Optical fiber distributed sound wave sensing method and system based on optical fiber end face phase control

Abstract

The invention discloses an optical fiber distributed sound wave sensing system based on optical fiber end face phase control, which is characterized by comprising a narrow linewidth light source, a first optical coupler, an optical modulator, a circulator, a spatial light modulation component, a second optical coupler, a pulse modulation driving circuit, a beam expander, an image collector and a data collection and signal processing module, specific phase regulation and control of backscattering light are realized by utilizing a spatial light modulation component, so that controllable phase shift is introduced in the transmission process of an optical signal, specifically, local oscillation light and the backscattering light subjected to phase modulation interfere in a second coupler, and vibration information and a strain signal are demodulated by analyzing an interference pattern or outputting a plurality of paths of signals. Through the accurate control of the optical fiber end surface phase, the system structure is simplified, the calculation complexity of signal processing is remarkably reduced, the reliability and easy integration of the system are improved, and the system is suitable for high-precision distributed sound wave sensing in a complex environment.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber sensing, and in particular relates to an optical fiber distributed acoustic wave sensing method and system based on optical fiber end face phase control. Background Art

[0002] Fiber-optic distributed acoustic sensing (DAS) technology obtains environmental vibration and strain information by detecting the backscattered light excited in the optical fiber. Due to the advantages of optical fiber such as low loss, anti-electromagnetic interference and tolerance to harsh environments, DAS technology has been widely used in oil and gas pipeline monitoring, border security, railway and tunnel structural health monitoring and other fields. However, most of the existing phase demodulation methods rely on complex interferometer structures or multi-channel detectors, which not only increases the complexity of the system, hardware cost and maintenance difficulty, but also has high computational complexity in high-frequency signal detection, making it difficult to meet real-time requirements.

[0003] Phase demodulation is a key technology in fiber optic sensing technology. Common phase detection methods include heterodyne detection and homodyne detection. Heterodyne detection detects external events by interfering backscattered light with local reference light and then analyzing the phase of the interfered signal. Homodyne detection relies on pulse pair detection or delayed interferometry to obtain the required phase information by introducing a phase shift at the receiving end. Although high-precision detection can be achieved, its limitation is that it relies on multi-path signal detection and complex interference structure, which increases the system design and maintenance costs. Summary of the invention

[0004] In view of the above, the purpose of the present invention is to provide a fiber optic distributed acoustic wave sensing method and system based on fiber end face phase control, which utilizes fiber end face phase control technology to achieve system simplification and performance optimization. It is particularly suitable for vibration and strain monitoring application scenarios that require high precision, high integration, and high stability, including but not limited to energy pipeline monitoring, infrastructure health monitoring and other fields.

[0005] To achieve the above-mentioned purpose of the invention, an embodiment provides an optical fiber distributed acoustic wave sensing system based on optical fiber end face phase control, including a narrow linewidth light source, a first optical coupler, an optical modulator, a circulator, a spatial light modulation component, a second optical coupler, a pulse modulation drive circuit, a beam expander, an image collector, and a data acquisition and signal processing module;

[0006] The output light of the narrow linewidth light source is divided into detection light and local oscillator light through an optical coupler. The detection light passes through an optical modulator and is modulated under the control of a modulated electrical signal of a pulse modulation drive circuit to output a detection light pulse. The detection light pulse is input into the sensing optical fiber through a circulator. The backscattered light returned by the sensing optical fiber is input into a spatial light modulation component through a circulator. The spatial light modulation component modulates the transverse cross-sectional phase distribution of the backscattered light and outputs it to a second optical coupler. The interference light signal formed by the second optical coupler and the local oscillator light is expanded by a beam expander and then collected by an image collector to obtain an interference pattern. The data acquisition and signal processing module processes the collected interference pattern to obtain a phase difference, thereby realizing fiber-optic distributed acoustic wave sensing.

[0007] Preferably, the spatial light modulation element modulates the transverse cross-sectional phase of the backscattered light. It shows multiple periodic changes with the spatial angle θ, that is, is a positive integer to ensure the periodicity of the phase.

[0008] Preferably, the spatial light modulation element modulates the transverse cross-sectional phase of the backscattered light. It changes in a step-by-step manner.

[0009] Preferably, the spatial light modulation element is of a phase type, including a phase type mask, a spatial light modulator, or a light-transmitting medium whose thickness varies with angle.

