Optical frequency domain reflectometer grating array multiplexing demodulation method based on peak position estimation

Abstract

The invention provides an optical frequency domain reflectometer grating array multiplexing demodulation method based on peak position estimation, and the method comprises the steps: in quasi-distributed sensing based on optical frequency domain reflectometer grating array multiplexing, in order to obtain an acceptable grating spectral measurement resolution ratio, carrying out peak position estimation; in the prior art, dense interpolation needs to be carried out on a spectrum of which the spectral resolution is lower than a threshold value required by demodulation, but the data volume is greatly increased, so that the calculation rate is slowed down, and the demodulation efficiency is low. According to the method, peak position estimation is carried out on the cross-correlation spectrum of the grating spectrum of which the spectral resolution is lower than the threshold required by demodulation, so that dense interpolation is avoided, and the problem of low demodulation efficiency caused by dense interpolation is improved; meanwhile, in order to overcome the problems of spectrum distortion caused by grating inscribing defects and spectrum splitting caused by gaps between gratings in the traditional demodulation process, the robustness of system demodulation is ensured to the maximum extent on the basis of improving the system demodulation efficiency.

Description

Technical Field

[0001] This invention belongs to the field of quasi-distributed optical fiber sensing, and particularly relates to a method for multiplexing and demodulating a grating array of an optical frequency domain reflectometer based on peak position estimation. Background Technology

[0002] Optical frequency domain reflectometry (OFDR) is a preferred technology in the field of distributed / quasi-distributed fiber optic sensing. Compared with traditional optical time domain reflectometry (OTDR), OFDR offers advantages such as high spatial resolution and a large dynamic range. The low-reflectivity grating array multiplexing method based on OFDR possesses quasi-distributed sensing capabilities and, compared with distributed sensing based on Rayleigh backscattering, offers advantages in measurement signal-to-noise ratio, measurement resolution, and measurable signal range. It has been widely applied in areas such as aircraft structural deformation and shape sensing, becoming one of the research hotspots in recent years.

[0003] OFDR-based grating array multiplexing technology, whether in academic research or industrial applications, generally relies on spectral demodulation. Compared to other demodulation schemes such as phase demodulation, spectral demodulation methods offer advantages such as high robustness and a large measurable signal range. However, due to the unique distributed / quasi-distributed sensing method and signal characteristics of OFDR, the demodulation process typically involves using a sliding window to capture the signal along the range domain. In this process, the spectral sampling rate is limited by the sliding window length. A shorter window length results in a lower sampling rate; however, increasing the window length reduces the system's spatial resolution. In practical applications, the spectral sampling rate determined by the window length is generally insufficient to achieve a sufficiently high spectral resolution while maintaining the required spatial resolution. Therefore, digital interpolation of the spectrum is necessary to improve spectral resolution. However, interpolation significantly increases the data volume, leading to low demodulation efficiency, reduced system refresh rate, and hindering real-time measurement.

[0004] Existing research has explored methods such as spectral analysis or machine learning to improve demodulation rates. However, these methods generally suffer from limited applicability, being effective only for certain application scenarios or signals with specific characteristics. Other studies have employed peak estimation schemes for efficient demodulation of individual fiber gratings. However, this approach is unsuitable for demodulation of OFDR-based grating arrays with multiplexing. In cases of high-volume multiplexing, individual grating writing defects inevitably lead to spectral distortion, and spacing between gratings causes spectral splitting. These issues result in demodulation errors or decreased accuracy when directly estimating peak positions for gratings with spectral resolution below the required demodulation threshold. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a multiplexing demodulation method for optical frequency domain reflectometer grating arrays based on peak position estimation. This method estimates the peak position of cross-correlation spectra of gratings with spectral resolution lower than the threshold required for demodulation. This method can not only reduce the amount of spectral interpolation data, but also completely avoid the spectral interpolation process, thereby solving the problem of reduced demodulation efficiency. At the same time, it retains the benefits of cross-correlation algorithms in improving the robustness of spectral demodulation to the greatest extent, effectively overcoming the problems of demodulation errors or decreased demodulation accuracy caused by spectral distortion and spectral splitting.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: The optical frequency domain reflectometer system is used to detect the grating array and obtain OFDR spectral domain data before and after the signal is generated. Fourier transform is then performed on the data to convert the spectral domain data to the distance domain, thus obtaining the complex reflectance curves of the grating array before and after the signal is generated, which are distributed with distance.

