A long-distance optical fiber distributed vibration sensing device and working method
By detecting and demodulating the backscattered signals of the two polarization channels, the polarization fading phenomenon is suppressed by using a 90° optical mixer, the problem of decreasing signal-to-noise ratio in the existing technology is solved, and high-precision long-distance vibration monitoring is achieved.
Patent Information
- Application Number
- CN202210853380.8
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2022-07-20
- Publication Date
- 2025-06-27
- Estimated Expiration
- 2042-07-20
AI Technical Summary
The existing fiber distributed vibration sensing system is difficult to suppress polarization fading during long-distance detection, resulting in a decrease in signal-to-noise ratio and making it difficult to achieve high-precision long-distance vibration monitoring.
By detecting and demodulating the backscattered signals of the two polarization channels, the 90° optical mixer makes the system signal-to-noise ratio independent of the polarization state of the fiber backscattered signal, thereby suppressing the polarization fading phenomenon.
It effectively suppresses the polarization fading phenomenon caused by long-distance detection, improves the signal-to-noise ratio of the system, and enhances the accuracy and reliability of long-distance vibration monitoring.
Smart Images
Figure CN115112219B_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of optical fiber sensing, and particularly to a long-distance optical fiber distributed vibration sensing device and a working method thereof. Background Technique
[0002] The statements in this part only provide background technical information related to the present invention and do not necessarily constitute prior art.
[0003] An optical fiber distributed vibration sensing (DVS) system uses interferometric measurement technology to detect the spatially distributed backward Rayleigh scattering signals generated when light waves propagate along an optical fiber link. By measuring the changes in the backward scattering signals along the optical fiber, the system can obtain the stress or refractive index changes that are spatially distributed along the optical fiber link. Stress changes are usually caused by vibrations, so the system can measure the vibration signals that are spatially distributed along the optical fiber link.
[0004] DVS has been widely used in oil and gas well monitoring, underground pipeline monitoring, and border control and security fields. With the expansion of the application fields, the requirements for the long-distance detection performance of DVS systems are getting higher and higher.
[0005] In a DVS system based on φ-OTDR (phase-sensitive optical time domain reflectometry), a coherent laser emits short pulses into the optical fiber. When the short pulses propagate along the optical fiber link, backward Rayleigh scattering signals are generated, and the backward scattering responses are received by the photodetectors of the system. The spatial resolution of the system is limited by the pulse length. When stress acts on a part of the optical fiber, the phase delays of the interference signals of multiple Rayleigh scattering points will change, causing fluctuations in the backward scattering responses, making it difficult to achieve long-distance monitoring. Summary of the Invention
[0006] In order to solve at least one of the technical problems existing in the above background technique, the present invention provides a long-distance optical fiber distributed vibration sensing device and a working method thereof, which detect and demodulate the backward scattering signals of two polarization channels. The signal powers of the two polarization channels obtained are independent of the polarization state of the backward scattering signals input to the 90° optical mixer, making the signal-to-noise ratio of the system independent of the polarization state of the optical fiber backward scattering signals, thereby being able to suppress the polarization fading phenomenon caused by long-distance detection and contributing to the realization of long-distance vibration monitoring.
[0007] In order to achieve the above object, the present invention adopts the following technical solutions:
[0008] The first aspect of the present invention provides a long-distance optical fiber distributed vibration sensing device, including:
[0009] A laser connected to the first port of the first coupler. The second port of the first coupler is sequentially connected to an acousto-optic modulator, an erbium-doped fiber amplifier, and the first port of an optical switch. The third port of the first coupler is connected to the second port of an optical mixer. The processor is connected to the acousto-optic modulator through a digital-to-analog converter and a radio frequency driver. The second port of the optical switch is connected to the first port of a circulator. The third port of the optical switch is connected to the third port of the second coupler. The second port of the circulator is connected to a sensing optical fiber, and the third port is connected to the second port of the second coupler. The first port of the second coupler is connected to the first port of the optical mixer. The third port of the optical mixer is connected to the processor through a balun receiver and an analog-to-digital converter.
