Spatial domain measurement distributed optical fiber sound wave sensing system and measurement method thereof

Abstract

The invention relates to a spatial domain measurement distributed optical fiber acoustic wave sensing system and a measurement method thereof, the system comprises a light source, and an optical phase modulator, an optical amplifier, an optical circulator and an optical fiber for distributed sensing which are sequentially arranged on a light path of incident light of the light source, and the optical circulator is also located on a light path of light returned from the full length range of the optical fiber. The optical circulator is sequentially connected with a photoelectric detector, a data acquisition card and a computer on the light path of the return light; the optical phase modulator comprises a first radio frequency source, a second radio frequency source, a first electronic switch, a second electronic switch, a frequency mixer and an acousto-optic modulator. According to the invention, a double heterodyne pulse modulation injection technology is combined with a spatial domain phase demodulation method, so that continuous distributed synchronous measurement of a vibration field / sound wave field in a full length range of the optical fiber is realized, and the sensing precision and the spatial resolution are remarkably improved; the core requirements of geophysical exploration, energy industry monitoring, large-scale structure health monitoring and other scenes on spatial domain signal analysis can be met.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of distributed optical fiber sensing technology, and particularly relates to a spatial domain measurement distributed optical fiber acoustic sensing system and a measurement method thereof. BACKGROUND

[0002] Distributed fiber-optic acoustic sensing (DAS) technology uses Rayleigh backscattering in an optical fiber to achieve sensing and positioning. During operation, high-coherence pulsed light is injected into the optical fiber, and Rayleigh scattering light at all positions within the pulse width is detected. The sensing unit of the DAS system is an optical cable, which has a simple structure and can significantly reduce system costs and facilitate installation and laying. Currently, DAS technology is widely used in geophysical exploration and energy industries. In geophysical exploration, DAS is used for monitoring the interaction between the ocean and the solid earth, monitoring seismic and volcanic events, etc. In the energy industry, DAS is used for vertical seismic profile exploration, hydraulic fracturing monitoring, downhole fluid monitoring, oil well integrity assessment, and pipeline intrusion monitoring, etc. In addition, DAS can also use vehicle vibrations on urban roads as a seismic source to characterize the near-surface.

[0003] Currently, DAS technology mainly obtains external vibration fields / acoustic fields through time-domain data processing. In the time-domain phase demodulation process, the interference signal is represented as a function of the amplitude changing over time. Two orthogonal time-domain signals are generated by processing, and the phase information is finally demodulated by arctangent operation.

[0004] However, the processing method has significant limitations: first, the spatial domain signal along the length of the optical fiber is converted into a fast time domain signal, so that the optical fiber is divided into several discrete equivalent sensing elements according to the distance corresponding to the propagation time, for example, if the optical pulse propagates for 1 nanosecond, the distance corresponding to 0.2 meters, then the optical fiber is divided into equivalent sensing elements (discrete sensing segments) with a spacing of 0.2 meters, resulting in a loss of continuity in spatial measurement, such as monitoring pipeline leaks, which can only be located in a certain equivalent sensing element interval (such as 10-10.2 meters), and cannot be accurately located at a specific leak point (such as 10.1 meters), and the equivalent sensing element spacing is limited by the optical pulse width, which is difficult to reduce indefinitely to improve resolution; second, in existing time domain distributed optical fiber sensing technology, spatial position information is completely dependent on the derivation of the time difference of optical signal transmission, resulting in strong coupling between time domain and spatial domain information - both share the same physical parameter (time), which cannot be decoupled. This coupling makes it impossible for the system to analyze the spatial characteristics of the full length of the optical fiber at the same time, such as obtaining the amplitude distribution of seismic waves at different positions of the optical fiber at a certain time to infer the structure of the rock layer, but this method can only output discrete data from different positions in time sequence, and cannot splice a synchronous spatial map; third, time domain noise (such as transient circuit interference, light source power fluctuations) will directly translate into spatial positioning errors, for example, a 1 nanosecond time measurement error will result in a 0.2 meter spatial error, which will accumulate and cannot meet the high-precision requirements of oil well monitoring, precision geological exploration, and other high-precision requirements, posing a great challenge to the accurate analysis of spatial domain signals. SUMMARY

[0005] The present application is directed to the problem that the existing DAS technology relies on time domain processing, resulting in spatial measurement discretization, inability to analyze spatial characteristics synchronously, susceptibility to time noise interference, and difficulty in directly conducting high-precision spatial domain signal analysis. A spatial domain measurement distributed optical fiber acoustic wave sensing system is proposed, which realizes continuous distributed measurement of vibration field / acoustic field spatial domain, accurately captures spatial domain information such as seismic wave propagation trajectory and pipeline leak sound wave diffusion range, significantly improves sensing accuracy, and meets the core needs of spatial domain signal analysis in geophysical exploration, energy industry monitoring, and large-scale structure health monitoring. The present application also relates to a spatial domain measurement distributed optical fiber acoustic wave sensing method.

[0006] The technical solutions of the present application are as follows:

[0007] A spatial domain measurement distributed optical fiber acoustic wave sensing system, characterized in that it comprises a light source and, sequentially arranged on the light path of the light source incident light, a light phase modulator, an optical amplifier, an optical circulator and an optical fiber for distributed sensing, the optical circulator is also located on the light path of the feedback light from the full length range of the optical fiber, and the optical circulator is sequentially connected with a photodetector, a data acquisition card and a computer on the light path of the feedback light; wherein the light phase modulator comprises a first radio frequency source, a second radio frequency source, a first electronic switch, a second electronic switch, a frequency mixer and an acousto-optic modulator, the first radio frequency source is connected with the first electronic switch, the second radio frequency source is connected with the second electronic switch, the first electronic switch and the second electronic switch are both connected with the frequency mixer, and the frequency mixer is also connected with the acousto-optic modulator;