[0007] A sliding window approach is used to segment the complex reflectance curve along the distance axis. If the window length is L, then L points are extracted from the complex reflectance curve before and after the signal generation, and an inverse Fourier transform is performed to convert them back to the spectral domain, yielding the local spectral data S_r(λ) and Sm(λ) before and after the signal generation, respectively. Cross-correlation is then performed on the obtained local spectral data S_r(λ) and Sm(λ) to obtain the cross-correlation spectrum S_c(λ) where the spectral resolution of the grating spectrum is lower than the demodulation threshold, i.e.: in This represents the cross-correlation operation. Since S_c(λ) is not sufficiently interpolated, it is called the cross-correlation spectrum when the spectral resolution is lower than the threshold required for demodulation. Directly finding the peak position of S_c(λ) cannot obtain an accurate signal, leading to a spectral shift in the grating spectrum and thus a decrease in demodulation accuracy. Therefore, peak position estimation is performed on the aforementioned S_c(λ). Because traditional local spectral interpolation methods involve padding the local range domain data with zeros before performing an inverse Fourier transform, its cross-correlation spectrum is mathematically equivalent to sinc interpolation.

[0008] Therefore, a more accurate method for estimating the peak position is determined by the following formula: This invention uses a peak position estimation algorithm based on this formula as an example, but it is clear that the scope of protection of this patent is not limited to this. To implement the estimation method determined by the above formula, we first find the light wavelength λ corresponding to the peak of the cross-correlation spectrum S_c(λ) with a spectral resolution lower than the threshold required for demodulation.m N is the data length of S_c(λ), λ r The spectral resolution, which is lower than the threshold cross-correlation spectrum S_c(λ) required for demodulation, is determined by the OFDR light source wavelength scanning range Δλ and the window length L (assuming no interpolation is performed on the local grating spectrum; if finite interpolation is performed, then L is the length of the interpolated local spectral data). I(λ m Let λ be the peak value of S_c(λ). The wavelength λ corresponding to the peak value of S_c(λ) is then obtained. m Then, determine the relationship with λ. m Two adjacent sampling points, the first point is λ m-1 The next point is λ. m+1 The corresponding amplitude value in S_c(λ) is I(λ). m-1 ) and I(λ m+1 ). I bias The bias is calculated as follows: λ is calculated based on the above parameters. c This is the wavelength corresponding to the true peak value of the cross-correlation spectrum obtained by the algorithm. This value corresponds to the accurate grating spectral shift caused by the signal. Finally, based on the linear mapping relationship between the spectral shift and the signal, this value λ is... c You can convert it to a signal value (temperature, strain, etc.).

[0009] wavelength λ c The sensor signal M to be measured is calculated using the following formula: In the formula, λ c Let α be the spectral shift, and α be the linear correlation coefficient between the spectral shift and the signal. The measured sensing signal M includes, but is not limited to, temperature and strain; when the measured signal is temperature, the temperature sensing coefficient α is 0.1℃ / pm; when the measured signal is strain, the strain sensing coefficient α is 1.2u / pm.

[0010] After demodulation of the first position (the first N points of data) of the window length, the sliding window is moved back N points along the distance axis to extract the distance domain data of the second position. The same processing is performed until the entire complex reflectivity curve is extracted and processed, thus realizing the demodulation of the grating array multiplexing of the entire sensing fiber and finally obtaining the curve of the sensing signal distribution with distance.

[0011] The present invention also provides an optical frequency domain reflectometer device, which includes: a light source module, an interference module, a detection module, and a signal processing module, wherein the signal processing module is configured to perform the above-described grating array multiplexing and demodulation method.

[0012] The present invention also provides a computer-readable storage medium having a program stored thereon, which, when executed by a processor, implements the above-described grating array multiplexing and demodulation method.