[0010] Preferably, the interference light signal formed by the second optical coupling is:

[0011]

[0012] Among them, E0 is the local oscillator light, E2 is the backscattered light after the transverse cross-section phase modulation, A0 and A2 are the two amplitude factors, The optical path difference introduced by the local oscillator light transmitted to the second coupler and the backscattered light transmitted to the spatial light modulation component, The phase difference introduced when the backscattered light after lateral cross-sectional phase modulation is transmitted to the second coupler.

[0013] Preferably, the collected interference pattern is processed to obtain the phase difference, including:

[0014] Extract feature points or lines from the interference pattern, track the movement of feature points or lines at each moment, and calculate the relationship between the angular velocity of the interference pattern and time t. t , angular velocity and signal phase The angular frequencies are completely synchronized, that is The phase difference is calculated based on this

[0015] Preferably, the collected interference pattern is processed to obtain the phase difference, including:

[0016] The collected interference pattern is evenly divided into m regions. Each signal corresponding to each region undergoes different modulations to generate phase offsets. The phase difference is calculated based on each signal after modulation.

[0017] Preferably, the system also includes an amplifier. The detection light pulse output by the optical modulator is amplified by the amplifier and then input into the sensing optical fiber through a circulator. The optical amplifier is an erbium-doped fiber amplifier, a semiconductor optical amplifier or other types of amplifiers that can increase the energy of the input optical signal.

[0018] Preferably, the system further comprises a bandpass filter introduced after the amplifier, and the amplified detection light pulse is filtered by the bandpass filter and then input into the sensing optical fiber through a circulator.

[0019] To achieve the above-mentioned object of the invention, an embodiment of the present invention further provides a fiber-optic distributed acoustic wave sensing method based on fiber end face phase control, wherein the method adopts the above-mentioned fiber-optic distributed acoustic wave sensing system and comprises the following steps:

[0020] The narrow line width light source generates a light pulse and emits it to the optical coupler to be divided into a detection light and a local oscillator light. The detection light passes through the optical modulator and is modulated under the control of the modulation electrical signal of the pulse modulation driving circuit and outputs the detection light pulse.

[0021] The detection light pulse is input into the sensing optical fiber through the circulator, and the backscattered light returned by the sensing optical fiber is input into the spatial light modulation component through the circulator;

[0022] The spatial light modulation element modulates the transverse cross-sectional phase distribution of the backscattered light and then outputs it to the second optical coupler. The second optical coupler interferes with the local oscillator light to form an interference light signal, which is then expanded by a beam expander and collected by an image collector to obtain an interference pattern.

[0023] The data acquisition and signal processing module processes the collected interference pattern to obtain the phase difference, thereby realizing fiber optic distributed acoustic wave sensing.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] The present invention introduces a spatial light modulation component at the output end face of the optical fiber, and uses the spatial light modulation component to achieve specific phase control of the backscattered light, so that the optical signal introduces a controllable phase shift during the transmission process. Specifically, the local oscillator light and the phase-modulated backscattered light interfere in the second coupler, and the vibration information and strain signal are demodulated by analyzing the interference pattern or outputting multiple signals. Through the precise control of the phase of the optical fiber end face, the present invention simplifies the system structure while significantly reducing the computational complexity of signal processing, improving the reliability and ease of integration of the system, and is suitable for high-precision distributed acoustic wave sensing in complex environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0027] Figure 1 is a schematic structural diagram of an optical fiber distributed acoustic wave sensing system based on optical fiber end face phase control provided in an embodiment;

[0028] Figure 2 is a schematic diagram of the mode distribution of a single-mode optical fiber provided in an embodiment, showing the cross-sectional distribution characteristics of the optical field at the end face of the optical fiber;

[0029] Figure 3 is a schematic diagram of a phase modulation method of a spatial light modulator provided in an embodiment, showing an implementation form of linear phase modulation;

[0030] Figure 4 is a schematic diagram of another phase modulation method of the spatial light modulator provided in the embodiment, showing the implementation form of step phase modulation;

[0031] Figure 5 is an example of light intensity distribution of the interference signal pattern provided by the embodiment;

[0032] Figure 6 is a flow chart of signal interference pattern demodulation provided by an embodiment, illustrating phase demodulation through image processing;

[0033] Figure 7 FIG. 1 is a flow chart of signal interference pattern demodulation provided in an embodiment, illustrating phase demodulation of multi-channel signals. DETAILED DESCRIPTION

[0034] To make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific implementation methods described herein are only used to explain the present invention and do not limit the scope of protection of the present invention.

