A method, apparatus and storage device for suppressing fading interference
By setting the threshold range using Fourier transform and rotating vector summation, interference fading in the Φ-OTDR system is eliminated, solving the problems of sensing quality and detection distance limitations, and achieving more efficient phase demodulation and reduced costs.
Patent Information
- Application Number
- CN202310647092.1
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2023-06-01
- Publication Date
- 2025-12-09
- Estimated Expiration
- 2043-06-01
AI Technical Summary
In existing Φ-OTDR systems, interference fading phenomena lead to phase demodulation errors, affecting sensing quality, limiting detection distance, and increasing system costs. Existing elimination methods are complex or computationally intensive, making them difficult to apply effectively in practical engineering.
Fourier transform is used to extract the main lobe and first-order side lobes to form three sub-bands. By using the rotation vector summation method and amplitude demodulation to set the threshold range, erroneous phase points are eliminated, the phase image is corrected, and the number of calculation points is reduced to speed up demodulation.
It effectively eliminates interference fading, improves sensing accuracy, extends detection range, reduces system cost, reduces computational load, and increases computational speed.
Smart Images
Figure CN116818082B_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The present application relates to the field of optical fiber sensing technology, and in particular to a method and device for suppressing interference fading and a storage device. BACKGROUND
[0002] In 1976, Barnoski and Jensen proposed the optical time domain reflectometer (OTDR), which is the earliest device using distributed optical fiber sensing technology. OTDR uses a non-coherent light source to measure the length, attenuation and breakpoints of the optical fiber, and uses the characteristics of backscattered Rayleigh light to locate faults. Its structure is simple, the cost is low, and it has the advantages of non-destructive testing, so it is widely used in communication and sensing links. However, with the development and progress of society, OTDR has been unable to meet the detection needs in many aspects. For this reason, researchers have proposed a distributed sensing system of phase-sensitive optical time domain reflectometer (Φ-OTDR). This emerging technology not only expands the measurable physical quantities, but also improves the accuracy and range of measurement.
[0003] Φ-OTDR is a high-sensitivity distributed optical fiber sensing technology that uses a highly coherent narrow-linewidth laser as a light source and uses the interference phase of backscattered Rayleigh light to sense the fluctuations in the external environment. When the environment of the optical fiber changes in temperature or stress, the change in the intensity of the scattered light is not linearly related to the change in the environment, so the intensity demodulation type Φ-OTDR can only make qualitative measurements. Since the phase change of the scattered light is proportional to the change in the refractive index of the optical fiber caused by strain, the phase demodulation type Φ-OTDR can quantitatively recover various characteristics of the original disturbance signal. As one of the most sensitive distributed optical fiber sensing systems, Φ-OTDR has broad application prospects in the fields of quantitative monitoring of optical fiber sensors, detection of optical fiber damage, detection of pipeline leaks, etc.
[0004] In the Φ-OTDR system, a highly coherent laser is used as the light source. When this highly coherent light passes through the optical fiber, the Rayleigh backscattered light generated at different positions will overlap with each other. Within the coherence distance of the laser, these highly coherent Rayleigh backscattered lights will interfere with each other. This causes the photoelectric current signal output by the photoelectric detector to have strong amplitude fluctuations, and this phenomenon is called interference fading (coherent fading). Interference fading can cause phase demodulation errors, leading to distortion of the recovered disturbance signal and false alarms in practical applications. In actual engineering, it may have the following consequences: 1. Reduce the quality of sensing and produce false alarms during monitoring, affecting the efficiency of sensing. 2. Affect the sensing distance. Interference fading will increase with the increase of the transmission distance of light in the optical fiber, so it will limit the detection distance of the optical fiber sensor. 3. Increase the system cost. In order to overcome interference fading, it may be necessary to use higher quality optical fibers and more advanced equipment, which will increase the construction and operation cost of the system. Therefore, it is of great significance to solve the problem of interference fading.
[0005] In the prior art, methods for eliminating coherent fading include collecting scattered light signals of two orthogonal polarization states, synthesizing the intensities of the two polarization states to suppress polarization fading technology, frequency domain regulation anti-interference fading technology, etc. These methods can eliminate coherent fading through experiments, but also have some limitations: complex experimental device, complex algorithm, etc. Intra-frequency pulse extraction and rotation vector summation technology, single pulse spectrum extraction and combination interference fading suppression technology can well eliminate interference fading, but the calculation amount is large and the calculation speed is slow, so the application in practical engineering will increase the cost.
[0006] In order to make the Φ-OTDR system have higher accuracy, reduce data calculation amount, increase sensing distance and reduce working cost, the existing scheme needs to be further improved. SUMMARY
[0007] In view of the above problems and deficiencies, the present application provides a method, device and storage device for suppressing interference fading, which can well eliminate coherent fading and speed up the calculation speed.