[0013] The present invention also provides a computer program product, including a computer program / instruction, which, when executed by a processor, implements the above-described grating array multiplexing and demodulation method.

[0014] The beneficial effects of this invention are: The method of this invention avoids the problem of low demodulation efficiency caused by the surge in data volume due to the need for dense interpolation of local grating spectra in traditional OFDR grating array multiplexing and demodulation.

[0015] The method of this invention estimates the peak position of the cross-correlation spectrum of gratings with spectral resolution lower than the threshold required for demodulation, instead of directly estimating the peak position of the spectrum of gratings with spectral resolution lower than the threshold required for demodulation. This can overcome the problems of spectral distortion caused by individual grating writing defects in the grating array, as well as spectral splitting caused by the gap between gratings when performing distance domain truncation by sliding window, which leads to demodulation errors and reduced demodulation accuracy. It preserves the ability of cross-correlation operation to improve demodulation robustness to the greatest extent.

[0016] (3) The method of the present invention is for OFDR grating array multiplexing system. While ensuring the demodulation robustness of the system, it improves the demodulation efficiency of the system, which is beneficial to improving the practicality of the system in practical applications. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 A flowchart illustrating a high-efficiency, high-robust demodulation method for OFDR grating array multiplexing provided in an embodiment of the present invention; Figure 2The figures show a comparison of demodulation results based on the demodulation algorithm described in this invention, specifically a demodulation method that estimates the peak position of a cross-correlation spectrum with a spectral resolution lower than the threshold required for demodulation, and a demodulation method that directly estimates the peak position of a grating spectrum with a spectral resolution lower than the threshold required for demodulation. (a) shows a comparison of the demodulation results of estimating the peak position of a grating spectrum with a spectral resolution lower than the threshold required for demodulation and estimating the peak position of a cross-correlation spectrum with a spectral resolution lower than the threshold required for demodulation. (b) is a partial magnification of (a). Here, Strain represents the measured strain signal, and Distance represents the distance, i.e., the position corresponding to the signal demodulation result. Figure 3 This diagram illustrates how spectral distortion caused by grating writing defects and spectral splitting caused by grating spacing can lead to decreased demodulation accuracy or demodulation errors when directly estimating the peak position of grating spectra with spectral resolution lower than the required demodulation threshold. (a1) shows the position of a normal fiber grating captured by a sliding window, and (a2) shows the corresponding grating spectrum; (b1) shows the position of the grating gap captured by the sliding window, and (b2) shows the corresponding split spectrum; (c1) shows the position of the fiber with writing defects captured by the sliding window, and (c2) shows the corresponding distorted grating spectrum; where Reflectivity represents the fiber grating reflectivity and Intensity represents the grating spectral intensity. Figure 4 This is a schematic diagram comparing the demodulation results based on the method of this invention and the traditional dense interpolation method; Figure 5 The graph shows a comparison of the demodulation efficiency of the algorithm based on this invention, the traditional dense interpolation algorithm, and the method of directly estimating the peak position of the grating spectrum under different grating array sizes. Time consumption represents the time overhead of the algorithm. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] It should be noted that, unless otherwise specified, the features in the following embodiments and implementation methods can be combined with each other.

