CN110207804B - Discretization processing method based on single-mode fiber distributed acoustic sensing - Google Patents
Discretization processing method based on single-mode fiber distributed acoustic sensing Download PDFInfo
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Abstract
The invention discloses discretization processing based on single-mode fiber distributed acoustic sensing, which comprises the following steps: dividing the single-mode optical fiber into M continuous sensing sections; injecting light detection pulses into the single mode fiber at the time t, and collecting the distribution of beat frequency signals of different positions of the single mode fiber; extracting the envelope of the beat frequency signal, and solving the distribution of the scattered light intensity of the single-mode fiber at different positions; dynamically searching peaks for the scattered light intensity distribution to obtain effective scattering points of the single-mode optical fiber; calculating phases of adjacent effective scattering points; obtaining a phase distribution function by adopting an interpolation method according to the phase of each effective scattering point; calculating the phase difference between two adjacent end points of each sensing section of the single-mode optical fiber at the time t according to the phase distribution function; and when T is equal to T', the change of the phase difference between two end points of each sensing interval along with time is taken as the sound wave information of the sensing interval. The invention solves the problem of phase demodulation point drift caused by fading, restrains low-frequency noise caused by fading and obtains stable sound wave signals.
Description
Technical Field
The invention belongs to the field of distributed acoustic wave optical fiber sensing, and particularly relates to a discretization processing method based on single-mode optical fiber distributed acoustic sensing.
Background
In the field of optical fiber distributed acoustic sensing, the phase sensitive optical time domain reflection technology has the characteristics of high signal-to-noise ratio, long detection distance and simple demodulation method, and meanwhile, coherent detection has the characteristics of high signal-to-noise ratio and high stability, so the phase sensitive optical time domain reflection technology based on coherent detection is the technology with the best development prospect and the strongest practicability in the field of optical fiber distributed acoustic sensing.
The phase sensitive optical time domain reflection technology based on coherent detection uses a single mode fiber as a sensing medium, and the traditional acoustic wave sensing principle is as follows:
when sound wave acts on the single-mode optical fiber, the optical fiber is stressed to stretch in the axial direction, so that the optical path of light propagating in the optical fiber is changed, the phase of the light wave is changed accordingly, and the phase-sensitive optical time domain reflection technology is used for sensing the sound wave by detecting the phase change of the light in the optical fiber.
The narrow-band light source emits continuous light and is divided into two paths, one path of the continuous light is used as detection light and is modulated into pulses, the pulses are incident into the single-mode optical fiber after being amplified, due to the fact that optical fiber core media are not uniform, backward Rayleigh scattering light can be generated when the optical pulses are transmitted in the single-mode optical fiber, the scattering light is transmitted back to the host along the optical fiber, the scattering light is coupled with the other path of the light source to generate beat frequency signals. Because the beat frequency signal contains the phase information of the reflected light, the beat frequency signal is demodulated to obtain the phase information of the light waves at different positions of the optical fiber. In the demodulation process, the optical fiber is divided into a plurality of sensing intervals, and the phase difference of each sensing interval is demodulated to represent the optical path of the detection optical pulse in the sensing interval, namely the length of the sensing interval. The sensing host sends a detection light pulse sequence to the single-mode fiber, so that the length change of each sensing section can be obtained, and the acoustic wave signal can be recovered.
Due to interference fading effects and polarization fading effects, the beat signal light intensity varies with time and distance. When the beat frequency light intensity is weak, the demodulated phase information has large noise, even correct phase information cannot be obtained, and stable sound wave measurement is difficult to realize.
