includes, in the propagation direction of the probe light, a single-mode optical fiber wound on a piezoelectric transducer, and an ultra-weak reflective fiber grating array, one of the grating arrays being pasted on a piezoelectric ceramic stack;

[0016] S11. A first signal driving the piezoelectric transducer is applied to make the single-mode optical fiber produce Doppler frequency shift; a second signal driving the piezoelectric ceramic stack is applied to make the grating pasted on it produce strain;

[0017] S12. The photoelectric detector and the oscilloscope are used to collect the frequency domain distortion discrete beat frequency signal backscattered by the measured optical fiber and reflected by the grating.

[0018] As a possible implementation, any one of the rectangular window function or the Hanning window function is used to intercept the frequency domain distortion discrete beat frequency signal.

[0019] As a possible implementation, when the rectangular window function is used to intercept the frequency domain distortion discrete beat frequency signal, the frequency correction factor is calculated by the following method:

[0020]

[0021] wherein, represents the frequency correction factor corresponding to the mth reflection peak in the nth measurement period; represents the frequency correction factor corresponding to the mth reflection peak in the nth measurement period; represents the maximum amplitude of the mth reflection peak in the nth measurement period, is the corresponding frequency point, is the corresponding frequency point, is the corresponding frequency point, is the corresponding frequency point, is the corresponding frequency point, is the corresponding frequency point, The second largest amplitude in the two adjacent frequency points of the maximum amplitude of the reflection peak.

[0022] As a possible implementation, when the frequency domain distortion discrete beat frequency signal is intercepted using the Hann window function, the frequency correction factor is calculated as follows:

[0023]

[0024] wherein, represents the frequency correction factor corresponding to the i-th reflection peak in the j-th measurement period; represents the maximum amplitude of the i-th reflection peak in the j-th measurement period, is the corresponding frequency point, is the second largest amplitude in the two adjacent frequency points of the maximum amplitude of the i-th reflection peak in the j-th measurement period.

[0025] As a possible implementation, the corrected phase value is calculated as follows:

[0026]

[0027] wherein, represents the phase value at the maximum amplitude frequency point of the i-th reflection peak in the j-th measurement period; is the corrected phase calculated in combination with the corresponding frequency correction factor

[0028] As a possible implementation, the differential calculation of the corrected array phase spectrum on the time axis and the distance axis specifically includes:

[0029] On the distance axis, the corrected phase value corresponding to each reflection peak of the reflection peak sequence is calculated by discrete difference;

[0030] On the time axis, the corrected phase values of all measurement periods of the reflection peak sequence are subtracted by the corrected phase value of the first measurement period.

[0031] In the second aspect, the application provides an anti-distortion distributed optical fiber sensing system based on array phase spectrum demodulation, comprising:

[0032] A frequency domain distortion discrete beat frequency signal acquisition module is configured to acquire a frequency domain distortion discrete beat frequency signal.

[0033] ​​​​​​​​​​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.

[0034] 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.

[0035] 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;

[0036] 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;

[0037] 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.

[0038] As one possible implementation, the frequency domain distortion discrete beat frequency signal acquisition module specifically includes:

[0039] 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;

[0040] 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.

[0041] A circulator is used to input the split probe light into the optical fiber to be measured, and is used to transmit the sensing optical signal composed of the back Rayleigh scattering signal and the grating reflection signal with frequency domain distortion to the second coupler and the photoelectric detector in turn;

[0042] The optical fiber to be measured comprises a single-mode optical fiber wound on a piezoelectric transducer and an ultra-weak reflection fiber grating array in the propagation direction of the probe light, and one grating in the grating array is pasted on a piezoelectric ceramic stack.

[0043] A 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 optical fiber generates a Doppler frequency shift.

[0044] A second signal generator generates a low-frequency vibration signal, and applies the low-frequency vibration signal to drive the piezoelectric ceramic stack, so that the grating pasted thereon generates a strain.

[0045] A second coupler is used for beam combination, a first input end is connected to the circulator to receive the sensing signal, a second input end is connected to the first coupler to receive the reference signal, and an output end is connected to a balanced photoelectric detector.

[0046] And a balanced photoelectric detector and an oscilloscope are used to convert the optical signal into an electrical signal and collect the frequency domain distortion discrete beat frequency signal.

