A distributed optical fiber vibration vector sensing system

Abstract

The application provides a distributed optical fiber vibration vector sensing system, which is characterized by being composed of a four-core optical fiber and a demodulation device; the demodulation device comprises a narrow line width laser, two acousto-optic modulators, two optical fiber amplifiers, five 1*2 optical fiber couplers, four optical fiber circulators, two photoelectric detectors, a double-channel data synchronous acquisition unit, a signal generation unit, a data processing unit and a four-core optical fiber fan-in fan-out device. The application can detect the vibration in the radial direction of the optical fiber in real time, including the vibration position, direction, frequency and amplitude and the like, and is suitable for application fields such as structural health monitoring of roads, railways and large building bodies.

Description

(I) Technical Field

[0001] This invention relates to the field of fiber optic sensing, and more specifically to a distributed fiber optic vibration vector sensing technology. (II) Background Technology

[0002] Distributed fiber optic vibration sensing (DVS) uses optical fiber as the sensing medium, treating each point on the fiber as an independent sensor to detect the vibration distribution in and around the fiber in real time. Phase-sensitive optical time-domain reflectometry (φ-OTDR) is currently the most commonly used method for distributed fiber optic vibration sensing. In this method, highly coherent light pulses are injected into the fiber, and the backscattered Rayleigh light excited by the light pulses is detected at the fiber's incident end. The Rayleigh scattered light interferes with each other, forming interference fringes. At the location of the vibration, the refractive index and length of the fiber undergo minute changes. These minute changes cause phase shifts, resulting in changes in the brightness of the interference fringes at the corresponding locations. Therefore, this technique can determine the location of vibration on the fiber, but not the intensity and frequency information. Building upon φ-OTDR, phase demodulation technology is introduced to reconstruct the phase difference distribution on the fiber and its changes over time. Based on the linear relationship between phase difference and vibration intensity, quantitative vibration intensity and frequency information at various points on the fiber can be obtained. This technology is also known as distributed fiber optic acoustic sensing (DAS). DAS technology boasts advantages such as long detection range and high sensitivity, and has been applied in many fields. However, this technology still has some limitations, the most prominent being that it can only obtain scalar vibration information, detecting only vibrations that cause changes along the fiber optic axis, and cannot detect vibrations from the radial direction. In practical applications, especially in the health monitoring of large buildings, it is usually necessary to detect vibrations from all directions. If the aforementioned DAS technology were used to perform vibration detection in all directions, the sensing fiber would be deployed in a very complex manner.

[0003] Using optical frequency domain reflectometer (OFDR) technology to monitor the strain distribution of different cores in a multi-core optical fiber, the three-dimensional shape of the fiber can be reconstructed in real time. This method can realize distributed optical fiber vibration vector sensing, but it still has three shortcomings: First, the detection distance of OFDR is short, generally only tens of meters, which cannot meet the measurement requirements of large buildings; second, the strain resolution of OFDR is on the order of με, resulting in low vibration detection sensitivity achieved by this method; and third, the vibration response frequency of this method is low, generally within 10Hz. (III) Summary of the Invention

[0004] The purpose of this invention is to provide a distributed optical fiber acoustic vector sensing system that can detect vibrations in the radial direction of an optical fiber in real time, including information such as the location, orientation, amplitude, and frequency of the vibrations.

[0005] The objective of this invention is achieved as follows:

[0006] The distributed fiber optic vibration vector sensing system consists of a four-core fiber and a demodulation device. The demodulation device includes: a narrow-linewidth laser, two acousto-optic modulators, two fiber amplifiers, five 1×2 fiber couplers, four fiber circulators, two photodetectors, a dual-channel data synchronous acquisition unit, a signal generation unit, a data processing unit, and a four-core fiber fan-in / fan-out converter. Its working process is as follows: the continuous light output by the narrow-linewidth laser (1) is transmitted through the fiber coupler (201... The light is divided into two paths. One path is modulated into a pulse by an acousto-optic modulator (301), amplified by an optical fiber amplifier (401), and then split into two paths by an optical fiber coupler (202). These paths pass through optical fiber circulators (501) and (502) and enter the two fiber cores (701) and (702) of the four-core optical fiber fan-in fan-out unit (6) through the input ports (601) and (602) of the four-core optical fiber fan-in fan-out unit (6). The second path of light output from the optical fiber coupler (201) is modulated into a pulse by the acousto-optic modulator (302). The light is amplified by an optical fiber amplifier (402) and split into two paths by an optical fiber coupler (203). These paths then pass through optical fiber circulators (503) and (504) respectively, entering the two cores (703) and (704) of the four-core optical fiber (7) through the input ports (603) and (604) of the four-core optical fiber fan-in fan-out unit (6). The backscattered light from the two cores (701) and (703) of the four-core optical fiber (7) passes through circulators (501) and (503) respectively and then into the optical fiber coupler (204). The backscattered light from the two cores (702) and (704) of the four-core optical fiber (7) forms an interference and is converted into an electrical signal by the photodetector (801). The backscattered light from the two cores (702) and (704) of the four-core optical fiber (7) passes through the circulator (502) and (504) respectively to the fiber coupler (205) to form an interference and is converted into an electrical signal by the photodetector (802). The two electrical signals are synchronously acquired by the dual-channel data synchronous acquisition unit (9), and the two phase demodulations are completed in the data processing unit (10) to finally obtain the distributed vibration vector measurement result.

