Complex multi-mode phase noise suppression method for phase-sensitive optical time domain reflectometer

Abstract

The invention belongs to the technical field of distributed optical fiber sound wave sensing, and relates to a complex multi-mode phase noise suppression method for a phase-sensitive optical time domain reflectometer, which comprises the following steps of: performing I / Q demodulation and unwinding treatment on two beat frequency electric signals output by a balanced photoelectric detector to obtain noise-containing signals; sVMD decomposition is carried out to obtain a decomposed mode, and the correlation between the mode and a noise-containing signal is judged through an SWPCC method so as to retain high-correlation components of a window position corresponding to the mode; and traversing all modes, and correspondingly accumulating all components needing to be reserved according to window positions to obtain a phase de-noising result. According to the process of calculating the correlation between each mode and the original signal, the influence of irrelevant components in the modes on the correlation coefficient is eliminated, and the signal-to-noise ratio of the DAS signal is remarkably improved.

Description

Technical Field

[0001] The invention belongs to the technical field of distributed optical fiber acoustic wave sensing, and in particular relates to a complex multi-modal phase noise suppression method for a phase-sensitive optical time domain reflectometer. Background Art

[0002] Distributed optical fiber sensing (DAS) is characterized by low cost, corrosion resistance, and anti-interference, and can achieve long-distance, long-term, and large-scale sensing. It is widely used in various fields such as oil and gas exploration, pipeline monitoring, traffic detection, disaster prediction, and geophysical applications. Phase-sensitive optical time domain reflectometer ( -OTDR) system is one of the key research directions in this field. The system injects high-coherence laser into the optical fiber under test, and the change of Rayleigh backscattered light (RBS) reflects the vibration of the medium under test. These vibration signals are often accompanied by the influence of various noises. In DAS systems, common noises include light source frequency deviation (LFD), beat frequency noise, electrical noise and fading noise. In addition, in practical applications, the noise of the environment is often at a relatively high level. In order to achieve more accurate sensing, it is necessary to suppress the noise in the signal to improve the signal-to-noise ratio.

[0003] In order to suppress the noise in the signal and improve the signal-to-noise ratio of the DAS signal, the existing methods only consider signals with simple frequency components. When the frequency components of the signal become complex, the results will deteriorate seriously. The reason is that the effective components of the signal may be randomly and irregularly distributed in all modes, and the amount of information contained in each mode is unknown. This results in that for a certain mode, it may only have a very short section of useful signal, resulting in the calculated correlation coefficient being lower than the threshold and being discarded, causing the signal component to be missing. Summary of the invention

[0004] The present invention provides a complex multi-modal phase noise suppression method for a phase-sensitive optical time domain reflectometer, which solves the problem of missing signal components and difficulty in applying to complex engineering applications, improves the signal-to-noise ratio of the signal, and is beneficial to accurate distributed acoustic sensing applications.

[0005] The present invention is implemented as follows: a complex multi-modal phase noise suppression method for a phase-sensitive optical time domain reflectometer comprises the following steps: S1: The two beat frequency electrical signals output by the balanced photodetector are subjected to I / Q demodulation and deconvolution to obtain a noisy signal , For time; S2: For noisy signals Perform SVMD decomposition to obtain the decomposed mode ,in, Indicates modal, represents the total number of modes; S3: Determine the mode by SWPCC method With noisy signal to preserve the modality Highly correlated components corresponding to window positions; traverse all modes , accumulate all the components that need to be retained according to the window position to obtain the phase denoising result .

