A signal processing host for a fiber optic acoustic sensor

Abstract

The application provides a signal processing host for a fiber acoustic wave sensor, and relates to the technical field of fiber sensors.The host comprises a polarization processing device, which is used for decomposing received back Rayleigh scattering light into two linearly polarized lights with perpendicular polarization directions; a 3*3 coupler, which is used for receiving two linearly polarized lights and reference light which are perpendicular to each other, and performing coupling coherent mixing of the linearly polarized light and the reference light to obtain interference light signals; a photoelectric detector, which is used for receiving the interference light signals and converting the interference light signals into electric signals; a digital signal processor, which is used for converting the electric signals into digital signals; and a three-dimensional filter, which is used for performing time dimension filtering, space dimension filtering and frequency-space joint dimension filtering on the digital signals.The application provides a signal processing host for a fiber acoustic wave sensor, and can increase the signal-to-noise ratio of the fiber acoustic wave sensor.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensor technology, and in particular to a signal processing host for fiber optic acoustic sensors. Background Technology

[0002] In underground mines, sound wave monitoring is necessary. Mine tunnels are interconnected and stretch for kilometers, making distributed fiber optic acoustic sensors crucial for monitoring sound waves over large areas. Distributed fiber optic acoustic sensing (DAS) technology is based on phase-sensitive optical time-domain reflectometry (OTDR). The principle is to transform ordinary communication optical fibers, ranging from several kilometers to hundreds of kilometers, into a continuously distributed array of acoustic wave sensors. However, its practical application is severely limited by inherent noise.

[0003] Therefore, there is an urgent need for a closed-loop processing device that can combine physical layer suppression and data layer intelligent enhancement to increase the signal-to-noise ratio of fiber optic acoustic sensors. Summary of the Invention

[0004] This invention provides a signal processing host for fiber optic acoustic sensors, which can increase the signal-to-noise ratio of the fiber optic acoustic sensors. The technical solution of this invention is as follows: A signal processing host for fiber optic acoustic sensors, comprising: A polarization processing device for decomposing received backscattered Rayleigh light into two linearly polarized lights with perpendicular polarization directions; A 3×3 coupler is used to receive two mutually perpendicular linearly polarized beams and a reference beam, and to perform coherent mixing of the linearly polarized beams and the reference beam to obtain an interference light signal; A photodetector is used to receive the interference optical signal and convert it into an electrical signal; A digital signal processor is used to convert the electrical signal into a digital signal, the digital signal including phase, time, and position information; A three-dimensional filter is used to perform time-dimensional filtering, spatial-dimensional filtering, and frequency-space joint-dimensional filtering on the digital signal.

[0005] Preferably, the polarization processing device includes a beam splitter, a linear polarizer, a first reflecting mirror, and a quarter-wave plate; The beam splitter is used to split the backscattered Rayleigh light into two incident beams. The two incident beams are perpendicular to each other and both are incident on the linear polarizer at a 45° angle. The projection of the linear polarizer's grating onto the quarter-wave plate forms a 45° angle with the fast or slow axis of the quarter-wave plate. The angle between the linear polarizer and the incident beam is 45°. The first reflector is located downstream of the quarter-wave plate and parallel to the quarter-wave plate.

[0006] Preferably, the beam splitter includes a transmission mirror and a second mirror, the second mirror being located in the direction of the reflected light propagation of the linear polarizer and perpendicular to the direction of the reflected light of the linear polarizer, and the transmission mirror being parallel to the linear polarizer and located upstream of the linear polarizer.

[0007] Preferably, a frequency selection layer is disposed upstream of the linear polarizer, the frequency selection layer being used to allow high-frequency backscattered Rayleigh light to pass through while shielding low-frequency Brillouin scattered light.

[0008] Preferably, the frequency selection layer includes dielectric layers on both sides and a periodic unit in the middle. The periodic unit is periodically distributed with periodic metal layers. Each periodic metal layer has a square metal plate in shape. The square metal plate has a circular cutout at the center and four square cutouts around its perimeter.

[0009] Preferably, the side length of the square metal plate is 60~90μm, the diameter of the circular cutout is 45~85μm, and the side length of the square cutout is 8~15μm.

[0010] Preferably, the time-dimensional filtering includes using recursive least squares adaptive filtering with quiet channel or historical background data as reference noise.

[0011] Preferably, the spatial dimension filtering includes moving average or median filtering along the fiber distance axis to suppress isolated, spatially uncorrelated shot noise and spikes.