[0007] The four cores of the four-core optical fiber (7) used in this invention have the same refractive index, the core diameter is the same as that of ordinary single-mode optical fiber, and they are distributed in a cross-shaped symmetrical distribution around the center of the optical fiber. The core spacing is greater than 30 micrometers, and the crosstalk between each core is less than -30dB / km.

[0008] The signal generation unit (11) used in this invention generates pulse control signals for the acoustic-optic modulators (301) and (302) and acquisition trigger signals for the dual-channel data synchronous acquisition unit (9) under a clock circuit.

[0009] The radio frequency drive signals of the two acousto-optic modulators (301) and (302) used in this invention have a frequency difference, which is between 20MHz and 100MHz.

[0010] The data processing unit (10) used in this invention first uses the heterodyne method to demodulate the phase of the two orthogonal interference signals, and then determines the vibration direction by the ratio of the amplitudes of the two phase signals.

[0011] Compared with existing technologies, the advantages of this invention are: it can obtain vibration information of various locations on the optical fiber in the radial direction, including vibration orientation, vibration intensity, and vibration frequency, while being insensitive to vibration in the axial direction. Therefore, this system can realize the measurement of deflection or lateral strain of large buildings. (iv) Description of the attached drawings

[0012] Figure 1 This is a schematic diagram of the structure of the present invention;

[0013] Figure 2 This is a schematic diagram of the four-core optical fiber used in this invention;

[0014] Figure 3 This is a schematic diagram showing the bending of a four-core optical fiber under radial force.

[0015] In the diagram: 1-Narrow linewidth laser; 201, 202, 203, 204, 205-1×2 fiber couplers; 301, 302-Acousto-optic modulators; 401, 402-Fiber amplifiers; 501, 502, 503, 504-Fiber circulators; 6-Four-core fiber fan-in / fan-out; 7-Four-core fiber; 801, 802-Photodetectors; 9-Dual-channel synchronous data acquisition unit; 10-Data processing unit; 11-Signal generation unit. (V) Detailed Implementation

[0016] The present invention will be further illustrated below with reference to specific embodiments.

[0017] Figure 1 The diagram shows the structure of a distributed fiber optic vibration vector sensing system, which consists of a four-core fiber and a demodulation device. Figure 2The specification specifies the four-core optical fiber used. This four-core fiber acts as the sensing element, deployed in the area to be measured, such as the roadbed along highways or railways, or inside large buildings, requiring close integration with the object being measured. The demodulation device includes: a narrow-linewidth laser, two acousto-optic modulators, two fiber amplifiers, five 1×2 fiber couplers, four fiber circulators, two photodetectors, a dual-channel data synchronization acquisition unit, a signal generation unit, a data processing unit, and a four-core fiber fan-in / fan-out converter. Its operation can... The description is as follows: The continuous light output from the narrow linewidth laser (1) is split into two paths by the fiber coupler (201). One path of light is modulated into a pulse by the acousto-optic modulator (301), amplified by the fiber amplifier (401), and then split into two paths by the fiber coupler (202). These paths pass through the fiber circulators (501) and (502) respectively and enter the two fiber cores (701) and (702) of the four-core fiber fan-in fan-out unit (6) through the input ports (601) and (602) of the four-core fiber fan-in fan-out unit (6). The output of the fiber coupler (201) is as follows: The second light path is modulated into pulses by an acousto-optic modulator (302), amplified by an optical fiber amplifier (402), and split into two paths by an optical fiber coupler (203). These paths then pass through optical fiber circulators (503) and (504) respectively, entering the two cores (703) and (704) of the four-core optical fiber (7) through the input ports (603) and (604) of the four-core optical fiber fan-in fan-out unit (6). The backscattered light from the two cores (701) and (703) of the four-core optical fiber (7) passes through circulators (501) and (503) respectively. The backscattered light from the two cores (702) and (704) of the four-core fiber (7) forms interference in the fiber coupler (204) and is converted into an electrical signal by the photodetector (801). The backscattered light from the two cores (702) and (704) of the four-core fiber (7) forms interference in the fiber coupler (205) through the circulator (502) and (504) respectively and is converted into an electrical signal by the photodetector (802). The two electrical signals are synchronously acquired by the dual-channel data synchronous acquisition unit (9) and phase demodulation is completed in the data processing unit (10). Finally, the distributed vibration vector measurement result is obtained.

[0018] The measurement method of the present invention is described below.