[0006] Furthermore, S1 specifically includes: The two beat frequency electrical signals output by the balanced photodetector are expressed as: (1), in, is the photoelectric conversion coefficient of the balanced photodetector; and represents the two orthogonal components of the local oscillator light, For the local oscillator Quantity, For the local oscillator Quantity, and represents the two orthogonal components of the signal light, For signal light Quantity, For signal light Quantity; is the angular frequency, where is the modulation frequency, is the sampling frequency, is the phase of the signal light, is the phase of the local oscillator light, Represents the beat frequency electrical signal Quantity, Represents the beat frequency electrical signal Quantity; Perform I / Q quadrature demodulation on the two beat frequency electrical signals to obtain Two orthogonal components of polarization direction , and Polarization direction Orthogonal components , , expressed as: (2), in, represents the local oscillator light, Represents the signal light, and the orthogonal components in the two polarization directions are superimposed to obtain the in-phase signal component and quadrature phase signal components , is the orthogonal projection angle: , According to the in-phase signal component and quadrature phase signal components Get the winding phase : (4), The unwinding process is as follows: (5), (6), in, for The difference between the original phase and the wrapped phase of time, express The difference between the original phase and the wrapped phase of time, for The phase after time unwrapping is the noisy signal, ; express The winding phase of time, express The winding phase of time, is the unwinding threshold.

[0007] Furthermore, the SVMD decomposition process in S2 includes: Divided into two parts: (7) in, Represents the residual signal, and the residual signal Divide into two parts to get: (8), in, Indicates the front -1 modal sum, express The unprocessed part Represents the frequency variable of Fourier transform, for noisy signals Perform Fourier transform to obtain the unwrapped phase in the frequency domain , modal Transform to obtain the mode in the frequency domain According to Parseval's theorem, the sum of the frequency domain energies of the decomposed parts is equal to the energy of the original signal. ,set up is the sampling point, Indicates Mode of sampling point , hour, Expressed as , Indicates Mode of sampling point , is the center frequency of the mode, express The center frequency of the mode, express The center frequency of the mode, express The center frequency of the mode; Indicates The modal Lagrange multiplier, Indicates The Lagrange multiplier at the sampling points, and let , , , respectively, based on the iterative calculation of formula (9), formula (10), and formula (11) , and : (9), (10), (11), in, For The result after Fourier transformation; For The result after Fourier transformation; is the balance factor; After variable substitution Modal Lagrange multipliers at the sampling points; express The Fourier transform result at the nth sampling point is, After variable substitution Modal The Lagrange multiplier at the sampling point, To update the coefficients, finally, the mode is calculated according to the inverse Fourier transform .

[0008] Furthermore, S3 specifically includes: Set the window's starting position to , the width of the sliding window is , the PCC threshold is , calculated from Location to Position range mode With noisy signal PCC value , remember the mode The arithmetic mean within this position range is , containing noisy signals The arithmetic mean within this position range is , represents the signal containing noise at the nth sampling point in the window, then the PCC value The calculation formula is as follows: , The PCC value With the set PCC threshold Compare, if , indicating the modality of the current window location should be discarded; if , indicating the modality of the current window location is retained, that is: , in, is the phase denoising result; represents the denoising result at the window position, that is Sampling point to De-noising results of sampling points; Indicates the window position mode, that is Sampling point to Modal components at the sampling points.

[0009] Compared with the prior art, the present invention has the following beneficial effects: The method proposed in the present invention solves the problem that existing methods are difficult to apply to complex engineering applications, which enables the "modal decomposition-correlation coefficient" denoising strategy to be applied to more DAS applications with complex signal components. Traditional methods such as EMD-PCC and VMD-PCC are effective for simple signals. When the frequency components of the signal become complex, a certain mode after decomposition does not necessarily contain all the valid information. In a certain mode, valuable signal components often only appear in a few small segments. At this time, calculating the correlation based on all the information of a certain mode for judgment will obviously cause errors. In this regard, the method of the present invention optimizes the process of calculating the correlation between each mode and the original signal, eliminates the influence of irrelevant components in the mode on the correlation coefficient, and significantly improves the signal-to-noise ratio of the DAS signal.