[0008] A method for suppressing interference fading mainly comprises:
[0009] S1: Fourier transform is performed on the original beat frequency signal to obtain a frequency spectrum of the beat frequency signal, a main lobe and two first-order side lobes are selected from the frequency spectrum, a filter with the same width as the main lobe is used to extract the main lobe and the two first-order side lobes to obtain three subbands;
[0010] S2: Fourier inverse transform is performed on the three subbands extracted in step S1 to obtain three curves;
[0011] S3: the three curves obtained in step S2 are added by using a rotation vector summation method to obtain a new signal;
[0012] S4: the phase information of each vibration point on the new signal curve is extracted by amplitude demodulation of the new signal, according to the corresponding relationship between the amplitude and the phase of Rayleigh scattering, the threshold range of the fading area is set, it is judged whether the phase information of each vibration point is within the threshold range, if yes, the phase information of the vibration point is the false phase information, if not, the phase information of the vibration point is the correct phase information, the phase is selected by this method, the phase outside the threshold range is retained, and the phase within the threshold range is averaged in front and back, so as to avoid the false phase point;
[0013] S5: after the error position of the phase image is corrected by average processing, the final phase demodulation image of the vibration point eliminating interference fading is obtained.
[0014] Further, the three sub-bands are uniformly spaced in frequency.
[0015] Further, the three curves have different intensity of jitter in time domain.
[0016] Further, the rotation vector summation method eliminates the continuous phase of each vector curve caused by the modulation frequency, so that the phase change of the vector curve is only caused by the disturbance; then the scattered signals with the continuous phase rotated to zero are added, the original interference fading point intensity after recombination is improved, and the coherent fading not existing in the three curves is eliminated.
[0017] A storage device stores instructions and data for implementing a method for suppressing interference fading.
[0018] A device for suppressing interference fading includes a processor and the storage device; the processor loads and executes the instructions and data in the storage device to implement a method for suppressing interference fading.
[0019] The technical scheme provided by the present application has the following beneficial effects:
[0020] (1) The present application extracts three sub-bands by selecting a main lobe and two first-order side lobes in the frequency domain, and using a filter with the same width as the main lobe; without multiple frequency data, the coherent fading can be well eliminated, and the number of detections can be reduced.
[0021] (2) The present application uses a rotation vector summation method to add the three curves obtained to obtain a new signal; the fading that is not completely eliminated by the RVS (vector rotation summation) method can be completely eliminated, and the misaligned phase demodulation can be well avoided.
[0022] (3) The present application extracts the phase information of each vibration point on the new signal curve by performing amplitude demodulation on the new signal, sets a threshold range of the fading area according to the corresponding relationship between the amplitude and the phase of Rayleigh scattering, judges whether the phase information of each vibration point is within the threshold range, if yes, the phase information of the vibration point is incorrect, if no, the phase information of the vibration point is correct, selects the phase by this method, retains the phase outside the threshold range, and averages the phase within the threshold range, so as to avoid the incorrect phase point; after the above down-sampling of whether falling into the threshold of the fading area, the number of calculation points is greatly reduced, the cost is reduced in engineering application, and the real-time demodulation speed is also accelerated. BRIEF DESCRIPTION OF DRAWINGS
[0023] The present application will be further described below with reference to the accompanying drawings and embodiments; in the drawings:
[0024] Figure 1 is an experimental device block diagram of an embodiment of the present application.
[0025] Figure 2 is a phase detection effect diagram of direct demodulation in an embodiment of the present application.
[0026] Figure 3 is a schematic diagram of demodulation by a single-pulse spectrum extraction and combination algorithm in an embodiment of the present application.
[0027] Figure 4 is a phase detection effect diagram by a wheel search algorithm in an embodiment of the present application.
[0028] Figure 5 is a phase detection effect diagram of the technical solution provided by the present application.
[0029] Figure 6 is a schematic diagram of hardware device operation in an embodiment of the present application. DETAILED DESCRIPTION
[0030] In order to have a clearer understanding of the technical features, objectives and effects of the present application, the specific embodiments of the present application will now be described in detail with reference to the drawings.
[0031] Embodiments of the present application provide a method, device and storage device for suppressing interference fading.