[0008] The light source is a narrow linewidth laser for outputting direct current laser, and the direct current laser is transmitted to the acousto-optic modulator of the light phase modulator; the light phase modulator adopts double heterodyne pulse technology, outputs a first radio frequency signal through the first radio frequency source, outputs a second radio frequency signal through the second radio frequency source, inputs the frequency mixer to generate a double heterodyne modulation signal after controlling the pulse timing of the two radio frequency signals through the first electronic switch and the second electronic switch, drives the acousto-optic modulator to modulate the input direct current laser into modulated light waves containing two spatial characteristic pulses and having a certain interval; the optical amplifier amplifies the power of the modulated light waves to ensure that the feedback light signals in the full length range are excited when the modulated light waves are transmitted in the optical fiber; the optical circulator injects the amplified modulated light waves into the optical fiber and directionally transmits the feedback light signals in the full length range of the optical fiber to the photodetector; the photodetector converts the feedback light signals into electrical signals carrying information of different spatial positions of the optical fiber; the data acquisition card collects the electrical signals and transmits them to the computer;

[0009] The computer performs spatial domain demodulation on the collected electrical signals: based on the wave number difference of the two spatial characteristic pulses, generates an orthogonal digital reference signal with the spatial position of the optical fiber as the variable, mixes the electrical signals with the orthogonal digital reference signal, performs spatial domain low-pass filtering, and then demodulates the light wave phase change amount caused by the vibration of the optical fiber in the full length range of the optical fiber due to external acoustic waves through arctangent operation; based on the light wave phase change amount, the distributed optical fiber acoustic wave sensing is realized.

[0010] Preferably, the feedback light signal is a reflected light signal, and a plurality of reflection points are inscribed on the optical fiber for distributed sensing along the length direction at equal intervals, the reflection points are weak reflection gratings or microstructure reflection points, used for reflecting the incident modulated light waves to form the reflected light signals carrying corresponding spatial position information.

[0011] Preferably, the feedback optical signal is a back Rayleigh scattering optical signal, the optical fiber for distributed sensing is a single-mode optical fiber, and the back Rayleigh scattering optical signal is formed by natural Rayleigh scattering of a modulated optical wave when the modulated optical wave is transmitted in the single-mode optical fiber, and carries information of each spatial position in the full length range of the optical fiber.

[0012] Preferably, the direct-current laser linewidth output by the narrow-linewidth laser is ≤1 kHz, the coherence length is ≥10 km, and the wavelength range is 1520 nm-1620 nm.

[0013] Preferably, in the optical phase modulator, a first radio frequency source outputs a first radio frequency signal of a first frequency, the wave number of the first radio frequency signal is determined based on the first frequency and the speed of light in the optical fiber, the first radio frequency signal is modulated into a pulse signal of a preset pulse width by a first electronic switch to form a first pulse modulation signal; a second radio frequency source outputs a second radio frequency signal of a second frequency, the wave number of the second radio frequency signal is determined based on the second frequency and the speed of light in the optical fiber, the second radio frequency signal is modulated into the pulse signal of the preset pulse width by a second electronic switch, and the second pulse modulation signal is time-delayed relative to the first pulse modulation signal by delay control to form a second pulse modulation signal; the first pulse modulation signal and the second pulse modulation signal are processed by a mixer to generate a double-heterodyne modulation signal, and an acousto-optic modulator is driven to modulate the input direct-current laser into a modulated optical wave containing two spatial feature pulses and having a certain interval; wherein the spatial distance corresponding to the time delay is a certain interval of the two spatial feature pulses, and the wave number difference of the two spatial feature pulses is the difference between the wave number of the first radio frequency signal and the wave number of the second radio frequency signal.

[0014] Preferably, when the computer performs spatial domain demodulation, the generated quadrature digital reference signal is a sine reference signal and a cosine reference signal related to the spatial position of the optical fiber; after the electrical signal is mixed with the sine reference signal and the cosine reference signal respectively, and then subjected to spatial domain low-pass filtering, the sine signal and the cosine signal after mixing and spatial domain low-pass filtering are subjected to arctangent operation, and the amount of change in the phase of the optical wave caused by the vibration of the optical fiber caused by the external acoustic wave at different spatial positions in the full length range of the optical fiber is demodulated.

[0015] A spatial domain measurement distributed optical fiber acoustic wave sensing method, characterized in that it comprises the following steps:

[0016] S1. An optical source outputs a direct-current laser, and the optical source is a narrow-linewidth laser;

[0017] S2. Modulating the direct-current laser by using a double heterodyne pulse technology: outputting a first radio frequency signal by a first radio frequency source, outputting a second radio frequency signal by a second radio frequency source, inputting a mixer after controlling the pulse timing of the two radio frequency signals by an electronic switch to generate a double heterodyne modulation signal, and driving an acousto-optic modulator to modulate the direct-current laser into a modulation light wave containing two spatial characteristic pulses and having a certain interval between each other;

[0018] S3. Power amplifying the modulation light wave, and injecting the power amplified modulation light wave into an optical fiber for distributed sensing through an optical circulator to excite a feedback light signal in the full length range of the optical fiber;

[0019] S4. Directing the feedback light signal returned by the optical fiber to a photodetector through the optical circulator to convert the feedback light signal into an electrical signal carrying information of different spatial positions of the optical fiber;

[0020] S5. Collecting the electrical signal and transmitting the electrical signal to a computer, and performing spatial domain demodulation by the computer: generating a quadrature digital reference signal with the spatial position of the optical fiber as a variable based on the wave number difference of the two spatial characteristic pulses, mixing the electrical signal with the quadrature digital reference signal, performing spatial domain low-pass filtering, and then performing arctangent operation to demodulate the phase change amount of the light wave caused by the vibration of the optical fiber at different spatial positions in the full length range of the optical fiber due to external sound waves; and realizing distributed optical fiber sound wave sensing based on the phase change amount of the light wave.

[0021] Preferably, in step S3, a plurality of reflection points are inscribed on the optical fiber for distributed sensing at equal intervals along the length direction, and the reflection points are weak reflection gratings or microstructure reflection points; and the feedback light signal is a reflection light signal formed by reflection of the modulation light wave by the reflection points and carrying information of corresponding spatial positions.

[0022] Preferably, in step S3, the optical fiber for distributed sensing is a single-mode optical fiber; and the feedback light signal is a backscattering Rayleigh scattering light signal formed by natural Rayleigh scattering of the modulation light wave with molecules, atoms and / or small particles of the single-mode optical fiber during transmission of the modulation light wave in the single-mode optical fiber, and carrying information of each spatial position in the full length range of the optical fiber.