[0010] The present invention adopts successive variational mode decomposition (SVMD) as the mode decomposition method, which solves the problem that the VMD method is difficult to determine the number of modes when the signal components are unknown; on the other hand, by introducing a Pearson correlation coefficient (PCC) calculation method with a moving window, the correlation coefficient calculation strategy is improved, and the local correlation between each mode and the corresponding position of the original signal is judged segment by segment, and the local highly correlated components are accurately retained without being affected by the irrelevant components in the mode, thereby achieving accurate phase denoising. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 An overall flow chart of the method provided by an embodiment of the present invention; Figure 2 A schematic diagram of the structure of a phase-sensitive optical time domain reflectometer provided in an embodiment of the present invention; Figure 3 A 5 Hz noisy phase signal provided in an embodiment of the present invention; Figure 4 This is the 5 Hz signal denoising result provided by the embodiment of the present invention. DETAILED DESCRIPTION

[0012] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present invention.

[0013] It should be noted that the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or apparatus comprising a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to these processes, methods, products or apparatus.

[0014] like Figure 1 As shown, a complex multi-modal phase noise suppression method for a phase-sensitive optical time domain reflectometer comprises the following steps: S1: The two beat frequency electrical signals output by the balanced photodetector (DAS system) are subjected to I / Q demodulation and deconvolution to obtain a noisy signal , For time; S2: For noisy signals Perform SVMD decomposition to obtain the decomposed mode ,in, Indicates modal, represents the total number of modes; SVMD refers to successive variational mode decomposition; S3: Determine the mode by SWPCC method With noisy signal to preserve the modality Highly correlated components corresponding to window positions; traverse all modes , accumulate all the components that need to be retained according to the window position to obtain the phase denoising result ; SWPCC refers to the Pearson correlation coefficient processing method.

[0015] See also Figure 2 As shown, the phase-sensitive optical time-domain reflectometer based on the present invention is composed of multiple components. Figure 2 The solid line in the figure is the optical path, and the dotted line is the circuit, including a narrow linewidth laser 1, a fiber coupler 2, an acousto-optic modulator 3, an erbium-doped fiber amplifier 4, a fiber circulator 5, a polarization diversity module 6, a balanced photodetector 1 7, a balanced photodetector 2 8, a data acquisition module 9, a data demodulation module 10, a signal generator 11, a fiber 1 12, a fiber 2 13, and a piezoelectric transducer 14. The signal generator 11 is connected to the piezoelectric transducer 14. In the phase-sensitive optical time domain reflectometer, the center frequency of the narrow linewidth laser 1 is 1550nm, and the emitted laser is divided into two parts through a 90:10 fiber coupler 2, which are 90% signal light and 10% local oscillator light. The signal light is modulated into pulse light by the acousto-optic modulator 3, and then amplified by the erbium-doped fiber amplifier 4 at the front end. The amplified pulse light enters the tested optical fiber 1 12 through the optical fiber circulator 5, and a piezoelectric transducer 14 is connected between the tested optical fiber 1 12 and the tested optical fiber 2 13. The signal light is backscattered in the tested optical fiber 1 12, the tested optical fiber 2 13 and the piezoelectric transducer 14, generating Rayleigh scattered signal light, which is input into the polarization diversity module 6 together with the local oscillator light after passing through the optical fiber circulator 5, forming beat frequency signals in two polarization directions, which are converted into electrical signals through the balanced photodetector 1 7 and the balanced photodetector 2 8, and finally enter the data acquisition module 9 and the data demodulation module 10; In this embodiment, the repetition frequency of the narrow linewidth laser 1 is set to 2000 Hz, the length of the tested optical fiber 13 and the tested optical fiber 2 14 is 1 km, the piezoelectric transducer 12 is arranged between the tested optical fiber 12 and the tested optical fiber 2 13, and its driving voltage is set to a 5 Hz sinusoidal signal containing Gaussian noise of 2 Vpp, which is used to generate a 5 Hz vibration signal on the optical fiber; The method of the present invention is applied to a processor that can receive the output of a balanced photodetector, including but not limited to a mobile phone, a laptop computer, a desktop computer, etc. This embodiment provides a complex multimodal phase noise suppression method for a phase-sensitive optical time domain reflectometer. Specifically, in S1, the beat frequency electrical signal output by the balanced photodetector is expressed as: (1), in, is the photoelectric conversion coefficient of the balanced photodetector; and represents the two orthogonal components of the local oscillator light, For the local oscillator Quantity, For the local oscillator Quantity, and represents the two orthogonal components of the signal light, For signal light Quantity, For signal light Quantity; is the angular frequency, where is the modulation frequency, is the sampling frequency, is the phase of the signal light, is the phase of the local oscillator light, Represents the beat frequency electrical signal Quantity, Represents the beat frequency electrical signal Quantity; Perform I / Q quadrature demodulation on the two beat frequency electrical signals to obtain Two orthogonal components of polarization direction , and Polarization direction Orthogonal components , , expressed as: (2), in, represents the local oscillator light, Represents the signal light, and the orthogonal components in the two polarization directions are superimposed to obtain the in-phase signal component and quadrature phase signal components , is the orthogonal projection angle: (3), According to the in-phase signal component and quadrature phase signal components The resulting winding phase : (4), The unwinding process is as follows: (5), (6), in, for The difference between the original phase and the wrapped phase of time, express The difference between the original phase and the wrapped phase of time, for The phase after time unwrapping is the noisy signal, ; express The winding phase of time, express The winding phase of time, is the unwinding threshold.

