Tilted fiber grating demodulation method based on continuous wavelet transform

Abstract

The invention discloses a tilted fiber grating demodulation method based on continuous wavelet transform. The method comprises the following steps: reading spectral data and focusing a cladding mode area; performing continuous wavelet transform on the spectrum to generate a scale-wavelength domain coefficient matrix, performing visualization processing on the scale-wavelength domain coefficient matrix, and positioning a cut-off mode through an energy gathering area in a graph; extracting a coefficient curve amplification peak value characteristic under the characteristic scale; detecting a first peak wavelength as a cut-off wavelength by adopting a multi-scale automatic peak searching algorithm; calculating the RMS of the overall matrix to verify the accuracy of the cut-off wavelength under the single scale; calculating sensitivity and goodness of fit based on the linear relationship between the cut-off wavelength and the refractive index; cascade TFBG and FBG are adopted to compensate the spectrum and eliminate external environment interference. According to the method, traditional complex preprocessing such as denoising and envelope fitting is abandoned, accurate and rapid positioning of the cut-off mode is achieved directly through the CWT and the AMPD algorithm, the demodulation efficiency is remarkably improved, and the method is suitable for high-precision real-time refractive index monitoring.

Description

Technical Field

[0001] The present invention relates to the technical field of fiber Bragg grating sensing, and particularly to a demodulation method for tilted fiber Bragg gratings based on continuous wavelet transform. Background Technique

[0002] In the past few decades, fiber Bragg grating sensors have developed rapidly due to their unique advantages. At present, a series of sensors that can be used to measure various physical quantities such as temperature, pressure, and refractive index have been successfully developed with fiber Bragg grating sensing technology. Among them, the tilted fiber Bragg grating (TFBG), as a fiber Bragg grating with a special structure and multifunctional characteristics, has received extensive attention in recent years. Essentially, the TFBG still belongs to a short-period fiber Bragg grating with uniform refractive index modulation. However, due to the introduction of an axial angle during its inscription, the symmetric structure of the traditional fiber Bragg grating (FBG) is destroyed, resulting in a decrease in reflectivity. The incident light is coupled into backward-propagating cladding modes and core modes in the grating region, thus forming multiple coupling modes. It is precisely due to this characteristic that the TFBG exhibits extremely high sensitivity to changes in refractive index in the external environment.

[0003] In recent years, in order to improve the accuracy and sensitivity of tilted fiber Bragg grating detection, the demodulation methods for tilted fiber Bragg grating spectra have become increasingly rich and diverse. Among them include the conventional wavelength demodulation method and intensity demodulation method. These two methods require selecting specific peaks in this spectrum and demodulating by measuring the wavelength shift or intensity change under different refractive index environments. However, there are still difficulties in identifying peaks with better performance and the sensitivity is too low. In addition, it is to demodulate by detecting the wavelength shift of the cutoff mode of the cladding mode under different refractive indices. Existing cutoff mode detection methods such as the threshold method, envelope derivative method, and cumulative length method. However, these existing methods currently all rely on multi-stage preprocessing steps such as spectral denoising, envelope fitting, and interpolation smoothing, making the preprocessing process too complex. These complex preprocessing steps not only increase the time cost of data processing, but also may introduce additional errors and reduce the accuracy and stability of demodulation.

[0004] Therefore, the present application designs a demodulation method for tilted fiber Bragg gratings based on continuous wavelet transform, which can directly capture the cutoff mode characteristics without complex preprocessing to solve the above problems. Summary of the Invention

[0005] The present invention provides a demodulation method for tilted fiber Bragg gratings based on continuous wavelet transform to make up for the deficiencies in the prior art.