[0023] Preferably, in step S5, the quadrature digital reference signal includes a sine reference signal and a cosine reference signal related to the spatial position of the optical fiber; and the spatial domain demodulation specifically includes: mixing the electrical signal with the sine reference signal and the cosine reference signal respectively, performing spatial domain low-pass filtering, and then performing arctangent operation on the sine signal and the cosine signal after mixing and spatial domain low-pass filtering to demodulate the phase change amount of the light wave at each spatial position.

[0024] The technical effects of the present application are as follows:

[0025] This invention relates to a spatial domain distributed fiber optic acoustic wave sensing system. Unlike traditional time-domain distributed fiber optic sensing that relies on the correlation between time difference and spatial location (which has limited spatial resolution), this system directly establishes a wavenumber difference-spatial location correspondence in the spatial domain by using the wavenumber difference of two spatial characteristic pulses generated through double heterodyne pulse technology. A computer generates an orthogonal digital reference signal with the fiber's spatial location as the variable, making the demodulation process entirely based on spatial domain parameters. This allows for the simultaneous acquisition of acoustic wave vibration information at all spatial locations along the entire length of the fiber, overcoming the efficiency bottleneck of point-by-point scanning in time-domain measurements and achieving truly distributed, parallel spatial domain measurement. The light source uses a narrow-linewidth laser (high coherence, long coherence length), combined with power amplification by an optical amplifier, ensuring that the modulated light wave can still excite feedback light signals across the entire length of the fiber after long-distance transmission. Simultaneously, the spatial characteristic pulse spacing and wavenumber difference of the double heterodyne pulses can be precisely controlled, achieving spatial resolution at the meter or even sub-meter level, thus resolving the technical contradiction of balancing spatial resolution and transmission distance in traditional long-distance sensing. During spatial domain demodulation, the combination of orthogonal digital reference signal mixing and spatial domain low-pass filtering effectively filters out system noise (such as spontaneous emission noise from optical amplifiers and thermal noise from photodetectors) and external environmental interference. The arctangent operation demodulates the phase change, achieving milliradian-level phase measurement accuracy. This allows for precise capture of minute fiber vibrations caused by external sound waves (vibration amplitudes can be as low as nanometers), making it suitable for high-precision acoustic sensing scenarios (such as pipeline leak monitoring and geological microseismic detection). Furthermore, the system architecture is modular and highly scalable. Each module (light source, optical phase modulator, optical amplifier, optical circulator, photodetector, and computer) is functionally independent and has standardized interfaces, allowing for flexible configuration according to different application scenarios: for high spatial resolution, the wavenumber difference and pulse spacing of the double heterodyne pulses can be adjusted; for longer transmission distances, high-power optical amplifiers or low-loss optical fibers can be replaced; it is compatible with both "written reflection point fiber" and "ordinary single-mode fiber" solutions, offering extremely high scalability. The spatial domain measurement distributed fiber optic acoustic wave sensing system proposed in this invention directly processes the spatial domain interference signals at each physical location of the fiber, eliminating the need to convert spatial information into temporal information. This enables continuous distributed measurement of the vibration field / acoustic field across the entire length of the fiber. It can acquire the phase change of each spatial location (rather than discrete sensing elements), significantly improving spatial resolution (e.g., positioning accuracy within 1 meter); it can also achieve synchronous analysis of the spatial characteristics of the entire fiber length at the same time, accurately capturing spatial domain information such as seismic wave propagation trajectories and the acoustic wave diffusion range of pipeline leaks; simultaneously, it avoids the influence of temporal domain noise on spatial positioning, significantly improving sensing accuracy and meeting the core requirements for spatial domain signal analysis in scenarios such as geophysical exploration, energy industry monitoring, and large-scale structural health monitoring.

[0026] Furthermore, by inscribing weak reflection gratings or microstructure reflection points at equal intervals along the length of the optical fiber, the feedback optical signal is made into a reflected optical signal carrying clear spatial location information. The equal spacing of the reflection points ensures the uniformity of spatial sampling, and the weak reflection characteristic avoids interference of the incident light with excessively strong reflected light. At the same time, the intensity of the reflected light signal is much higher than that of the naturally scattered light, which improves the signal-to-noise ratio during long-distance transmission, making the system perform better in scenarios that require high signal strength and accurate spatial positioning (such as monitoring of large building structures).

[0027] Furthermore, by employing single-mode optical fiber and utilizing the backscattered Rayleigh light generated by its molecules, atoms, and / or tiny particles as the feedback light signal, no additional processing of the optical fiber (such as marking reflection points) is required, reducing the cost and complexity of optical fiber manufacturing. This makes it suitable for long-distance, large-scale distributed sensing scenarios (such as monitoring of oil and gas pipelines and power cables). The all-fiber distribution characteristics of natural Rayleigh scattering enable theoretically continuous spatial position monitoring, breaking through the spatial resolution limitations caused by the interval between reflection points, and capturing more subtle changes in spatial position.

[0028] Furthermore, by limiting the linewidth of the output laser from the narrow-linewidth laser to ≤1kHz, the coherence length to ≥10km, and the wavelength range of 1520nm~1620nm, the ultra-narrow linewidth and ultra-long coherence length ensure that the double heterodyne pulses can maintain stable interference characteristics after long-distance transmission, avoiding demodulation errors caused by insufficient coherence. The 1520nm~1620nm band is within the low-loss window of optical fiber, reducing the attenuation of optical signals during transmission, enabling the system to achieve long-distance distributed sensing of more than 50km, while balancing signal strength and transmission distance.

[0029] Furthermore, by clarifying the frequency-wavenumber correlation, pulse width consistency, and time delay control mechanism of the first and second radio frequency signals, the wavenumber difference and spacing of the two spatial characteristic pulses can be precisely controlled through radio frequency signal parameters, realizing the designability of spatial characteristic pulses. The same preset pulse width ensures the signal integrity of the two pulses, the spatial spacing converted by time delay avoids pulse overlap interference, and the accurate definition of the wavenumber difference provides a stable reference for subsequent spatial domain demodulation, improving the adaptability of the system under different spatial resolution and measurement range requirements.

[0030] Furthermore, the orthogonal digital reference signal is concretized into sine and cosine reference signals related to the spatial position of the optical fiber. Through a demodulation process of mixing, filtering, and then performing arctangent operation, the phase ambiguity problem during demodulation of a single reference signal is effectively eliminated by utilizing the phase complementarity of the orthogonal signals. The step-by-step mixing and filtering operations make noise suppression more thorough, and the arctangent operation directly maps the phase change, further improving the linearity and accuracy of phase measurement, enabling the system to more accurately reproduce the subtle optical fiber vibration information caused by external sound waves.

