OFDR system vibration detection method based on reference vibration compensation

Abstract

The invention belongs to the technical field of distributed optical fiber sensing, and discloses an OFDR system vibration detection method based on reference vibration compensation. Comprising the following steps: collecting a reference signal under a vibration-free condition, and then enabling a vibration generator to work to collect a measurement signal; dividing the reference signal and the measurement signal into N equal parts in a time domain, wherein each part is a segmented signal; performing fast Fourier transform on the first segment signals of the reference signal and the measurement signal, and segmenting the first segment signals in a distance domain; performing inverse Fourier transform on the segmented signals at the same position and then performing cross-correlation operation; repeating the above steps on the remaining segmented signals, and splicing cross-correlation results to obtain a three-dimensional vibration signal; obtaining the measurement frequency of the reference vibration according to the three-dimensional vibration signal; the actual time resolution is calculated, the time axis of the three-dimensional vibration signal is calibrated, and then the time domain waveform signal of the vibration to be measured is subjected to FFT conversion to obtain the accurate frequency of the vibration to be measured.

Description

Technical Field

[0001] The present invention belongs to the technical field of distributed optical fiber sensing, and in particular is a vibration detection method for an OFDR system based on reference vibration compensation. Background Art

[0002] Vibration detection is of great significance in areas such as national defense security, infrastructure monitoring, and industrial equipment operation and maintenance. For example, in perimeter security, real-time perception of intrusion behavior is required; in the aerospace field, high-precision assessment of the structural health of aircraft is required. The current mainstream distributed fiber optic vibration sensing technologies mainly include distributed acoustic sensing (DAS) and optical frequency domain reflectometry (OFDR). DAS can achieve ultra-long-distance monitoring of tens or even hundreds of kilometers, but its spatial resolution is usually at the meter level, and its dynamic range and signal-to-noise ratio are limited, making it difficult to meet the needs of high-precision vibration detection (such as precision equipment monitoring, local structural health diagnosis, laboratory research, etc.). Therefore, OFDR has irreplaceable advantages. Its high-resolution characteristics enable it to accurately identify the position and frequency characteristics of tiny vibration events, further expanding its application potential in high-end fields such as aerospace, precision manufacturing, and scientific research experiments.

[0003] The principle of OFDR is as follows: the light emitted by the light source is a narrow-linewidth laser with linear frequency tuning (in practice, it is generally wavelength linear tuning, so there is a nonlinear effect). After passing through the coupler, it is divided into two beams, one as the reference light and the other into the test fiber. Rayleigh scattering occurs at any position in the test fiber (as the signal light). After this part of the light returns, it interferes with the reference light and outputs an electrical signal called the beat signal. At this time, the frequency of the signal and the position of the test fiber are linear functions. Therefore, through Fourier transform, the beat signal is converted to the frequency domain, which can be equivalent to converting it to the distance domain, thereby realizing distributed fiber optic sensing. Due to the nonlinear effect of the laser, hardware compensation is needed to remove the nonlinearity of the laser in order to ensure measurement accuracy. However, in OFDR vibration measurement experiments based on hardware compensation, the stability of the laser sweep speed will affect the calculation of the time resolution. If the sweep speed cannot be accurately given, the time axis of the vibration information will be deviated, which will eventually lead to deviation in the vibration frequency measurement.

[0004] Therefore, how to solve the nonlinear effect of the laser has become an important issue in improving the accuracy of OFDR system measurements. Summary of the Invention

[0005] In order to solve the problem that when measuring vibration in the existing OFDR system based on hardware compensation, the time resolution cannot be accurately calculated due to the unstable laser sweep speed, which in turn causes deviation in vibration measurement, the present invention proposes a vibration detection method for an OFDR system using reference vibration compensation to achieve accurate measurement of vibration along the optical fiber.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is: a vibration detection method of an OFDR system based on reference vibration compensation, comprising the following steps:

[0007] Step 1: A vibration generator is placed at any position on the sensing fiber of the OFDR system to apply a reference vibration signal to the sensing fiber. Under vibration-free conditions, the OFDR system collects the reference signal, then operates the vibration generator and collects the measurement signal through the OFDR system. The reference signal and the measurement signal are collected at the same time.

