CN106052842B - Distributed optical fiber vibration sensing system capable of eliminating fading noise and its demodulation method - Google Patents
Distributed optical fiber vibration sensing system capable of eliminating fading noise and its demodulation method Download PDFInfo
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- 239000000835 fiber Substances 0.000 claims abstract description 26
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Abstract
一种可消衰落噪声的分布式光纤振动传感系统及其解调方法，包括：信号发生模块、窄线宽激光光源模块、扫频切光脉冲模块、环形器、传感光纤、相干接收模块、光电转换模块和数字信号处理模块；计算机通过产生多个频段不同且无重叠的数字带通滤波器，结合来自多个扫频探测光脉冲的原始数据段，得到传感光纤的反射率曲线；对反射率曲线进行移相平均处理，得到无干涉和偏振衰落的多条综合反射率曲线；对综合反射率曲线作延迟差分处理，得到多条相位差分曲线；对相位差分曲线求方差，得到一条相位方差曲线；根据相位方差曲线中的方差作振动点的判定，得到振动点的位置和振动波形；本发明设计合理，定位准确度高，可同时获得高空间分辨率和长探测距离。
A distributed optical fiber vibration sensing system capable of eliminating fading noise and a demodulation method thereof, comprising: a signal generation module, a narrow linewidth laser light source module, a sweep frequency cut light pulse module, a circulator, a sensing fiber, and a coherent receiving module , photoelectric conversion module and digital signal processing module; the computer obtains the reflectivity curve of the sensing fiber by generating multiple digital band-pass filters with different frequency bands and without overlapping, and combining the original data segments from multiple frequency-sweeping detection light pulses; Perform phase-shift averaging processing on the reflectivity curve to obtain multiple comprehensive reflectivity curves without interference and polarization fading; perform delay differential processing on the comprehensive reflectivity curve to obtain multiple phase difference curves; calculate the variance of the phase difference curves to obtain one Phase variance curve; the vibration point is determined according to the variance in the phase variance curve, and the position and vibration waveform of the vibration point are obtained; the present invention has reasonable design, high positioning accuracy, and can simultaneously obtain high spatial resolution and long detection distance.
Description
Technical Field
The invention relates to a technology in the field of optical fiber sensing, in particular to a distributed optical fiber vibration sensing system capable of eliminating fading noise and a demodulation method thereof.
Background
Vibration sensing technology is widely used in the engineering field, such as boundary intrusion monitoring, structural health monitoring, petroleum pipeline safety monitoring, seismic wave monitoring and the like. Conventional vibration sensors are generally mechanical or electromagnetic and have many problems, such as: in a strong electric field environment, the traditional vibration sensor is easy to be interfered by electromagnetic waves and cannot work normally; in a flammable and explosive environment, electric sparks can be generated to cause accidents; in remote locations, there are power supply problems; distributed sensing cannot be achieved. These drawbacks severely limit the practical application of conventional vibration sensors.
Since the invention of optical fiber in the 70's of the 20 th century, the development of optical fiber sensing technology has been vigorous. In addition to being useful for high-speed communications over long distances, optical fibers also have the ability to sense external physical parameters. With this sensitive characteristic, researchers have invented a range of fiber optic sensing devices. Distributed fiber optic vibration sensors have been a focus of research in recent years. It has many advantages over conventional vibration sensors, such as: waterproof and moistureproof; anti-electromagnetic interference; the use is safe; most importantly, distributed sensing and remote sensing capabilities are provided.
The most studied and widely used distributed fiber vibration sensor at present is the fiber vibration sensor based on phase sensitive optical time domain reflectometry (phase sensitive optical time domain reflectometry). The method detects and positions vibration according to the interference result of Rayleigh back scattering light of each reflection point in the sensing optical fiber by transmitting optical pulses with extremely strong coherence to the sensing optical fiber, and can obtain the time domain waveform of the vibration. The method has the advantages of compact system structure, simple demodulation algorithm, accurate positioning, high signal-to-noise ratio and high sensitivity. But has a serious inherent disadvantage: the spatial resolution (i.e. the minimum distance at which two vibration events occur simultaneously and can be resolved) and the maximum measurement length are contradictory. As known from the principle of phi-OTDR, high spatial resolution requires short detection pulse length, but short pulse power, which makes the detection distance limited; and vice versa.