[0016] like Figure 3 As shown. Figure 3 It can be concluded that In addition to the 5 Hz sinusoidal signal, there is a lot of random noise.

[0017] In S2: The SVMD decomposition process includes: Divided into two parts: (7), in, Represents the residual signal, and the residual signal Divide into two parts to get: (8), in, Indicates the front -1 modal sum, express The unprocessed part Represents the frequency variable of Fourier transform, for noisy signals Perform Fourier transform to obtain the noisy signal in the frequency domain , modal Transform to obtain the mode in the frequency domain According to Parseval's theorem, the sum of the frequency domain energies of the decomposed parts is equal to the energy of the original signal. ,set up is the sampling point, Indicates Mode of sampling point , hour, Expressed as , Indicates Mode of sampling point , is the center frequency of the mode, express The center frequency of the mode, express The center frequency of the mode, express The center frequency of the mode; Indicates The modal Lagrange multiplier, Indicates The Lagrange multiplier at the sampling points, and let , , , respectively, based on the iterative calculation of formula (9), formula (10), and formula (11) , and : (9), (10), (11), in, For The result after Fourier transformation; For The result after Fourier transformation; is the balance factor; After variable substitution Modal Lagrange multipliers at the sampling points; express The Fourier transform result at the nth sampling point is, After variable substitution Modal The Lagrange multiplier at the sampling point, To update the coefficients, finally, the mode is calculated according to the inverse Fourier transform .

[0018] S3: The purpose of the SWPCC method is to determine With noisy signal The correlation of the window is to retain the components with high correlation. Suppose the starting position of the window is , the width of the sliding window is , the PCC (Pearson correlation coefficient) threshold is , calculated from Location to Position range mode With noisy signal PCC value , remember the mode The arithmetic mean within this position range is , containing noisy signals The arithmetic mean within this position range is , represents the signal containing noise at the nth sampling point in the window, then the PCC value The calculation formula is as follows: , The PCC value With the set PCC threshold Compare, if , indicating the modality of the current window location should be discarded; if , indicating the modality of the current window location is retained, that is: , in, is the phase denoising result; represents the denoising result at the window position, that is Sampling point to De-noising results of sampling points; Indicates the window position mode, that is Sampling point to Modal components at the sampling points.

[0019] like Figure 4 The following is the result after noise removal. Figure 3 and Figure 4 It can be concluded that after the noise reduction process of the present invention, the noise signal The noise in the image is effectively eliminated.