[0008] Step 2: Split the reference signal and the measurement signal into N equal parts in the time domain to obtain N segmented signals;

[0009] Step 3: Extract the first segmented signals of the reference signal and the measurement signal, perform fast Fourier transform on each of them to obtain a first range domain reference signal and a first range domain measurement signal, and then divide the first range domain reference signal and the first range domain measurement signal into M equal parts;

[0010] Step 4: performing inverse Fourier transform on the segmented signals at the same position of the first distance domain reference signal and the first distance domain measurement signal to obtain respective wavelength domain data;

[0011] Step 5: performing a cross-correlation operation on the wavelength domain data at each position corresponding to the reference signal and the measurement signal;

[0012] Step 6: Repeat steps 2 to 5 for the remaining N-1 groups of segmented signals, and concatenate the cross-correlation results obtained from each group of segmented signals to obtain a three-dimensional vibration signal;

[0013] Step 7: Find and extract the time domain waveform signal of the reference vibration in the distance domain of the three-dimensional vibration signal, perform FFT transformation to the frequency domain, and obtain the measurement frequency f0 of the reference vibration;

[0014] Step 8: Calculate the actual time resolution. Calibrate the time axis of the obtained three-dimensional vibration signal using the actual time resolution. The calculation formula is:

[0015]

[0016] in, represents the actual time resolution, represents the actual applied frequency of the reference vibration signal, f0 represents the measured frequency of the reference vibration; Δt represents the time resolution corresponding to the sweep speed set by the laser;

[0017] Step 9: Extract the time domain waveform signal of the vibration to be measured in the distance domain of the three-dimensional vibration signal after time axis calibration, and perform FFT transformation to obtain the accurate frequency of the vibration to be measured.

[0018] The calculation formula for the time resolution corresponding to the sweep speed set by the laser is:

[0019]

[0020] Where c represents the speed of light in vacuum, N sample represents the total number of sampling points, L represents the length of the delay fiber in the Mach-Zehnder interferometer, N represents the number of equal parts of the reference signal and the measurement signal, n' represents the refractive index of the fiber, and γ represents the set laser sweep speed.

[0021] The vibration generator is piezoelectric ceramic.

[0022] The vibration generator is arranged at the head end of the sensing optical fiber.

[0023] The OFDR system includes a laser. The continuous laser output by the laser is divided into two beams by a first coupler, one of which is divided into a first beam and a second beam by a second coupler. The first beam directly enters a third coupler, and the second beam enters a sensing optical fiber through a circulator. The back Rayleigh scattering signal generated in the sensing optical fiber enters the third coupler after passing through the circulator. The interference signal generated by the interaction with the first beam is converted into an electrical signal by a first balanced photodetector, and then collected by an acquisition card and sent to a computer; the other beam is incident on an unbalanced Mach-Zehnder interferometer. The optical signal output by the Mach-Zehnder interferometer is received by a second balanced photodetector, converted into an electrical signal, and sent to the acquisition card, providing a trigger signal for the acquisition card.

[0024] The laser is a tunable laser.

[0025] The first coupler is a 20:80 1×2 coupler, wherein 20% of the first coupler is incident on the unbalanced Mach-Zehnder interferometer.

[0026] The second coupler is a 50:50 1×2 coupler.

[0027] The third coupler is a 50:50 2×2 coupler.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] The present invention provides an OFDR system vibration detection method based on reference vibration compensation. The method accurately calculates the actual average sweep frequency speed of the laser within a sampling period through FFT transformation, and corrects the time axis of the vibration signal according to the actual sweep frequency speed of the laser. This solves the problem of deviation in vibration frequency measurement based on external clock sampling due to unstable laser sweep frequency speed when measuring vibration in existing OFDR systems. The method can improve the measurement accuracy of vibration signals of the OFDR system and realize accurate measurement of vibration along the optical fiber. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 A schematic flow chart of a vibration detection method for an OFDR system based on reference vibration compensation provided by an embodiment of the present invention;

[0031] Figure 2 Schematic diagram of the structure of the OFDR system used in the embodiment of the present invention;

[0032] Figure 3 Schematic diagram of the principle of using reference vibration to correct vibration measurement in an embodiment of the present invention;