[0031] This invention also relates to a spatial domain measurement distributed fiber optic acoustic wave sensing method, corresponding to the spatial domain measurement distributed fiber optic acoustic wave sensing system described above. It can be understood as a method for implementing the aforementioned spatial domain measurement distributed fiber optic acoustic wave sensing system. This method, through the steps of "narrow linewidth laser output → double heterodyne pulse modulation → optical signal amplification and transmission → feedback photoelectric conversion → spatial domain demodulation," forms a precise synergy with the corresponding system's hardware architecture. On the one hand, operations such as "double heterodyne pulse timing control," "feedback light type adaptation," and "orthogonal reference signal generation and arctangent demodulation" in the method directly implement the system's... The core technology concept of the system ensures a logical closed loop from hardware composition to functional implementation. On the other hand, the method clarifies the key points of operation in each step, such as selecting reflected light or back Rayleigh scattering light as feedback light according to the type of optical fiber, and generating spatial domain reference signals based on wavenumber difference. This approach takes into account both long-distance transmission and meter-level spatial resolution, improves anti-interference capability and phase measurement accuracy, and has flexible adaptability. The operating parameters can be adjusted according to different application scenarios (such as pipeline monitoring, geological early warning, etc.), ultimately achieving a high-precision, distributed optical fiber acoustic wave sensing effect consistent with the system. Moreover, the steps are clear and easy to implement in actual engineering. Attached Figure Description

[0032] Figure 1 This is a preferred structural block diagram of the spatial domain measurement distributed fiber optic acoustic wave sensing system of the present invention.

[0033] Figure 2 This is a block diagram of an optical phase modulator.

[0034] Figure 3 This is a schematic diagram of the modulated light wave output by the optical phase modulator.

[0035] Figure 4 This is another preferred structural block diagram of the spatial domain measurement distributed fiber optic acoustic wave sensing system of the present invention.

[0036] Figure 5The simulation results are shown in the following figures: (a) is the spatial domain waveform of the phase change along the fiber caused by the vibration of the fiber optic cable due to external sound waves; (b) is the wavenumber spectrum of the phase change along the fiber optic cable caused by the vibration of the fiber optic cable due to external sound waves; (c) is the spatial domain waveform of the phase change along the fiber optic cable obtained by demodulation; and (d) is the wavenumber spectrum of the phase change along the fiber optic cable obtained by demodulation.

[0037] The labels in the diagram are listed below:

[0038] 101—Light source; 102—Optical phase modulator; 1021—First radio frequency source; 1022—Second radio frequency source; 1023—First electronic switch; 1024—Second electronic switch; 1025—Mixer; 1026—Acousto-optic modulator; 103—Optical amplifier; 104—Optical circulator; 105—Photodetector; 106—Data acquisition card; 107—Computer; 201—Optical fiber; 301—Reflection point. Detailed Implementation

[0039] The present invention will now be described with reference to the accompanying drawings.

[0040] This invention provides a spatial domain measurement distributed fiber optic acoustic wave sensing system. Its core lies in extending time-domain signal processing methods to the spatial domain and introducing double heterodyne pulse technology to optimize the quality of spatial domain signals, such as... Figure 1 The preferred structure shown includes a light source 101 and an optical phase modulator 102, an optical amplifier 103, an optical circulator 104, and an optical fiber 201 for distributed sensing, sequentially arranged on the optical path of the incident light from the light source 101. The optical circulator 104 is also located on the optical path of the return light along the entire length of the optical fiber 201. On the return light optical path, the optical circulator 104 is sequentially connected to a photodetector 105, a data acquisition card 106, and a computer 107. The structure of the optical phase modulator is as follows: Figure 2 As shown, it includes a first radio frequency source 1021, a second radio frequency source 1022, a first electronic switch 1023, a second electronic switch 1024, a mixer 1025, and an acousto-optic modulator 1026. The first radio frequency source 1021 is connected to the first electronic switch 1023, the second radio frequency source 1022 is connected to the second electronic switch 1024, and both the first electronic switch 1023 and the second electronic switch 1024 are connected to the mixer 1025. The mixer 1025 is also connected to the acousto-optic modulator 1026.

[0041] The light source 101 employs a narrow-linewidth laser to output a DC laser with a wavelength of λ0. Preferably, the DC laser has a linewidth ≤1kHz, a coherence length ≥10km, and a wavelength range of 1520nm~1620nm. It is then transmitted to an optical phase modulator 102 for modulation. The optical phase modulator 102 uses double heterodyne pulse technology to modulate the DC laser into a modulated light wave containing two spatial characteristic pulses with a certain distance between them. This is then amplified by an optical amplifier 103 and finally injected into the optical fiber 201 via an optical circulator 104. Reflection points 301 (which can be weak reflection gratings or microstructure reflection points) are etched at equal intervals on the optical fiber 201 to reflect the incident modulated light wave, forming a reflected light signal carrying corresponding spatial position information. The distance between two adjacent reflection points is d. R The reflectivity of each reflection point is r R The reflected light from reflection point 301 passes through circulator 104 to photodetector 105 and is converted into an electrical signal. Then, it is collected by data acquisition card 106 and transmitted to computer 107. Finally, the collected data is demodulated in computer 107.

[0042] like Figure 2 As shown, the optical phase modulator 102 adopts double heterodyne pulse technology. It outputs a first radio frequency signal through the first radio frequency source 1021 and a second radio frequency signal through the second radio frequency source 1022. After the pulse timing of the two radio frequency signals is controlled by the first electronic switch 1023 and the second electronic switch 1024 respectively, the signals are input to the mixer 1025 to generate a double heterodyne modulation signal, which drives the acousto-optic modulator 1026 to modulate the input DC laser into a modulated light wave containing two spatial characteristic pulses with a certain distance between them. The optical amplifier 103 amplifies the power of the modulated light wave to ensure that it excites a feedback light signal across the entire length of the optical fiber during transmission, supporting distributed measurement. The optical circulator 104 injects the amplified modulated light wave into the optical fiber and directionally transmits the reflected light signal returned along the entire length of the optical fiber to the photodetector 105. The photodetector 105 converts the reflected light signal into an electrical signal carrying information about different spatial positions of the optical fiber. The data acquisition card 106 acquires the electrical signal and transmits it to the computer 107. The computer 107 performs spatial domain demodulation on the acquired electrical signal: based on the wavenumber difference between the two spatial characteristic pulses, it generates an orthogonal digital reference signal with the spatial position of the optical fiber as the variable; after mixing the electrical signal with the orthogonal digital reference signal, it performs spatial domain low-pass filtering; and then demodulates the phase change of the optical wave caused by the vibration of the optical fiber due to external sound waves at different spatial positions along the entire length of the optical fiber through arctangent operation. Based on the phase change of the optical wave, distributed optical fiber acoustic wave sensing is realized.