[0020] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A complex multimodal phase noise suppression method for a phase-sensitive optical time domain reflectometer, characterized in that: The steps include: S1: The two beat frequency electrical signals output by the balanced photodetector are subjected to I / Q demodulation and deconvolution to obtain a noisy signal , For time; S2: For noisy signals Perform SVMD decomposition to obtain the decomposed mode ,in, Indicates modal, represents the total number of modes; S3: Determine the mode by SWPCC method With noisy signal to preserve the modality Highly correlated components corresponding to window positions; traverse all modes , accumulate all the components that need to be retained according to the window position to obtain the phase denoising result .

2. The complex multimodal phase noise suppression method of a phase-sensitive optical time domain reflectometer according to claim 1, characterized in that: S1 specifically includes: The two beat frequency electrical signals output by the balanced photodetector are expressed as: （1）， in, is the photoelectric conversion coefficient of the balanced photodetector; and represents the two orthogonal components of the local oscillator light, For the local oscillator Quantity, For the local oscillator Quantity, and represents the two orthogonal components of the signal light, For signal light Quantity, For signal light Quantity; is the angular frequency, where is the modulation frequency, is the sampling frequency, is the phase of the signal light, is the phase of the local oscillator light, Represents the beat frequency electrical signal Quantity, Represents the beat frequency electrical signal Quantity; Perform I / Q quadrature demodulation on the two beat frequency electrical signals to obtain Two orthogonal components of polarization direction , and Polarization direction Orthogonal components , , expressed as: （2）， in, represents the local oscillator light, Represents the signal light, and the orthogonal components in the two polarization directions are superimposed to obtain the in-phase signal component and quadrature phase signal components , is the orthogonal projection angle: , According to the in-phase signal component and quadrature phase signal components Get the winding phase : （4）， The unwinding process is as follows: （5）， （6）， in, for The difference between the original phase and the wrapped phase of time, express The difference between the original phase and the wrapped phase of time, for The phase after time unwrapping is the noisy signal, ; express The winding phase of time, express The winding phase of time, is the unwinding threshold.

3. The complex multimodal phase noise suppression method of a phase-sensitive optical time domain reflectometer according to claim 2, characterized in that: The SVMD decomposition process in S2 includes: Divided into two parts: （7）， in, Represents the residual signal, and the residual signal Divide into two parts to get: （8）， in, Indicates the front -1 modal sum, express The unprocessed part Represents the frequency variable of Fourier transform, for noisy signals Perform Fourier transform to obtain the unwrapped phase in the frequency domain , modal Transform to obtain the mode in the frequency domain According to Parseval's theorem, the energy of each part in the frequency domain after decomposition is equal to the energy of the original signal. ,set up is the sampling point, Indicates Mode of sampling point , hour, Expressed as , Indicates Mode of sampling point , is the center frequency of the mode, express The center frequency of the mode, express The center frequency of the mode, express The center frequency of the mode; Indicates The modal Lagrange multiplier, Indicates The Lagrange multiplier at the sampling points, and let , , , respectively, based on the iterative calculation of formula (9), formula (10), and formula (11) , and : （9）， （10）， （11）， in, For The result after Fourier transformation; For The result after Fourier transformation; is the balance factor; After variable substitution Modal Lagrange multipliers at the sampling points; express The Fourier transform result at the nth sampling point is, After variable substitution Modal The Lagrange multiplier at the sampling point, To update the coefficients, finally, the mode is calculated according to the inverse Fourier transform .

4. The complex multi-modal phase noise suppression method of a phase-sensitive optical time domain reflectometer according to claim 1, characterized in that: S3 specifically includes: Set the window's starting position to , the width of the sliding window is , the PCC threshold is , calculated from Location to Position range mode With noisy signal PCC value , remember the mode The arithmetic mean within this position range is , containing noisy signals The arithmetic mean within this position range is , represents the signal containing noise at the nth sampling point in the window, then the PCC value The calculation formula is as follows: ， The PCC value With the set PCC threshold Compare, if , indicating the modality of the current window location should be discarded; if , indicating the modality of the current window location is retained, that is: ， in, is the phase denoising result; represents the denoising result at the window position, that is Sampling point to De-noising results of sampling points; Indicates the window position mode, that is Sampling point to Modal components at the sampling points.

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