In order to solve the above interference fading effect and polarization fading effect, patent CN201610635522.8 discloses that optical pulses with different frequencies are detected for multiple times, and then phase shift averaging is performed to suppress the interference fading effect and the polarization fading effect, but this method sacrifices the sampling frequency, and increases the complexity of the optical path and the cost of the device. Patent CN201711067989.8 discloses that interference fading and polarization fading can be directly eliminated by using chirped light pulse as detection pulse and diversity receiving and processing back scattered light in different directions for superposition, but the complexity of the system is greatly increased, so that the cost is increased and the stability of the system is reduced. Patent cn201410177760.x discloses that acoustic detection using a randomly polarized light source can only suppress polarization fading, and increases system cost. Patent CN201410105302.5 discloses a method for collecting 4 backscattered light signals with a polarization state having a phase difference of pi/2 and performing superposition processing on the four signals to suppress polarization fading, which sacrifices the sampling frequency and can only suppress interference fading noise. Patent CN201711104429.5 discloses a method for suppressing polarization fading by using a QPSK coherent receiver for polarization diversity reception, which increases system cost and cannot suppress interference fading noise.
In summary, the conventional method has the disadvantage of sacrificing other performance and increasing system cost, and a fading suppression method without increasing system cost and sacrificing other performance is urgently needed to be proposed.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide a discretization processing method based on single-mode optical fiber distributed acoustic sensing, and aims to solve the problem that phase demodulation is unstable due to interference fading and polarization fading of scattered light in the prior art.
In order to achieve the above object, the present invention provides a discretization processing method based on single-mode fiber distributed acoustic sensing, which includes:
s1: dividing the single-mode optical fiber into M continuous sensing sections;
s2: injecting light detection pulses into the single mode fiber at the time t, and collecting the distribution of beat frequency signals of different positions of the single mode fiber;
s3: extracting the envelope of the beat frequency signal, and solving the distribution of the scattered light intensity of the single-mode fiber at different positions;
s4: dynamically searching peaks for the scattered light intensity distribution to obtain effective scattering points of the single-mode optical fiber;
s5: calculating the phase difference between adjacent effective scattering points and the phase of each effective scattering point;
s6: obtaining a phase distribution function by adopting an interpolation method according to the phase of each effective scattering point;
s7: calculating the phase difference between two adjacent end points of each sensing section of the single-mode optical fiber at the time t according to the phase distribution function;
s8: judging whether T is equal to T', if not, T is equal to T + T, and turning to the step (2); otherwise, the change of the phase difference between two end points of each sensing interval along with time is used as the sound wave information of the sensing interval;
wherein T' is the last moment of a preset transmission pulse; t is the period of injecting emission detection pulse; the initial value of t is 0; m is an integer greater than 1.
Preferably, the relationship between the interval of the light detection pulse and the sampling frequency of the beat signal in step S2 is:
wherein T is the period of injecting the emission detection pulse; f is the sampling frequency of the beat frequency signal;
preferably, the relationship between the number of photo-detection pulses and the sampling frequency and sampling time of the beat signal in step S2 is:
Q＝f×t
wherein Q is the number of the photo-detection pulses; f is the sampling frequency of the beat frequency signal; t is the beat frequency signal sampling time;
preferably, the length of each sensing section in step S1 is:
wherein τ is the pulse width of the photodetection pulse; c is the speed of light in vacuum; NN is a gain coefficient of spatial resolution, which represents that the resolution limited by the pulse width is improved by N times; n is the refractive index of the fiber.
Preferably, the method for obtaining the scattered light intensity distribution of the single-mode optical fiber in step S3 includes:
performing Hilbert transform on the beat frequency signal to obtain an imaginary part of the beat frequency signal;
and taking the real part of the beat frequency signal as the real part of the envelope signal, taking the imaginary part of the beat frequency signal as the imaginary part of the envelope signal, and calculating the mode of the envelope signal as the scattering light intensity distribution of the single-mode optical fiber.
Preferably, the dynamic peak searching method of step S4 is:
determining the maximum value of the scattered light intensity distribution, and setting the threshold value to 1/10-1/5 of the maximum value;
setting the minimum position interval of the maximum value points of the collected scattered light intensity distribution, and searching all the maximum value points larger than a threshold value;
and if the position coordinate of the current maximum value point is more than half of the position minimum interval and the interval with the last maximum value point is more than the position minimum interval, the current maximum value point is an effective scattering point.