[0043] Specifically, in the optical phase modulator 102, the first radio frequency source 1021 generates a first radio frequency signal with a frequency of f1 (where f1 is the first frequency and its corresponding wavenumber is k1=f1 / v, where v is the speed of light in the optical fiber), which is then modulated by the first electronic switch 1023 into a preset pulse width of w. P The pulse signal is generated to form a first pulse modulation signal; the second radio frequency source 1022 generates a second radio frequency signal with a frequency of f2 (f2 is the second frequency, and its corresponding wavenumber is k2=f2 / v, where v is the speed of light in the optical fiber), which is then modulated by the second electronic switch 1024 to the preset pulse width w. P The pulse signal, and simultaneously add a delay distance d to the pulse signal. P In other words, the second pulse modulation signal is delayed by a time Δt relative to the first pulse modulation signal through delay control, with a delay distance d. P In reality, it is the product of the time delay Δt and the speed of light v (d P =v×Δt), forming a second pulse modulation signal; the two signals (the first pulse modulation signal and the second pulse modulation signal) are combined into one by the mixer 1025 to generate a double heterodyne modulation signal, which is finally loaded onto the acousto-optic modulator 1026 to drive the acousto-optic modulator 1026 to perform phase modulation on the input laser, that is, to drive the acousto-optic modulator to modulate the DC laser into a modulated light wave containing two spatial characteristic pulses with a certain distance between them. Figure 3 The image shows the modulated light wave output by the optical phase modulator 102. The modulated light wave consists of two pulses (two spatial characteristic pulses), each with a width of w. P The pulse spacing is d P The wave number of the first pulse is k1, and the wave number of the second pulse is k2.

[0044] The modulated light wave will be reflected by reflection point 301 in optical fiber 201, and the light waves arriving at the injection end of optical fiber 201 at the same time will superimpose and interfere. Optical fiber 201 can be divided into N=L / d sections according to reflection point 301. R Segment, where L is the length of fiber 201, d R The distance between reflection points is the spatial distance between adjacent reflection points on fiber 201. The number of reflection point segments N covered by the pulse width w is... w =w / d R Segment, pulse spacing d P Number of reflection points N covered d =d P / d R The nth segment. The phase change φ(n) of the light wave caused by external vibration. The superimposed light field of the reflected light is:

[0045] (1)

[0046] Where z=nd R This represents the position of the nth segment; E1 and E2 are the amplitudes of the two light pulses, respectively. Let be the reflection coefficient at the reflection point on the optical fiber; x and y are index variables for spatial segments, used to mark different spatial segments (i.e., segment x and segment y) after the optical fiber is divided by the reflection point. These are used to traverse and accumulate the phase changes of different spatial segments. The interference intensity of the above optical field can be expressed as:

[0047] (2)

[0048] Where Δk = k1 - k2 is the difference between the two wave numbers; The superimposed light field of the reflected light describes the complex amplitude distribution of all reflected light at spatial position z after reflection at the optical fiber reflection point, including the amplitude and phase information of the light. for The complex conjugate of the , when calculating the interference light intensity, is obtained by... Multiplying it by its complex conjugate yields a real value for the interference light intensity.

[0049] When computer 107 performs spatial domain demodulation, the generated orthogonal digital reference signals are a sine reference signal and a cosine reference signal related to the spatial position of the optical fiber. The electrical signal is mixed with the sine and cosine reference signals respectively, then subjected to spatial domain low-pass filtering. Finally, by performing an arctangent operation on the mixed and spatially low-pass filtered sine and cosine signals, the phase change of the optical wave caused by optical fiber vibration due to external sound waves at different spatial positions along the entire length of the optical fiber is demodulated. Specifically, computer 107 generates the following two orthogonal digital reference signals:

[0050] (3)

[0051] (4)

[0052] By mixing equation (2) and equation (3) and then passing the signals through a low-pass filter in the spatial domain, the following signal can be obtained:

[0053] (5)

[0054] By mixing equation (2) and equation (4) and then passing the signals through a low-pass filter in the spatial domain, the following signal can be obtained:

[0055] (6)

[0056] The cutoff frequency k of the low-pass filter in the above steps cut Satisfying the relation: 0 <k cut <Δk.

[0057] Dividing equation (5) by equation (6) and performing an arctangent operation yields the change in optical wave phase caused by optical fiber vibration induced by external sound waves at different locations in the optical fiber:

[0058]

[0059] The above process can obtain the phase change of all reflection segments of optical fiber 201 caused by external vibration by injecting a set of modulated light waves into optical fiber 201. By continuously injecting modulated light waves into optical fiber 201 at certain time intervals and demodulating the reflected signals, the distributed vibration field of optical fiber vibration caused by external sound waves can be obtained.

[0060] Another preferred structure of the spatial domain measurement distributed fiber optic acoustic wave sensing system of the present invention is as follows: Figure 4 As shown, in this embodiment, the optical fiber 201 is a common optical fiber, which eliminates the need to write reflection points on the optical fiber 201, thereby simplifying the complexity of the system.

[0061] Figure 4 The spatial domain measurement distributed fiber optic acoustic wave sensing system of the present invention, as shown, includes: a light source 101, an optical phase modulator 102, an optical amplifier 103, an optical circulator 104, a photodetector 105, a data acquisition card 106, a computer 107, and an optical fiber 201. The optical fiber 201 is a common optical fiber (or a common single-mode optical fiber), utilizing its Rayleigh scattering for sensing. The system uses a narrow-linewidth laser as the light source 101, outputting a DC laser with a wavelength of λ0, which is then modulated by the optical phase modulator 102 (the structure of the optical phase modulator and its output modulated light wave are respectively...). Figure 2 and Figure 3 The light wave (same as in the previous example) is then amplified by optical amplifier 103 and finally injected into sensing fiber 201 via optical circulator 104. The light wave will induce Rayleigh scattering in fiber 201, and the backscattered Rayleigh light will reach photodetector 105 via circulator 104 and be converted into an electrical signal. This signal is then acquired by data acquisition card 106 and transmitted to computer 107, where it is finally demodulated.