[0033] Among them, 1-tunable laser; 2-first coupler; 3-unbalanced Mach-Zehnder interferometer; 4-second balanced photodetector; 5-second coupler; 6-circulator; 7-third coupler; 8-first balanced photodetector; 9-acquisition card; 10-computer; 11-vibration generator; 12-sensing optical fiber; 13-piezoelectric ceramic. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, not all the embodiments; based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0035] like Figure 1 and Figure 3 As shown, an embodiment of the present invention provides an OFDR system vibration detection method based on reference vibration compensation, comprising the following steps:

[0036] Step 1: A vibration generator is set at any position on the sensing fiber of the OFDR system to apply a reference vibration signal with a known vibration frequency to the sensing fiber; under vibration-free conditions, the OFDR system collects the reference signal, and then the vibration generator is activated to collect the measurement signal through the OFDR system; the reference signal and the measurement signal are collected at the same time.

[0037] like Figure 2 As shown, in this embodiment, the OFDR system includes a laser 1. The continuous laser output by the laser 1 is divided into two beams by a first coupler, one of which is divided into a first beam and a second beam by a second coupler 5. The first beam directly enters the third coupler 7, and the second beam enters the sensing fiber 12 through a circulator 6. The back Rayleigh scattered signal generated in the sensing fiber 12 passes through the circulator 6 and enters the third coupler 7. The interference signal generated by the interaction with the first beam is converted into an electrical signal by a first balanced photodetector 8 and then collected by an acquisition card 9 and sent to a computer 10; the other beam is incident on an unbalanced Mach-Zehnder interferometer 3. The optical signal output by the Mach-Zehnder interferometer 3 is received by the second balanced photodetector 4 and then converted into an electrical signal and sent to the acquisition card 9, providing a trigger signal for the acquisition card 9.

[0038] Specifically, in this embodiment, the vibration generator 11 is piezoelectric ceramic.

[0039] Specifically, in this embodiment, the vibration generator 11 is arranged at the head end.

[0040] Specifically, in this embodiment, the laser 1 is a tunable laser.

[0041] Specifically, in this embodiment, the first coupler is a 20:80 1×2 coupler, wherein 20% of the light is incident on the unbalanced Mach-Zehnder interferometer 3 .

[0042] Specifically, in this embodiment, the second coupler 5 is a 50:50 1×2 coupler.

[0043] Specifically, in this embodiment, the third coupler is a 2×2 coupler with a ratio of 750:50.

[0044] In addition, in this embodiment, a piezoelectric ceramic 13 is provided at the end of the sensing optical fiber 12 for simulating the vibration to be measured.

[0045] Step 2: Split the reference signal and the measurement signal into N equal parts in the time domain, each part being a segmented signal, to obtain the first to Nth segmented reference signals and the first to Nth segmented measurement signals.

[0046] Specifically, the reference signal and the measurement signal can be segmented in the time domain using a window of a certain size. Assuming that each segmented signal has n data points, the duration of n data points is Δt, and the laser sweep speed is the set sweep speed γ, the time resolution of the system is:

[0047]

[0048] Where T represents the sampling time of the reference signal and the measurement signal, N represents the number of equal parts of the reference signal and the measurement signal, c represents the speed of light in vacuum, and N sample represents the total number of sampling points, L represents the length of the delay fiber in the Mach-Zehnder interferometer, n' represents the refractive index of the fiber, and γ represents the set laser sweep speed.

[0049] Step 3: Perform a fast Fourier transform on the first segmented reference signal and the first segmented measurement signal to obtain a first range domain reference signal and a first range domain measurement signal. Then, the first range domain reference signal and the first range domain measurement signal are divided into M equal parts. Assume that each part has m data points and the continuous distance is Δx. Δx is the spatial resolution of the system, which is expressed as:

[0050]

[0051] Wherein, L represents the length of the delay fiber in the Mach-Zehnder interferometer, and n represents the number of data points of each segment signal.

[0052] Step 4: Perform inverse Fourier transform on the segmented signals at the same position of the first distance domain reference signal and the first distance domain measurement signal to obtain local spectrum information of the reference signal and the test signal, that is, their respective wavelength domain data.

