Low-frequency noise suppression method for distributed optical fiber vibration sensing system based on spatial dispersion

Abstract

A method for suppressing low-frequency noise in a distributed optical fiber vibration sensing system based on spatial dispersion includes: obtaining a time domain signal; selecting multiple spatial channels along the axis of the sensing optical fiber, and constructing a sample sequence z1, z2, z3, ..., z N ; obtain low-frequency self-referenced noise; and obtain a signal after eliminating the influence of the system's low-frequency noise. This method does not change the structure of the distributed fiber optic vibration sensing system, imposes no restrictions on the choice of sensing fiber or application scenarios, and allows for diverse and flexible sample sequence construction. This method can also effectively reduce the impact of the system's low-frequency noise on the performance of the distributed fiber optic vibration sensing system, improving the system's ability to detect low-frequency signals. This has important implications for fields such as seismic wave detection and ocean hydroacoustic detection.

Description

Technical Field

[0001] The present invention relates to the suppression of low-frequency noise, in particular to a method for suppressing low-frequency noise in a distributed optical fiber vibration sensing system based on spatial dispersion. Background Art

[0002] Distributed fiber-optic vibration sensing technology, due to its high sensitivity, fast response speed, and long sensing distance, has been widely used in structural health monitoring, mine safety, geological exploration, pipeline safety, and perimeter intrusion detection, becoming a hot topic of research both domestically and internationally, with a particular focus on high-frequency applications. In recent years, researchers have focused on monitoring weak, low-frequency signals in fields such as seismic wave detection and ocean hydroacoustic detection. These applications place higher demands on the low-frequency detection capabilities of distributed fiber-optic vibration sensing technology. However, the conflicting relationship between the narrow linewidth of the laser and the stability of the wavelength, as well as the asynchrony between the optical frequency and the system electronics clock signal, can affect the system's low-frequency detection capabilities.

[0003] Prior Art 1 [K. Ying, D. Chen, H. Pi, Z. Wang, and X. Li, Ultra-Stable Fiber Laser Based on Intracavity Dual Mode Self-Reference Mechanism, JOURNAL OF LIGHTWAVE TECHNOLOGY, 40(12): 3923–3929, 2022] This method starts with the structure of a distributed fiber vibration sensing system and realizes a wavelength-stabilized DFB fiber laser with an intracavity frequency reference mechanism, reducing the laser frequency drift from 50 MHz to less than 5 kHz. However, there is a contradiction between the narrow linewidth of the laser and the absolute wavelength, and the optical frequency cannot be synchronized with the system's electrical clock signal. Therefore, this method has limited effect on suppressing the system's low-frequency noise.

[0004] Prior Art 2 [F. Zhu, X. Zhang, L. Xia, and Y. Zhang, Active compensation method for light source frequency drifting in phi-OTDR sensing system, IEEE Photonics Technol. Lett., 27(24):2523–2526, 2015] This method first predicts the backscattered signal at different laser frequencies and then tracks and compensates for the laser frequency drift based on the relationship between the real-time signal and the predicted signal. However, changes in the overall environment (such as changes in ambient temperature) may invalidate the prediction results, resulting in the inability to always correctly compensate for the low-frequency noise caused by the laser frequency drift.

[0005] Prior Art 3 [M.Wu, X.Fan, Q.Liu, and Z.He, Highly sensitive quasi-distributed fiber-optic acoustic sensing system by interrogating a weak reflector array, Optics Letter, 43(15):3594–3597, 2018] successfully reduced the laser phase noise of the weak reflector array in the Φ-OTDR by using an auxiliary interferometer to monitor laser frequency drift and compensate for phase noise. However, when using ordinary single-mode fiber, this method may have uneven effects along the sensing distance and is not universally applicable.

[0006] Prior Art 4 [Q. Yuan, F. Wang, T. Liu, et al., Compensating for influence of laser-frequency-drift in phase-sensitive OTDR with twice differential method, Optics Express, 27(3):3664–3671, 2019] proposed a quadratic differential method to compensate for the influence of laser frequency drift in Φ-OTDR. It uses the differential signal between two points on the sensing fiber as a reference signal, and then subtracts the reference signal from the main signal to obtain the final result. However, when selecting the differential signal between two points on the sensing fiber as the reference signal, the selected area must be a specially set fiber area isolated from the outside world, which places high demands on the test environment and is not suitable for application scenarios with complex environments. Summary of the Invention

[0007] In order to overcome the shortcomings of the above-mentioned prior technologies, the present invention proposes a low-frequency noise suppression method for a distributed optical fiber vibration sensing system based on spatial dispersion, in order to break through the key problems in the current distributed optical fiber vibration sensing system, such as the limited low-frequency detection capability of the system and the restricted application scenarios caused by the contradictory relationship between the narrow linewidth of the laser and the absolute wavelength and the asynchrony between the optical frequency and the electrical clock signal of the system.