[0062] Rayleigh scattering is an inherent physical phenomenon in optical fibers. When light propagates through the medium, it interacts with the atoms and molecules that make up the medium, inducing polarized dipoles through their electric field. These dipoles then generate secondary waves, which constitute the scattering of light. Rayleigh scattering occurs on a scale much smaller than the wavelength of light (typically 1.55 micrometers), thus Rayleigh scattering in optical fibers can be considered spatially continuous. The optical field of the backscattered Rayleigh light at position z in the optical fiber can be expressed as:

[0063] (8)

[0064] Where E1 and E2 are the amplitudes of the two optical pulses, r(p) is the Rayleigh scattering rate at position p on the optical fiber, θ(p) is the Rayleigh scattering phase at position p on the optical fiber, r(p) and θ(p) are random quantities along the spatial position p; φ(l) is the change in the phase of the optical wave at position l on the optical fiber caused by external vibration.

[0065] The interference intensity of the above light field can be expressed as:

[0066] (9)

[0067] The following two orthogonal digital reference signals are generated in computer 107:

[0068] (10)

[0069] (11)

[0070] By mixing equation (9) and equation (10) and then passing the signals through a low-pass filter in the spatial domain, the following signal can be obtained:

[0071] (12)

[0072] By mixing equation (9) and equation (11) and then passing the signals through a low-pass filter in the spatial domain, the following signal can be obtained:

[0073] (13)

[0074] The cutoff frequency k of the low-pass filter in the above steps cut Satisfying the relation: 0 <k cut <Δk.

[0075] Dividing equation (12) by equation (13) and performing an arctangent operation yields the change in optical wave phase caused by optical fiber vibration induced by external sound waves at different locations in the optical fiber:

[0076]

[0077] The above process can obtain the phase change caused by external vibration at all positions of the optical fiber 201 by injecting a set of modulated light waves into the optical fiber 201. By continuously injecting modulated light waves into the optical fiber 201 at certain time intervals and demodulating the Rayleigh backscatter signal, the distributed vibration field of the optical fiber vibration caused by external sound waves can be obtained.

[0078] Figure 5 The simulation results of the spatial domain measurement distributed fiber optic acoustic wave sensing system using the present invention are shown. In the simulation, the fiber length was set to 10000 m, and the width of a single spatial characteristic pulse output by the optical phase modulator was w. P=6m, the physical distance between two spatial characteristic pulses (the spatial distance corresponding to the inter-pulse delay) is d P =10m, the wavenumber corresponding to the first radio frequency signal is k1=1m -1 The wavenumber corresponding to the second radio frequency signal is k2 = 0.5m. -1 The resulting heterodyne wavenumber is Δk = k1 - k2 = 0.5m. -1 To address the optical fiber vibration interference caused by external sound waves, the simulated phase change of the optical wave in the fiber is given by φ(z) = Asin(2πk). sig z), where A = 0.2 rad is the amplitude of the signal, k sig =0.002m -1 The wavenumber of the signal. Figure 5 (a) shows the spatial domain waveform of the phase change caused by the vibration of the optical fiber due to the external sound wave along the fiber length, and (b) shows its corresponding wavenumber spectrum. After processing the electrical signal output by the photodetector using the spatial domain demodulation method of this invention, the demodulation result is as follows: Figure 5 As shown in (c) and (d) in the figure. Comparing the spatial domain waveforms of Figures (a) and (c) and the wavenumber spectra of Figures (b) and (d), it can be clearly seen that the demodulated phase change is completely consistent with the original vibration signal in terms of amplitude and wavenumber characteristics. This proves that the system of the present invention can completely and accurately recover the optical wave phase change caused by the optical fiber vibration caused by external sound waves along the optical fiber, and fully verifyes the effectiveness and accuracy of the spatial domain demodulation mechanism.

[0079] This invention also relates to a spatial domain measurement distributed fiber optic acoustic wave sensing method, corresponding to the spatial domain measurement distributed fiber optic acoustic wave sensing system described above. It can be understood as a method for implementing the aforementioned spatial domain measurement distributed fiber optic acoustic wave sensing system, comprising the following steps:

[0080] S1. The light source outputs DC laser light, and the light source is a narrow linewidth laser;

[0081] S2. The DC laser is modulated using double heterodyne pulse technology: a first radio frequency signal is output from a first radio frequency source and a second radio frequency signal is output from a second radio frequency source. After the pulse timing of the two radio frequency signals is controlled by an electronic switch, they are input into a mixer to generate a double heterodyne modulation signal, which drives an acousto-optic modulator to modulate the input DC laser into a modulated light wave containing two spatial characteristic pulses with a certain distance between them.

[0082] S3. The modulated light wave is amplified in power and then injected into the optical fiber used for distributed sensing through an optical circulator to excite the feedback light signal over the entire length of the optical fiber.

[0083] S4. The feedback optical signal returned from the optical fiber is directionally transmitted to the photodetector through the optical circulator and converted into an electrical signal carrying information about the different spatial positions of the optical fiber;

[0084] S5. Acquire the electrical signal and transmit it to the computer, where the computer performs spatial domain demodulation: Based on the wavenumber difference between the two spatial characteristic pulses, generate an orthogonal digital reference signal with the spatial position of the optical fiber as the variable; mix the electrical signal with the orthogonal digital reference signal and perform spatial domain low-pass filtering; then demodulate the optical wave phase change caused by the vibration of the optical fiber caused by external sound waves at different spatial positions along the entire length of the optical fiber through arctangent operation; based on the optical wave phase change, realize distributed optical fiber acoustic wave sensing.

[0085] Furthermore, in step S3, several reflection points are etched at equal intervals along the length direction on the optical fiber used for distributed sensing. The reflection points are weak reflection gratings or microstructure reflection points. The feedback optical signal is a reflected optical signal, which is formed by the modulation light wave being reflected by the reflection points and carries information about the corresponding spatial position.