[0006] A demodulation method for tilted fiber Bragg gratings based on continuous wavelet transform, characterized by including the following steps: S1, Hardware preprocessing, TFBG cascaded with FBG to enhance the reflected spectrum; S2, Data preprocessing, reading the spectral data and screening the wavelength range to focus on the cladding mode region; S3, CWT, performing continuous wavelet transform to generate the coefficient matrix in the scale - wavelength domain; S4, Visualization processing, selecting the coefficient curve at a specific scale, which amplifies the peak characteristics of the cutoff mode; S5, AMPD peak searching, detecting the peak and defining the wavelength corresponding to the first peak as the "cutoff wavelength"; S6, RMS, calculating the RMS of the wavelet transform coefficient matrix to prove the consistency between a single scale and the overall change; S7, Temperature stress compensation is carried out through TFBG cascaded with FBG to eliminate external interference; S8, Linearly fitting the compensated cutoff wavelength and refractive index to obtain the sensitivity and goodness - of - fit indicators.

[0007] Further, to better implement the present invention, in S1, during the detection process, if the light intensity of the TFBG reflected spectrum weakens, the spectrum is enhanced by cascading FBG.

[0008] Further, to better implement the present invention, in S2, according to the difference in the propagation mode in TFBG, its spectrum can be divided into two regions: the core mode and the cladding mode. Research shows that only the cladding mode region will respond sensitively to the change of the external refractive index, and the core mode remains basically stable. Therefore, the analysis focus is concentrated on the cladding mode region.

[0009] Further, to better implement the present invention, the transformation formula of the continuous wavelet transform in S3 is:

[0010] where, is the mother wavelet function; a is the scale parameter, b is the translation parameter, is the wavelength corresponding to the spectrum, is the normalization factor, represents the complex conjugate of the mother wavelet function, is the intensity corresponding to the spectrum.

[0011] Further, to better implement the present invention, each element of the coefficient matrix in the scale - wavelength domain in S4 corresponds to the signal energy intensity at the scale and wavelength. The visual heat map of the matrix can intuitively show the energy aggregation region, where the region with significant energy intensity change corresponds to the position of the cutoff mode. To accurately determine the cutoff wavelength, select the coefficient curve at the characteristic scale .

[0012] Further, to better implement the present invention, in S5 for the selected coefficient curve, an automatic peak detection is performed using the Automatic Multiscale Peak Detection (AMPD) algorithm. The AMPD algorithm adaptively determines the optimal window length by calculating the number of local maxima of the signal under different window lengths, and then accurately locates the peak position and defines the wavelength of the first detected peak as the cut-off wavelength.

[0013] Further, to better implement the present invention, in S6, by calculating the root mean square (RMS) of the coefficient matrix after wavelet transform the changing trend of the energy distribution in the two-dimensional wavelength-scale space can be intuitively displayed, the energy distribution characteristics of the signal at different wavelengths corresponding to different scales can be quantified, and it is helpful to discover the correlation characteristics of specific wavelengths and scales. Comparing the coefficients of a single scale with the overall RMS trend can verify the rationality of the cut-off wavelength selection. If both show obvious changes at the position of the mutation region (cut-off wavelength), it indicates the accuracy of the cut-off wavelength position at this location.

[0014] Further, to better implement the present invention, in S7, the central wavelength of the FBG has good sensitivity to strain and temperature, and the FBG does not change with the change of the external refractive index; by virtue of this characteristic, the interference caused by temperature and strain to the TFBG spectrum is corrected and compensated, so that the TFBG maintains a more stable and accurate state during the refractive index measurement process.

[0015] Further, to better implement the present invention, in S8, a linear fitting is performed on the compensated cut-off wavelength and the refractive index to obtain sensitivity and goodness-of-fit indicators; A tilted fiber Bragg grating demodulation system based on continuous wavelet transform, comprising a computer: the computer is used to execute the above-mentioned tilted fiber Bragg grating refractive index sensing demodulation method based on continuous wavelet transform; the computer is connected to a fiber Bragg grating demodulator for obtaining the reflection spectrum of the tilted fiber Bragg grating; the tilted fiber Bragg grating is placed in the solution to be measured.