Besides the defects in principle, the existing phi-OTDR algorithm for detecting and positioning vibration also has certain problems. The initial algorithm is to detect and locate vibrations based on the intensity variations of the rayleigh backscattered light, which is known as intensity demodulation. Although the algorithm is simple and easy to implement, the algorithm is easy to be interfered by noise, the signal-to-noise ratio and the sensitivity are low, the intensity of the reflected light and the amplitude of the vibration are in a nonlinear relation, the measured vibration waveform is distorted, and in addition, the light polarization state has a great influence on the detection result. Then, an algorithm based on the phase change of the rayleigh backscattered light, i.e., a phase demodulation method, is proposed. The algorithm has extremely high signal-to-noise ratio and sensitivity, and because the phase change and the vibration amplitude are in a linear relation, the phase demodulation method can obtain a vibration waveform without distortion. However, the phase demodulation method still has problems: under the influence of interference fading and polarization fading, the power of the backscattered light of many reflection points can be very low, which causes the phase demodulation of these points to be wrong, and the point with wrong phase can be misjudged as a vibration point. This problem leads to the phase demodulation method not being able to accurately determine the position of the vibration point.
Disclosure of Invention
The invention provides a distributed optical fiber vibration sensing system capable of eliminating fading noise and a demodulation method thereof, aiming at the defects that the signal-to-noise ratio is low, the polarization fading and the interference fading noise cannot be eliminated and the like in the prior art.
The invention is realized by the following technical scheme:
the invention relates to a distributed optical fiber vibration sensing system capable of eliminating fading noise, which comprises: signal generation module, narrow linewidth laser light source module, sweep frequency cut light pulse module, circulator, sensing fiber, coherent receiving module, photoelectric conversion module and digital signal processing module, wherein: the signal generation module inputs the amplified sweep frequency radio frequency pulse train signal to the sweep frequency light-cutting pulse module, and simultaneously the signal generation module sends a trigger signal to the digital signal processing module; the ultra-narrow line width laser generated by the narrow line width laser light source module is divided into two paths, one path is a probe light input sweep frequency light cutting pulse module, and the other path is a reference light input coherent receiving module; the frequency sweep light cutting pulse module outputs an amplified frequency sweep detection light pulse string, and the amplified frequency sweep detection light pulse string is input into the sensing optical fiber through the circulator; rayleigh back scattering light generated by the sensing optical fiber is input into the coherent receiving module through the circulator, and is subjected to beat frequency with reference light in the coherent receiving module, and a generated beat frequency optical signal is input into the photoelectric conversion module; the photoelectric conversion module converts the beat frequency optical signal into an electric signal and inputs the electric signal into the digital signal processing module for phase demodulation.
The signal generation module comprises: a signal generator and a radio frequency signal amplifier connected.
The frequency sweep radio frequency pulse train signal output by the signal generator comprises: a plurality of sweep-frequency radio frequency pulse signals with equal time interval, same sweep-frequency range and same pulse width.
The narrow linewidth laser light source module comprises: the device comprises a narrow linewidth fiber laser, a fiber coupler and a polarization controller which are connected in sequence.
Preferably, the splitting ratio of the optical fiber coupler is 90: 10.
The sweep frequency light-cutting pulse module comprises: an acousto-optic modulator/single sideband modulator and an erbium-doped fiber amplifier connected.
The sensing optical fiber is a common single-mode communication optical fiber.
The coherent receiving module is a 50:50 optical fiber coupler.
The photoelectric conversion module is a balanced detector.
The digital signal processing module comprises: the data acquisition card and the computer that link to each other, wherein: the data acquisition card samples the input electric signal and inputs the original data into the computer for phase demodulation.
The invention relates to a demodulation method based on the system, wherein a computer obtains a plurality of reflectivity curves of a sensing optical fiber by generating a plurality of digital band-pass filters with different frequency bands and without overlapping and combining original data segments of Rayleigh back scattering light from a plurality of sweep frequency detection optical pulses; carrying out fading elimination treatment on the reflectivity curves by adopting a phase-shift averaging method to obtain a plurality of comprehensive reflectivity curves without interference fading and polarization fading; carrying out delay difference processing on the comprehensive reflectivity curve to obtain a plurality of phase difference curves; solving the variance of the phase difference curve to obtain a phase variance curve; and finally, judging the vibration point according to the variance in the phase variance curve to obtain the position and the vibration waveform of the vibration point.
The reflectivity curve is obtained by the following method: the computer generates a plurality of digital band-pass filters with different frequency bands and without overlapping, divides an original data segment from a plurality of sweep frequency detection light pulses into sub-data segments with the same number as the digital band-pass filters, and performs cross-correlation operation on the sub-data segments and the digital band-pass filters with matched marks to obtain a reflectivity curve set of the sensing optical fiber.