[0086] Furthermore, in step S3, the optical fiber used for distributed sensing is a single-mode optical fiber; the feedback optical signal is a backscattered Rayleigh optical signal, which is formed by the natural Rayleigh scattering of the modulated light wave with the molecules, atoms and / or tiny particles of the single-mode optical fiber when the modulated light wave is transmitted in the single-mode optical fiber, and carries information about the spatial position of each part of the optical fiber along its entire length.

[0087] Further, in step S5, the orthogonal digital reference signal includes a sine reference signal and a cosine reference signal related to the spatial position of the optical fiber; the spatial domain demodulation specifically includes: mixing the electrical signal with the sine reference signal and the cosine reference signal respectively, performing spatial domain low-pass filtering, and then performing arctangent operation on the mixed and spatially low-pass filtered sine signal and the cosine signal to demodulate the phase change of the optical wave at each spatial position.

[0088] This invention relates to a spatial domain measurement distributed optical fiber acoustic wave sensing system and its measurement method, including key technical points such as spatial domain phase demodulation method, double heterodyne pulse modulation injection, and applicability to two optical fiber structures. Spatial domain phase demodulation method: This invention extends the traditional time domain phase demodulation method to the spatial domain. By generating two orthogonal digital reference signals (Equations (3) and (4)) or (Equations (10) and (11)), the acquired spatial domain interference signal (Equation (2)) or (Equation (9)) is mixed with it and subjected to spatial domain low-pass filtering. Finally, the phase change of the optical wave caused by the external vibration field / acoustic field at different positions of the optical fiber is demodulated by arctangent operation (Equation (7)) or (Equation (14)), realizing continuous measurement of the spatial distributed vibration field / acoustic field. Double heterodyne pulse modulation injection: Two pulses with a width of w are injected into the optical fiber through optical phase modulator 102.P The spacing is d P The two pulsed light waves, with wavenumbers k1 and k2 respectively, are used to form a double heterodyne interference optical field in the optical fiber with a wavenumber difference of Δk = k1 - k2. This double heterodyne pulse injection method is key to achieving spatial domain phase demodulation. Applicability to two optical fiber structures: This invention provides... Figure 1 and Figure 4 Two preferred structures are proposed: one is to write reflection points 301 at equal intervals on the optical fiber 201 and use the reflected light for interference demodulation; the other is to use ordinary optical fiber 201 and use the inherent Rayleigh scattering of the optical fiber for sensing. The light field of the back Rayleigh scattered light can be represented by equation (8). Both structures can obtain the phase change through the same spatial domain demodulation method, which improves the flexibility and universality of the scheme.

[0089] This invention relates to a spatial domain distributed optical fiber acoustic wave sensing system and its measurement method, realizing spatial domain distributed acoustic wave sensing. By processing and demodulating the reflected light or Rayleigh backscattered light of modulated light waves in the optical fiber in the spatial domain, this invention overcomes the limitations of traditional techniques that primarily rely on time domain analysis. It achieves direct measurement and analysis of phase changes caused by vibration / acoustic fields in the spatial dimension, thus obtaining more intuitive and accurate distributed sensing results. It improves signal quality and resolvability: This invention employs a double heterodyne pulse injection scheme, which effectively suppresses random signal fluctuations and wide-range noise caused by Rayleigh scattering. Especially in the presence of Rayleigh fading, simulation verification shows that this method can completely recover the phase change of the optical wave along the fiber caused by external vibration, improving signal resolvability and the system's signal-to-noise ratio. It offers structural simplification and flexibility: This invention not only provides a structure for marking reflection points on the optical fiber but also proposes a scheme using ordinary optical fiber for sensing via Rayleigh scattering. The latter scheme eliminates the need for marking reflection points on the fiber, significantly simplifying the complexity of the scheme while ensuring sensing performance, thus improving the flexibility and applicability of the scheme.

[0090] It should be noted that the specific embodiments described above enable those skilled in the art to more fully understand the present invention, but do not limit the present invention in any way. Therefore, although the present invention has been described in detail with reference to the accompanying drawings and embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention. In short, all technical solutions and improvements that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the present invention patent.

Claims

1. A spatial domain measurement distributed optical fiber acoustic wave sensing system, characterized by, The application relates to a distributed optical fiber acoustic sensor, which comprises a light source, a light phase modulator, a light amplifier, a light circulator and an optical fiber for distributed sensing which are sequentially arranged on the light path of the light source and the incident light of the light source, the light circulator is also arranged on the light path of the feedback light from the full length range of the optical fiber, and the light circulator is sequentially connected with a photodetector, a data acquisition card and a computer on the light path of the feedback light; wherein the light phase modulator comprises a first radio frequency source, a second radio frequency source, a first electronic switch, a second electronic switch, a mixer and an acousto-optic modulator, the first radio frequency source is connected with the first electronic switch, the second radio frequency source is connected with the second electronic switch, the first electronic switch and the second electronic switch are both connected with the mixer, and the mixer is further connected with the acousto-optic modulator. The light source is a narrow linewidth laser, which is used for outputting direct current laser, and the direct current laser is transmitted to the acousto-optic modulator of the light phase modulator; the light phase modulator adopts double heterodyne pulse technology, a first radio frequency signal is outputted through the first radio frequency source, a second radio frequency signal is outputted through the second radio frequency source, after the two radio frequency signals are controlled by the first electronic switch and the second electronic switch respectively, the double heterodyne modulation signal is generated in the mixer, the acousto-optic modulator is driven to modulate the inputted direct current laser into modulated light waves containing two spatial characteristic pulses and having a certain interval; the light amplifier is used for power amplifying the modulated light waves to ensure that the modulated light waves excite feedback light signals in the full length range of the optical fiber during transmission in the optical fiber; the light circulator is used for injecting the amplified modulated light waves into the optical fiber and for directing the feedback light signals in the full length range of the optical fiber to the photodetector; the photodetector is used for converting the feedback light signals into electrical signals carrying the information of different spatial positions of the optical fiber; and the data acquisition card is used for collecting the electrical signals and transmitting the electrical signals to the computer. The computer is used for performing spatial domain demodulation on the collected electrical signals: based on the wave number difference of the two spatial characteristic pulses, a quadrature digital reference signal with the spatial position of the optical fiber as a variable is generated, the electrical signals are mixed with the quadrature digital reference signal, then spatial domain low-pass filtering is performed, and the optical wave phase change amount caused by the vibration of the optical fiber in different spatial positions in the full length range of the optical fiber due to external sound waves is demodulated through arctangent operation; and based on the optical wave phase change amount, distributed optical fiber acoustic sensing is realized.

