A kind of distributed acoustic sensing all-in-one machine device and method based on five-mode optical fiber

Abstract

This invention discloses an integrated distributed acoustic wave sensing device and method based on five-mode optical fiber, belonging to the field of optical fiber sensing technology. The device includes an integrated distributed acoustic wave sensing chassis (DAS chassis) and a light source module, an optical modulation module, and a photoelectric detection module integrated and packaged inside the DAS chassis. It also includes an external optical fiber sensing module and a data acquisition and signal processing module. This invention uses five-mode optical fiber as the sensing medium, significantly improving the system's detection sensitivity to low-frequency vibration signals. Simultaneously, through a self-designed integrated optical path structure and compact layout, the core optical modules are integrated into a single chassis, with each optical component connected via a flange on the outer wall of the chassis, achieving miniaturization and high reliability. This invention has advantages such as high low-frequency sensitivity, high integration, good stability, and simple operation, and is suitable for applications such as perimeter security, pipeline monitoring, railway traffic monitoring, and power equipment monitoring.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, specifically to a distributed acoustic wave sensing integrated device and method based on five-mode optical fiber. This invention is applicable to applications requiring long-distance, distributed, and high-sensitivity vibration signal detection, such as perimeter security, pipeline monitoring, railway traffic monitoring, power equipment vibration monitoring, and geological exploration. Background Technology

[0002] Distributed Acoustic Sensing (DAS) is a novel sensing technology that uses optical fiber as the sensing medium. It achieves continuous distributed detection and localization of vibration and acoustic signals distributed along the fiber path by demodulating the phase, polarization, or intensity changes of backscattered Rayleigh light in the fiber. Compared to traditional point sensors, DAS systems offer significant advantages such as long sensing distances (tens to hundreds of kilometers), resistance to electromagnetic interference, passive intrinsic safety, and ease of remote operation and maintenance. They have been widely applied in fields such as safety monitoring of oil and gas pipeline corridors, health diagnosis of high-speed railway track structures, and perimeter security in important areas.

[0003] Most current mainstream DAS systems are built on single-mode fiber, among which phase-sensitive optical time-domain reflectometry (Φ-OTDR) technology has become the most widely used DAS implementation scheme due to its high sensitivity to phase changes and fast response capability. However, in practical engineering applications, traditional DAS equipment based on single-mode fiber still faces the following technical bottlenecks: First, low-frequency response sensitivity is inherently insufficient. Single-mode fiber only supports the fundamental mode (LP). 01 The interference phase change of the backscattered Rayleigh light in single-mode DAS transmission has physical limitations in its response to low-frequency vibration components (the low-frequency band referred to in this invention refers to the vibration frequency band with a frequency below 10Hz). In typical monitoring scenarios such as pipeline leaks, mechanical excavation, slow personnel intrusion, and geological microseismic events, the effective energy spectrum of vibration events is often concentrated in the low-frequency range. The single-mode DAS system has a low signal-to-noise ratio for detecting such signals, which directly leads to an increased false alarm rate and makes it difficult to meet the requirements of high-reliability monitoring.

[0004] Secondly, the system architecture is fragmented and has low integration. Existing DAS equipment is typically composed of cascaded discrete modules such as narrow-linewidth light sources, pulse modulators, optical amplifiers, circulators, coherent receivers, high-speed acquisition cards, and industrial control computers. These modules are interconnected via fiber optic patch cords and RF cables. This "building block" structure not only results in a large overall size and high power consumption, but also leads to complex field deployment wiring and an increase in long-term failure points, severely restricting its engineering applicability in harsh environments such as unattended field stations and mobile monitoring platforms.

[0005] Third, the optical path stability is poor, resulting in significant measurement drift. In discrete structures, there are numerous flanges and weld points between optical components. Changes in ambient temperature and mechanical vibrations can easily cause random fluctuations in the coupling efficiency of these connection points, thereby inducing time-varying drift in the system's background noise. This optical path instability directly degrades the accuracy and repeatability of phase demodulation, leading to baseline drift in the DAS system during long-term continuous monitoring, affecting the accuracy of vibration localization and the reliability of alarms.