[0012] Preferably, the frequency-space joint dimensional filtering includes performing a two-dimensional Fourier transform on the spatiotemporal data of the digital signal.

[0013] Compared with the prior art, the present invention has at least the following beneficial effects: This invention utilizes a polarization processing device to obtain two linearly polarized lights with perpendicular polarization directions, and then allows each component to interfere with a reference light. In this way, regardless of the polarization rotation of the signal light, at least one interference channel maintains a high signal strength. By processing the signal into two paths, phase information can be reliably demodulated, thus overcoming fading. After demodulation with a 3×3 coupler, a two-dimensional data matrix relating time and fiber position is obtained, containing the target signal and a significant amount of noise. The combined space-time-frequency three-dimensional filtering effectively separates the signal and noise from various dimensions. Therefore, this invention increases the signal-to-noise ratio of fiber optic acoustic sensors. Attached Figure Description

[0014] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0015] Figure 1 This is a schematic diagram of an optical structure of the polarization processing device in this invention; Figure 2 This is a schematic diagram of one structure of the frequency selection layer in this invention; Figure 3 This is a schematic diagram of the structure of the periodic metal in this invention.

[0016] In the picture: 1-Linear polarizer; 2-First reflecting mirror; 3-1 / 4 wave plate; 4-Second reflecting mirror; 5-Reflective mirror; 6-Frequency Selection Layer. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0018] like Figure 1 As shown, this embodiment provides a signal processing host for an optical fiber acoustic sensor, including: A polarization processing device for decomposing received backscattered Rayleigh light into two linearly polarized lights with perpendicular polarization directions; A 3×3 coupler is used to receive two mutually perpendicular linearly polarized beams and a reference beam, and to perform coherent mixing of the linearly polarized beams and the reference beam to obtain an interference light signal; A photodetector is used to receive the interference optical signal and convert it into an electrical signal; A digital signal processor is used to convert the electrical signal into a digital signal, the digital signal including phase, time, and position information; A three-dimensional filter is used to perform time-dimensional filtering, spatial-dimensional filtering, and frequency-space joint-dimensional filtering on the digital signal.

[0019] In this embodiment, a polarization processing device can be used to obtain two linearly polarized lights with perpendicular polarization directions, which helps to solve polarization fading noise. In DAS, after the signal light propagates in a long-distance optical fiber, its polarization state rotates randomly. When it interferes with the reference light, if the polarization directions of the two are inconsistent, the interference signal will weaken or even disappear ("fading"). Therefore, the light is split into two orthogonal components, and then each component interferes with the reference light separately. In this way, no matter how the polarization of the signal light rotates, at least one interference channel can maintain a high signal strength. By processing it into two signals, phase information can be reliably demodulated, thereby overcoming fading.

[0020] In conventional interferometers (such as Mach-Zehnder interferometers using 50 / 50 couplers), the system output is... ,in, I Light intensity signal The phase difference to be measured. This method has two fundamental weaknesses: signal fading and operating point drift. The 3×3 coupler scheme completely solves these two problems at the physical level. Specifically, the 3×3 coupler scheme outputs three optical intensity signals. I 1. I 2. I 3: ; ; ; in, The DC component includes the non-interfering background light intensity and the DC bias of the detector. This refers to the amplitude.

[0021] Even if polarization fading affects the amplitude of a certain path Attenuation is extremely rare; the probability of all three light intensity signals simultaneously attenuating to zero is very low. By combining the three light intensity signals (such as through differential cross-multiplication), the attenuation of amplitude can be eliminated. The dependence is directly calculated from the phase relationship of the three light intensity signals. This allows it to work stably even when the polarization state changes arbitrarily.

[0022] In addition, its demodulation algorithm (such as (Or based on a combination of arctangent operations) can directly output The linear value of the signal is limited only by the electronic system and algorithm, theoretically reaching tens to hundreds of radians. This allows the system to measure large-amplitude vibrations without signal saturation or ambiguity. Differential processing of the three optical intensity signals effectively suppresses common-mode noise caused by laser intensity fluctuations (relative intensity noise RIN), further improving the signal-to-noise ratio.

[0023] In summary, the 3×3 coupler is a passive optical device that does not require external power supply or active feedback control (such as an optical phase-locked loop), resulting in a simpler system structure and better long-term stability.

[0024] After demodulation by a 3×3 coupler, a time-related parameter can be obtained. t and fiber optic location z Two-dimensional data matrix This contains the target signal and a large amount of noise. The three-dimensional joint filtering of space, time, and frequency, formed by time-dimensional filtering, spatial-dimensional filtering, and frequency-space joint filtering, acts like a "smart filter," separating the signal from the noise from various dimensions.

