CN110207803B - Distributed optical fiber sensing gain improving method based on variable integration window - Google Patents
Distributed optical fiber sensing gain improving method based on variable integration window Download PDFInfo
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Abstract
The invention discloses a distributed optical fiber sensing gain improving method based on a variable integration window, which comprises the steps of collecting a back scattering optical signal in a sensing optical fiber; carrying out complex phase demodulation on the collected backward scattering light signals to obtain light intensity phase complex numbers of all scattering points; obtaining the argument principal value of the light intensity phase complex number; integrating the argument main value of the optical fiber section in the integration window along the axial direction of the optical fiber according to the preset length of the integration window by preset step length to obtain the signal-to-noise ratio of the signal to be measured; and judging whether the signal-to-noise ratio exceeds a threshold value, if not, establishing a relation model of the length of an integration window and the axial distance of the optical fiber according to the mean value distribution of all the integration window values of the sensing optical fiber, and performing phase re-demodulation by adopting the integration window with variable length to realize the improvement of the signal-to-noise ratio. Compared with the traditional demodulation mode that the length of the phase difference optical fiber is fixed and unchanged, the method adopts the variable integration window to realize phase demodulation, thereby flexibly improving the signal-to-noise ratio of the measured external sound wave signal, namely improving the sensing gain.
Description
Technical Field
The invention belongs to the field of optical fiber sensing, and particularly relates to a distributed optical fiber sensing gain improving method based on a variable integration window.
Background
The distributed optical fiber sensing utilizes optical fibers as sensing media, each position on the optical fibers can sense external signals, the distributed optical fiber sensing has the advantages of electromagnetic interference resistance, small mass, light weight and high sensitivity of full distribution, can be applied to multiple engineering application fields, and has been popularized and applied particularly in the aspects of oil gas logging and shallow surface geological detection in recent years. In such applications, seismic sound waves are typically detected and analyzed. However, due to different media for seismic acoustic wave transmission at different depths of different layers or different transverse positions of a shallow earth surface, the acoustic wave speed is different from the acoustic wave wavelength, and under the same demodulation mode, the signal-to-noise ratios at different optical fiber sensing distances are inconsistent, and the signal-to-noise ratio balance of long-distance detection signals is poor. Therefore, a signal-to-noise ratio enhancement method is needed to optimize the detection quality of the seismic acoustic wave.
The existing common distributed optical fiber acoustic wave sensing technology is generally based on a phase-sensitive optical time domain reflectometer, and phase change in a sensing optical fiber is demodulated to obtain an external signal to be measured acting on the sensing optical fiber, in order to realize optical phase demodulation, a 3 × 3 coupler demodulation and phase carrier generation method is adopted in the existing common method, phase information of the optical fiber section can be obtained by calculating the phase difference between two points of the optical fiber, and further the external signal to be measured is obtained, the optical fiber between the two points of the optical fiber is used as a medium for sensing the external signal to be measured, and the length of the optical fiber directly determines the sensing signal to noise ratio.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide a distributed optical fiber sensing gain improving method based on a variable integration window, and aims to solve the problem that the sensing signal-to-noise ratio of the existing long-distance acoustic detection optical fiber is low.
In order to achieve the above object, the present invention provides a distributed optical fiber sensing gain improving method based on a variable integration window, which includes the following steps:
s1, carrying out complex phase demodulation on the collected backscattered light signals along the axial direction of the optical fiber by using an integration window with a preset length and a preset step length to obtain the signal-to-noise ratio of the signals to be detected;
s2, if the integration window reaches the tail end of the sensing optical fiber, the demodulation is finished, and S3 is entered, otherwise, S1 is repeated;
s3, if the signal-to-noise ratio of the signal to be detected of the optical fiber section in the integration window exceeds a preset threshold value, ending demodulation, otherwise, entering S4;
s4, establishing a relation model of the length of the integration window and the axial distance of the optical fiber according to the mean value distribution of all the integration window mode values of the optical fiber, and performing phase re-demodulation along the axial direction of the optical fiber by adopting the length-variable integration window to obtain an updated signal-to-noise ratio so as to realize the improvement of the sensing gain.