Preferably, the minimum interval of the positions of the maximum points of the collected scattered light intensity distribution is:
wherein τ is the pulse width of the photodetection pulse; n is the refractive index of the optical fiber; c is the speed of light in vacuum;
preferably, in step S5, the method for obtaining the phase difference between adjacent effective scattering points in the beat signal includes:
(5.1) mixing
Beat frequency signal E betweenk(l) As a beat signal of the kth effective scattering point, k is 1, 2, 3 … … N;
(5.2) adding Ek(l) And Ek+1(l) Multiplying and low-pass filtering to obtain constant A1；
(5.3) adding Ek(l) Hilbert transform followed by Ek+1(l) Multiplying and low-pass filtering to obtain constant A2；
(5.4) to A1/A2Performing arc tangent operation, wherein the operation result is the phase difference between the kth effective scattering point and the (k + 1) th effective scattering point;
(5.5) judging whether k is equal to N-1; if not, making k equal to k +1, and returning to the step (5.2); if yes, ending;
wherein the initial value of k is set to be 1, N is the number of effective scattering points, LkIs the position of the kth effective scattering point; and D is the position minimum interval of the maximum value points of the collected scattered light intensity distribution.
Preferably, the interpolation in step S6 is a piecewise linear interpolation or a cubic polynomial interpolation.
Through the technical scheme, compared with the prior art, the invention can obtain the following advantages
Has the advantages that:
1. according to the invention, firstly, a dynamic peak searching mode is adopted to obtain the effective scattering point of the single-mode optical fiber, noise generated by interference fading effect and polarization fading effect is avoided, then the phase of the effective scattering point is obtained by a high-precision demodulation method, and finally the phase of a set sensing interval is obtained by difference and resampling, so that the phase change of each sensing interval at different moments is obtained, and finally, acoustic wave measurement is realized.
2. The existing distributed acoustic wave sensing technology detects the scattering distribution of an optical fiber by emitting a single light pulse, divides the scattering distribution into equal intervals, processes and demodulates the equal intervals, and the spatial resolution is limited by the pulse width of the detected pulse light.
Drawings
FIG. 1 is a schematic diagram of the interference fading effect provided by the present invention;
FIG. 2 is a flow chart of a discretization processing method provided by the present invention;
FIG. 3 is a beat signal generated by emitting a photo-detection pulse according to the present invention;
FIG. 4 is a schematic diagram of the peak-finding-demodulating-interpolating-resampling process provided by the present invention;
FIG. 5 is an effective scattering point obtained by envelope extraction and peak searching of the beat signal provided by the present invention;
FIG. 6 is a schematic diagram of the beat signal interception of the phase demodulation point provided by the present invention;
FIG. 7 is a phase and interpolation result for a phase demodulation point provided by the present invention;
fig. 8 is a schematic diagram of resampling provided by the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
Fig. 1 is a schematic diagram of interference fading effect and polarization fading provided by the present invention, in which a beat signal envelope diagram generated by four optical detection pulses transmitted at an interval of 0.5s is shown, and it can be known from fig. 1 that, due to the influence of the interference fading effect, the beat signal generates large fluctuation in a short time, and in the conventional method, it cannot be ensured that the beat signal at a phase demodulation point has large light intensity, so that low-frequency noise is generated, and stable acoustic detection cannot be achieved.