[0053] Step 5: Perform a cross-correlation operation on the wavelength domain data at each position corresponding to the reference signal and the measurement signal. After the cross-correlation operation, there will be a shift in the cross-correlation peak at the data where vibration information exists.

[0054] Step 6: Repeat steps 2 to 5 for the remaining N-1 groups of segmented signals, and concatenate the cross-correlation results obtained from each group of segmented signals to obtain a three-dimensional vibration signal.

[0055] At this time, the temporal resolution and spatial resolution are respectively the above equations (1) and (2).

[0056] Step 7: Find and extract the time domain waveform signal of the reference vibration in the distance domain of the three-dimensional vibration signal, perform FFT transformation to the frequency domain, and obtain the frequency f0 of the reference vibration.

[0057] Step 8: Calculate the time resolution after calibration, and calibrate the time axis of the obtained three-dimensional vibration signal using the time resolution after calibration.

[0058] The time domain waveform of the reference vibration is found and extracted in the distance domain of the three-dimensional vibration information, and FFT is performed to transform it into the frequency domain to obtain the measured frequency f0 of the reference vibration. The actual applied frequency of the reference vibration is Since the laser sweep speed cannot be strictly swept according to the set sweep speed, the sweep speed actually fluctuates. If the time resolution obtained according to the laser sweep speed γ set is different from the actual time resolution, and the time axis is different from the actual time resolution, then the measured frequency f0 of the reference vibration obtained after FFT transformation is different from the actual time resolution. There is a deviation. Therefore, obtaining the actual equivalent sweep frequency speed is crucial for vibration demodulation. The following is the principle of obtaining the actual average sweep frequency speed in this embodiment.

[0059] Assume that the total number of sampling points is N sample , the sweep speed fluctuates in one sampling period, so N sample The following relationship is satisfied:

[0060]

[0061] Where T is the total acquisition time of the reference and measurement signals, and are the actual average sampling rate and average sweep speed of the entire sampling period, respectively. γ is the set laser sweep speed, L is the length of the delay fiber in the Mach-Zehnder interferometer, and n′ is the refractive index of the fiber. b (t) is the beat frequency of the Mach-Zehnder interferometer, which is also the external clock frequency of the acquisition card 9, and γ(t) is the actual scanning speed of the laser.

[0062]

[0063] Where n0 represents the number of data points included in one time period of the reference vibration on the time axis, represents the actual time resolution, and Δt represents the time resolution corresponding to the sweep speed set by the laser. Indicates the actual average sweep speed obtained by calibration.

[0064] visible The relationship with f0 satisfies:

[0065]

[0066] Therefore, the sweep speed γ set by the laser is determined by the measured frequency f0 of the reference vibration and the actual applied frequency The actual average sweep speed of the laser can be calculated Then we can get a more accurate actual time resolution By calibrating the time axis of the obtained three-dimensional vibration signal according to the actual time resolution, a vibration waveform with accurate time information can be obtained, and then a more accurate vibration frequency can be obtained.

[0067] Therefore, the actual time resolution is calculated as:

[0068]

[0069] in, represents the actual time resolution, represents the actual applied frequency of the reference vibration signal, f0 represents the measured frequency of the reference vibration; Δt represents the time resolution corresponding to the sweep speed set by the laser.

[0070] Step 9: Extract the time domain waveform signal of the vibration to be measured in the distance domain of the three-dimensional vibration signal after time axis calibration, perform FFT transformation to the frequency domain, and obtain the accurate frequency of the vibration to be measured.

[0071] The present invention provides an OFDR system vibration detection method based on reference vibration compensation. The method accurately calculates the actual average sweep frequency speed of the laser within a sampling period through FFT transformation, and corrects the time axis of the vibration signal according to the actual sweep frequency speed of the laser. This solves the problem of deviation in vibration frequency measurement based on external clock sampling due to unstable laser sweep frequency speed when measuring vibration in existing OFDR systems. The method can improve the measurement accuracy of vibration signals of the OFDR system and realize accurate measurement of vibration along the optical fiber.