[0047] Compared with the prior art, the present application has the following beneficial effects:

[0048] 1. Compared with the prior art, the anti-distortion distributed optical fiber sensing method and system based on array phase spectrum demodulation proposed by the present application uses an ultra-weak reflection fiber Bragg grating array as a sensing optical fiber, generates a series of reflection peaks with a signal-to-noise ratio RBS higher than that of a RBS, forms a reflection peak sequence, increases the spectral amplitude of the beat frequency signal, provides a basis for realizing reflection peak sequence tracking and Doppler frequency shift and other frequency domain distortion suppression, and can realize multi-point quasi-distributed demodulation.

[0049] 2. The anti-distortion distributed optical fiber sensing method and system based on array phase spectrum demodulation proposed by the present application uses the linear phase frequency relationship in the main lobe of the frequency spectrum of the sensing signal after being intercepted by a symmetric window function and subjected to FFT processing, and the spectral centroid theorem of adjacent frequency points, to give a clear interpolation calculation formula. Without the need for time domain zero padding, only a limited amount of arithmetic operations are needed to break the limitation of frequency resolution, reduce the amount of calculation caused by spectral refinement, realize high-precision phase extraction and correct strain demodulation while effectively suppressing large Doppler frequency shifts, and have the potential for fast real-time demodulation.

[0050] 3. Experiments show that the anti-distortion distributed optical fiber sensing method based on array phase spectrum demodulation proposed in the application can inhibit Doppler frequency shift, restore signal fidelity, realize correct strain demodulation, and greatly reduce demodulation time. BRIEF DESCRIPTION OF DRAWINGS

[0051] The accompanying drawings, which are included to provide a further understanding of the application and are incorporated in and constitute a part of this application, illustrate embodiments of the application and together with the description serve to explain the application. In the drawings:

[0052] Figure 1 The anti-distortion distributed optical fiber sensing method based on array phase spectrum demodulation proposed in the embodiment of the application has the flow chart shown in the figure;

[0053] Figure 2 The amplitude and the corresponding phase diagram under different distortion conditions in the embodiment of the application; wherein, (a) corresponds to (d), (b) corresponds to (e), and (c) corresponds to (f);

[0054] Figure 3 The anti-distortion distributed optical fiber sensing system structure schematic diagram based on array phase spectrum demodulation proposed in the embodiment of the application has the structure schematic diagram shown in the figure;

[0055] Figure 4 The three-dimensional strain demodulation result comparison diagram of the traditional phase demodulation method and the amplitude interpolation method of the present scheme in the embodiment of the application;

[0056] Figure 5 The demodulation time comparison diagram of the time domain zero padding method and the amplitude interpolation method of the present scheme when demodulating the same data in the embodiment of the application.

[0057] Reference signs

[0058] 1 - frequency domain distortion discrete beat frequency signal acquisition module, 10 - probe light generation unit, 11 - first coupler, 12 - circulator, 13 - optical fiber to be measured, 14 - first signal generator, 15 - second signal generator, 16 - second coupler, 17 - balanced photodetector, 18 - piezoelectric transducer, 19 - piezoelectric ceramic stack, 20 - oscilloscope;

[0059] 2 - intercepting module, 3 - transforming module, 4 - correction factor array calculation module, 5 - array phase spectrum calculation module, 6 - demodulation module. DETAILED DESCRIPTION

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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:

[0065] S1. Acquire frequency-domain distorted discrete beat frequency signals;

[0066] As one possible implementation, the frequency-domain distortion discrete beat frequency signal is generated in the following way:

[0067] S10. The light frequency-modulated continuous wave with a preset sweeping range and a preset sweeping period is used as the probe light to enter the non-equilibrium Mach-Zehnder interferometer for sensing; the to-be-detected optical fiber in the OFDR sensing system sequentially includes a single-mode optical fiber wound on a piezoelectric transducer and an ultra-weak reflective fiber grating array in the propagation direction of the probe light, and one grating in the grating array is pasted on a piezoelectric ceramic stack;

[0068] S11. A first signal driving the piezoelectric transducer is applied to make the single-mode optical fiber produce Doppler frequency shift; and a second signal driving the piezoelectric ceramic stack is applied to make the grating pasted thereon produce strain;

[0069] S12. A photodetector and an oscilloscope are applied to collect the frequency-domain distortion discrete beat frequency signals of the back Rayleigh scattering and grating reflection after the to-be-detected optical fiber.

