CN113237431A - Measurement method for improving distributed spatial resolution of OFDR system - Google Patents
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Abstract
The invention discloses a measuring method for improving distributed spatial resolution of an OFDR system, which comprises the steps of firstly converting a traditional one-dimensional cross-correlation processing result signal into a two-dimensional image signal, carrying out denoising processing on an image through a Gaussian filtering denoising algorithm on the basis of the two-dimensional image, then carrying out next operation on the processed image, and finally obtaining a result with high spatial resolution. The OFDR sensing system of the Gaussian filtering denoising algorithm provided by the invention can improve the spatial resolution of the measurement system, so that the OFDR sensing system has greater advantages and wider application in the high-precision monitoring fields of aerospace, machine equipment and the like.
Description
Technical Field
The invention relates to a measuring method for improving distributed spatial resolution of an OFDR system, which is applied to the field of strain measurement, improves the spatial resolution of the system and belongs to the technical field of optical fiber sensing detection.
Background
The optical fiber in the distributed optical fiber sensing is used as a sensing medium and a measured transmission medium, and real-time monitoring on the external environment along the length direction of the optical fiber is realized by using the transmission characteristics of the optical wave in the optical fiber, including Raman scattering, Rayleigh scattering and Brillouin scattering. The distributed optical fiber sensing technology has the advantages of strong anti-electromagnetic interference capability, relatively simple structure, high spatial resolution, long sensing distance and the like. Based on the above-mentioned advantages, the technology is being gradually applied to more and more fields, such as detection of bridge safety, detection of civil engineering, underground fire alarm of tunnels and the like, exploration of geology and the like, and plays an important role in social construction. As a representative example of a distributed optical fiber sensing system, the Optical Frequency Domain Reflectometry (OFDR) has the advantages of light weight, small size, high sensitivity, strong electromagnetic interference resistance, high spatial resolution, and the like, and can continuously measure external physical quantity changes such as strain, vibration, temperature, and the like along the distance of an optical fiber. In recent years, with the development of OFDR technology, the application of shape sensing and acoustic sensing has also been realized.
The OFDR principle is: linear frequency-sweeping light emitted by the tunable laser source is divided into two beams by the coupler, one beam enters the optical fiber to be detected, and backward Rayleigh scattering light of the optical fiber to be detected returns to form signal light and generates beat frequency interference with the other beam of reference light. And acquiring the beat frequency signal and performing fast Fourier transform processing to obtain the distance domain information constructed along the sensing optical fiber. In the measurement of the OFDR system, a reference signal without external influence and a test signal with an influenced optical fiber need to be collected once, and the test signal and the reference signal are subjected to cross-correlation calculation to obtain the change of external information. The OFDR system has the characteristic of high spatial resolution, and the spatial resolution of the OFDR system can reach millimeter magnitude, so that the OFDR system has very important application in the high-precision monitoring fields of aerospace and aviation and the like. However, when the measurement spatial resolution is increased, the cross-correlation between the reference signal and the test signal is greatly reduced, so that the cross-correlation result generates multiple peaks and false peaks, and a correct result is not obtained. Therefore, how to effectively improve the spatial resolution of the OFDR system is a very important research direction.
Disclosure of Invention
The invention aims to provide a measuring method for improving the distributed spatial resolution of an OFDR system, which can effectively improve the spatial resolution of the system. The technical proposal is that the method comprises the following steps,
a measuring method for improving the distributed spatial resolution of an OFDR system comprises the following steps,
s1, respectively collecting signals twice, wherein the signals which do not contain strain information are taken as reference signals once; the other time is a signal containing strain information, which is a test signal;
s2, mapping the reference signal and the test signal to a distance domain through fast Fourier change, and dividing the signal into N equal parts according to the window size C, wherein the window size C determines the spatial resolution of the system;
and S3, carrying out fast inverse Fourier transform on the local distance domain information of the first reference signal and the measurement signal.
S4, performing cross-correlation calculation on the reference signal and the measurement signal after the fast inverse Fourier transform to obtain a one-dimensional cross-correlation result;
s5, repeating the steps S3-S4 to obtain a cross-correlation result of each corresponding position of the optical fiber, reconstructing all the obtained one-dimensional cross-correlation results into a two-dimensional image signal on the optical fiber distance, and denoising the image on the basis of the two-dimensional image through a Gaussian filtering denoising algorithm;
s6, reconstructing the image processed by the Gaussian filtering denoising algorithm to obtain the spectral offset of each position of the optical fiber, so that the measurement result under high spatial resolution can be obtained, and the measurement accuracy is improved.