The phase shift averaging method comprises the following steps: and taking the conjugate of one reflectivity curve as a reference, multiplying the conjugate of the reflectivity curve by other reflectivity curves to obtain a reflectivity curve set with the phase return to zero, and carrying out average operation on the reflectivity curves with the phase return to zero to obtain a comprehensive reflectivity curve without interference fading and polarization fading.
The delay difference processing means: and taking the phase item of each comprehensive reflectivity curve as a phase curve, delaying the phase curve, and differentiating the phase curves before and after time shifting to obtain a differential phase curve.
The judgment of the vibration point is as follows: if the variance of a point in the phase variance curve is greater than 0.02, the point is a vibration point.
The corresponding positions of the vibration points on the sensing optical fiber are as follows:
wherein: c' is the propagation velocity of light in the fiber, tsIs the sampling rate, k, of the data acquisition card0Is a vibration point.
The vibration waveform of the vibration point is a new sequence formed by the differential phases at the vibration point in the differential phase curve.
Technical effects
Compared with the prior art, the invention designs a new reflectometer structure, emits the detection light pulse with long pulse width and large sweep frequency range, and can simultaneously obtain large spatial resolution and long detection distance; the phase demodulation algorithm of 'frequency conversion-phase shift averaging' can effectively eliminate the extremely weak points caused by interference fading and polarization fading on the reflectivity curve of the sensing optical fiber, further eliminate phase demodulation errors, facilitate accurate detection and positioning of vibration and have high signal-to-noise ratio.
Drawings
FIG. 1 is a schematic diagram of a distributed optical fiber vibration sensing system;
FIG. 2 is a time-frequency spectrum of a probe light pulse;
FIG. 3 is a graph of an embodiment output;
in the figure: (a) a mode curve of the integrated reflectivity, (b) a differential phase curve of the integrated reflectivity, (c) a mode curve of the integrated reflectivity near the vibration point, and (d) a differential phase curve of the integrated reflectivity near the vibration point;
FIG. 4 is a vibration signal spectrum of vibration points output by the embodiment;
in the figure: (a) time domain graph of a first vibration point, (b) power spectrum of the first vibration point, (c) time domain graph of a second vibration point, and (d) power spectrum of the second vibration point;
in the figure: the device comprises a signal generator 1, a radio frequency signal amplifier 2, a narrow-linewidth fiber laser 3, a fiber coupler 4, an acousto-optic modulator 5, a erbium-doped fiber amplifier 6, a circulator 7, a sensing fiber 8, a polarization controller 9, a fiber coupler 10 in a ratio of 50:50, a balance detector 11, a data acquisition card 12 and a computer 13.
Detailed Description
The following examples are given for the detailed implementation and specific operation of the present invention, but the scope of the present invention is not limited to the following examples.
Example 1
As shown in fig. 1, the present embodiment includes: signal generation module, narrow linewidth laser light source module, sweep frequency cut light pulse module, circulator 7, sensing fiber 8, coherent receiving module, photoelectric conversion module and digital signal processing module, wherein: the signal generation module inputs the amplified sweep frequency radio frequency pulse train signal to the sweep frequency light-cutting pulse module, and simultaneously the signal generation module sends a trigger signal to the digital signal processing module; the ultra-narrow linewidth laser generated by the narrow linewidth laser light source module enters an a port and is divided into two paths, wherein one path is that probe light is input into the sweep frequency light cutting pulse module through a b port, and the other path is that reference light is input into the coherent receiving module through a c port; the sweep frequency light cutting pulse module outputs an amplified sweep frequency detection light pulse string, and the sweep frequency detection light pulse string is input through an a port of the circulator 7 and output to the sensing optical fiber 8 from a b port; rayleigh back scattering light generated by the sensing optical fiber 8 is input through a port b of the circulator 7 and output to the coherent receiving module through a port c, and is subjected to beat frequency with reference light in the coherent receiving module, and a generated beat frequency light signal is input to the photoelectric conversion module; the photoelectric conversion module converts the beat frequency optical signal into an electric signal and inputs the electric signal into the digital signal processing module for phase demodulation.
The total length of the sensing optical fiber 8 is 10 km.
The signal generation module comprises: a signal generator 1 and a radio frequency signal amplifier 2 connected.
As shown in fig. 2, the swept rf burst signal output by the signal generator 1 includes: multiple equal time intervals T, same sweep range F and same pulse width tauPThe swept radio frequency pulse signal of (1).