[0008] The technical solutions of the present invention are as follows:

[0009] A method for suppressing low-frequency noise in a distributed optical fiber vibration sensing system based on spatial dispersion is characterized in that the method comprises the following steps:

[0010] 1) Obtaining time domain signal: The time-space distribution of the demodulated time domain signal obtained by the distributed optical fiber vibration sensing system is expressed as V(z,t), where z is the axial position along the sensing optical fiber, i.e., the spatial channel, and t is the sampling time of the distributed optical fiber vibration sensing system signal; the zth position on the sensing optical fiber is obtained. i The time domain signal V(z i ,t)=V n (z i ,t)+V LFD (z i ,t)+V s (z i ,t), where V n (z i ,t) is introduced by environmental noise, V LFD (z i ,t) is introduced by the system low-frequency noise, V s (z i ,t) introduced by external disturbance;

[0011] 2) Select multiple spatial channels along the axial direction of the sensing fiber and construct the sample sequence z1, z2, z3, ..., z N , where N is the number of samples: First, the impact of external disturbance signals on the system's low-frequency noise must be eliminated. The spatial channel length must be much longer than the sensing fiber length affected by external fiber disturbances, thereby determining the minimum number of spatial channel samples. Second, based on the fact that the low-frequency noise introduced by laser frequency drift varies in spatial scale along the axial direction of the sensing fiber, the spatial channel is divided into X segments. When constructing the sample sequence, any sample can be selected from the X-segment spatial channel interval while ensuring that the minimum number of samples is met.

[0012] 3) Obtain low-frequency self-reference noise: For the sample sequence z1, z2, z3, ..., z constructed in step 2) N The corresponding time domain signal is further synthesized to obtain the low-frequency self-reference noise. Suppose that there are m1, m2, m3, ..., m in the X-segment spatial channel interval. X samples, i.e. m1+m2+m3+…+m X =N, then the low-frequency self-referenced noise can be expressed as:

[0013]

[0014] where k X Indicates the proportional coefficient of the Xth spatial channel interval, which is determined according to the changing trend of the low-frequency noise in each spatial channel interval. When the environmental noise is considered to be Gaussian white noise in space and time, According to the sample sequence constructed in step 2), we get and Where M represents the center channel of the sample sequence; thus, the low-frequency self-reference noise V REF (z,t)=V LFD (z M ,t);

[0015] 4) Obtain the signal after eliminating the influence of the system low-frequency noise: Difference the time domain signal obtained in step 1) and the low-frequency self-reference noise obtained in step 3) to obtain V real (z i ,t)=V(z i ,t)-V LFD (z M ,t), then for the obtained signal V real (z i ,t) is filtered to obtain the signal after eliminating the influence of the system's low-frequency noise, thereby improving the low-frequency detection capability of the distributed optical fiber vibration sensing system.

[0016] The distributed optical fiber vibration sensing system in step 1) is any one of an optical frequency domain reflectometer (OFDR), a phase-sensitive optical time domain reflectometer (Φ-OTDR), a chirped pulse phase-sensitive optical time domain reflectometer (CP-ΦOTDR), a coherent optical time domain reflectometer (COTDR), a distributed vibration sensor (DVS), and a distributed acoustic sensor (DAS).

[0017] The sensing optical fiber is one or more of common single-mode optical fiber, few-mode optical fiber, multi-mode optical fiber, multi-core optical fiber, and wound acoustic sensitive cable.

[0018] The filtering in step 4) includes smoothing filtering, mean filtering or linear filtering.

[0019] The characteristics and advantages of the present invention are as follows:

[0020] (1) The spatial deviation method is used to improve the performance of the distributed optical fiber vibration sensing system. The present invention can not only suppress the low-frequency signal drift caused by the laser frequency drift, effectively reduce the impact of frequency drift noise on the performance of the distributed optical fiber vibration sensing system, but also solve the problem of low-frequency noise introduced by the asynchronous system clock, thereby improving the system's overall detection capability of low-frequency signals.