[0019] The principle of Rayleigh scattering interference between two fiber cores:

[0020] As is well known, when two beams of light interfere without considering polarization, the intensity of the light after interference... In the formula, I1 and I2 represent the intensities of the two interfering light beams, respectively, and Δφ is the phase difference between the two beams. In this invention, the two beams used for interference originate from backscattered Rayleigh light from two different fiber cores. According to the φ-OTDR principle, a highly coherent optical pulse is injected into the optical fiber and propagates within it. The backscattered Rayleigh light within the fiber segment illuminated by the optical pulse interferes with each other. When the optical fiber is not subjected to external disturbance, this interference result can be considered as weak reflection, and its reflectivity and phase remain stable over a relatively long period. Therefore, the interference result of the backscattered Rayleigh light from the two fiber cores can be expressed as: In the formula, P1(t) and P2(t) represent the scattered light from two different fiber cores at time t, and they can be expressed as P i (r)=P0η i (z)exp(-2αz), where P0 represents the peak power of the incident light pulse, η i (z) represents the reflectivity at position z of the i-th fiber core, α is the attenuation coefficient of the optical fiber, and Δφ(t) represents the phase difference between the two interfering beams.

[0021] Heterodyne-based phase demodulation methods:

[0022] The radio frequency drive signals of the two acousto-optic modulators 301 and 301 have a frequency difference Δf, therefore the interference result can be expressed as follows: The intermediate frequency term can be extracted from it using a bandpass filter. After mixing the intermediate frequency term with sin(Δf t) and then low-pass filtering, the result is... After mixing the intermediate frequency term with cos(Δf t), a low-pass filter is obtained. After dividing the two, taking the arctangent and unwinding the phase, Δφ(t) is recovered.

[0023] Methods for determining the direction of vibration:

[0024] like Figure 3 As shown, suppose that at a certain moment, a four-core optical fiber of length L bends with curvature ρ under vibration. If the angle between one set of diagonal cores and the vibration direction is θ, and the distance between the core and the center of the fiber is r, then due to the bending, the length difference of the diagonal cores is ΔL1 = 2ρrL cosθ, and the angle between the second set of diagonal cores and the vibration direction is (π / 2 - θ). Due to the bending, the length difference of the second set of diagonal cores is ΔL2 = 2ρrL sinθ. According to the relationship between phase difference and length difference Δφ = ΔL / λ, as long as the phase difference of the two sets of diagonal cores is obtained, the direction of vibration can be obtained by θ = atan(Δφ1 / Δφ2).

Claims

1. A distributed fiber optic vibration vector sensing system, characterized in that: It consists of a four-core optical fiber and a demodulation device, which includes a narrow linewidth laser, two acousto-optic modulators, two fiber amplifiers, five 1×2 fiber couplers, four fiber circulators, two photodetectors, a dual-channel data synchronous acquisition unit, a signal generation unit, a data processing unit, and a four-core optical fiber fan-in and fan-out unit, etc. The continuous light output from the narrow linewidth laser (1) is split into two paths by the fiber coupler (201). One path is modulated into a pulse by the acousto-optic modulator (301), amplified by the fiber amplifier (401), and split into two paths by the fiber coupler (202). The two paths then pass through the fiber circulators (501) and (502) respectively, and enter the two cores (701) and (702) of the four-core fiber fan-in fan-out (6) through the input ports (601) and (602) of the four-core fiber fan-in fan-out (6). The second path of light output from the fiber coupler (201) is modulated into a pulse by the acousto-optic modulator (302), amplified by the fiber amplifier (402), and split into two paths by the fiber coupler (203). The two paths then pass through the fiber circulators (503) and (504) respectively, and enter the two cores (701) and (702) of the four-core fiber fan-in fan-out (6). The input ports (603) and (604) of the four-core fiber 7 enter the two cores 703 and 704 of the four-core fiber 7. The backscattered light from the two cores 701 and 703 of the four-core fiber 7 passes through the circulator (501) and (503) to the fiber coupler (204) to form interference, and is converted into an electrical signal by the photodetector (801). The backscattered light from the two cores (702) and (704) of the four-core fiber (7) passes through the circulator (502) and (504) to the fiber coupler (205) to form interference, and is converted into an electrical signal by the photodetector (802). The two electrical signals are synchronously acquired by the dual-channel data synchronous acquisition unit (9), and the two phase demodulations are completed in the data processing unit (10) to finally obtain the distributed vibration vector measurement result.

2. The distributed fiber optic vibration vector sensing system according to claim 1, characterized in that: The four cores of the four-core optical fiber (7) have the same refractive index, the core diameter is the same as that of ordinary single-mode optical fiber, and they are distributed in a cross-shaped symmetrical distribution around the center of the optical fiber. The core spacing is greater than 30 micrometers, and the crosstalk between each core is less than -30dB / km.

3. The distributed optical fiber vibration vector sensing system according to claim 1, characterized in that: The signal generation unit (11) generates pulse control signals for the acoustic-optic modulators (301) and (302) and acquisition trigger signals for the dual-channel data synchronous acquisition unit (9) under the same clock circuit.

4. The distributed optical fiber vibration vector sensing system according to claim 1, characterized in that: The radio frequency drive signals of the two acousto-optic modulators (301) and (302) have a frequency difference between 20MHz and 100MHz.

5. The distributed optical fiber vibration vector sensing system according to claim 1, characterized in that: The data processing unit (10) first uses the heterodyne method to demodulate the phase of the two orthogonal interference signals, and then determines the vibration direction by the ratio of the amplitudes of the two phase signals.

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