[0006] In summary, there is an urgent need to develop a novel distributed fiber optic vibration sensing device that combines high and low frequency sensitivity, high system integration, and high optical path stability in order to overcome the application limitations of current single-mode DAS technology in specific engineering scenarios. Summary of the Invention

[0007] To address the aforementioned shortcomings of existing technologies, the present invention aims to provide a DAS integrated device and method based on five-mode optical fiber. This invention uses five-mode optical fiber instead of traditional single-mode optical fiber as the sensing medium. By having five modes work together, the effective mode field area is increased, significantly improving the system's sensitivity to low-frequency vibration signals. Simultaneously, through a self-designed integrated optical path structure and compact layout, core optical modules such as the light source, modulation, coupling, and detection are integrated into a single chassis, achieving miniaturization, high reliability, and plug-and-play functionality.

[0008] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: A DAS (Digital Optical Array) integrated device based on five-mode fiber includes an integrated chassis (DAS integrated unit) and a light source module, an optical modulation module, and a photoelectric detection module integrated and packaged inside the DAS integrated unit. Its features are: The light source module includes a narrow linewidth laser and an optical splitter, which are used to generate a coherent continuous optical signal and split it into two paths: a reference light and a signal light. The optical modulation module includes an acousto-optic modulator and an optical fiber amplifier, used to modulate a reference continuous optical signal into an optical pulse signal with appropriate power. The photoelectric detection module includes a tunable optical attenuator and a coherent detector. The tunable optical attenuator processes the reference light and then sends it to the coherent detector. The coherent detector collects the processed reference light and the backscattered Rayleigh light, and performs frequency mixing and demodulation on the two. Optical signals are converted into electrical signals; As a core technical feature of the present invention, the device further includes an optical fiber sensing module and a data acquisition and signal processing module, characterized in that; The fiber optic sensing module uses five-mode fiber as the sensing medium, and the five-mode fiber supports LP. 01 LP 11 LP 21 LP02 LP 31 Five linear polarization modes are used for transmission. The signal light is transmitted in a five-mode fiber and generates back Rayleigh scattered light. The scattered light is transmitted to the photoelectric detection module through a circulator in the fiber amplifier. The five-mode optical fiber is laid out and fixed in a specific manner on the surface or inside of the vibration substrate to be tested. The vibration substrate to be tested includes at least one of the following: geological soil and rock, bridge structure, oil and gas pipeline, power equipment, and sleeping mattress. The data acquisition and signal processing module is set outside the integrated chassis and mounted on a computer. It is connected to the photoelectric detection module via a data cable and includes a data acquisition card and host computer software.

[0009] The data acquisition card converts the electrical signal generated by the coherent detector into a digital signal, and the host computer software processes the digital signal processed by the data acquisition card to display the waveform of the vibration signal acquired by the optical fiber.

[0010] The present invention has the following beneficial effects: The low-frequency sensitivity is significantly improved. By using few-mode fiber as the sensing medium, the combined operation of multiple modes increases the effective mode field area, resulting in improved vibration detection sensitivity in the low-frequency range (10Hz~30Hz) compared to traditional single-mode fiber systems. Laboratory tests have shown that while traditional single-mode fiber cannot identify low-frequency vibration signals, this invention can effectively identify them, effectively detecting low-frequency vibration events such as minor external interference, and significantly reducing the false negative rate.

[0011] The system boasts high integration and compact size. This invention employs an integrated chassis design, consolidating all core optical and electronic modules within a single chassis. The overall dimensions are 400mm × 290mm × 74mm, facilitating transportation and on-site deployment.

[0012] The system has a low technical threshold and good stability. The integrated optical path connects various optical components through the flange port on the outer wall of the integrated machine, which facilitates the functional testing and troubleshooting of the components and effectively reduces the technical threshold. At the same time, the compact layout design is conducive to temperature uniformity, which improves the long-term stability and reliability of the system.