[0070] S2. The frequency-domain distortion discrete beat frequency signals are continuously intercepted using a symmetric window function with a window length of a single measurement period of the OFDR system and are subjected to Fourier transform to obtain the frequency spectrum corresponding to the intercepted frequency-domain distortion discrete beat frequency signals;

[0071] As a possible implementation manner, the frequency-domain distortion discrete beat frequency signals are intercepted using any one of a rectangular window function or a Hanning window function.

[0072] S3. A sequence of reflection peaks in the obtained frequency spectrum is tracked and identified, and a maximum amplitude of a maximum amplitude frequency point of a first reflection peak in a first measurement period is extracted , a maximum amplitude of a second reflection peak in a second measurement period is extracted , and a maximum amplitude of a third reflection peak in a third measurement period is extracted . As a possible implementation manner, the frequency-domain distortion discrete beat frequency signals are intercepted using a rectangular window function. As a possible implementation manner, the frequency-domain distortion discrete beat frequency signals are intercepted using a Hanning window function. As a possible implementation manner, the frequency-domain distortion discrete beat frequency signals are intercepted using a rectangular window function, and the frequency correction factor is obtained by the following method:

[0073] As a possible implementation manner, the frequency-domain distortion discrete beat frequency signals are intercepted using a Hanning window function, and the frequency correction factor is obtained by the following method: Figure 2

[0074]

[0075] wherein, f represents the frequency correction factor corresponding to the first reflection peak in the first measurement period; and f represents the frequency correction factor corresponding to the second reflection peak in the second measurement period. ​​​​​​​the maximum amplitude of the first reflection peak in the first measurement period, the second largest amplitude of the two adjacent frequency points of the maximum amplitude of the first reflection peak in the first measurement period, the corresponding frequency point, the first measurement period, the maximum amplitude of the first reflection peak in the first measurement period, the second largest amplitude of the two adjacent frequency points of the maximum amplitude of the first reflection peak in the first measurement period.

[0076] Referring to Figure 2 , as a possible implementation manner, when the frequency domain distortion discrete beat frequency signal is intercepted using the Hann window function, the frequency correction factor is obtained by the following manner:

[0077]

[0078] wherein, the frequency correction factor corresponding to the first reflection peak in the first measurement period; the frequency correction factor corresponding to the first reflection peak in the first measurement period; the maximum amplitude of the first reflection peak in the first measurement period, the corresponding frequency point, the first measurement period, the maximum amplitude of the first reflection peak in the first measurement period, the corresponding frequency point, the first measurement period, the maximum amplitude of the first reflection peak in the first measurement period, the second largest amplitude of the two adjacent frequency points of the maximum amplitude of the first reflection peak in the first measurement period.

[0079] S4. Based on the first measurement period, the maximum amplitude frequency point of the first reflection peak, the phase value at the maximum amplitude frequency point and the corresponding frequency correction factor , the corresponding corrected phase value is obtained by calculation, and all the corrected phase values constitute a corrected array phase spectrum; As a possible implementation manner, the corrected phase value is obtained by the following manner:

[0080]

[0081]

[0082] wherein, the phase value at the maximum amplitude frequency point of the first reflection peak in the first measurement period; the phase value at the maximum amplitude frequency point of the first reflection peak in the first measurement period; the corrected phase calculated in combination with the corresponding frequency correction factor

[0083] ​​​​S5. The corrected array phase spectrum is differentially calculated on the time axis and the distance axis to obtain a corrected phase change value array, that is, a correction signal for suppressing frequency domain distortion, and a corrected demodulation strain can be further obtained through a phase strain relationship to realize quasi-distributed demodulation of multiple points.

[0084] As a possible implementation manner, the differential calculation of the corrected array phase spectrum on the time axis and the distance axis specifically includes:

[0085] On the distance axis, the corrected phase value corresponding to each reflection peak of the reflection peak sequence is discretely differentially calculated.

[0086] On the time axis, the corrected phase value of all measurement periods of the reflection peak sequence is subtracted by the corrected phase value of the first measurement period.