[0010] The second aspect of the present invention provides a working method for the above device, including the following steps:
[0011] The laser generates a low-noise light wave that enters the first port of the first coupler and is divided into two paths. One path exits from the second port as a modulated light wave and enters the acousto-optic modulator, and the other path exits from the third port as a local oscillator light wave and enters the second port of the optical mixer. The acousto-optic modulator shapes the modulated light wave to generate a short pulse light wave, which is amplified by the erbium-doped fiber amplifier and then transmitted to the circulator through the second port of the optical switch. The short pulse light wave exits from the second port of the circulator and propagates in the sensing optical fiber.
[0012] The short pulse light wave propagates in the sensing optical fiber, generates a backward Rayleigh scattering signal that returns to the second port of the circulator, exits from the third port of the circulator and enters the second port of the second coupler, then exits from the first port of the second coupler and enters the first port of the optical mixer. The third port of the optical mixer outputs four-path interference light intensity signals and enters the balun receiver. The balun receiver converts the interference light signal into an electrical signal and outputs four-path analog electrical signals to the analog-to-digital converter. The analog-to-digital converter converts the four-path analog electrical signals into digital signals and sends them to the processor, so that the processor obtains the complex polarization signal of the backward scattering signal of the sensing optical fiber.
[0013] The processor controls the acousto-optic modulator through a digital-to-analog converter and a radio frequency driver, and sets the short pulse duration, linear sweep bandwidth, and step number.
[0014] The processor performs pulse compression processing on the complex polarization signal, performs convolution processing on the backward scattering signal and the matched filtering signal, and obtains the response signal after pulse compression. Specifically:
[0015]
[0016] In the formula, the convolution result S c (τ) is a function of the optical fiber delay τ, the optical pulse response r(t), the propagated short pulse optical field E t (t), and the matched filtering signal are all functions of τ.
[0017] The processor performs pulse compression processing to obtain the autocorrelation function of the propagation signal:
[0018]
[0019] Based on the backscattered digital signal after pulse compression processing, the processor demodulates the measurement result of the optical pulse response r(τ) related to the stress of the sensing optical fiber, realizing vibration monitoring along the sensing optical fiber.
[0020] Compared with the prior art, the above one or more technical solutions have the following beneficial effects:
[0021] 1. Detect and demodulate the backscattered signals of two polarization channels. The signal powers of the two polarization channels obtained are independent of the polarization state of the backscattered signal input to the optical mixer, making the system signal-to-noise ratio independent of the polarization state of the optical fiber backscattered signal. Thus, it can suppress the polarization fading phenomenon caused by long-distance detection, which helps to realize long-distance vibration monitoring.
[0022] 2. Short propagation pulse signals with a high time-bandwidth product can be used to improve the signal detection energy of the system, thereby increasing the system signal-to-noise ratio and extending the detectable distance of the sensing optical fiber. On the other hand, in the processing, the backscattered signal is convolved with the matched filtering signal to obtain a distance resolution proportional to the reciprocal of the frequency bandwidth. While increasing the detection signal energy, a good spatial resolution can still be obtained, enhancing the practicability of the long-distance vibration monitoring system. BRIEF DESCRIPTION OF THE DRAWINGS
[0023] The specification drawings forming a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention.
[0024] Figure 1 is a schematic structural diagram of a long-distance fiber distributed vibration sensing device provided by one or more embodiments of the present invention;
[0025] Figure 2 is a schematic diagram of pulse compression when the long-distance fiber distributed vibration sensing device provided by one or more embodiments of the present invention is working. DETAILED DESCRIPTION OF THE EMBODIMENTS
[0026] The present invention will be further described below in conjunction with the drawings and embodiments.
[0027] It should be noted that the following detailed description is exemplary and is intended to provide a further description of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.
[0028] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular forms are also intended to include the plural forms. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they specify the presence of features, steps, operations, devices, components, and / or combinations thereof.
[0029] DVS, Distributed Vibration Sensing.
[0030] φ-OTDR, Phase Sensitive Optical Time Domain Reflectometry.