[0032] In a very low amplitude, an error demodulation occurs when I / Q arctangent calculation is performed, and through the difference between adjacent points, a cluster of protrusions appears on the image, which is coherent fading. The method proposed in the present application uses a filter with a width consistent with the main lobe width to uniformly separate three sub-bands. Secondly, Fourier inverse transform is performed on the three sub-bands to obtain three curves. Since the interference fading points of the three curves appear at different positions, the signal is a complex signal, and if direct intensity addition is performed, the phase information may be lost. In order to solve this problem, a rotating vector summation method can be used. The rotating vector summation method eliminates the continuous phase of each vector curve caused by the modulation frequency, so that the phase change of the vector curve is only caused by the disturbance. Then, the scattered signals with continuous phase rotated to zero are added to realize the enhancement of the intensity of the original interference fading points after recombination, thereby eliminating the coherent fading that does not exist at the same time in the three curves. Then, it is analyzed that the width of the place where the error demodulation occurs is generally not more than 5 points. Therefore, according to the amplitude, the maximum amplitude point is selected as the representative of the 5 points, and a threshold value for determining whether the signal falls into the fading area is set. Therefore, it is known whether the maximum value falls into the fading area, so that the error phase point can be well avoided. This operation is equivalent to downsampling with a size of 5. The phase of the maximum amplitude point after selection is a correct phase point, and after phase image correction, the final phase demodulation image of the vibration point with interference fading eliminated is obtained.
[0033] Adopting such Figure 1 The experimental setup shown detects beat frequency signals. Specifically, a narrow-linewidth laser is used as the light source. The laser beam is split into two parts by a 90:10 coupler. 10% is used as the local oscillator and enters the polarization controller, while the other 90% enters the acousto-optic modulator. Driven by a signal generator, a 100ns pulse width, 200MHz frequency shift, and 25kHz repetition rate detection pulse is generated. This pulse is then input into the fiber under test (FUT) via an erbium-doped fiber amplifier (EDFA). The backscattered Rayleigh light returns through a circulator and beats with the local oscillator at a 2×2 coupler. A balanced detector (BPD) is used for detection, and the signal is acquired by a data acquisition (DAQ) device. The final output signal is digitized using an 8-bit data acquisition card. Orthogonal signals I and Q containing phase information are obtained through detection. These orthogonal signals I and Q are multiplied by a pair of standard orthogonal signals with the same frequency as the beat frequency signal generated by the computer, followed by low-frequency filtering. Finally, the filtered orthogonal signals are demodulated to obtain the phase signal. The first step in phase demodulation is to calculate the arctangent of the quadrature signal. The phase at this point includes both the vibration of the PZT (piezoelectric ceramic transformer) and the original phase of the signal. The range of the arctangent function is... In practical applications, the demodulated PZT signal may exceed [a certain value]. Therefore, the result of the arctangent operation is no longer the actual phase demodulation result. The next step is to extend the value range. Based on the sign characteristics of the cos(Φ(t)) and sin(()) functions in each coordinate quadrant of the orthogonal signals I and Q, the value range of the phase signal Φ(t) is extended to (-,π). If the actual phase signal exceeds this range, the result of the phase demodulation algorithm will exhibit jumps from - to π, or from π to -, and the phase signal will no longer be a continuously changing curve, i.e., phase winding occurs. Finally, a phase dewinding process is performed to demodulate the vibration information of the PZT.
[0034] The image still exhibits interference fading at this point, therefore a method to suppress interference fading is proposed, specifically including:
[0035] S1: Perform a Fourier transform on the original beat frequency signal to obtain the spectrum of the beat frequency signal. Select a main lobe and two first-order side lobes from the spectrum and extract the main lobe and two first-order side lobes with a filter of the same width as the main lobe to obtain three sub-bands.
[0036] S2: Perform an inverse Fourier transform on the three sub-bands extracted in step S1 to obtain three curves with different intensities of jitter in the time domain.
[0037] S3: using the rotation vector summation method to add the three curves obtained in step S2 to obtain a new signal to eliminate most of the coherent fading;
[0038] S4: by amplitude demodulation of the new signal, the phase information of each vibration point on the new signal curve is extracted, the signal-to-noise ratio of the point with very low amplitude is low, and therefore, according to the corresponding relationship between the amplitude and the phase of Rayleigh scattering, the amplitude is taken as a reference to set the threshold range of the fading area to determine whether fading still exists at the place, and the phase is selected, the phase outside the threshold range is retained, and the method is used to verify whether the phase at a certain place is wrong, if the amplitude at a certain place is too low, the signal-to-noise ratio is low, and therefore, the phase demodulation error occurs, the correct phase data is retained, and the error phase is averaged, that is, the phase within the threshold range is averaged before and after, so that the purpose of avoiding the error phase point is achieved.
[0039] S5: after the error position of the phase image is averaged and corrected, a correct phase demodulation image of the vibration point after the interference fading is eliminated is obtained.
[0040] In order to verify the effectiveness and accuracy of the method disclosed in the application, the results of the phase demodulation directly, the coherent fading elimination by the single pulse spectrum extraction and combination algorithm, and the phase detection by the over wheel algorithm are compared with the demodulation results of the method proposed in the application.