[0087] Compared with the prior art, the anti-distortion distributed fiber sensing method based on array phase spectrum demodulation provided by the application uses an ultra-weak reflection fiber Bragg grating array as a sensing fiber to generate a series of reflection peaks with a higher signal-to-noise ratio than RBS, form a reflection peak sequence, increase the spectral amplitude of the beat frequency signal, and provide a basis for realizing reflection peak sequence tracking and suppression of frequency domain distortion such as Doppler frequency shift, so that quasi-distributed demodulation of multiple points can be realized.

[0088] In a second aspect, the embodiments of the application provide an anti-distortion distributed fiber sensing system based on array phase spectrum demodulation, referring to Figure 3 , comprising:

[0089] The frequency domain distortion discrete beat frequency signal acquisition module 1 is configured to acquire the frequency domain distortion discrete beat frequency signal.

[0090] As a possible implementation manner, the frequency domain distortion discrete beat frequency signal acquisition module 1 specifically includes:

[0091] The probe light generation unit 10 is configured to generate an optical frequency modulation continuous wave with a preset sweep frequency range and a preset sweep frequency period.

[0092] The first coupler 11 is configured to split light, and the input end is connected to the probe light generation unit 10, the first output end is connected to the circulator to form a sensing arm, and the second output end is connected to the second coupler to form a reference arm.

[0093] The circulator 12 is configured to input the split probe light to the fiber to be measured, and configured to sequentially transmit the sensing optical signal composed of the backward Rayleigh scattering and the grating reflection frequency domain distortion discrete beat frequency signal to the second coupler and the photodetector.

[0094] The to-be-measured optical fiber 13 includes, in the propagation direction of the probe light, a single-mode optical fiber wound on the piezoelectric transducer 18, and an ultra-weak reflective fiber grating array, one grating in the grating array being pasted on the piezoelectric ceramic stack 19;

[0095] As a possible implementation, the length of the single-mode optical fiber is 60 m; the grating interval is 2.5 cm, and the grating length is 1.2 m.

[0096] 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;

[0097] 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 pasted thereon generates a strain;

[0098] The second coupler 16 is used for beam combination, the first input end is connected to the circulator to receive a sensing signal, the second input end is connected to the first coupler to receive a reference signal, and the output end is connected to the balanced photodetector;

[0099] The balanced photodetector 17 and the oscilloscope 20 convert the optical signal into an electrical signal and collect a frequency-domain distortion discrete beat frequency signal.

[0100] The intercepting module 2 continuously intercepts the frequency-domain distortion discrete beat frequency signal using a symmetric window function with a window length of a single measurement period of the OFDR system;

[0101] The transforming module 3 performs Fourier transform on the intercepted frequency-domain distortion discrete beat frequency signal to obtain a frequency spectrum corresponding to the intercepted frequency-domain distortion discrete beat frequency signal;

[0102] The correction factor array calculation module 4 tracks and identifies a reflection peak sequence in the obtained frequency spectrum, and extracts a maximum amplitude of a maximum amplitude frequency point of a maximum amplitude of a first reflection peak in a first measurement period, a second reflection peak in a second measurement period, a maximum amplitude of a maximum amplitude frequency point of a third reflection peak in a third measurement period, a second maximum amplitude of two frequency points adjacent to the maximum amplitude frequency point of the third reflection peak in the third measurement period, and a second maximum amplitude of two frequency points adjacent to the maximum amplitude frequency point of the third reflection peak in the third measurement period, according to a spectral centroid theorem of the symmetric window function, interpolates a frequency correction factor between a frequency point corresponding to the maximum amplitude of the first reflection peak in the first measurement period and a true peak frequency, and calculates the frequency correction factor between the frequency point corresponding to the maximum amplitude of the first reflection peak in the first measurement period and the true peak frequency, and calculates the frequency correction factor between the frequency point corresponding to the maximum amplitude of the first reflection peak in the first measurement period and the true peak frequency, to obtain a frequency correction factor array.

[0103] The array phase spectrum calculation module 5 calculates a phase spectrum of the frequency correction factor array based on the first measurement period and the first reflection peak, the second measurement period and the second reflection peak, and the third measurement period and the third reflection peak. phase values of the array phase spectrum and corresponding frequency correction factors , the corresponding corrected phase values are calculated , the corrected array phase spectrum is composed of all the corrected phase values;

[0104] and the demodulation module 6, the corrected array phase spectrum is differentially calculated on the time axis and the distance axis to obtain the corrected phase change value array, which is the correction signal for suppressing the frequency domain distortion, and the corrected demodulation strain can be further obtained through the phase strain relationship to realize the quasi-distributed demodulation of multiple points.