[0021] Figure 1 This is a schematic flowchart illustrating a high-efficiency, high-robust demodulation method for OFDR grating array multiplexing provided in an embodiment of the present invention. Figure 1 As shown, the demodulation optimization method in this embodiment of the invention may include the following steps: Step S1: The OFDR system probes the grating array to acquire spectral domain data before and after signal generation; Step S2: Perform Fourier transform on the two sets of data to convert the spectral domain data to the distance domain, and obtain two curves showing the distribution of complex reflectance of the grating array with distance before and after the signal is generated; Step S3: Select the sliding window length L, and slide the window to capture the corresponding positions of the two complex reflectivity curves before and after the signal is generated to obtain two local range domain data. Step S4: Perform inverse Fourier transform on the two local distance domain data (finite zero padding can be performed first to achieve finite interpolation in the spectral domain) to obtain local grating spectral data with spectral resolution lower than the demodulation threshold before and after the signal generation, namely S_r (λ) and Sm (λ). Step S5: Perform cross-correlation on S_r (λ) and Sm (λ) to obtain the cross-correlation spectrum S_c (λ) with a spectral resolution lower than the threshold required for demodulation, i.e., S_c (λ) = S_r (λ). Sm (λ); Step S6: After estimating the peak position of the cross-correlation spectrum S_c(λ) with a spectral resolution lower than the threshold required for demodulation, the wavelength λ corresponding to the accurate peak position is obtained. c ; Step S7: Wavelength λ c This corresponds to the accurate grating spectral shift caused by the signal. Based on the linear mapping relationship between the spectral shift and the signal, this value can be calculated into the sensor signal M to be measured (temperature, strain, etc.). Step S8: Repeat steps S3 to S7 until all the sensing grating arrays are demodulated to obtain the curve of the sensing signal distribution with distance.

[0022] Steps S3-S4 can be performed without interpolating the spectral data, or by padding the truncated range domain data with zeros before the inverse Fourier transform, thus completing a limited number of interpolations on the spectral data. Unlike traditional demodulation methods, this interpolation is not a full interpolation; it is non-dense. The spectral resolution after interpolation is still far from sufficient to meet general needs. Instead, it is a limited number of interpolations performed to better estimate peak positions.

[0023] Directly finding the peak position of S_c(λ) cannot obtain an accurate signal, resulting in a spectral shift in the grating spectrum and thus a decrease in demodulation accuracy. Therefore, peak position estimation of S_c(λ) is performed. Since traditional local spectral interpolation methods involve padding the local range domain data with zeros before performing an inverse Fourier transform, its cross-correlation spectrum is mathematically equivalent to sinc interpolation. Therefore, a more accurate peak position estimation method described in step S6 is determined by the following formula: In the formula, λ m Let N be the wavelength of light corresponding to the peak of the cross-correlation spectrum S_c(λ), and N be the data length of S_c(λ). r For spectral resolutions lower than the threshold cross-correlation spectrum S_c(λ) required for demodulation, the spectral resolution is determined by the OFDR light source wavelength scanning range Δλ and the window length L (assuming no interpolation of the local grating spectrum is performed here; if finite interpolation is performed, then L is the length of the interpolated local spectral data), I(λ m ) represents the peak value of S_c(λ), λ r The spectral resolution is lower than the threshold cross-correlation spectrum S_c(λ) required for demodulation. This value corresponds to the accurate grating spectral shift caused by the signal. Finally, the value is converted to the signal value (temperature, strain, etc.) according to the correspondence between the spectral shift and the signal.

[0024] λ r It is calculated using the following formula: In the formula, Δλ is the scanning range of the OFDR light source wavelength, and L is the length of the local spectral data after interpolation.

[0025] I(λ m Let λ be the peak value of S_c(λ). The wavelength λ corresponding to the peak value of S_c(λ) is then obtained. m Next, determine the two adjacent sampling points, with the first point being λ. m-1 The next point is λ. m+1 The corresponding amplitude value in S_c(λ) is I(λ). m-1 ) and I(λ m+1 ). I bias The bias is calculated as follows: wavelength λ c The sensor signal M to be measured is calculated using the following formula: In the formula, λ c Let α be the spectral shift, and α be the linear correlation coefficient between the spectral shift and the signal. The measured sensing signal M includes, but is not limited to, temperature and strain; when the measured signal is temperature, the temperature sensing coefficient α is 0.1℃ / pm; when the measured signal is strain, the strain sensing coefficient α is 1.2u / pm.

[0026] Throughout the demodulation process, steps S4 to S6 are the main operational flows that significantly distinguish this invention from traditional demodulation algorithms, and they are marked in red in the flowchart.