[0031] As described in the background art, a DVS system based on φ-OTDR (Phase Sensitive Optical Time Domain Reflectometry) uses a coherent laser to emit short pulses into an optical fiber. When the short pulses propagate along the optical fiber link, a backward Rayleigh scattering signal is generated, and the backward scattering response is received by the optoelectronic detector of the system. The spatial resolution of the system is limited by the pulse length. When stress acts on a part of the optical fiber, the phase delay of the interference signals of multiple Rayleigh scattering points will change, causing fluctuations in the backward scattering response, thus making it difficult to achieve long-distance monitoring.
[0032] The following embodiments provide a long-distance fiber optic distributed vibration sensing device and working method, which detect and demodulate the backward scattering signals of two polarization channels. The signal powers of the two polarization channels obtained are independent of the polarization state of the backward scattering signal input to the 90° optical mixer, so that the signal-to-noise ratio of the system is independent of the polarization state of the optical fiber backward scattering signal, thereby being able to suppress the polarization fading phenomenon caused by long-distance detection and contributing to the realization of long-distance vibration monitoring.
[0033] Embodiment 1:
[0034] As Figure 1-2As shown in the figure, the purpose of this embodiment is to provide a long-distance fiber optic distributed vibration sensing device. A laser connected to the first port of the first coupler (coupler 1), the second port of the first coupler is sequentially connected to an acousto-optic modulator (AOM), an erbium-doped fiber amplifier (EDFA), and the first port of an optical switch. The third port of the first coupler is connected to the second port of an optical mixer (90° optical mixer); the processor is connected to the acousto-optic modulator through a digital-to-analog converter (DAC) and a radio frequency driver; the second port of the optical switch is connected to the first port of a circulator, and the third port of the optical switch is connected to the third port of the second coupler (coupler 2); the second port of the circulator is connected to the sensing fiber, the third port is connected to the second port of the second coupler, the first port of the second coupler is connected to the first port of the optical mixer (90° optical mixer), and the third port of the optical mixer (90° optical mixer) is connected to the processor through a balun receiver and an analog-to-digital converter.
[0035] In this embodiment, the device includes: a low-noise fiber laser with a wavelength of 1550 nm (model Origami-15), a 1×2 fiber coupler 1 (brand Senko), an ultrafast acousto-optic modulator (AOM) (brand TeraXion), a radio frequency driver (brand iXblue, model DR-PL-10-MO), a digital-to-analog converter (DAC) (brand Texas Instruments, USA, model DAC12DL3200), an erbium-doped fiber amplifier (EDFA) (brand Suzhou Bofu Optoelectronics, model AEDFA-23), an optical switch (brand Senko), a circulator (brand Senko), a sensing fiber (brand Jiangsu Hengtong, single-mode with a wavelength of 1550 nm), a 1×2 fiber coupler 2 (brand Senko), a 90° optical mixer (brand Suzhou Bofu Optoelectronics, model COH24), a balun receiver (brand Guangyi Intelligence, model PBD), an analog-to-digital converter (brand Texas Instruments, USA, model ADC12QJ1600), and a processor (a control board with an FPGA chip as the core).
[0036] The laser generates low-noise light waves that enter port 1 of coupler 1 and are divided into two paths. One path exits from port 2 as modulated light waves and enters the acousto-optic modulator, and the other path exits from port 3 as local oscillator light waves and enters port 2 of the 90° optical mixer.
[0037] The acousto-optic modulator shapes the modulated light waves to generate short pulse light waves and enters the erbium-doped fiber amplifier.
[0038] The processor controls the acousto-optic modulator through a digital-to-analog converter and a radio frequency driver to set the short pulse duration, linear sweep bandwidth, and number of steps.
[0039] The erbium-doped fiber amplifier amplifies the power of short-pulse light waves and enters ports 3 of coupler 2 and port 1 of the circulator in a time-division multiplexing manner through a 1×2 optical switch.
[0040] The short-pulse light wave exits from port 2 of the circulator and propagates in the sensing optical fiber.
[0041] The short-pulse light wave propagates in the sensing optical fiber and generates a backward Rayleigh scattering signal.