The time interval T of the sweep frequency radio frequency pulse signal is 120 mu s, the sweep frequency range F is 150-250 MHz, and the pulse width tauPWas 4. mu.s.
The narrow linewidth laser light source module comprises: the device comprises a narrow linewidth optical fiber laser 3, an optical fiber coupler 4 and a polarization controller 9 which are connected in sequence.
The splitting ratio of the optical fiber coupler 4 is 90: 10.
The sweep frequency light-cutting pulse module comprises: an acousto-optic modulator 5 and an erbium doped fibre amplifier 6 connected.
The sensing optical fiber 8 is a common single-mode communication optical fiber.
The coherent receiving module is a 50:50 optical fiber coupler 10.
The photoelectric conversion module is a balanced detector 11.
The bandwidth of the balanced detector 11 is 400 MHz.
The digital signal processing module comprises: a data acquisition card 12 and a computer 13 connected thereto, wherein: the data acquisition card 12 samples the input electrical signal and inputs the raw data into the computer 13 for phase demodulation.
The sampling rate t of the data acquisition card 12s1GSa/s and 8bit resolution.
The embodiment relates to a demodulation method of the system, which is a frequency conversion-phase shift averaging method and comprises the following steps:
The reflectivity curve is as follows:
κ denotes an index symbol, denotes a conjugate, and the reflectance is a complex number.
Interference fading and polarization fading exist on the reflectivity curve.
Take n as 1 as an example, L reflectivity curves
Demodulating L parts of Rayleigh back scattering light of a detection light pulse from the same sweep frequency, wherein serious interference fading and polarization fading points exist on L reflectivity curves, and the modulus of the reflectivity of the fading points is very small, so that the phase demodulation of the points can be wrong. However, since the frequencies of the L portions are different, the L reflectivity curves are also different, i.e., the positions of the weak points caused by interference fading and polarization fading on the L reflectivity curves are also different. Averaging the L reflectivity curves eliminates these weak points, thereby eliminating phase demodulation errors at these points. However, since the reflectance is a complex number, it is known from the knowledge of complex addition that the modulus of the result of the complex addition does not necessarily become large, and may become small. To maximize the summed moduli of the reflectivities, the reflectivities need to be rotated to zero their included angle before being summed.
And 6, for the N phase curves obtained in the previous step, performing time delay M units, and then differentiating the phase curves before and after time shift to obtain N differential phase curves: { Delta phi [ [ alpha ] ]n(k)＝φn(k)-φn(k-M)；k＝1,…,K}；n＝1,…,N。
Step 7, solving the variance of the N differential phase curves obtained in the previous step to obtain a phase variance curve:
The spatial resolution Δ z of this embodiment is determined by the size Δ F of the swept range of the swept probe light pulse, i.e.
Wherein: and gamma is the sweep speed.
As shown in fig. 3 and 4, the present embodiment provides two vibration points, wherein a single-frequency vibration with a frequency of 1kHz occurs at 9.83km of the sensing fiber 8, a single-frequency vibration with a frequency of 4kHz occurs at 9.93km, and the vibration coverage of the two vibration points is 10 m.
In this embodiment, N is 80, L is 4, and the delay unit M is 50; the four frequency bands are respectively 150-170 MHz, 175-195 MHz, 200-220 MHz and 225-245 MHz; the raw data segments for the 80 swept probe light pulses are chronologically labeled xn(k) (ii) a K ═ 1,. K }; n 1., 80, the divided 80 × 4 320 sub-data segments are labeled as { x }n,l(k)；k＝1,…,K}；n＝1,…,80；l＝1,…,4。
The reflectivity curve obtained in this example is:
{φn(k)＝angle[rn(k)](ii) a K ═ 1, …, K }; n is 1, …,80, and the differential phase curve is:
{Δφn(k)＝φn(k)-φn(k-50); k ═ 1, …, K }; n is 1, …,80, and the phase variance curve is:
as shown in FIG. 3(d), k is at [98200, 98400 ]]And [99200, 99300]The variance in the range is far more than 0.02, and the two sections can be judged to vibrate, and the vibration position z is 9830m and 9930m and is matched with the set vibration position; the vibration waveforms of the two vibration points are respectively { delta phi98300(n); n is 1, …,80, and { Δ φ99300(n); n is 1, …,80, and the signal-to-noise ratio of the obtained vibration waveform reaches 30dB, as shown in fig. 4(b) and (d).