2. The spatial domain measurement distributed optical fiber acoustic wave sensing system of claim 1, wherein, The feedback light signal is a reflected light signal, a plurality of reflection points are inscribed on the optical fiber for distributed sensing along the length direction at equal intervals, the reflection points are weak reflection gratings or microstructure reflection points, which are used for reflecting the incident modulated light waves to form the reflected light signals carrying the information of corresponding spatial positions.

3. The spatial domain measurement distributed optical fiber acoustic wave sensing system of claim 1, wherein, The feedback light signal is a backscattering Rayleigh scattering light signal, the optical fiber for distributed sensing is a single mode optical fiber, the backscattering Rayleigh scattering light signal is formed by natural Rayleigh scattering of the modulated light waves in the single mode optical fiber with the molecules, atoms and / or small particles of the single mode optical fiber, and carries the information of each spatial position in the full length range of the optical fiber.

4. The spatial domain measurement distributed optical fiber acoustic wave sensing system according to one of claims 1 to 3, characterized in that, The direct current laser outputted by the narrow linewidth laser has a linewidth of less than or equal to 1 kHz, a coherence length of greater than or equal to 10 km, and a wavelength range of 1520nm-1620nm.

5. The spatial domain measurement distributed optical fiber acoustic wave sensing system according to one of claims 1 to 3, characterized in that, The first radio frequency source outputs a first radio frequency signal of a first frequency, a wave number of the first radio frequency signal being determined based on the first frequency and a speed of light in the optical fiber, the first radio frequency signal being modulated by a first electronic switch into a pulse signal of a preset pulse width, forming a first pulse modulation signal; The second radio frequency source outputs a second radio frequency signal of a second frequency, a wave number of the second radio frequency signal being determined based on the second frequency and the speed of light in the optical fiber, the second radio frequency signal being modulated by a second electronic switch into the pulse signal of the preset pulse width, and the second pulse modulation signal is time-delayed relative to the first pulse modulation signal by delay control, forming a second pulse modulation signal; the first pulse modulation signal and the second pulse modulation signal are processed by a mixer to generate a double heterodyne modulation signal, driving an acousto-optic modulator to modulate an input direct-current laser into a modulated light wave containing two spatial feature pulses and having a certain interval; wherein the time delay corresponds to a spatial distance of the certain interval of the two spatial feature pulses, and a wave number difference of the two spatial feature pulses is a difference between the wave number of the first radio frequency signal and the wave number of the second radio frequency signal.

6. The spatial domain measurement distributed optical fiber acoustic wave sensing system of claim 1, wherein, When the computer performs spatial domain demodulation, the generated orthogonal digital reference signal is a sine reference signal and a cosine reference signal related to the spatial position of the optical fiber; after the electrical signal is mixed with the sine reference signal and the cosine reference signal respectively, and then subjected to spatial domain low-pass filtering, the sine signal and the cosine signal after mixing and spatial domain low-pass filtering are subjected to arctangent operation, and the amount of phase change of the light wave caused by the vibration of the optical fiber at different spatial positions in the full length range of the optical fiber due to external sound waves is demodulated.

7. A spatial domain measurement distributed optical fiber acoustic wave sensing method, characterized by, The method comprises the following steps: S1. A light source outputs a direct-current laser, and the light source is a narrow linewidth laser; S2. The direct-current laser is modulated by double heterodyne pulse technology: a first radio frequency source outputs a first radio frequency signal, and a second radio frequency source outputs a second radio frequency signal, the pulse timing of the two radio frequency signals is controlled by electronic switches, and then a double heterodyne modulation signal is generated by a mixer, driving an acousto-optic modulator to modulate the direct-current laser into a modulated light wave containing two spatial feature pulses and having a certain interval; S3. The modulated light wave is power amplified, and then injected into an optical fiber for distributed sensing through an optical circulator, exciting feedback light signals in the full length range of the optical fiber; S4. The feedback light signals returned by the optical fiber are transmitted to a photodetector through the optical circulator, and converted into electrical signals carrying information of different spatial positions of the optical fiber; S5. The electrical signals are collected and transmitted to a computer, and the computer performs spatial domain demodulation: based on the wave number difference of the two spatial feature pulses, an orthogonal digital reference signal with the spatial position of the optical fiber as a variable is generated, the electrical signal is mixed with the orthogonal digital reference signal, and then subjected to spatial domain low-pass filtering, and the amount of phase change of the light wave caused by the vibration of the optical fiber at different spatial positions in the full length range of the optical fiber due to external sound waves is demodulated by arctangent operation; and based on the amount of phase change of the light wave, distributed optical fiber sound wave sensing is realized.

8. The spatial domain measurement distributed optical fiber acoustic wave sensing method according to claim 7, characterized in that, In step S3, the optical fiber for distributed sensing is marked with a plurality of reflection points at equal intervals along the length direction, the reflection points being weak reflection gratings or microstructure reflection points; the feedback optical signal is a reflection optical signal, formed by reflection of the modulated optical wave by the reflection points, and carrying information of the corresponding spatial position.

9. The spatial domain measurement distributed optical fiber acoustic wave sensing method of claim 7, wherein, In step S3, the optical fiber for distributed sensing is a single-mode optical fiber; the feedback optical signal is a back Rayleigh scattering optical signal, formed by natural Rayleigh scattering of the modulated optical wave with molecules, atoms and / or small particles of the single-mode optical fiber during transmission in the single-mode optical fiber, and carrying information of each spatial position in the full length range of the optical fiber.

10. The spatial domain measurement distributed optical fiber acoustic wave sensing method according to one of claims 7 to 9, characterized in that, In step S5, the quadrature digital reference signal includes a sine reference signal and a cosine reference signal related to the spatial position of the optical fiber; and the spatial domain demodulation specifically includes: mixing the electrical signal with the sine reference signal and the cosine reference signal respectively, performing spatial domain low-pass filtering, and then performing arctangent operation on the mixed and spatial domain low-pass filtered sine signal and cosine signal to demodulate the phase change amount of the optical wave at each spatial position.

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