[0042] The backward scattering signal generated by the sensing optical fiber returns to port 2 of the circulator, exits from port 3 of the circulator and enters port 2 of coupler 2, then exits from port 1 of coupler 2 and enters port 1 of a 90° optical mixer. Four interference light intensity signals exit from port 3 of the 90° optical mixer and enter a balun receiver.
[0043] The balun receiver converts the interference optical signal into an electrical signal, realizes the balanced detection of the four interference light intensity signals, and the converted electrical signal removes the DC component. The balun receiver outputs four analog electrical signals to an analog-to-digital converter in total.
[0044] The analog-to-digital converter converts the four analog electrical signals into digital signals and sends them to a processor, so that the processor obtains the complex polarization signal of the backward scattering signal of the sensing optical fiber.
[0045] The processor performs pulse compression processing on the new digital signal, performs convolution processing on the backward scattering signal and the matched filtering signal (the complex conjugate of the time-reversed short-pulse light field propagation), and obtains the response signal after pulse compression.
[0046] Based on the backward scattering digital signal after pulse compression processing, the processor can demodulate the measurement result of the optical pulse response r(τ) related to the stress of the sensing optical fiber, so as to realize the function of vibration monitoring along the sensing optical fiber.
[0047] Embodiment 2:
[0048] This embodiment provides a working method of the device in Embodiment 1:
[0049] The laser generates a low-noise light wave and enters the first port of the first coupler, which is divided into two paths. One path exits from the second port as a modulated light wave and enters the acousto-optic modulator, and the other path exits from the third port as a local oscillator light wave and enters the second port of the optical mixer;
[0050] The acousto-optic modulator shapes the modulated light wave, generates a short-pulse light wave, which is amplified by the erbium-doped fiber amplifier and then transmitted to the circulator through the second port of the optical switch. The short-pulse light wave exits from the second port of the circulator and propagates in the sensing optical fiber.
[0051] Specifically:
[0052] In the first step, a laser generates a low-noise light wave that enters port 1 of coupler 1 and is split into two paths. One path exits from port 2 as a modulated light wave and enters an acousto-optic modulator, and the other path exits from port 3 as a local oscillator light wave and enters port 2 of a 90° optical mixer.
[0053] In the second step, the acousto-optic modulator shapes the modulated light wave to generate a short pulse light wave that enters an erbium-doped fiber amplifier. The short pulse duration is denoted as T sw = 100 μs, and a frequency component with a linear frequency sweep characteristic is superimposed on the light wave frequency.
[0054] The processor controls the acousto-optic modulator through a digital-to-analog converter and a radio frequency driver, setting the short pulse duration to 100 μs, the linear frequency sweep bandwidth to 80 MHz, and the number of steps to 10,000 times.
[0055] In the third step, the erbium-doped fiber amplifier amplifies the power of the short pulse light wave and enters port 3 of coupler 2 and port 1 of the circulator in a time-division multiplexing manner through a 1×2 optical switch.
[0056] In the fourth step, the short pulse light wave exits from port 2 of the circulator and propagates in the sensing fiber. The backward Rayleigh scattering generated when the short pulse light wave propagates in the sensing fiber can be described using the optical pulse response r(τ) related to the scattering delay parameter τ. The original waveform of the optical pulse response r(τ) is related to the thermal noise determined by the material density and structure of the fiber itself. When the short pulse light wave propagates in the sensing fiber, the backward scattering signal generated by the sensing fiber can be expressed as the convolution of the propagating short pulse optical field E t (t) and the fiber optical pulse response r(t), that is, r(t)*E t (t). When a certain area of the sensing fiber is deformed under stress, the optical pulse response r(τ) also changes in the corresponding delay area, thereby causing a phase change in the backward scattering signal in the corresponding space (time) area.
[0057] In the fifth step, the backward scattering signal generated by the sensing fiber returns to port 2 of the circulator, exits from port 3 of the circulator and enters port 2 of coupler 2, and then exits from port 1 of coupler 2 and enters port 1 of a 90° optical mixer. The 90° optical mixer realizes the coherent detection of the local oscillator light wave exiting from port 3 of coupler 1 and the backward scattering signal of the sensing fiber. Inside the 90° optical mixer, the light wave is split into two orthogonally polarized light waves by a polarization beam splitter. The two orthogonally polarized light waves pass through a 2×4 beam splitter and are split into four paths to be mixed with the local oscillator optical signal E LO (t), and four interference light intensity signals with a phase shift of 90° are obtained for output. Four interference light intensity signals exit from port 3 of the 90° optical mixer and enter a balun receiver.