[0013] Plug and play, easy to operate. The integrated design of this invention allows the system to work immediately upon power-on, with fewer steps for optical path debugging and parameter configuration, reducing the difficulty of use and making it suitable for non-professionals. Attached Figure Description

[0014] Figure 1 This is a block diagram of the overall structure of the DAS all-in-one device of the present invention; Figure 2 This is a schematic diagram of the internal optical path layout of the DAS all-in-one machine of the present invention; Figure 3 This is a comparison curve of the low-frequency sensitivity of the five-mode fiber and the single-mode fiber of the present invention. Detailed Implementation

[0015] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0016] Combination Figure 1 and Figure 2 The working steps of the DAS integrated device based on five-mode optical fiber described in this invention are as follows: Step 1: After the system is powered on and debugged, the narrow linewidth laser (UNL) in the light source module outputs a continuous optical signal. A 1*2 optical splitter splits the optical signal into two paths: a local oscillator beam and a reference beam. The NUNL laser outputs a highly coherent continuous optical signal with a typical wavelength of 1550nm. The 1550nm wavelength falls within the third low-loss window of silica optical fiber, and its transmission loss is the lowest among all communication bands, making it the preferred wavelength for long-distance, high-capacity transmission. Simultaneously, the continuous optical signal's narrow linewidth characteristic ensures the accuracy of subsequent phase detection.

[0017] Step Two: The optical modulation module generates an optical pulse signal. The acousto-optic modulator (AOM) in the optical modulation module receives the continuous optical signal output from the light source module, modulates the continuous light into pulsed light, and causes a frequency shift of approximately 80MHz, resulting in a frequency of (ν+80)MHz. The modulated optical pulse signal is then amplified by an optically activated amplifier (EDFA) to compensate for transmission losses in subsequent optical paths, ensuring sufficient signal strength during long-distance fiber optic transmission.

[0018] Step 3: Transmission and Scattering of the Sensing Optical Path Module. The signal light is injected into the five-mode fiber sensing optical path via a circulator (placed inside the EDFA). The five-mode fiber, acting as the sensing medium, is fixed to the surface or interior of the vibrating substrate under test in a specific manner. While the signal light propagates in the few-mode fiber, backscattered Rayleigh light is generated along various points in the fiber due to the slight inhomogeneity of the refractive index. When the optical signal wavelength is in the 1550nm band, the LP in the five-mode fiber... 01 LP 11 LP 21 LP 02 LP 31 The modes work together, and the backscattered Rayleigh light contains the scattering components of each mode, effectively increasing the effective mode field area.

[0019] Assume the backscattered Rayleigh light signal returned in the sensing fiber is (Equation 1)

[0020] In the formula Indicates the power of the scattered signal. The signal light field strength is represented by ν, which is different at each point on the optical fiber due to the coherence effect; ν is the optical frequency of the laser; Δυ is the frequency shift caused by the acousto-optic modulator; φ0 is the initial phase; and j represents the imaginary unit.

[0021] Local oscillator light is continuous light; its amplitude and power are determined by the laser and can be considered constant, as shown below: (Equation 2)

[0022] in Indicates the local oscillator power. The initial phase of the reference light.

[0023] Because of losses within optical fibers, light waves continuously attenuate as they propagate through the fiber. Therefore, the backscattered Rayleigh signals generated at different locations within the fiber carry information about the fiber's losses along its path. Furthermore, since backscattering retains the polarization state of the light wave before scattering, the backscattered Rayleigh signal also contains information about the light wave's polarization state. Therefore, when the backscattered Rayleigh light returns to the fiber's incident end, by detecting the power and polarization state of the Rayleigh scattering signal, phenomena such as defects appearing in the fiber after external factors have acted upon it can be detected. This allows for the sensing of relevant parameters acting on the fiber, such as pressure and bending.