[0035] like Figure 1 As shown, an embodiment provides an optical fiber distributed acoustic wave sensing system based on optical fiber end face phase control, including a narrow linewidth light source, a first optical coupler (referred to as optical coupler 1), an optical modulator, an optical amplifier, a circulator, a spatial light modulator, a second optical coupler (referred to as optical coupler 2), a pulse modulation drive circuit, a beam expander, an image collector, and a data acquisition and signal processing module.

[0036] In the embodiment, the narrow linewidth light source outputs coherent light with a frequency of f0, which enters the 90:10 optical coupler 1 and is divided into two paths, one of which is the local oscillator light connected to the guide coupler 2 (spatial optical coupling), and the other is the detection light, which passes through the optical modulator to generate a repetition frequency of f s , the detection light pulse with a pulse width of Δτ, the specific optical modulator is modulated by the modulation electrical signal output by the pulse modulation driving circuit, and the pulse modulation driving circuit is also under the control of the signal acquisition and processing module to ensure the synchronization of the signal.

[0037] In the embodiment, the detection light pulse output by the optical modulator enters the circulator port 1 after being amplified by the optical amplifier, and is injected into the sensing optical fiber through port 2. When transmitting in the sensing optical fiber, it excites Rayleigh backscattered light, and the backscattered light carries the vibration sensing information back, enters the circulator port 2, and enters the spatial light modulator through port 3 to modulate the transverse phase distribution of the optical fiber end cross section. Among them, the optical amplifier can be an erbium-doped fiber amplifier, a semiconductor optical amplifier, or other types of amplifiers that can increase the energy of the input optical signal.

[0038] In the embodiment, the sensing optical fiber can be an ordinary single-mode optical fiber, an optical fiber with continuous enhancement of scattered signals after special processing, a point-scattering enhanced optical fiber with discrete reflection points or fiber gratings, or other types of optical fibers that can generate backscattered light signals. The optical field distribution characteristics of this type of optical fiber end face are that the central optical power is the strongest and gradually weakens outwards, such as Figure 2 As shown, the shadow represents weak optical power and the phase is evenly distributed on the cross section.

[0039] The spatial light modulator is a phase-type spatial light modulator SLM, or a phase-type mask, or a light-transmitting medium whose thickness varies with the angle. Its function is to modulate the transverse cross-sectional phase distribution of the scattered signal light output from the circulator port 3 to The lateral cross-sectional phase It shows multiple periodic changes with the spatial angle θ. Figure 3As shown, when θ rotates from 0 to 2π, the phase They can undergo changes of 2π, 4π, 6π, ..., more specifically but not limited to, they can satisfy Where s=1,2,3,…is a positive integer to ensure the periodicity of the phase.

[0040] The embodiment also provides a method for modulating the transverse cross-sectional phase of the optical fiber end face. There are step-by-step changes, such as Figure 4 As shown, phase shift is applied in different regions. Figure 4 The left figure shows the application of The phase shift of Figure 4 The middle picture shows the application of The phase shift, Figure 4 The figure on the right shows the application of phase shift.

[0041] In the embodiment, the backscattered light after the transverse cross-section phase modulation enters the coupler 2, and spatially interferes with the local oscillator light from the coupler 1. The local oscillator light, the backscattered light output by the circulator, and the backscattered light modulated by the spatial light modulator are denoted as E0, E1, and E2 respectively:

[0042]

[0043] in, The optical path difference introduced by the local oscillator light transmitted to the coupler 2 and the backscattered light transmitted to the spatial light modulation component, The phase difference is introduced when the backscattered light after the transverse cross-section phase modulation is transmitted to the coupler 2. The local oscillator light and the modulated scattered light interfere with each other in the coupler 2 to obtain the interference light signal:

[0044]

[0045] The interference light signal has the following characteristics:

[0046] 1. Figure 3 Taking the linear modulation of as an example, at a specific moment, the pattern imaging of the interference light signal presents a change from light to dark. Figure 5 As shown, the three figures from left to right correspond to s = 1, 2, and 3. At a specific moment, when s = 2, the pattern of the interference light signal rotates with the spatial angle to present a bright-dark-bright-dark pattern, and when s = 3, it presents a bright-dark-bright-dark-bright-dark pattern;

[0047] 2. The interference light signal is affected by the phase of the vibration sensing information Influence, the overall pattern rotates over time, the angular velocity and the phase of the signal to be measured The angular velocity is consistent;

[0048] 3. The overall brightness of the interference light signal is affected by the intensity of the scattered light, namely the two amplitude factors A0 and A2. After normalization, the pattern only rotates and does not deform.