[0025] In some embodiments of the present invention, the polarization processing device includes a beam splitter, a linear polarizer 1, a first reflecting mirror 2, and a quarter-wave plate 3; The beam splitter is used to split the backscattered Rayleigh light into two incident beams. The two incident beams are perpendicular to each other and both are incident on the linear polarizer 1 at a 45° angle. The projection of the grating of the linear polarizer 1 onto the quarter-wave plate forms a 45° angle with the fast axis or slow axis of the quarter-wave plate 3. The angle between the linear polarizer 1 and the incident beam is 45°. The first reflector 2 is located downstream of the quarter-wave plate 3 and is parallel to the quarter-wave plate.

[0026] As mentioned earlier, comparing two linearly polarized beams with a reference beam, each with a perpendicular polarization direction, can overcome polarization fading. However, this requires designing two separate optical path comparison systems. In this embodiment, a beam splitter is used to split the backscattered Rayleigh light into two beams that are incident on the linear polarizer 1 at different angles. One of the incident beams, after entering the linear polarizer 1, forms linearly polarized light. The polarization direction of this linearly polarized light has a 45° angle with the fast or slow axis of the quarter-wave plate 3. This causes the linearly polarized light to form circularly polarized light after passing through the quarter-wave plate 3. After being reflected by the first reflecting mirror 2, the phase of the circularly polarized light changes, causing its rotation direction to change. The changed circularly polarized light then passes through the quarter-wave plate 3 again, forming linearly polarized light with a polarization direction perpendicular to the initially formed linearly polarized light. This linearly polarized light is then reflected after entering the linear polarizer 1. When another incident beam of light enters linear polarizer 1, it also forms linearly polarized light. Thus, the two beams of linearly polarized light with orthogonal polarization directions that pass through and are reflected by linear polarizer 1 coincide. Therefore, only one interference system is needed to form interference fringes.

[0027] The specific process of linearly polarized light passing through linear polarizer 1, quarter-wave plate 3, and first reflecting mirror 2 is as follows: The polarization direction of linearly polarized light forms a 45° angle with the fast axis (or slow axis) of the quarter-wave plate 3.

[0028] The fast axis of the quarter-wave plate 3 is along the x-direction, and the slow axis is along the y-direction. Its Jones matrix is... (This indicates that the x component leads the y component by π / 2 in phase), where i is the imaginary unit.

[0029] The first reflecting mirror 2 is perpendicular to the incident light and has a reflection coefficient of r = −1 (that is, reflection from a less dense medium to a denser medium introduces a π phase jump, commonly known as "half-wave loss").

[0030] We ignore the phase of the geometric path of light as it travels through the air (this phase is related to the optical path, but we will focus on the phase changes introduced by the device here).

[0031] Incident ray: The incident ray is linearly polarized, making a 45° angle with the x-axis, and its Jones vector... .

[0032] First pass through 1 / 4 wave plate 3: This produces right-handed circularly polarized light; in, The Jones vector of the light after passing through the 1 / 4 wave plate 3.

[0033] reflection: After reflection, the rotation direction of circular polarization is reversed (becomes left-handed), and an overall phase π is introduced; in, Let Jones vector be the light reflected by the first reflecting mirror 2. This is a reflection operator.

[0034] Second pass through 1 / 4 wave plate 3: ; in, The Jones vector of the light after it passes through the 1 / 4 wave plate 3 for the second time.

[0035] The outgoing ray is linearly polarized and propagates in a direction at a -45° angle to the x-axis. It is rotated 90° relative to the incident ray and has an overall factor of −1, meaning that the phase increases by π.

[0036] In some embodiments of the present invention, the beam splitter includes a transflector 5 and a second reflector 4. The second reflector 4 is located in the direction of propagation of the reflected light of the linear polarizer 1 and is perpendicular to the direction of the reflected light of the linear polarizer 1. The transflector 5 is parallel to the linear polarizer 1 and is located upstream of the linear polarizer (1) (preferably the two are attached together).

[0037] In this embodiment, the phase of the two linearly polarized beams can be controlled at 180°. Specifically, the light is described using an electric field vector. Assuming the two linearly polarized beams propagate along the z-axis, their electric fields are as follows: Horizontal components ; Vertical component (phase difference) That is, 180 degrees); where, For phase, For time, , These are the amplitudes of two orthogonally polarized linearly polarized lights, respectively.