[0024] Step 4: Photoelectric Detection Module Reception and Conversion. The reference light generated by the light source module is processed by the optical attenuator (VOA) and then enters the coherent detector (CD). In addition, the backscattered Rayleigh light from the optical fiber also enters the coherent detector. Ignoring polarization dependence, the backscattered Rayleigh signal light and the intrinsic light will interfere upon reaching the coupler. The superimposed interference signal can be represented as: (Equation 3)

[0025] The optical power is the square of the electric field intensity modulus, which can be expressed as: (Equation 4)

[0026] Assume the responsivity of the photodetector is R. When the detector is AC-coupled, the DC term in the signal can be ignored, and the current generated by the detector can be expressed as: (Equation 5)

[0027] Analysis of Equation 5 shows that the power of the backscattered Rayleigh light is entirely concentrated at the frequency offset Δυ. Therefore, by detecting the difference frequency signal at frequency Δυ, effective detection of the backscattered Rayleigh light can be achieved. In practical systems, bandpass filtering can be applied to the specific frequency band containing this difference frequency signal (i.e., the narrow band interval centered at Δυ) to effectively filter out noise power outside the effective frequency band and significantly improve signal quality. Furthermore, Equation 5 also shows that the amplitude of the local reference light is equivalent to amplifying the power of the backscattered Rayleigh light, which helps to improve the power level of the returned signal light.

[0028] Next, the coherent detector performs frequency mixing and demodulation on the signal light, converting the optical signal into an analog electrical signal. The present invention mainly uses the quadrature demodulation method.

[0029] Step 5: The data acquisition and signal processing module processes the electrical signals to visualize the vibration signals on the optical fiber. The data acquisition card performs analog-to-digital conversion on the analog electrical signals output from the coherent detector in the DAS all-in-one machine. Then, the host computer software processes the digital signals and displays the specific vibration waveform of the tested substrate on the screen. When abnormal vibration occurs in the tested substrate, the vibration waveform displayed by the host computer software will change significantly, thus providing an early warning of abnormal conditions.

[0030] Since the aforementioned test substrate includes at least one of the following: geological strata, bridge structures, oil and gas pipelines, power equipment, and mattresses, the abnormal conditions corresponding to different types of test substrates include: 1. When the stratum rock and soil is the matrix to be tested for vibration, the abnormal vibration includes seismic stratum vibration, slope slip vibration, and underground collapse disturbance vibration; 2. When the bridge structure is the vibration matrix to be tested, the abnormal vibration includes bridge overload resonance, bearing loosening vibration, abnormal vibration caused by structural crack propagation, and vehicle impact overload vibration; 3. When the oil and gas pipeline is the vibration substrate to be tested, the abnormal vibration includes vibration caused by third-party excavation and impact, vibration caused by water hammer of fluid inside the pipe, vibration caused by corrosion and damage deformation of the pipe, and resonance caused by the pipe being suspended in the air. 4. When the power equipment is the vibration substrate to be tested, the abnormal vibration includes transformer core loosening vibration, winding deformation resonance, cooling fan failure vibration, high voltage contact poor contact vibration, and micro-vibration accompanied by partial discharge of equipment; 5. When the sleep mattress is the vibration substrate to be tested, the abnormal vibration includes abnormal movement of the human body turning over at night, abnormal vibration of breathing rhythm, limb spasm and shaking, and mattress self-vibration interference when there is no human activity.

[0031] like Figure 3The figure shows a comparison curve of the vibration detection sensitivity of the five-mode fiber optic system of this invention and the traditional single-mode fiber optic system at different frequencies. The test conditions were: vibration frequency range 0~40Hz, sensing distance 10km.

[0032] The comparison curves show that in the low-frequency range (0~18Hz), the detection sensitivity of the five-mode fiber optic system of this invention is significantly higher than that of the single-mode fiber optic system. In the low-frequency range, the single-mode fiber optic system has virtually no response, while the few-mode fiber optic system can detect vibration signals. In other frequency ranges (18~40Hz), the sensitivity of the two systems is roughly equivalent. This result verifies the advantages of five-mode fiber optics in low-frequency vibration detection and effectively solves the problem of insufficient sensitivity of traditional single-mode fiber optic systems for detecting low-frequency vibration signals.

[0033] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A distributed acoustic wave sensing integrated device based on five-mode optical fiber, characterized in that, include: Distributed acoustic wave sensor integrated chassis (DAS all-in-one machine); The light source module, the light modulation module, and the photoelectric detection module are all integrated into the integrated chassis. The fiber optic sensing module is fixed to the outside or inside of the vibration substrate to be measured and connected to the DAS all-in-one machine. A data acquisition and signal processing module is located outside the integrated chassis and is communicatively connected to the photoelectric detection module.