Preferably, the gaussian filtering is used to perform weighted average on the whole reconstructed two-dimensional image in step S5, and each pixel point is obtained by performing weighted average on itself and other pixel values in the neighborhood.
Preferably, in step S6, the spectral shift amount is obtained by,
scanning each pixel in the image by using a specific template, and replacing the value of the central pixel point of the template by using the weighted average gray value of the pixels in the neighborhood determined by the template; selecting a gaussian low-pass filter, using a fspecial function, and selecting a template as an M × M matrix with a standard deviation of M; and (4) processing the two-dimensional image signal reconstructed in the step (S5) by Gaussian filtering, decomposing the processed image to the corresponding position of the optical fiber again, and searching the shift of the main peak to obtain the shift of the spectrum at the corresponding optical fiber position.
Advantageous effects
1) The OFDR sensing system of the Gaussian filtering denoising algorithm provided by the invention can improve the spatial resolution of the measurement system, so that the OFDR sensing system has greater advantages and wider application in the high-precision monitoring fields of aerospace, machine equipment and the like.
2) The OFDR sensing system based on the Gaussian filtering denoising algorithm provided by the invention not only can improve the spatial resolution of the system, but also can effectively remove abnormal values of measurement results and improve the accuracy of measurement by denoising the two-dimensional image information.
Drawings
FIG. 1 is a high spatial resolution OFDR system processing flow based on a Gaussian filtering denoising algorithm.
Fig. 2 is a schematic diagram of an OFDR system.
Wherein 1-is a tunable laser; 2-is a coupler I; 3-is a coupler II; 4-is a circulator; 5-is a Mach-Zehnder interferometer; 6 is a first polarization controller; 7-is a second polarization controller; 8-is a coupler III; 9-is a polarization beam splitter; 10-is a balance detector; 11-is a collection card; 12-a sensing fiber; 13-phenanthrene ring.
FIG. 3 is a graph of the results of the sensing fiber 10.1-10.7m subjected to 100. mu. epsilon. without the use of this method, with a spatial resolution of 0.4 mm.
FIG. 4 is a graph of the results of this method at 10.1-10.7m of the sensing fiber at 100 μ ε with a spatial resolution of 0.4 mm.
Detailed Description
The following further description of the technology, in conjunction with the accompanying figures 1-4 and the specific embodiments, is provided to assist in understanding the present invention.
A measuring method for improving the distributed spatial resolution of an OFDR system comprises the following steps,
s1, respectively collecting signals twice, wherein the signals which do not contain strain information are taken as reference signals once; the other time is a signal containing strain information, which is a test signal.
S2, dividing the reference signal and the test signal into N equal parts on a distance domain according to a certain window size C;
s3, performing fast inverse Fourier transform on each distance domain information of the reference signal and the test signal;
s4, performing cross-correlation operation on the reference signal subjected to the inverse Fourier transform and the test signal to obtain a one-dimensional cross-correlation result;
s5, repeating the steps S3-S4 to obtain the cross-correlation result of each corresponding position of the optical fiber, and reconstructing all the obtained one-dimensional cross-correlation results into a two-dimensional image signal on the optical fiber distance;
and S6, Gaussian filtering is linear smooth filtering and is suitable for eliminating Gaussian noise.
The principle is that the two-dimensional image reconstructed in step S5 is weighted-averaged over the entire reconstructed two-dimensional image, and each pixel point is obtained by weighted-averaging itself and other pixel values in the neighborhood. Each pixel in the image is scanned using a particular template, and the weighted average gray value of the pixels in the neighborhood determined by the template is used to replace the value of the pixel in the center of the template. And (3) selecting a gaussian low-pass filter by using a fspecial function in matlab software, wherein the selected template is a 100 x 100 matrix, the standard deviation is 2, processing the two-dimensional image signal reconstructed in the step S5 by gaussian filtering, decomposing the processed image to the corresponding position of the optical fiber again, and searching the deviation of the main peak to obtain the offset of the spectrum corresponding to the position of the optical fiber.