[0058] In Step 6, the balun receiver converts the interference optical signal into an electrical signal, and the internal circuit bandwidth can meet the working requirements of the detection signal bandwidth while obtaining the best dynamic range. At the same time, the balun receiver realizes the balanced detection of 4-channel interference optical intensity signals, and the converted electrical signal removes the DC component. For each polarization optical field, the two electrical signals output by the balun receiver differ by 180°, and can be used as the in-phase signal and the quadrature signal of the polarization optical field respectively. The balun receiver outputs 4-channel analog electrical signals to the analog-to-digital converter in total.
[0059] In Step 7, the analog-to-digital converter converts the 4-channel analog electrical signals into digital signals and sends them to the processor, so that the processor obtains the complex polarization signal of the backscattered signal of the sensing optical fiber.
[0060] For the in-phase signal and the quadrature signal of each polarization optical field, they can be expressed as the correlation expressions of the optical pulse response r(t), the propagated short pulse optical field E t (t), and the local oscillator optical signal E LO (t):
[0061]
[0062] Since the acousto-optic modulator superimposes a frequency component with a linear frequency sweep characteristic on the basis of the laser output optical wave frequency, the frequency difference between the local oscillator optical signal E LO (t) and the propagated short pulse optical field E t (t) is usually greater than 100 MHz. Therefore, after the processor receives the complex polarization digital signal of the backscattered signal of the sensing optical fiber, it first uses digital mixing technology to reduce the signal center frequency to 0 Hz (DC), and then uses an anti-aliasing filter and sampling technology to obtain the digital signal equivalent to the sampling rate f ds obtained.
[0063] In Step 8, the processor performs pulse compression processing on the new digital signal, performs convolution processing on the backscattered signal and the matched filtering signal (the complex conjugate of the time reversal of the propagated short pulse optical field), and obtains the response signal after pulse compression:
[0064]
[0065] In the above formula, since the convolution result S c (τ) is a function of the optical fiber delay τ, the optical pulse response, the propagated short pulse optical field, and the matched filtering signal are all expressed as functions of τ.
[0066] The pulse compression processing of the processor is equivalent to performing a correlation operation on the propagated short pulse optical field to obtain the autocorrelation function of the propagated signal:
[0067]
[0068] Therefore, in the long-distance fiber-optic distributed vibration sensing device in this embodiment, the distance resolution is determined by A(τ), that is, the reciprocal of the bandwidth of the linearly chirped signal superimposed on the acousto-optic modulator.
[0069] Each group of backscattering responses after pulse compression processing by the processor contains multiple sampling points. The number of sampling points is usually greater than 10,000, and the specific value depends on the short pulse duration and the sampling rate of the analog-to-digital converter.
[0070] Based on the backscattering digital signal after pulse compression processing, the processor can demodulate the measurement result of the optical pulse response r(τ) related to the stress of the sensing fiber, so as to realize the vibration monitoring function along the sensing fiber.
[0071] On the one hand, short propagation pulse signals with a high time-bandwidth product can be used to improve the signal detection energy of the system, thereby improving the signal-to-noise ratio of the system and extending the detectable distance of the sensing fiber. On the other hand, in the processing, the backscattering signal is convolved with the matched filtering signal to obtain a distance resolution proportional to the reciprocal of the frequency bandwidth. While increasing the energy of the detection signal, a good spatial resolution can still be obtained, enhancing the practicability of the long-distance vibration monitoring system.
[0072] By detecting and demodulating the backscattering signals of the two polarization channels, the signal powers of the two polarization channels obtained are independent of the polarization state of the backscattering signal input to the 90° optical mixer, making the signal-to-noise ratio of the system independent of the polarization state of the fiber backscattering signal, so as to be able to suppress the polarization fading phenomenon caused by long-distance detection, which helps to realize long-distance vibration monitoring.