[0021] (2) The present invention has diversity in constructing sample sequences, which improves the flexibility and applicability of the method; at the same time, the present invention does not change the structure of the distributed optical fiber sensing system, does not interfere with the sensing information, and can eliminate the influence of low-frequency noise in the system, which can more effectively improve the sensing performance and enhance the detection capability of the target signal.

[0022] (3) By making full use of the distributed sensing advantages of distributed optical fiber vibration sensing, it can detect over long distances, has good environmental adaptability, is small in size, and has low cost, providing a new idea for monitoring low-frequency weak signals in the fields of seismic wave detection and ocean hydroacoustic detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a flow chart of an embodiment of a method for suppressing low-frequency noise in a distributed optical fiber vibration sensing system based on spatial dispersion of the present invention;

[0024] Figure 2 is a sample sequence construction flow chart of an embodiment of the present invention;

[0025] Figure 3 4 is a flow chart of low-frequency self-reference noise synthesis according to an embodiment of the present invention. DETAILED DESCRIPTION

[0026] The present invention is further described below with reference to the accompanying drawings and examples, but is not intended to be limiting. Several implementation methods are possible based on the principles of the present invention. The following solutions are merely illustrative of the principles of the present invention and are not intended to be limiting. Furthermore, for ease of description, the accompanying drawings only illustrate portions relevant to the present invention, rather than the entire process.

[0027] Embodiment 1 of the low-frequency noise suppression method of the distributed optical fiber vibration sensing system based on spatial dispersion of the present invention is as follows: Figure 1 As shown, the method mainly includes:

[0028] 1) Obtaining time domain signal: The time-space distribution of the demodulated time domain signal obtained by the distributed optical fiber vibration sensing system is expressed as V(z,t), where z is the axial position along the sensing optical fiber, i.e., the spatial channel, and t is the sampling time of the distributed optical fiber vibration sensing system signal; the zth position on the sensing optical fiber is obtained. i The time domain signal V(z i ,t)=V n (z i ,t)+V LFD (z i ,t)+V s (z i ,t), where V n (z i ,t) is introduced by environmental noise, V LFD (z i ,t) is introduced by the system low-frequency noise, V s (z i ,t) introduced by external disturbance;

[0029] In this embodiment, the distributed optical fiber vibration sensing system adopts a phase-sensitive optical time domain reflectometer (Φ-OTDR); the sensing optical fiber adopts an ordinary single-mode optical fiber.

[0030] 2) Select multiple spatial channels along the axial direction of the sensing fiber and construct the sample sequence z1, z2, z3, ..., z N , where N is the number of samples: First, the influence of external disturbance signals on the low-frequency noise of the system must be eliminated. The length of the spatial channel should be much longer than the length of the sensing fiber affected by external fiber disturbances, so as to determine the minimum number of samples of the spatial channel; secondly, the low-frequency noise introduced by the laser frequency drift is different in the spatial scale along the axial direction of the sensing fiber, and the spatial channel is divided into X segments; when constructing the sample sequence, any selection can be made in the X-segment spatial channel interval under the premise of ensuring that the minimum number of samples is met. The flow chart is as follows: Figure 2 As shown;

[0031] In this embodiment, the length of the sensing fiber affected by external fiber disturbance is 12 m, and the sample sequences all belong to one of the X-segment spatial channel intervals and are continuous, which is 640-800 m.

[0032] 3) Obtain low-frequency self-reference noise: For the sample sequence z1, z2, z3, ..., z constructed in step 2) N The corresponding time domain signal is further synthesized to obtain the low-frequency self-reference noise. Suppose that there are m1, m2, m3, ..., m in the X-segment spatial channel interval. X samples, i.e. m1+m2+m3+…+m X =N, then the low-frequency self-referenced noise can be expressed as:

[0033]

[0034] where k X Indicates the proportional coefficient of the Xth spatial channel interval, which is determined according to the changing trend of the low-frequency noise in each spatial channel interval. When the environmental noise is considered to be Gaussian white noise in space and time, According to the sample sequence constructed in step 2), we get and Where M represents the center channel of the sample sequence; thus, the low-frequency self-reference noise V REF (z,t)=V LFD (z M ,t), the synthesis flow chart is as follows Figure 3 As shown;

[0035] 4) Obtain the signal after eliminating the influence of the system low-frequency noise: Difference the time domain signal obtained in step 1) and the low-frequency self-reference noise obtained in step 3) to obtain V real (z i,t)=V(z i ,t)-V LFD (z M ,t), then for the obtained signal V real (z i ,t) is filtered to obtain the signal after eliminating the influence of the system's low-frequency noise, thereby improving the low-frequency detection capability of the distributed optical fiber vibration sensing system.