[0016] The beneficial effects of the present invention are: In terms of demodulation efficiency, the present invention abandons the cumbersome and complicated preprocessing process in the traditional method and directly performs continuous wavelet transform on the original spectral data. There is no need to perform time-consuming operations such as spectral denoising, envelope fitting, interpolation smoothing, etc., which greatly shortens the data processing time and significantly improves the demodulation efficiency. It can meet the needs of quickly obtaining measurement results in real-time monitoring scenarios, and the performance indicators have outstanding advantages. It has been verified that the technical solution of the present invention has excellent performance in sensitivity, resolution and goodness of fit. The sensitivity can reach 544.34 nm / RIU, which is significantly improved compared with traditional methods and can capture tiny changes in physical quantities; the goodness of fit R² is as high as 0.999, indicating that the demodulation result is highly consistent with the actual situation, and the measurement accuracy is effectively guaranteed. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a structural diagram of the tilted fiber Bragg grating demodulation system based on continuous wavelet transform of the present invention; Figure 2 is a graph showing the relationship between sucrose concentration and refractive index of the present invention; Figure 3 The optical spectrum of the tilted fiber grating of the present invention in solutions with different refractive indices; Figure 4 It is a flow chart of the tilted fiber Bragg grating demodulation method based on continuous wavelet transform of the present invention; Figure 5 It is a heat map after continuous wavelet transformation of the present invention; Figure 6 A coefficient curve diagram of selecting a specific scale and automatically finding a peak using multiple scales according to the present invention; Figure 7 A comparison chart of a single scale and overall RMS of the present invention; Figure 8 The function model of the present invention after continuous wavelet transform demodulation; Figure 9 This is a spectrum diagram of the tilted fiber Bragg grating cascaded fiber Bragg grating of the present invention.

[0018] In the figure, 1. Computer, 2. Fiber Bragg grating demodulator, 3. Tilt fiber Bragg grating, 4. Solution to be tested. DETAILED DESCRIPTION

[0019] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. The components of the embodiments of the present invention described and indicated in the drawings here can be arranged and designed in various different configurations.

[0020] Accordingly, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0021] Figures 1-9 A specific embodiment of the present invention is a method and system for demodulating tilted fiber Bragg gratings based on continuous wavelet transform.

[0022] As Figure 1 shown, in this embodiment, an experimental system is built to collect spectral data of the TFBG at different refractive indices. Among them, an SM125 fiber optic sensing demodulator is used to obtain and record the TFBG spectrum. This device has a scanning range of 1510 - 1590 nm, a wavelength resolution of 0.1 pm, a wavelength accuracy of ±1 pm, and a full-spectrum sampling rate of 2 Hz, and can support high-precision and continuous monitoring of fast dynamic processes. In the experiment, a reflective tilted fiber Bragg grating structure is adopted, that is, after the end of the optical fiber is cut to form a vertical end face, the collection of the reflection spectrum is realized. During the experiment, the prepared reflective TFBG probe is vertically inserted into sucrose solutions with different mass fractions, ensuring that the sensitive area of the TFBG is completely immersed and the insertion depth is the same each time, so as to eliminate the influence of position factors on the experimental results and ensure that the ambient temperature is stably controlled at 25 ± 1 °C to avoid interference with the spectral data caused by temperature fluctuations.

[0023] As Figure 2 shown, the test solutions are a series of sucrose solutions prepared according to mass fractions of 6%, 10%, 14%, 18%, 22%, 26%, 30%, 34%, 38%, 42%, 46%, and 50% respectively, and their corresponding refractive indices are measured as 1.3417 RIU, 1.3478 RIU, 1.3541 RIU, 1.3606 RIU, 1.3674 RIU, 1.3743 RIU, 1.3814 RIU, 1.3887 RIU, 1.3962 RIU, 1.4039 RIU, 1.4118 RIU, 1.4199 RIU, to construct a linear relationship model between concentration and refractive index.

[0024] As Figure 3 shown, after the collection is completed, the spectral data of each group is fitted, and it can be seen that as the concentration increases, the cut-off mode shifts to longer wavelengths.