[0041] Figure 2 The phase is directly demodulated, and it can be seen that there are many coherent fadings. Figure 3 The coherent fading is eliminated by the single pulse spectrum extraction and combination algorithm, and it can be seen that most of the coherent fadings have been eliminated, but there are still some fading points that have not been eliminated. According to the analysis, it is because the same position in the extracted spectrum simultaneously exists interference fading, and these places cannot eliminate the coherent fading when combining, and the positioning of the vibration point still has uncertainty. Figure 4 The phase detection effect diagram by the over wheel algorithm can be seen, although some fadings are eliminated, but there are still many burrs at other points, and it is not very beautiful.
[0042] Figure 5 The method proposed in the application is used for spectrum extraction and vector rotation summation, and then the phase selection is performed, the amplitude is taken as a reference to set the threshold to determine whether the signal falls into the fading area, the point with error phase demodulation is discarded, the coherent fading is well eliminated, the calculation speed is also accelerated, and the position of the vibration point can be well found.
[0043] Please refer to Figure 6 , Figure 6It is a hardware device working schematic diagram of the embodiment of the present application, and the hardware device specifically comprises: a device for suppressing fading interference 401, a processor 402 and a storage device 403.
[0044] The device for suppressing fading interference 401: the device for suppressing fading interference 401 implements the method for suppressing fading interference.
[0045] The processor 402: the processor 402 loads and executes instructions and data in the storage device 403 for implementing the method for suppressing fading interference.
[0046] The storage device 403: the storage device 403 stores instructions and data; the storage device 403 is used for implementing the method for suppressing fading interference.
[0047] The beneficial effects of the present application are:
[0048] (1) The present application selects one main lobe and two first-order side lobes in the frequency domain, extracts three sub-bands with a filter with the same width as the main lobe; without multiple frequency data, the coherent fading can be well eliminated, and the detection times can be reduced.
[0049] (2) The present application uses a rotating vector summation method to add the three curves obtained to obtain a new signal; the fading that is not completely eliminated by the RVS (vector rotating summation) method can be completely eliminated, and the misaligned phase demodulation can be well avoided.
[0050] (3) The present application extracts the phase information of each vibration point on the new signal curve by amplitude demodulation of the new signal, sets the threshold range of the fading area according to the corresponding relationship between the amplitude and the phase of Rayleigh scattering, judges whether the phase information of each vibration point is within the threshold range, if yes, the phase information of the vibration point is the wrong phase information, if no, the phase information of the vibration point is the correct phase information, selects the phase through the above-mentioned method, retains the phase outside the threshold range, and averages the phase within the threshold range, so as to avoid the wrong phase point; after the above-mentioned decimation whether falling into the threshold of the fading area, the number of calculation points is greatly reduced, the cost is reduced in engineering application, and the speed of real-time demodulation is also accelerated.
[0051] The above only describes the preferred embodiments of the present application and is not used to limit the present application, and any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.
Claims
1. A method for suppressing interference fading, characterized in that: include: S1: Perform a Fourier transform on the original beat frequency signal to obtain the spectrum of the beat frequency signal. Select a main lobe and two first-order side lobes from the spectrum and extract the main lobe and two first-order side lobes with a filter of the same width as the main lobe to obtain three sub-bands. S2: Perform an inverse Fourier transform on the three sub-bands extracted in step S1 to obtain three curves; S3: Use the rotation vector summation method to add the three curves obtained in step S2 to obtain a new signal; S4: By performing amplitude demodulation on the new signal, the phase information of each vibration point on the new signal curve is extracted. Based on the correspondence between the amplitude and phase of Rayleigh scattering, a threshold range for the fading region is set. It is then determined whether the phase information of each vibration point is within the threshold range. If it is, the phase information of that vibration point is incorrect; otherwise, the phase information of that vibration point is correct. By selecting the phase in this way, the phase outside the threshold range is retained, and the phase within the threshold range is averaged before and after, thereby avoiding incorrect phase points. S5: After correcting the erroneous positions of the phase image by averaging, the final phase demodulated image of the vibration points with eliminated interference fading is obtained.
2. The method for suppressing interference fading as described in claim 1, characterized in that: The three sub-bands were extracted at uniform intervals in frequency.
3. The method for suppressing interference fading as described in claim 1, characterized in that: The three curves exhibit jitter of varying intensities in the time domain.
4. The method for suppressing interference fading as described in claim 1, characterized in that: In step S3, the rotating vector summation method eliminates the continuous phase of each vector curve caused by the modulation frequency, so that their phase changes are only caused by disturbances; then the scattered signals with continuous phases rotated to zero are added together to improve the intensity of the original interference fading points after recombination, thereby eliminating the coherent fading that does not exist simultaneously in the three curves.
5. A storage device, characterized in that: The storage device stores instructions and data for implementing the method for suppressing interference fading as described in any one of claims 1 to 4.
6. A device for suppressing interference fading, characterized in that: include: A processor and a storage device; the processor loads and executes instructions and data in the storage device to implement the method for suppressing interference fading as described in any one of claims 1 to 4.