[0072] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A vibration detection method for an OFDR system based on reference vibration compensation, characterized in that: The following steps are involved: Step 1: a vibration generator (11) is set at any position of the sensing optical fiber of the OFDR system to apply a reference vibration signal to the sensing optical fiber; under a vibration-free condition, a reference signal is collected by the OFDR system, and then the vibration generator is operated to collect a measurement signal by the OFDR system; the reference signal and the measurement signal are collected at the same time; Step 2: Split the reference signal and the measurement signal into N equal parts in the time domain to obtain N segmented signals; Step 3: Extract the first segmented signals of the reference signal and the measurement signal, perform fast Fourier transform on each of them to obtain a first range domain reference signal and a first range domain measurement signal, and then divide the first range domain reference signal and the first range domain measurement signal into M equal parts; Step 4: performing inverse Fourier transform on the segmented signals at the same position of the first distance domain reference signal and the first distance domain measurement signal to obtain respective wavelength domain data; Step 5: performing a cross-correlation operation on the wavelength domain data at each position corresponding to the reference signal and the measurement signal; Step 6: Repeat steps 2 to 5 for the remaining N-1 groups of segmented signals of the reference signal and the measurement signal in three dimensions, and concatenate the cross-correlation results obtained from each group of segmented signals to obtain a three-dimensional vibration signal; Step 7: Find and extract the time domain waveform signal of the reference vibration in the distance domain of the three-dimensional vibration signal, perform FFT transformation to the frequency domain, and obtain the measurement frequency f0 of the reference vibration; Step 8: Calculate the actual time resolution. Calibrate the time axis of the obtained three-dimensional vibration signal using the actual time resolution. The calculation formula is: in, represents the actual time resolution, represents the actual applied frequency of the reference vibration signal, f0 represents the measured frequency of the reference vibration; Δt represents the time resolution corresponding to the sweep speed set by the laser; Step 9: Extract the time domain waveform signal of the vibration to be measured in the distance domain of the three-dimensional vibration signal after time axis calibration, and perform FFT transformation to obtain the accurate frequency of the vibration to be measured.

2. The OFDR system vibration detection method based on reference vibration compensation according to claim 1, wherein: The calculation formula for the time resolution corresponding to the sweep speed set by the laser is: Where c represents the speed of light in vacuum, N sample represents the total number of sampling points, L represents the length of the delay fiber in the Mach-Zehnder interferometer, N represents the number of equal parts of the reference signal and the measurement signal, n' represents the refractive index of the fiber, and γ represents the set laser sweep speed.

3. The OFDR system vibration detection method based on reference vibration compensation according to claim 1, wherein: The vibration generator (11) is a piezoelectric ceramic.

4. The OFDR system vibration detection method based on reference vibration compensation according to claim 1, wherein: The vibration generator (11) is arranged at the head end of the sensing optical fiber.

5. The OFDR system vibration detection method based on reference vibration compensation according to claim 1, wherein: The OFDR system comprises a laser (1), wherein the continuous laser output by the laser (1) is divided into two beams by a first coupler, wherein one beam is divided into a first beam and a second beam by a second coupler (5), the first beam directly enters a third coupler (7), and the second beam enters a sensing optical fiber (12) by a circulator (6), a backward Rayleigh scattering signal generated in the sensing optical fiber (12) passes through the circulator (6) and enters the third coupler (7), an interference signal generated by the interaction with the first beam is converted into an electrical signal by a first balanced photoelectric detector (8), and then collected by an acquisition card (9) and sent to a computer (10); the other beam is incident on an unbalanced Mach-Zehnder interferometer (3), the optical signal output by the Mach-Zehnder interferometer (3) is received by a second balanced photoelectric detector (4), converted into an electrical signal, and sent to an acquisition card (9), providing a trigger signal for the acquisition card (9).

6. The OFDR system vibration detection method based on reference vibration compensation according to claim 5, characterized in that: The laser (1) is a tunable laser.

7. The OFDR system vibration detection method based on reference vibration compensation according to claim 5, characterized in that: The first coupler is a 20:80 1×2 coupler, wherein 20% of the first coupler is incident on the unbalanced Mach-Zehnder interferometer (3).

8. The OFDR system vibration detection method based on reference vibration compensation according to claim 5, characterized in that: The second coupler (5) is a 50:50 1×2 coupler.

9. The OFDR system vibration detection method based on reference vibration compensation according to claim 5, characterized in that: The third coupler (7) is a 50:50 2×2 coupler.

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