[0049] In the embodiment, the interference light signal is expanded by the beam expander and introduced into the image collector to obtain the interference image. The interference pattern is input into the data acquisition and signal processing module, and the signal phase demodulation calculation is performed. The phase information can be demodulated according to the change of the pattern over time. More specifically, there are two demodulation methods, which are described as follows:

[0050] Method 1: If Figure 6 As shown, the phase difference is obtained by analyzing the characteristic motion of the interference pattern:

[0051] According to the above features 2 and 3, the interference pattern can be normalized after deducting the DC component to extract feature points or lines from the pattern. Alternatively, the image algorithm can be used to directly extract features without DC filtering and normalization. By tracking the movement of feature points or lines at each moment, the relationship between the angular velocity of the pattern movement and time can be calculated, which is denoted as β t , this angular velocity and signal phase The angular frequencies are completely synchronized, that is From this we can calculate:

[0052]

[0053] Method 2: If Figure 7 As shown, the phase difference is solved by splitting the interference pattern:

[0054] The collected interference pattern is evenly divided into m regions, where m = 2, 3, ..., and each signal corresponding to each region undergoes different modulations to generate phase offsets. The phase difference is calculated based on each signal after modulation. Take three output signals as an example (m=3), assuming that the three signals are S1, S2, and S3, for the case of linear modulation, And when s=1, the three signals are:

[0055]

[0056] therefore,

[0057] More generally, when s and m are other positive integers, each output signal is:

[0058]

[0059] Where k = 1, 2, 3, ..., m, based on multi-channel signal inversion or linear fitting as well as Finally, the phase

[0060] The embodiment further provides a method of using the above-mentioned fiber distributed acoustic wave system based on fiber end face phase control, comprising the following steps:

[0061] The narrow line width light source generates a light pulse and emits it to the optical coupler to be divided into a detection light and a local oscillator light. The detection light passes through the optical modulator and is modulated under the control of the modulation electrical signal of the pulse modulation driving circuit and outputs the detection light pulse.

[0062] The detection light pulse is input into the sensing optical fiber through the circulator, and the backscattered light returned by the sensing optical fiber is input into the spatial light modulation component through the circulator;

[0063] The spatial light modulation element modulates the transverse cross-sectional phase distribution of the backscattered light and then outputs it to the second optical coupler. The second optical coupler interferes with the local oscillator light to form an interference light signal, which is then expanded by a beam expander and collected by an image collector to obtain an interference pattern.

[0064] The data acquisition and signal processing module processes the collected interference pattern to obtain the phase difference, thereby realizing fiber optic distributed acoustic wave sensing.

[0065] The method and system of the present invention simplifies the system structure and significantly reduces the computational complexity of signal processing by regulating the phase distribution of the light emitted from the end face of the optical fiber. It also has the advantages of small size, easy integration, and low computational complexity, especially in complex environments, such as energy pipeline monitoring, infrastructure health monitoring, and border security. It shows significant technical and economic advantages. Compared with traditional methods, the present invention provides a more efficient and stable fiber optic sensing solution, providing important technical support for the development of the next generation of fiber optic sensing technology.

[0066] The specific implementation methods described above provide a detailed description of the technical solutions and beneficial effects of the present invention. It should be understood that the above is only the most preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, supplements and equivalent substitutions made within the scope of the principles of the present invention should be included in the protection scope of the present invention.

Claims

1. An optical fiber distributed acoustic wave sensing system based on optical fiber end face phase control, characterized in that: It includes a narrow line width light source, a first optical coupler, an optical modulator, a circulator, a spatial light modulation component, a second optical coupler, a pulse modulation drive circuit, a beam expander, an image collector, and a data acquisition and signal processing module; The output light of the narrow linewidth light source is divided into detection light and local oscillator light through an optical coupler. The detection light passes through an optical modulator and is modulated under the control of a modulated electrical signal of a pulse modulation drive circuit to output a detection light pulse. The detection light pulse is input into the sensing optical fiber through a circulator. The backscattered light returned by the sensing optical fiber is input into a spatial light modulation component through a circulator. The spatial light modulation component modulates the transverse cross-sectional phase distribution of the backscattered light and outputs it to a second optical coupler. The interference light signal formed by the second optical coupler and the local oscillator light is expanded by a beam expander and then collected by an image collector to obtain an interference pattern. The data acquisition and signal processing module processes the collected interference pattern to obtain a phase difference, thereby realizing fiber-optic distributed acoustic wave sensing.