Further, the integration window performs complex phase demodulation on the collected backscattered light signals along the axial direction of the optical fiber by a preset length and a preset step length to obtain the signal-to-noise ratio of the signal to be detected, and the method specifically comprises the following steps:
s11, collecting the back scattering light signals of the m scattering points in the sensing optical fiber to the N emission light pulses;
s12, carrying out complex phase demodulation on the collected backward scattering light signals to obtain light intensity phase complex numbers of each scattering point;
s13, respectively carrying out conjugate multiplication on light intensity phase complex numbers of two adjacent scattering points to obtain intermediate complex numbers, carrying out conjugate multiplication on the intermediate complex numbers of two adjacent emitted light pulses at the same position of the optical fiber to obtain final complex numbers, and solving the argument main value of the final complex numbers;
s14, integrating the argument main value of the optical fiber section in the integration window along the axial direction of the optical fiber by a preset step length according to the preset length of the integration window to obtain the phase of the optical fiber section in the integration window, and further obtaining the signal-to-noise ratio of the signal to be measured of the optical fiber section in the integration window.
Further, the process of complex phase demodulation is as follows: and respectively multiplying the collected backscatter signals by a pair of orthogonal cosine signals with the same frequency as the backscatter signals to obtain cosine quantity as a real part of the complex number and sine quantity as an imaginary part of the complex number.
Furthermore, the signal-to-noise ratio of the signal measured by the optical fiber section in the integration window changes along with the change of the length of the integration window, and along with the increase of the length of the integration window, the signal-to-noise ratio is firstly enhanced and then weakened, and a signal-to-noise ratio peak exists.
Furthermore, the integration window slides along the axial direction of the optical fiber according to a preset length and a preset step length, the sensing optical fiber is divided into a plurality of sensing sections by the preset step length, the length of the integration window of each sensing section is the same, after the length of the integration window is changed, the lengths of the integration windows of the sensing sections are different, and the signal-to-noise ratio balance of the sensing optical fiber can be realized by utilizing different lengths of the integration windows.
Further, the obtaining of the mean distribution of the mode values of the integration window comprises recording the mean values of the complex mode values of the start point and the end point of the integration window and three adjacent scattering points of the integration window respectively to form the mean values of the start point and the end point, and then calculating the mean values of the start point and the end point to obtain the mean values of the mode values of the integration window distributed along the axial direction of the optical fiber.
Furthermore, a relation model of the length of the integration window and the axial distance of the optical fiber is determined by mode value mean distribution along the axial direction of the optical fiber, and the signal-to-noise ratio enhancement intensity is inversely proportional to the size of the mode value mean.
Through the technical scheme, compared with the prior art, the invention has the following beneficial effects:
1. the distributed optical fiber sensing gain improving method based on the variable integration window can perform complex phase demodulation on each sampling point of an optical fiber, retain original light intensity phase information of all the points, and further realize phase integration of any optical fiber length interval; compared with the traditional demodulation mode that the length of the phase difference optical fiber is fixed and unchanged, the method adopts a variable integral window to realize the phase demodulation with flexibly regulated and controlled signal-to-noise ratio, thereby flexibly improving the signal-to-noise ratio of the measured external sound wave signal, namely improving the sensing gain;
2. according to the relation model of the length of the integration window and the axial distance of the optical fiber, aiming at the optical fibers at different axial distances of the optical fiber, the light intensity states of corresponding scattering points at the distance are combined with the types of transmission media where the scattering points are located, the signal-to-noise ratio is flexibly enhanced by adopting different window lengths, and further the balance of the long-distance sensing signal-to-noise ratio can be realized;
3. the variable integration window processing method provided by the invention can effectively prolong the sensing distance, and can realize the ultra-long distance external signal detection by selecting the optimal integration window length according to the relation model of the integration window length and the signal-to-noise ratio aiming at the problem of the reduction of the demodulation signal-to-noise ratio caused by the fading of the long-distance back scattering light signal.
Drawings
FIG. 1 is a schematic structural diagram of a distributed sensing optical fiber based on a variable integration window according to an embodiment of the present invention;
fig. 2 is a schematic flowchart of a distributed optical fiber sensing gain increasing method based on a variable integration window according to an embodiment of the present invention;
fig. 3 is a schematic effect diagram of a distributed optical fiber sensing gain improving method based on a variable integration window according to an embodiment of 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.
The invention provides a distributed optical fiber sensing gain improving method based on a variable integration window, which comprises the following steps:
s1, carrying out complex phase demodulation on the collected backscattered light signals along the axial direction of the optical fiber by using an integration window with a preset length and a preset step length to obtain the signal-to-noise ratio of the signals to be detected;
s2, if the integration window reaches the tail end of the sensing optical fiber, the demodulation is finished, and S3 is entered, otherwise, S1 is repeated;
s3, if the signal-to-noise ratio of the signal to be detected of the optical fiber section in the integration window exceeds a preset threshold value, ending demodulation, otherwise, entering S4;
s4, establishing a relation model of the length of the integration window and the axial distance of the optical fiber according to the mean value distribution of all the integration window mode values of the optical fiber, and performing phase re-demodulation along the axial direction of the optical fiber by adopting the length-variable integration window to obtain an updated signal-to-noise ratio so as to realize the improvement of the sensing gain.