[0073] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.
Claims
1. A working method of a long-distance optical fiber distributed vibration sensing device, characterized in that: The long-distance optical fiber distributed vibration sensing device includes a laser connected to the first port of the first coupler. The second port of the first coupler is sequentially connected to an acousto-optic modulator, an erbium-doped fiber amplifier, and the first port of an optical switch. The third port of the first coupler is connected to the second port of an optical mixer. The processor is connected to the acousto-optic modulator through a digital-to-analog converter and a radio frequency driver. The second port of the optical switch is connected to the first port of a circulator, and the third port of the optical switch is connected to the third port of the second coupler. The second port of the circulator is connected to a sensing optical fiber, and the third port is connected to the second port of the second coupler. The first port of the second coupler is connected to the first port of the optical mixer. The third port of the optical mixer is connected to the processor through a balun receiver and an analog-to-digital converter. The laser generates a low-noise light wave that enters the first port of the first coupler and is split into two paths. One path exits from the second port as a modulated light wave and enters the acousto-optic modulator, and the other path exits from the third port as a local oscillator light wave and enters the second port of the optical mixer. The acousto-optic modulator shapes the modulated light wave to generate a short pulse light wave. After being amplified by the erbium-doped fiber amplifier, the short pulse light wave is transmitted through the second port of the optical switch to the circulator, and the short pulse light wave exits from the second port of the circulator and propagates in the sensing optical fiber. During propagation, the backward Rayleigh scattering signal generated by the sensing optical fiber returns to the circulator, and then passes through the second coupler, the optical mixer, the balun receiver, and the analog-to-digital converter in sequence and is sent to the processor, so that the processor obtains the complex polarization signal of the backward scattering signal of the sensing optical fiber. Inside the optical mixer, the light wave is split into two orthogonally polarized light waves by a polarization beam splitter, passes through the beam splitter and is split into four paths and mixed with the local oscillator light signal to obtain four interference light intensity signals with a phase shift of 90°. The four interference light intensity signals enter the balun receiver, and the balun receiver converts the four interference light signals into electrical signals and sends them to the analog-to-digital converter. The processor controls the acousto-optic modulator through a digital-to-analog converter and a radio frequency driver, and sets the short pulse duration, the linear sweep bandwidth, and the number of steps. The processor performs pulse compression processing on the complex polarization signal, performs convolution processing on the backward scattering signal and the matched filtering signal, and obtains the response signal after pulse compression. Specifically: ； In the formula, the convolution result is a function of the optical fiber delay , the optical pulse response , the propagating short-pulse optical field , and the matched-filtered signal are all functions of; The processor performs pulse compression processing to obtain the autocorrelation function of the propagation signal: ; The processor demodulates the optical pulse response related to the stress of the sensing optical fiber based on the backscattered digital signal after pulse compression processing Measurement results are used to achieve vibration monitoring along the sensing optical fiber; the backscattered signal generated by the sensing optical fiber is expressed as a propagating short pulse optical field And the optical pulse response of the optical fiber The convolution of, that is ; When the sensing optical fiber in a certain area is deformed under the influence of stress, the optical pulse response also changes in the corresponding delay area, causing a phase change of the backscattering signal in the corresponding space / time area.
2. The working method of a long-distance optical fiber distributed vibration sensing device according to claim 1, characterized in that: The short pulse light wave propagates in the sensing optical fiber, generates a backward Rayleigh scattering signal that returns to the second port of the circulator, exits from the third port of the circulator and enters the second port of the second coupler, then exits from the first port of the second coupler and enters the first port of the optical mixer. The third port of the optical mixer exits four interference light intensity signals and enters the balun receiver.
3. The working method of a long-distance optical fiber distributed vibration sensing device according to claim 1, characterized in that: The balun receiver converts the interference light signal into an electrical signal and outputs four analog electrical signals to the analog-to-digital converter.
4. The working method of a long-distance optical fiber distributed vibration sensing device according to claim 1, characterized in that: The analog-to-digital converter converts the four analog electrical signals into digital signals and sends them to the processor, so that the processor obtains the complex polarization signal of the backward scattering signal of the sensing optical fiber.