2. The optical fiber distributed acoustic wave sensing system based on optical fiber end face phase control according to claim 1, characterized in that: The spatial light modulation device modulates the transverse cross-sectional phase of the backscattered light It shows multiple periodic changes with the spatial angle θ, that is, is a positive integer to ensure the periodicity of the phase.

3. The optical fiber distributed acoustic wave sensing system based on optical fiber end face phase control according to claim 1, characterized in that: The spatial light modulation device modulates the transverse cross-sectional phase of the backscattered light It changes in a step-by-step manner.

4. The optical fiber distributed acoustic wave sensing system based on optical fiber end face phase control according to any one of claims 1 to 3, characterized in that: The spatial light modulation component is of phase type, including a phase type mask, a spatial light modulator or a light-transmitting medium whose thickness varies with angle.

5. The optical fiber distributed acoustic wave sensing system based on optical fiber end face phase control according to claim 1, characterized in that: The interference light signal formed by the second optical coupling is: Among them, E0 is the local oscillator light, E2 is the backscattered light after the transverse cross-section phase modulation, A0 and A2 are the two amplitude factors, The optical path difference introduced by the local oscillator light transmitted to the second coupler and the backscattered light transmitted to the spatial light modulation component, The phase difference introduced when the backscattered light after lateral cross-sectional phase modulation is transmitted to the second coupler.

6. The optical fiber distributed acoustic wave sensing system based on optical fiber end face phase control according to claim 1, characterized in that: The collected interference pattern is processed to obtain the phase difference, including: Extract feature points or lines from the interference pattern, track the movement of feature points or lines at each moment, and calculate the relationship between the angular velocity of the interference pattern and time t. t , angular velocity and signal phase The angular frequencies are completely synchronized, that is The phase difference is calculated based on this 7. The optical fiber distributed acoustic wave sensing system based on optical fiber end face phase control according to claim 1, characterized in that: The collected interference pattern is processed to obtain the phase difference, including: The collected interference pattern is evenly divided into m regions. Each signal corresponding to each region undergoes different modulations to generate phase offsets. The phase difference is calculated based on each signal after modulation.

8. The optical fiber distributed acoustic wave sensing system based on optical fiber end face phase control according to claim 1, characterized in that: The system also includes an amplifier. The detection light pulse output by the optical modulator is amplified by the amplifier and then input into the sensing optical fiber through a circulator. The optical amplifier is an erbium-doped fiber amplifier, a semiconductor optical amplifier or other types of amplifiers that can increase the energy of the input optical signal.

9. The optical fiber distributed acoustic wave sensing system based on optical fiber end face phase control according to claim 8, characterized in that: The system also includes introducing a bandpass filter after the amplifier, and the amplified detection light pulse is filtered by the bandpass filter and then input into the sensing optical fiber through a circulator.

10. A fiber-optic distributed acoustic wave sensing method based on fiber end face phase control, characterized in that: The method comprises the optical fiber distributed acoustic wave sensing system according to any one of claims 1 to 9, comprising the following steps: The narrow line width light source generates a light pulse and emits it to the optical coupler to be divided into a detection light and a local oscillator light. The detection light passes through the optical modulator and is modulated under the control of the modulation electrical signal of the pulse modulation driving circuit and outputs the detection light pulse. The detection light pulse is input into the sensing optical fiber through the circulator, and the backscattered light returned by the sensing optical fiber is input into the spatial light modulation component through the circulator; The spatial light modulation element modulates the transverse cross-sectional phase distribution of the backscattered light and then outputs it to the second optical coupler. The second optical coupler interferes with the local oscillator light to form an interference light signal, which is then expanded by a beam expander and collected by an image collector to obtain an interference pattern. The data acquisition and signal processing module processes the collected interference pattern to obtain the phase difference, thereby realizing fiber optic distributed acoustic wave sensing.

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