[0038] The vertical component is due to the phase difference Its vibration direction is always opposite to the horizontal component.

[0039] Instantaneous electric field of synthetic light It is the vector sum of the two, that is .

[0040] At any given moment, the direction of the resultant vector is fixed because it satisfies the following relationship: This is a constant.

[0041] Two orthogonally linearly polarized lights with a phase difference of 0° or 180° will always result in linearly polarized light, the angle θ between its polarization direction and the x-axis being determined by the amplitude ratio. .

[0042] This setup has the following specific advantages: Advantage 1: Generates stable and controllable linearly polarized light. The amplitudes of the two orthogonal polarization components can be controlled ( and This allows for precise and continuous rotation of the pointing angle of the output linearly polarized light without the need for mechanical rotation of the polarization element. This has important applications in high-speed polarization control systems.

[0043] Advantage 2: Provides ideal input for polarization diversity reception: This is its most important application in systems such as DAS and fiber optic communication. As mentioned earlier, the polarization state of signal light rotates randomly after long-distance transmission, leading to interference fading.

[0044] Advantage 3: Optimizing the operating point in zero-difference detection: In the interferometer, the photodetector outputs a signal of: This cosine function is in The point at which the slope is 90° or 270° (i.e., the point of maximum slope) is most sensitive to small phase changes and is called the optimal operating point.

[0045] By controlling the relative phase between the two polarization components of the signal light (for example, setting it to 180°), it is possible to indirectly help lock the phase operating point of the entire interferometer around the optimal 90°, thereby obtaining the highest detection sensitivity.

[0046] In this embodiment, the method to control the phase difference of the two polarized beams to 180° can be: adding a 1 / 2 waveplate to any one of the two beam paths.

[0047] It should be noted that the two beams of light undergo the same number of reflections at 0° and 45°, thus maintaining the same phase. When one of the linearly polarized beams passes through the quarter-wave plate 3, the first mirror 2, and the quarter-wave plate 3 in sequence, its phase only changes by 180° due to the 0° reflection from the first mirror 2. During the two passes through the quarter-wave plate 3, only the polarization direction changes.

[0048] In some embodiments of the present invention, a frequency selection layer 6 is provided upstream of the linear polarizer 1, the frequency selection layer 6 being used to shield low-frequency Brillouin scattering light through high-frequency backscattered Rayleigh light.

[0049] In this embodiment, in order to improve the signal-to-noise ratio of Rayleigh scattered light, the lower frequency Brillouin scattered light is shielded by the frequency selection layer 6 (high frequency is blocked by low frequency), allowing the higher frequency Rayleigh scattered light to pass through.

[0050] Please refer to Figure 2 and Figure 3 In some embodiments of the present invention, the frequency selection layer 6 includes dielectric layers on both sides and a periodic unit in the middle. The periodic unit is periodically distributed with periodic metal layers. The shape of each periodic metal layer includes a square metal plate. The center of the square metal plate is a circular cutout, and four square cutouts are processed around the square metal plate.

[0051] In this embodiment, Rayleigh scattered light is a very high-frequency light wave; therefore, a frequency-selective structure for high-frequency Rayleigh waves needs to be designed. Periodically arranged metal layers of a special shape can create narrow-band selective transmission for Rayleigh scattered light.

[0052] In some embodiments of the present invention, the side length of the square metal plate is 60~90μm, the diameter of the circular cutout is 45~85μm, and the side length of the square cutout is 8~15μm.

[0053] In some embodiments of the present invention, the time-dimensional filtering includes using recursive least squares adaptive filtering with quiet channel or historical background data as reference noise.

[0054] In this embodiment, time-dimensional filtering performs digital filtering (such as bandpass or high-pass) on the vibration time series of each point on the optical fiber. Specifically, it can: filter out low-frequency drift: remove extremely low-frequency (<0.1Hz) phase drift caused by temperature changes, which is one of the strongest interferences in DAS; focus on the target frequency band: retain only the characteristic frequencies and directly filter out out-of-band noise; suppress narrowband interference: eliminate fixed-frequency interference such as 50 / 60Hz power frequency and its harmonics through notch filters.

[0055] In some embodiments of the present invention, the spatial dimension filtering includes moving average or median filtering along the fiber distance axis to suppress isolated, spatially uncorrelated shot noise and spikes.