[0105] The anti-distortion distributed optical fiber sensing method and system based on array phase spectrum demodulation provided by the application utilize the linear phase frequency relationship in the spectrum main lobe of the sensing signal intercepted by the symmetric window function and subjected to FFT processing and the spectrum centroid theorem of adjacent frequency points to give an explicit interpolation calculation formula, so that the limitation of frequency resolution can be broken and the operation amount caused by spectrum refinement can be reduced only by adding limited arithmetic operations without time domain zero padding, while effectively suppressing larger Doppler frequency shifts, high-precision phase extraction and correct strain demodulation are realized, and the potential of fast real-time demodulation is possessed.

[0106] Referring to Figure 3 In order to more clearly introduce the sensing and demodulation principles of the anti-distortion distributed optical fiber sensing method and system based on array phase spectrum demodulation provided by the application, a linear sweep signal generated by an arbitrary waveform generator is used to drive a phase modulator to modulate the output of a fiber laser with a wavelength of 1554.4 nm, and then the optical frequency modulation continuous wave with a sweep range of 12.5 GHz and a sweep period of 1 ms obtained through a programmable optical filter is used as probe light to enter an OFDR interferometer for sensing, and a balanced photodetector 17 is used to receive the coherent beat frequency signal. The optical fiber to be measured of the OFDR includes a single-mode optical fiber with a length of 60 m wound on a piezoelectric transducer 18 and an ultra-weak reflective fiber grating array with a grating interval of 2.5 cm and a length of 1.2 m, and one of the gratings is pasted on a piezoelectric ceramic stack 19. A high-frequency vibration signal is applied to the piezoelectric transducer 18 through a first signal generator 14 to drive the piezoelectric transducer 18 to make the single-mode optical fiber segment produce Doppler frequency shift, and a low-frequency vibration signal is applied to the piezoelectric ceramic stack 19 through a second signal generator 15 to drive the gratings to produce strain. The frequency domain distortion discrete beat frequency signal collected by the balanced photodetector 17 and an oscilloscope 20 is subjected to FFT calculation, the maximum amplitude and the adjacent second maximum amplitude of the reflection peak sequence in the FFT spectrum at all measurement periods are tracked and identified, the frequency correction factor array and the corrected array phase spectrum are calculated, the corrected phase change value array and the corrected demodulation strain are differentially calculated, and high-fidelity demodulation is realized. The demodulation result of the traditional phase demodulation method is as shown in Figure 4As shown in (a), the Doppler frequency shift and other frequency domain distortion phenomena make signal demodulation fail, and correct strain cannot be obtained, resulting in a large error; the demodulation result of the application is as shown in (b). Figure 4 As shown in (a), the Doppler frequency shift and other frequency domain distortion phenomena make signal demodulation fail, and correct strain cannot be obtained, resulting in a large error; the demodulation result of the application is as shown in (b).

[0107] Referring to Figure 5 A comparison chart of demodulation time required for demodulating the same data by using the existing time domain zero padding method and the amplitude interpolation method of the present application is shown in the figure, and it can be seen from the figure that the present application greatly reduces the demodulation time and improves the operation efficiency.

[0108] Although the present application is described herein in conjunction with various embodiments, other variations of the disclosed embodiments can be understood and implemented by those skilled in the art with reference to the drawings, the disclosure, and the appended drawings. In the specification, the word "comprising" does not exclude other components or steps, and "a" or "one" does not exclude a plurality. A single processor or other unit can implement several functions listed in the specification. Some measures are described in different embodiments, but this does not mean that these measures cannot be combined to produce good results.

[0109] Although the present application is described in conjunction with specific features and embodiments thereof, it is obvious that various modifications and combinations can be made without departing from the spirit and scope of the present application. Accordingly, the specification and drawings are merely illustrative of the present application, and any and all modifications, variations, combinations or equivalents that fall within the scope of the present application are intended to be embraced by the present application. Obviously, those skilled in the art can make various modifications and variations to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application fall within the scope of the present application and its equivalent technology, the present application is intended to include these modifications and variations.

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 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 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 photodetector. In addition, photodetectors and oscilloscopes 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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