Specifically, the integration window performs complex phase demodulation on the collected backscattered light signals along the axial direction of the optical fiber by a preset length and a preset step length to obtain the signal-to-noise ratio of the signal to be detected, and specifically includes:
s11, collecting the back scattering light signals of the m scattering points in the sensing optical fiber to the N emission light pulses;
s12, carrying out complex phase demodulation on the collected backward scattering light signals to obtain light intensity phase complex numbers of each scattering point;
s13, respectively carrying out conjugate multiplication on light intensity phase complex numbers of two adjacent scattering points to obtain intermediate complex numbers, carrying out conjugate multiplication on the intermediate complex numbers of two adjacent emitted light pulses at the same position of the optical fiber to obtain final complex numbers, and solving the argument main value of the final complex numbers;
s14, integrating the argument main value of the optical fiber section in the integration window along the axial direction of the optical fiber by a preset step length according to the preset length of the integration window to obtain the phase of the optical fiber section in the integration window, and further obtaining the signal-to-noise ratio of the signal to be measured of the optical fiber section in the integration window.
Specifically, the process of complex phase demodulation is: and respectively multiplying the collected backscatter signals by a pair of orthogonal cosine signals with the same frequency as the backscatter signals to obtain cosine quantity as a real part of the complex number and sine quantity as an imaginary part of the complex number.
Specifically, the signal-to-noise ratio of a signal measured by the optical fiber section in the integration window changes along with the change of the length of the integration window, and the signal-to-noise ratio is firstly enhanced and then weakened along with the increase of the length of the integration window, so that a signal-to-noise ratio peak exists.
Specifically, the integration window slides along the axial direction of the optical fiber according to a preset length and a preset step length, the sensing optical fiber is divided into a plurality of sensing sections by the preset step length, the length of the integration window of each sensing section is the same, after the length of the integration window is changed, the lengths of the integration windows of the sensing sections are different, and the signal-to-noise ratio balance of the sensing optical fiber can be realized by utilizing different lengths of the integration windows.
Specifically, the obtaining of the mean distribution of the mode values of the integration window comprises the steps of respectively recording the mean values of the complex mode values of the starting point and the end point of the integration window and three adjacent scattering points of the integration window, forming the mean values of the starting point and the end point, and then calculating the mean values of the starting point and the end point to obtain the mean values of the mode values of the integration window distributed along the axial direction of the optical fiber.
Specifically, a relation model of the length of the integration window and the axial distance of the optical fiber is determined by mode value mean distribution along the axial direction of the optical fiber, and the signal-to-noise ratio enhancement intensity is inversely proportional to the size of the mode value mean.
The present invention will be described in further detail with reference to the present embodiment, as shown in fig. 1. A plurality of scattering points 2 are distributed on a sensing optical fiber 1, a sound wave signal acts on the sensing optical fiber to cause phase change, the phase is demodulated, detection and recovery of the sound wave signal are achieved, a system adopts coherent detection to collect beat frequency signals generated by coherent backward scattering light and local oscillator light, the frequency difference between coherent detection light and reference light in the embodiment is 200MHz, and the frequency of the collected beat frequency signals is 200 MHz. As shown in fig. 2, the method specifically includes the following steps:
step 4, integrating the argument main value of the optical fiber section in the integration window along the axial direction of the optical fiber according to the preset length L ═ 3.2m of the integration window and the preset step length x ═ 3.2m, so as to obtain the phase of the optical fiber section in the integration window, and further obtain the signal-to-noise ratio of the signal to be measured of the optical fiber section in the integration window;
step 7, if the signal-to-noise ratio of the signal to be measured of the optical fiber section in the integral window exceeds a preset threshold value, the demodulation is finished, otherwise, the step 8 is carried out;
and 8, establishing a relation model between the length of the integration window and the axial distance of the optical fiber according to the mode value mean distribution of all the integration windows of the sensing optical fiber, and performing phase re-demodulation by adopting the length-variable integration window to improve the signal-to-noise ratio.