[0036] The filtering described in this embodiment adopts a smooth filter. real (z i ,t) performs smoothing filtering to further suppress the low-frequency noise of the system.

[0037] While some embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to the implementations described in the aforementioned embodiments. Any modifications or variations made without departing from the spirit of the present invention are considered to fall within the scope of this patent and should not be relied upon to limit the scope of protection of the present invention.

Claims

1. A method for suppressing low-frequency noise in a distributed optical fiber vibration sensing system based on spatial dispersion, characterized in that: The method comprises the following steps: 1) Obtaining time domain signal: The time-space distribution of the demodulated time domain signal obtained by the distributed optical fiber vibration sensing system is expressed as V(z,t), where z is the axial position along the sensing optical fiber, i.e., the spatial channel, and t is the sampling time of the distributed optical fiber vibration sensing system signal; the zth position on the sensing optical fiber is obtained. i The time domain signal V(z i ,t)=V n (z i ,t)+V LFD (z i ,t)+V s (z i ,t), where V n (z i ,t) is introduced by environmental noise, V LFD (z i ,t) is introduced by the system low-frequency noise, V s (z i ,t) introduced by external disturbance; 2) Select multiple spatial channels along the axial direction of the sensing fiber and construct the sample sequence z1, z2, z3, ..., z N , where N is the number of samples: First, the impact of external disturbance signals on the system's low-frequency noise must be eliminated. The spatial channel length must be much longer than the sensing fiber length affected by external fiber disturbances, thereby determining the minimum number of spatial channel samples. Second, based on the fact that the low-frequency noise introduced by laser frequency drift varies in spatial scale along the axial direction of the sensing fiber, the spatial channel is divided into X segments. When constructing the sample sequence, any sample can be selected from the X-segment spatial channel interval while ensuring that the minimum number of samples is met. 3) Obtain low-frequency self-reference noise: For the sample sequence z1, z2, z3, ..., z constructed in step 2) N The corresponding time domain signal is further synthesized to obtain the low-frequency self-reference noise. Suppose that there are m1, m2, m3, ..., m in the X-segment spatial channel interval. X samples, i.e. m1+m2+m3+…+m X =N, and the ambient noise is considered to be Gaussian white noise in space and time, then the low-frequency self-referenced noise is: V REF (z,t)=V LFD (z M ,t) Where M represents the central channel of the sample sequence; 4) Obtain the signal after eliminating the influence of the system low-frequency noise: Difference the time domain signal obtained in step 1) and the low-frequency self-reference noise obtained in step 3) to obtain V real (z i ,t)=V(z i ,t)-V LFD (z M ,t), then for the obtained signal V real (z i ,t) is filtered to obtain the signal after eliminating the influence of the system's low-frequency noise, thereby improving the low-frequency detection capability of the distributed optical fiber vibration sensing system.

2. The method for suppressing low-frequency noise in a distributed optical fiber vibration sensing system based on spatial dispersion according to claim 1, characterized in that: The distributed optical fiber vibration sensing system in step 1) is any one of an optical frequency domain reflectometer (OFDR), a phase-sensitive optical time domain reflectometer (Φ-OTDR), a chirped pulse phase-sensitive optical time domain reflectometer (CP-ΦOTDR), a coherent optical time domain reflectometer (COTDR), a distributed vibration sensor (DVS), and a distributed acoustic sensor (DAS).

3. The method for suppressing low-frequency noise in a distributed optical fiber vibration sensing system based on spatial dispersion according to claim 1, characterized in that: The sensing optical fiber in step 1) is one or more of ordinary single-mode optical fiber, few-mode optical fiber, multi-mode optical fiber, multi-core optical fiber, and wound acoustic sensitive cable.

4. The method for suppressing low-frequency noise in a distributed optical fiber vibration sensing system based on spatial dispersion according to claim 1, wherein: The filtering in step 4) includes smoothing filtering, mean filtering or linear filtering.

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