As shown in fig. 2, the present invention provides a discretization processing method based on single-mode fiber distributed acoustic sensing, including:
s1: dividing the single-mode optical fiber into M continuous sensing sections;
specifically, the length of each sensing interval is:
wherein τ is the pulse width of the photodetection pulse; c is the speed of light in vacuum; n is a gain coefficient of spatial resolution, which represents that the resolution limited by the pulse width is improved by N times; n is the refractive index of the optical fiber;
in this example, τ is 20ns, M is 2, and the sensing interval has a length of 1M;
s2: injecting light detection pulses into the single mode fiber at the time t, and collecting the distribution of beat frequency signals of different positions of the single mode fiber;
wherein T is the period of injecting the emission detection pulse; (ii) a f is the sampling frequency of the beat frequency signal;
the relationship between the number of the photo-detection pulses and the sampling frequency and the sampling time of the beat signal in step S2 is:
Q＝f×t
wherein Q is the number of the photo-detection pulses; f is the sampling frequency of the beat frequency signal; t is the beat frequency signal sampling time;
fig. 3 shows a beat signal obtained by injecting a photodetection pulse with a pulse width τ into a single-mode fiber and then receiving a scattered signal;
FIG. 4 is a schematic diagram of four important steps of the present invention; the method comprises the following specific steps:
s3: extracting the envelope of the beat frequency signal, and solving the distribution of the scattered light intensity of the single-mode fiber at different positions;
specifically, the method for obtaining the scattered light intensity distribution of the single-mode fiber comprises the following steps:
performing Hilbert transform on the beat frequency signal to obtain an imaginary part of the beat frequency signal;
taking the real part of the beat frequency signal as the real part of the envelope signal, taking the imaginary part of the beat frequency signal as the imaginary part of the envelope signal, and calculating the mode of the envelope signal as the scattering light intensity distribution of the single-mode optical fiber;
as shown in FIG. 5, the beat signal E (l) is HillAfter the burt transform, the imaginary part is taken, and the imaginary part and the beat frequency signal E (l) are subjected to the square sum operation and then the root is obtained, so that the beat frequency envelope signal can be obtained, and the formula is shown as follows:
wherein E isc(l) Is the envelope of the beat signal; hilbert (x) is a hilbert transform on the x sequence, and the result is a complex sequence; imag (c) represents the imaginary part of the complex sequence c, and the curve in fig. 5 is the result of envelope extraction of the beat signal of fig. 4;
s4: dynamically searching peaks for the scattered light intensity distribution to obtain effective scattering points of the single-mode optical fiber;
specifically, the method for obtaining the effective scattering point of the single-mode fiber comprises the following steps:
determining the maximum value of the scattered light intensity distribution, and setting the threshold value to 1/10-1/5 of the maximum value;
setting the minimum position interval of the maximum value points for collecting the scattered light intensity distribution, and searching all the maximum value points larger than a threshold value;
and if the position coordinate of the current maximum value point is greater than half of the position minimum interval and the interval with the last phase extraction point is greater than the position minimum interval, the current maximum value point is an effective scattering point.
Preferably, the minimum position interval of the maximum value points of the collected scattered light intensity distribution is:
wherein τ is the pulse width of the photodetection pulse; n is the refractive index of the optical fiber; c is the speed of light in vacuum;
specifically, the positions of all maximum values are found in the curve shown in fig. 5, and points satisfying the threshold and interval conditions are selected from all maximum value points as phase extraction points; the threshold condition is 1/5 of the maximum value of the scattered light intensity distribution, and the purpose is to provide points with small assignment of beat signals and avoid noise interference caused by fading effect; the spacing condition being that the distance from the previous maximum point exceeds the minimum spacing and the distance from the signal start position exceeds the minimum spacingIn the present embodiment, the pulse width of the optical detection pulse is 20ns, and the light speed is 3 × 108m/s, refractive index of optical fiber is 1.5, minimum interval is 2m, and the positions of effective scattering points in the scattering light intensity distribution are L according to the setting of the parameters1、L2、……、LNCorresponding to the black dots in fig. 5;
s5: calculating the phase difference between adjacent effective scattering points in the beat frequency signal, and integrating the phase difference to obtain the phase of each effective scattering point;
specifically, the method for acquiring the phase difference between adjacent effective scattering points comprises the following steps:
(5.1) Single mode optical fiber
Beat frequency signal E betweenk(l) As a beat signal of the kth effective scattering point, k is 1, 2, 3 … … N;
(5.2) adding Ek(l) And Ek+1(l) Multiplying and low-pass filtering to obtain constant A1；
(5.3) adding Ek(l) Hilbert transform followed by Ek+1(l) Multiplying and low-pass filtering to obtain constant A2；
(5.4) to A1/A2Performing arc tangent operation, wherein the operation result is the phase difference between the kth effective scattering point and the (k + 1) th effective scattering point;
(5.5) judging whether k is equal to N-1; if not, making k equal to k +1, and returning to the step (5.2); if yes, ending;
wherein the initial value of k is set to be 1, N is the number of effective scattering points, LkIs the position of the kth effective scattering point; and D is the position minimum position interval of the maximum value points of the collected scattered light intensity distribution.