[0056] In this embodiment, spatial dimension filtering filters the spatial distribution of vibration at each moment (along the fiber) using methods such as moving average or median filtering. Specifically, it smooths random noise: shot noise, thermal noise, etc., are spatially uncorrelated random spikes. Spatial smoothing effectively suppresses these isolated noise points while preserving the true vibration events (which typically affect a continuous section of fiber). It sharpens event edges: differential filtering can enhance the abrupt changes in the start and end positions of vibration events, improving positioning accuracy from meter-level to sub-meter-level. It suppresses fixed-pattern noise related to spatial location.

[0057] In some embodiments of the present invention, the frequency-space joint dimensional filtering includes performing a two-dimensional Fourier transform on the spatiotemporal data of the digital signal to identify and filter out interference patterns with specific propagation speeds in the frequency-space plane.

[0058] In this embodiment, the frequency-space joint dimensionality filtering pair A two-dimensional Fourier transform of the matrix yields the frequency-space spectrum, allowing analysis of the propagation characteristics of different frequency components along the optical fiber. Specifically, it enables the separation of waves with different velocities: for example, Rayleigh waves and volume waves generated by ground vibrations have different propagation speeds, appearing as straight lines with different slopes in the frequency-space spectrum. Two-dimensional filters can be designed to retain only components with specific slopes, thereby separating waves caused by specific events (such as intrusions) and filtering out environmental background vibrations. It can also suppress interference with specific propagation patterns: such as mechanical waves propagating along pipelines or traffic vibrations from specific directions.

[0059] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A signal processing host for fiber optic acoustic sensors, characterized in that, include: A polarization processing device for decomposing received backscattered Rayleigh light into two linearly polarized lights with perpendicular polarization directions; A 3×3 coupler is used to receive two mutually perpendicular linearly polarized beams and a reference beam, and to perform coherent mixing of the linearly polarized beams and the reference beam to obtain an interference light signal; A photodetector is used to receive the interference optical signal and convert it into an electrical signal; A digital signal processor is used to convert the electrical signal into a digital signal, the digital signal including phase, time, and position information; A three-dimensional filter is used to perform time-dimensional filtering, spatial-dimensional filtering, and frequency-space joint-dimensional filtering on the digital signal. The polarization processing device includes a beam splitter, a linear polarizer (1), a first reflecting mirror (2), and a quarter-wave plate (3). The beam splitter is used to split the backscattered Rayleigh light into two incident beams. The two incident beams are perpendicular to each other and both are incident on the linear polarizer (1) at an angle of 45°. The projection of the grating of the linear polarizer (1) onto the quarter-wave plate (3) forms a 45° angle with the fast axis or slow axis of the quarter-wave plate (3). The angle between the linear polarizer (1) and the incident beam is 45°. The first reflector (2) is located downstream of the quarter-wave plate and parallel to the quarter-wave plate. The beam splitter includes a transflector (5) and a second reflector (4). The second reflector (4) is located in the direction of the reflected light propagation of the linear polarizer (1) and is perpendicular to the direction of the reflected light of the linear polarizer (1). The transflector (5) is parallel to the linear polarizer (1) and located upstream of the linear polarizer (1). A frequency selection layer (6) is provided upstream of the linear polarizer (1). The frequency selection layer (6) is used to shield low-frequency Brillouin scattering light while allowing high-frequency backscattered Rayleigh light to pass through.

2. The signal processing host for an optical fiber acoustic sensor according to claim 1, characterized in that, The frequency selection layer (6) includes dielectric layers on both sides and a periodic unit in the middle. The periodic unit is periodically distributed with a periodic metal layer. The shape of each periodic metal in the periodic metal layer includes a square metal plate. The center of the square metal plate is a circular cutout, and four square cutouts are processed around the square metal plate.

3. A signal processing host for an optical fiber acoustic sensor according to claim 2, characterized in that, The square metal plate has a side length of 60 μm, the circular cutout has a diameter of 45 μm, and the square cutout has a side length of 8 μm.

4. A signal processing host for an optical fiber acoustic sensor according to claim 1, characterized in that, The time-dimensional filtering includes using quiet channel or historical background data as reference noise and employing recursive least squares adaptive filtering.

5. A signal processing host for an optical fiber acoustic sensor according to claim 1, characterized in that, The spatial dimension filtering includes moving average or median filtering along the fiber distance axis to suppress isolated, spatially uncorrelated shot noise and spikes.

6. A signal processing host for an optical fiber acoustic sensor according to claim 1, characterized in that, The frequency-space joint dimensional filtering includes performing a two-dimensional Fourier transform on the spatiotemporal data of the digital signal.

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