[0027] according to Figure 1The operation process handles OFDR grating array multiplexed data, which can improve system demodulation efficiency while ensuring demodulation robustness. Figure 2 This is a comparison chart of demodulation results based on the demodulation algorithm described in this invention, specifically a demodulation method that estimates the peak position of cross-correlation spectra with spectral resolution below the required demodulation threshold, and a demodulation method that directly estimates the peak position of grating spectra with spectral resolution below the required demodulation threshold. Figure 2 As shown, based on the demodulation algorithm described in this invention, the demodulation result, i.e., the distribution of the strain signal with distance, appears as a smooth curve. At the strain application location (@9.3 meters, 2000 microstrain), the applied strain signal can be accurately demodulated. In contrast, the demodulation method based on directly estimating the peak position of the grating spectrum exhibits many "glitches." These "glitches" refer to demodulation points with suddenly large absolute values ​​on the curve of the demodulated signal changing with distance, indicating demodulation failure at that location and an incorrect demodulation result. This demonstrates its poor demodulation robustness. This figure illustrates the beneficial effect of the algorithm of this invention in terms of demodulation robustness.

[0028] Figure 3 This diagram illustrates how spectral distortion caused by grating writing defects and spectral splitting caused by grating spacing can lead to decreased demodulation accuracy or demodulation errors when directly estimating peak positions for gratings with spectral resolution below the required demodulation threshold. Figure 3 As shown, (a) is a schematic diagram of a normal grating spectrum, and (b) and (c) are schematic diagrams of spectral splitting caused by the spacing between grating arrays and grating spectral distortion caused by grating writing defects, respectively. The situations shown in Figures (b) and (c) are the reasons for demodulation errors and decreased demodulation accuracy when directly estimating the peak position of grating spectra with spectral resolution lower than the threshold required for demodulation. Figure 2 (The "burrs" in the text).

[0029] Figure 4 This diagram illustrates a comparison of demodulation results between the algorithm based on this invention and a traditional dense interpolation algorithm. Figure 4 As shown, the demodulation results of the two demodulation algorithms are basically the same, and even in the noisy region, they have basically the same results, indicating that the mathematical essence of the algorithm of the present invention is consistent with the traditional dense interpolation algorithm. However, the demodulation results of the algorithm of the present invention also have high robustness.

[0030] Figure 5 A comparison chart shows the demodulation efficiency of the algorithm based on this invention, the traditional dense interpolation algorithm, and the method of directly estimating the peak position of the grating spectrum under different grating array sizes, highlighting the beneficial effect of the algorithm of this invention in improving demodulation efficiency. Figure 5As shown, the demodulation scheme proposed in this invention, which estimates the peak position of cross-correlation spectra with spectral resolution below the threshold required for demodulation, significantly improves computational efficiency compared to traditional dense interpolation methods. With a grating array reuse scale of 10 meters and a spatial resolution of 5 millimeters, the computational efficiency is improved by 15.3 times. This indicates that the algorithm of this invention significantly improves demodulation efficiency while maintaining the same excellent demodulation robustness as traditional dense interpolation algorithms. Furthermore, compared to the demodulation method that directly estimates the peak position of grating spectra with spectral resolution below the threshold required for demodulation, the computational efficiency is not significantly reduced. This demonstrates that the algorithm of this invention improves demodulation robustness without sacrificing demodulation efficiency.

[0031] The present invention also provides an optical frequency domain reflectometer device, which includes: a light source module, an interference module, a detection module, and a signal processing module, wherein the signal processing module is configured to perform the above-described grating array multiplexing and demodulation method.

[0032] The present invention also provides a computer-readable storage medium having a program stored thereon, which, when executed by a processor, implements the above-described grating array multiplexing and demodulation method.

[0033] The present invention also provides a computer program product, including a computer program / instruction, which, when executed by a processor, implements the above-described grating array multiplexing and demodulation method.

[0034] The various embodiments in this invention are described in a progressive manner. Similar or identical parts between embodiments can be referred to mutually. Each embodiment focuses on describing the differences from other embodiments. In particular, the system embodiments are basically similar to the method embodiments, so the description is relatively simple; relevant parts can be referred to the descriptions in the method embodiments.

[0035] The above description is merely an embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principle of the present invention are protected. All of these should be included within the scope of the claims of this invention.