Specifically, the mathematical format is described as follows:
intercept each phase point
The beat frequency signal in the inner is used as the waveform for the demodulation phase of the k effective scattering point, FIG. 6 is the beat frequency signal intercepted by each effective scattering point, and the intercepted waveform of the k effective scattering point is Ek(l) The formula is adopted to be expressed as:
the phase difference between the kth effective scattering point and the k +1 effective scattering point is demodulated in the following manner:
where hilbert (x) denotes hilbert transform of the sequence x, and low _ pass _ filter (x) denotes low-pass filtering of the sequence x, in this example 50Hz low-pass filtering.
Through the method, the phase difference between every two adjacent effective scattering points is demodulated, and the phase of each effective scattering point is obtained through integral operation, as shown by the black dots in fig. 7.
S6: obtaining a phase distribution function at any position of the single-mode fiber by adopting an interpolation method according to the phase of each effective scattering point;
preferably, the interpolation method is a piecewise linear interpolation method or a cubic polynomial interpolation method;
the curves in fig. 8 are obtained by cubic spline interpolation of the phase of the effective scattering point.
S7: calculating the phase difference at the end point of each sensing section of the single-mode optical fiber at the time t according to the phase distribution function;
as shown in fig. 8, the phase information obtained by interpolation is resampled according to the divided sensing interval with 1m interval, so as to obtain the phase difference between two endpoints of the sensing interval, where fig. 8 shows the resampling process;
s8: judging whether T is equal to T', if not, T is equal to T + T, and turning to the step (2); otherwise, the change of the phase difference between two end points of each sensing interval along with time is used as the sound wave information of the sensing interval;
wherein T' is the last moment of a preset transmission pulse; t is the period of injecting emission detection pulse; the initial value of t is 0; m is an integer greater than 1.
The method is used for processing the detection pulses at different moments, and the waveforms are finally restored by arranging according to the pulse emission time and carrying out unwrapping operation.
In summary, the invention adopts a discretization data processing method of peak searching, demodulation, interpolation and resampling to solve the problem of unstable acoustic wave measurement, which specifically comprises the following steps:
firstly, the scattering intensity distribution of the optical fiber is obtained by detecting beat frequency signals generated by pulses, effective scattering points are obtained by peak value extraction, and the influence of fading effect on phase demodulation is effectively avoided. In the peak searching algorithm, the peak searching threshold value is set to 1/10 to 1/5 of the maximum value, so that the fading effect is avoided, and meanwhile, as many effective scattering points as possible are selected, and the high accuracy is ensured. In the peak searching algorithm, the peak searching interval is set as the minimum length which can be distinguished by the pulse width of the detection light, so that the aims of avoiding signal crosstalk and improving the operation speed can be achieved on the premise of not sacrificing the spatial resolution;
secondly, phase demodulation directly solves the phase difference of two adjacent effective scattering points, the cosine value of the phase difference of the two points can be obtained by multiplying the beat frequency signals of the two adjacent effective scattering points, one signal is subjected to 90-degree phase shift by using Hilbert transform and then multiplied by the other beat frequency signal, the sine value of the phase difference of the two points can be obtained, the sine value and the cosine value of the phase difference are divided, and arc tangent operation and unwrapping operation are carried out, so that the phase difference of the two points is finally obtained. The method directly solves the phase difference of two points, extracts the phase difference through trigonometric function operation, and has higher accuracy compared with a method for directly solving a single-point phase;
finally, due to the random effect of fading, the effective scattering point positions of the detection pulse beat frequency signals at different moments are different, the phase distribution of the optical fiber can be obtained by interpolating the phases, and then the phase distribution of the pulses transmitted at different moments is resampled according to the same sensing interval, so that the phase change of each sensing interval is obtained, and the acoustic wave measurement is realized.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (8)
1. A discretization processing method based on single-mode optical fiber distributed acoustic sensing is characterized by comprising the following steps:
(1) dividing the single-mode optical fiber into M continuous sensing sections;
(2) injecting light detection pulses into the single mode fiber at the time t, and collecting the distribution of beat frequency signals of different positions of the single mode fiber;
(3) extracting the envelope of the beat frequency signal, and solving the distribution of the scattered light intensity of the single-mode fiber at different positions;
(4) dynamically searching peaks for the scattered light intensity distribution to obtain effective scattering points of the single-mode optical fiber;
(5) calculating the phase difference between adjacent effective scattering points and the phase of each effective scattering point;
(6) obtaining a phase distribution function by adopting an interpolation method according to the phase of each effective scattering point;
(7) calculating the phase difference between two adjacent end points of each sensing section of the single-mode optical fiber at the time t according to the phase distribution function;
(8) judging whether T is equal to T', if not, T is equal to T + T, and turning to the step (2); otherwise, the change of the phase difference between two end points of each sensing interval along with time is used as the sound wave information of the sensing interval;
wherein T' is the last moment of a preset transmission pulse; t is the period of injecting emission detection pulse; the initial value of t is 0; m is an integer greater than 1.