The distributed optical fiber sensing system based on the novel reflectometer structure of the embodiment overcomes the contradiction between the spatial resolution and the dynamic range existing in the traditional phi-OTDR system, and simultaneously obtains a high dynamic range of 20dB and a high spatial resolution of 5m, as shown in (a) and (d) of FIG. 3; the phase demodulation algorithm of 'frequency conversion-phase shift averaging' is adopted, the problems of polarization fading and interference fading in the traditional phase demodulation method are solved, the vibration position is accurately positioned, and meanwhile, the high signal-to-noise ratio is kept.
Claims (5)
1. A demodulation method of a distributed optical fiber vibration sensing system based on fading noise elimination is characterized in that the sensing system comprises: signal generation module, narrow linewidth laser light source module, sweep frequency cut light pulse module, circulator, sensing fiber, coherent receiving module, photoelectric conversion module and digital signal processing module, wherein: the signal generation module inputs the amplified sweep frequency radio frequency pulse train signal to the sweep frequency light-cutting pulse module, and simultaneously the signal generation module sends a trigger signal to the digital signal processing module; the ultra-narrow line width laser generated by the narrow line width laser light source module is divided into two paths, one path is a probe light input sweep frequency light cutting pulse module, and the other path is a reference light input coherent receiving module; the frequency sweep light cutting pulse module outputs an amplified frequency sweep detection light pulse string, and the amplified frequency sweep detection light pulse string is input into the sensing optical fiber through the circulator; rayleigh back scattering light generated by the sensing optical fiber is input into the coherent receiving module through the circulator, and is subjected to beat frequency with reference light in the coherent receiving module, and a generated beat frequency optical signal is input into the photoelectric conversion module; the photoelectric conversion module converts the beat frequency optical signal into an electric signal and inputs the electric signal into the digital signal processing module for phase demodulation;
the demodulation method comprises the following steps: the computer obtains a plurality of reflectivity curves of the sensing optical fiber by generating a plurality of digital band-pass filters with different frequency bands and without overlapping and combining original data segments of Rayleigh back scattering light from a plurality of sweep frequency detection light pulses; carrying out fading elimination treatment on the reflectivity curves by adopting a phase-shift averaging method to obtain a plurality of comprehensive reflectivity curves without interference fading and polarization fading; carrying out delay difference processing on the comprehensive reflectivity curve to obtain a plurality of phase difference curves; solving the variance of the phase difference curve to obtain a phase variance curve; finally, judging a vibration point according to the variance in the phase variance curve to obtain the position and the vibration waveform of the vibration point;
the reflectivity curve is obtained by the following method: the computer generates a plurality of digital band-pass filters with different frequency bands and without overlapping, original data segments from a plurality of sweep frequency detection light pulses are divided into sub data segments with the same number as the digital band-pass filters, and the sub data segments and the digital band-pass filters with matched marks perform cross-correlation operation to obtain a reflectivity curve set of the sensing optical fiber;
the phase shift averaging method comprises the following steps: taking the conjugate of one reflectivity curve as a reference, performing multiplication operation with other reflectivity curves to obtain a reflectivity curve set with the phase return to zero, and performing average operation on the reflectivity curves with the phase return to zero to obtain a comprehensive reflectivity curve without interference fading and polarization fading;
the delay difference processing means: taking the phase item of each comprehensive reflectivity curve as a phase curve, delaying the phase curve, and differentiating the phase curves before and after time shifting to obtain a differential phase curve;
the judgment of the vibration point is as follows: when the variance of a certain point in the phase variance curve is more than 0.02, the point is a vibration point; the location of the vibration point is：
Wherein: c' is the propagation velocity of light in the fiber, tsIs the sampling rate, k, of the data acquisition card0Is a vibration point; the vibration waveform of the vibration point is a new sequence formed by the differential phases at the vibration point in the differential phase curve.
2. The demodulation method according to claim 1, wherein said signal generation module comprises: the sweep-frequency radio-frequency pulse train signal output by the signal generator comprises: a plurality of sweep-frequency radio frequency pulse signals with equal time interval, same sweep-frequency range and same pulse width.
3. The demodulation method according to claim 1, wherein the narrow linewidth laser light source module comprises: the device comprises a narrow linewidth fiber laser, a fiber coupler and a polarization controller which are connected in sequence.
4. The demodulation method of claim 1 wherein said swept-frequency optical pulse-chopping module comprises: an acousto-optic modulator/single sideband modulator and an erbium-doped fiber amplifier connected.
5. The demodulation method according to claim 1, wherein said digital signal processing module comprises: the data acquisition card and the computer that link to each other, wherein: the data acquisition card samples the input electric signal and inputs the original data into the computer for phase demodulation.
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