2. The DAS integrated device based on five-mode optical fiber according to claim 1, characterized in that: This invention adopts an integrated chassis design, integrating all core optical and electronic modules into a single chassis. The overall dimensions are 400mm×290mm×74mm, and the optical components are connected through the flange port on the outer wall of the integrated chassis.

3. The DAS integrated device based on five-mode optical fiber according to claim 1, characterized in that: The light source module includes a narrow linewidth laser and a 1*2 optical splitter. The narrow linewidth laser outputs a highly coherent continuous optical signal with a typical wavelength of 1550nm. The 1*2 optical splitter splits the continuous optical signal into two paths: a reference light and a signal light.

4. The DAS integrated device based on five-mode optical fiber according to claim 1, characterized in that: The optical modulation module includes an acousto-optic modulator (AOM) and an optical fiber amplifier. The AOM receives the continuous optical signal output from the light source module, modulates the continuous light into pulsed light, and causes a frequency shift of approximately 80 MHz, resulting in a frequency of (ν+80) MHz. The modulated optical pulse signal is then amplified by an optically induced frequency amplifier (EDFA).

5. The DAS integrated device based on five-mode optical fiber according to claim 1, characterized in that: The photoelectric detection module includes an adjustable optical attenuator and a coherent detector. The adjustable optical attenuator processes the reference light and sends it to the coherent detector. The coherent detector collects the processed reference light and the backscattered Rayleigh light, mixes and demodulates the two, and converts the optical signal into an electrical signal.

6. The DAS integrated device based on five-mode optical fiber according to claim 1, characterized in that: The fiber optic sensing module uses five-mode fiber as the sensing medium, and the five-mode fiber can support LP at a wavelength of 1550nm. 01 LP 11 LP 21 LP 02 LP 31 Five linearly polarized modes are used for transmission. The signal light is transmitted in the five-mode fiber and generates back Rayleigh scattered light, which is then transmitted to the photoelectric detection module through the circulator in the fiber amplifier.

7. The fiber optic sensing module according to claim 6, characterized in that: The five-mode optical fiber is laid out and fixed in a specific manner on the surface or inside of the vibration substrate to be tested. The vibration substrate to be tested refers to at least one of the following substrates, including strata soil and rock, bridge structures, oil and gas pipelines, power equipment, and sleeping mattresses, whose working conditions and health status can be monitored through vibration.

8. The DAS integrated device based on five-mode optical fiber according to claim 1, characterized in that: The data acquisition and signal processing module is set outside the integrated chassis and mounted on a computer. It is connected to the photoelectric detection module via an SMA radio frequency cable and includes a data acquisition card and host computer software.

9. The data acquisition and signal processing module according to claim 8, characterized in that: The data acquisition and signal processing module processes the electrical signals to visualize the vibration signals on the optical fiber. Specifically, the data acquisition card performs analog-to-digital conversion on the analog electrical signals output from the coherent detector in the DAS all-in-one machine. Then, the host computer software processes the digital signals and displays the specific vibration waveform of the tested substrate on the screen. When abnormal vibration occurs in the tested substrate, the vibration waveform displayed by the host computer software will change significantly, thus providing an early warning of abnormal conditions.

10. A DAS integrated device based on five-mode optical fiber according to claim 1, characterized in that: The specific connection process of the equipment is as follows: the narrow linewidth laser is connected to a 1*2 optical splitter. The optical splitter splits into two optical fibers, one of which is connected to the acousto-optic modulator and the other to the adjustable optical attenuator. The acousto-optic modulator is then connected to an optical fiber amplifier with a built-in circulator. After the pulsed light comes out of the optical fiber amplifier, it enters the five-mode optical fiber for sensing. Backscattered Rayleigh light is generated in the sensing optical fiber and enters the coherent detector through the other port of the circulator. That is, the optical fiber amplifier is connected to the coherent detector. At the same time, the adjustable optical attenuator is also connected to the coherent detector. Then the output of the coherent detector is connected to the data acquisition card. The output of the data acquisition card is connected to the host computer, and the signal is transmitted into the host computer software.

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