And S7, denoising the image through a Gaussian filtering denoising algorithm on the basis of the two-dimensional image, and reconstructing the image processed through the Gaussian filtering denoising algorithm to obtain the spectral offset of each position of the optical fiber, so that a measurement result under high spatial resolution can be obtained, and the measurement accuracy is improved.
FIG. 3 is a graph of the results of sensing fibers 10.1-10.7m subjected to 100 μ ε without using this method, with a spatial resolution of 0.4mm, and it can be seen that there are many outliers in the results and no correct strain distribution results along the length of the fiber can be obtained.
FIG. 4 is a graph of the results of the method applied to the sensing fiber 10.1-10.7m by 100 mu epsilon, the method well eliminates the abnormal value, improves the system resolution and obtains the correct strain distribution result with the 0.4mm spatial resolution.
Fig. 2 is a schematic diagram of an OFDR system. An OFDR sensing system based on distance domain compensation, comprising: the continuous laser output of the tunable laser source is divided into two parts by a first coupler 2(10/90 optical coupler), 10 percent of the continuous laser output enters an unbalanced Mach-Zehnder interferometer 5 to provide a trigger signal for an acquisition card 11, and the rest part of the continuous laser output enters a second coupler 3; the second coupler 3(1/99 optocoupler) is then split into two parts, where 1% of the output is adjusted by the first polarization controller 6 so that the "p" and "s" light components have the same power, 99% enters the sensing fiber 12 through the circulator 4 and the second polarization controller 7 for detection, and the fresnel ring 13 is used to suppress the fresnel reflection at the end of the fiber. (ii) a Then the interference signal obtained by combining the Rayleigh scattering signal and 1% laser output from a third coupler 8(50/50 optical coupler) is decomposed into components of 'p' and's' through a polarization beam splitter; and finally, collecting the light of the p and the light of the s by a collecting card.
The above description is only a preferred embodiment of the present application and is not intended to limit the present application, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, improvement and the like made within the spirit and principle of the present application shall be included in the protection scope of the present application.
Claims (3)
1. A measuring method for improving the distributed spatial resolution of an OFDR system is characterized by comprising the following steps,
s1, respectively collecting signals twice, wherein the signals which do not contain strain information are taken as reference signals once; the other time is a signal containing strain information, which is a test signal;
s2, mapping the reference signal and the test signal to a distance domain through fast Fourier change, and dividing the signal into N equal parts according to the window size C, wherein the window size C determines the spatial resolution of the system;
s3, carrying out fast inverse Fourier transform on the local distance domain information of the first reference signal and the measurement signal;
s4, performing cross-correlation calculation on the reference signal and the measurement signal after the fast inverse Fourier transform to obtain a one-dimensional cross-correlation result;
s5, repeating the steps S3-S4 to obtain a cross-correlation result of each corresponding position of the optical fiber, reconstructing all the obtained one-dimensional cross-correlation results into a two-dimensional image signal on the optical fiber distance, and denoising the image on the basis of the two-dimensional image through a Gaussian filtering denoising algorithm;
s6, reconstructing the image processed by the Gaussian filtering denoising algorithm to obtain the spectral offset of each position of the optical fiber, so that the measurement result under high spatial resolution can be obtained, and the measurement accuracy is improved.
2. The method as claimed in claim 1, wherein the gaussian filtering is used to perform weighted average on the whole reconstructed two-dimensional image in step S5, and each pixel is obtained by performing weighted average on itself and other pixel values in the neighborhood.
3. The measurement method for improving the distributed spatial resolution of the OFDR system as claimed in claim 1, wherein the spectral shift amount in step S6 is obtained by,
scanning each pixel in the image by using a specific template, and replacing the value of the central pixel point of the template by using the weighted average gray value of the pixels in the neighborhood determined by the template; selecting a gaussian low-pass filter, using a fspecial function, and selecting a template as an M × M matrix with a standard deviation of M; and (4) processing the two-dimensional image signal reconstructed in the step (S5) by Gaussian filtering, decomposing the processed image to the corresponding position of the optical fiber again, and searching the shift of the main peak to obtain the shift of the spectrum at the corresponding optical fiber position.
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