2. The discretization processing method of claim 1, wherein the relationship between the interval of the photodetection pulses and the sampling frequency of the beat signal in the step (2) is:
the relationship between the number of the light detection pulses in the step (2) and the sampling frequency and the sampling time of the beat frequency signal is as follows:
Q＝f×t
wherein Q is the number of the photo-detection pulses; f is the sampling frequency of the beat frequency signal; t is the beat frequency signal sampling time; t is the time interval between adjacent light detection pulses.
3. The discretization processing method according to claim 1 or 2, wherein the length of each sensing section in the step (1) is:
wherein τ is the pulse width of the photodetection pulse; c is the speed of light in vacuum; n is a gain coefficient of spatial resolution; n is the refractive index of the fiber.
4. The discretization processing method of claim 1, wherein the step (3) of obtaining the scattered intensity distribution of the single-mode fiber comprises:
performing Hilbert transform on the beat frequency signal to obtain an imaginary part of the beat frequency signal;
and taking the real part of the beat frequency signal as the real part of the envelope signal, taking the imaginary part of the beat frequency signal as the imaginary part of the envelope signal, and calculating the mode of the envelope signal as the scattering light intensity distribution of the single-mode optical fiber.
5. The discretization processing method according to claim 1 or 4, wherein the dynamic peak finding method in the step (4) is:
determining the maximum value of the scattered light intensity distribution, and setting the threshold value to 1/10-1/5 of the maximum value;
setting the minimum position interval of the maximum value points for collecting the scattered light intensity distribution, and searching all the maximum value points larger than a threshold value;
and if the position coordinate of the current maximum value point is larger than half of the position minimum interval and the interval with the last maximum value point is larger than the minimum interval, the current maximum value point is an effective scattering point.
6. The discretization processing method of claim 5, wherein the minimum spacing of the positions of the maxima of the distribution of the collected scattered intensities is:
wherein τ is the pulse width of the photodetection pulse; n is the refractive index of the optical fiber; c is the speed of light in vacuum.
7. The discretization processing method according to claim 1, wherein the phase difference obtaining method of the adjacent effective scattering points in the step (5) is:
(5.1) Single mode optical fiber
Beat frequency signal E betweenk(l) As a beat signal of the kth effective scattering point, k is 1, 2, 3 … … N;
(5.2) adding Ek(l) And Ek+1(l) Multiplying and low-pass filtering to obtain constant A1；
(5.3) adding Ek(l) Hilbert transform followed by Ek+1(l) Multiplying and low-pass filtering to obtain constant A2；
(5.4) to A1/A2Performing arc tangent operation, wherein the operation result is the phase difference between the kth effective scattering point and the (k + 1) th effective scattering point;
(5.5) judging whether k is equal to N-1; if not, making k equal to k +1, and returning to the step (5.2); if yes, ending;
wherein the initial value of k is set to be 1, N is the number of effective scattering points, LkIs the position of the kth effective scattering point; and D is the position minimum interval of the maximum value points of the collected scattered light intensity distribution.
8. The discretization processing method according to claim 1 or 7, wherein the interpolation in the step (6) is a piecewise linear interpolation or a cubic polynomial interpolation.
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