Claims

1. A method for multiplexing and demodulating a grating array of an optical frequency domain reflectometer based on peak position estimation, characterized in that, The method includes the following steps: S1. Obtain spectral domain data before and after signal generation by probing the grating array using an optical frequency domain reflectometer; S2. Perform Fourier transform on the two sets of spectral domain data to obtain two curves showing the distribution of complex reflectance of the grating array with distance before and after the signal is generated; S3. Select window length L, and slide the window length L to the corresponding positions of the two curves obtained in step S2 to obtain two local distance domain data. S4. Perform inverse Fourier transform on the two local range domain data or perform finite interpolation on them and then perform inverse Fourier transform to obtain the local grating spectrum data before and after the signal generation as S_r (λ) and Sm (λ), respectively. The length of the local grating spectrum data is still represented by L. S5. Perform cross-correlation operation on S_r (λ) and Sm (λ) to obtain the cross-correlation spectrum S_c (λ); S6. After estimating the peak position of the cross-correlation spectrum S_c(λ), the wavelength λ corresponding to the accurate peak position is obtained. c ; S7. Wavelength λ c This corresponds to the accurate grating spectral shift caused by the sensing signal. Based on the linear mapping relationship between the spectral shift and the sensing signal, the wavelength λ can be... c Solving for sensor signals; S8. Repeat steps S3 to S7 until all sensor grating arrays are demodulated, and obtain the curve of the sensor signal distribution with distance.

2. The grating array multiplexing and demodulation method according to claim 1, characterized in that, In step S5, the formula for calculating the cross-correlation spectrum S_c(λ) is: In the formula, S_r (λ) and Sm (λ) are the local grating spectral data before and after the signal generation, respectively. For cross-correlation operations.

3. The grating array multiplexing and demodulation method according to claim 1, characterized in that, In step S6, the wavelength λ corresponding to the accurate peak position is... c The estimation formula is as follows: In the formula, λ m The wavelength corresponding to the spectral peak is N, where N is the data length of S_c(λ), and λ is the wavelength of the peak. r I(λ) represents the spectral resolution when it is below the threshold required for demodulation. m ) represents the peak value of S_c(λ), λ m+1 For λ m The next adjacent sampling point, I(λ) m+1 ) represents the sampling point λ m+1 The corresponding spectral intensity, I bias This is the bias value.

4. The grating array multiplexing and demodulation method according to claim 3, characterized in that, The spectral resolution λ is lower than the threshold required for demodulation. r The specific calculation formula is as follows: In the formula, Δλ is the scanning range of the OFDR light source wavelength used, and L is the length of the local grating spectral data.

5. The grating array multiplexing and demodulation method according to claim 3, characterized in that, The bias amount I bias The calculation formula is as follows: In the formula, λ m λ represents the wavelength corresponding to the spectral peak. m-1 For λ m The adjacent previous sampling point, λ m+1 For λ m The next adjacent sampling point, I(λ) m-1 ) and I(λ m+1 ) represent the sampling points λ m-1 and λ m+1 The corresponding spectral intensity.

6. The grating array multiplexing and demodulation method according to claim 1, characterized in that, In step S7, the wavelength λ c The sensor signal M to be measured is calculated using the following formula: In the formula, λ c Let be the spectral shift, and a be the linear correspondence coefficient between the spectral shift and the signal.

7. The grating array multiplexing and demodulation method according to claim 6, characterized in that, The measured sensing signal M includes temperature and strain; when the measured signal is temperature, the temperature sensing coefficient a is 0.1℃ / pm; when the measured signal is strain, the strain sensing coefficient a is 1.2ue / pm.

8. An optical frequency domain reflectometer device, characterized in that, The device includes: a light source module, an interference module, a detection module, and a signal processing module; wherein the signal processing module is configured to perform the grating array multiplexing and demodulation method according to any one of claims 1-7.

9. A computer-readable storage medium, characterized in that, It stores a program that, when executed by a processor, implements the grating array multiplexing and demodulation method according to any one of claims 1-7.

10. A computer program product comprising a computer program / instructions, characterized in that, When the computer program / instruction is executed by the processor, it implements the grating array multiplexing and demodulation method according to any one of claims 1-7.

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