Specifically, the preset lengths of the integration windows in each sensing interval are equal to each other and are 3.2m, and when the lengths of the integration windows are changed, the lengths of the integration windows in the sensing intervals are different, L in fig. 11And L2Representing the lengths of the integration windows with different lengths, the signal-to-noise ratio balance of the sensing optical fiber can be realized. As shown in fig. 3, the measured signal is a weak acoustic signal acting on a section of optical fiber, and is demodulated by an initial integration window length of 3.2m to obtain a waveform before optimization, that is, at a position where the longitudinal axis is offset to 0, it can be seen that the measured waveform in the square frame is close to noise, and the signal-to-noise ratio is poor and is 3.52 dB; and re-demodulating the same optical fiber position by adopting a variable integration window, selecting the length of the integration window of 16m to obtain an optimized waveform, namely the position with the vertical axis offset of 5, wherein the waveform amplitude of the actually measured sound wave signal in the square frame is far beyond noise, the signal-to-noise ratio is greatly improved to 16.48dB, and the signal-to-noise ratio optimization is realized at the optical fiber position.
Specifically, a relation model of the length of the integration window and the axial distance of the optical fiber is determined by mode value mean distribution along the axial direction of the optical fiber, and the signal-to-noise ratio enhancement intensity is inversely proportional to the size of the mode value mean.
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 distributed optical fiber sensing gain improving method based on a variable integration window is characterized by comprising the following steps:
s1, carrying out complex phase demodulation on the collected backscattered light signals along the axial direction of the optical fiber by using an integration window with a preset length and a preset step length to obtain the signal-to-noise ratio of the signals to be detected;
step S1 specifically includes:
s11, collecting the back scattering light signals of the m scattering points in the optical fiber to the N emission light pulses;
s12, carrying out complex phase demodulation on the collected backward scattering light signals to obtain light intensity phase complex numbers of each scattering point;
s13, respectively carrying out conjugate multiplication on light intensity phase complex numbers of two adjacent scattering points to obtain intermediate complex numbers, carrying out conjugate multiplication on the intermediate complex numbers of two adjacent emitted light pulses at the same position of the optical fiber to obtain final complex numbers, and solving the argument main value of the final complex numbers;
s14, integrating the argument main value of the optical fiber section in the integration window along the axial direction of the optical fiber by a preset step length according to the preset length of the integration window to obtain the phase of the optical fiber section in the integration window, and further obtaining the signal-to-noise ratio of the signal to be measured of the optical fiber section in the integration window;
s2, if the integration window reaches the tail end of the sensing optical fiber, the demodulation is finished, and S3 is entered, otherwise, S1 is repeated;
s3, if the signal-to-noise ratio of the signal to be detected of the optical fiber section in the integration window exceeds a preset threshold value, ending demodulation, otherwise, entering S4;
s4, establishing a relation model of the length of the integration window and the axial distance of the optical fiber according to the mean value distribution of all the integration window mode values of the optical fiber, and performing phase re-demodulation along the axial direction of the optical fiber by adopting the length-variable integration window to obtain an updated signal-to-noise ratio so as to realize the improvement of the sensing gain.
2. The method of claim 1, wherein the complex phase demodulation process is: and respectively multiplying the collected backscatter signals by a pair of orthogonal cosine signals with the same frequency as the backscatter signals to obtain cosine quantity as a real part of the complex number and sine quantity as an imaginary part of the complex number.
3. The method of claim 1, wherein the signal-to-noise ratio of the measured signal of the fiber segment within the integration window varies with the length of the integration window, and wherein as the length of the integration window increases, the signal-to-noise ratio increases and then decreases, and wherein there is a peak signal-to-noise ratio.
4. The method of claim 1, wherein the integration window is axially slid along the sensing fiber by a predetermined length and a predetermined step size, the predetermined step size dividing the sensing fiber into a plurality of sensing zones, the integration window length of each sensing zone being the same.
5. The method of claim 4, wherein the integration window length of the sensing region is different after the integration window length is changed.
6. The method of claim 5, wherein the integration window lengths of the sensing regions are different, so as to achieve signal-to-noise ratio equalization of the whole sensing fiber.
7. The method of claim 1, wherein the obtaining of the mean distribution of the integrated window mode values comprises recording the mean values of the complex mode values of the three scattering points adjacent to the start point and the end point of the integrated window respectively to form the mean values of the start point and the end point, and averaging the mean values to obtain the mean values of the integrated window mode values distributed along the axial direction of the optical fiber.
8. The method of claim 1 or 7, wherein the model of the integration window length versus the axial distance of the optical fiber is determined by a mode mean distribution along the axial direction of the optical fiber, and the signal-to-noise ratio enhancement strength is inversely proportional to the magnitude of the mode mean.
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