[0025] As Figure 4 shown, the flowchart of the method for demodulating the refractive index of a tilted fiber Bragg grating based on continuous wavelet transform in this embodiment mainly realizes the accurate positioning of the cut-off mode through multi-scale feature extraction and automatic peak-seeking algorithms. The specific process is as follows: S1, Hardware preprocessing, TFBG cascaded with FBG to enhance the reflected spectrum; S2, Data preprocessing, read the spectral data and screen the wavelength range to focus on the cladding mode region; S3, CWT, perform continuous wavelet transform to generate a coefficient matrix in the scale - wavelength domain; S4, Visualization processing, select the coefficient curve at a specific scale, which amplifies the peak characteristics of the cut - off mode; S5, AMPD peak searching, detect the peak, and define the wavelength corresponding to the first peak as the "cut - off wavelength"; S6, RMS, calculate the RMS of the wavelet transform coefficient matrix to prove the consistency between a single scale and the overall change; S7, Perform temperature - stress compensation through TFBG cascaded with FBG to eliminate external interference; S8, Linearly fit the compensated cut - off wavelength and refractive index to obtain the sensitivity and goodness - of - fit indicators.

[0026] As Figure 5 shown, before performing the continuous wavelet transform, it is necessary to first select the demodulation range of the TFBG spectrum. Because the coupling mechanism of the TFBG structure is different from that of ordinary fiber Bragg gratings, the cladding mode is sensitive to the external refractive index while the core mode is not sensitive to the refractive index. Therefore, only the spectrum within the cladding mode range is selected for analysis. Use the continuous wavelet transform to decompose the original spectral data to generate a two - dimensional wavelet coefficient matrix in the scale - wavelength domain . Each element of this matrix represents the signal energy intensity at the corresponding scale and wavelength. The visual heat map of the matrix can intuitively show the energy aggregation region, where the boundary between strong and weak energy corresponds to the position of the cut - off mode.

[0027]

[0028] Among them, is the mother wavelet function; is the scale parameter, is the translation parameter corresponding to the wavelength of the spectrum , is the normalization factor, represents the complex conjugate of the mother wavelet function, is the intensity corresponding to the spectrum.

[0029] As Figure 6 shown, in order to accurately determine the cut - off wavelength, select a single scale (such as Scale5) of the coefficient curve , for the selected coefficient curve, use the AMPD algorithm for automatic peak detection. Here, is defined as having a length of Signal and define the window length as , where . By calculating the local maxima for each scale and :

[0030] Form into a matrix . Then, by summing each row of the matrix . Then, by summing each row of the matrix , the total number of local maxima at each window length is obtained:

[0031] The maximum scale is determined by the maximum value of , that is . Subsequently, remove the elements of in the matrix and form a new matrix . If all elements in a certain column of the matrix are 1, then the position of the sampling point corresponding to this column is the valid peak point. Among them, the wavelength value corresponding to the first valid peak is the cut-off wavelength. .

[0032] As Figure 7 shown, by calculating the root mean square (RMS) of the coefficient matrix after wavelet transform, the change trend of energy distribution in the wavelength-scale two-dimensional space can be intuitively displayed, the energy distribution characteristics of the signal at different wavelengths corresponding to different scales can be quantified, and the correlation characteristics of specific wavelengths and scales can be helped to be discovered. Further analyzing the RMS values corresponding to all scales helps to highlight the global characteristics of the signal. Comparing the coefficients of a single scale with the overall RMS trend can verify the rationality of the cut-off wavelength selection. If both show obvious changes at the position of the mutation region (cut-off wavelength), it indicates the accuracy of the cut-off wavelength position at this location. From Figure 7 , the RMS with a refractive index of 1.3417 in

[0033] As Figure 8 shown, based on the determined cut-off wavelength, combined with the pre-established function relationship model between the cut-off wavelength and the refractive index, the sensitivity is 544.34 nm / RIU and R² is 0.999.

[0034] As Figure 9As shown, if the light intensity of the TFBG weakens during the detection process, the spectrum can be compensated by cascading the FBG. Moreover, when the spectrum shape changes due to temperature and strain, the central wavelength of the FBG has good sensitivity to strain and temperature, and the FBG does not change with the change of the external refractive index. By virtue of this characteristic, the changes in the TFBG spectrum caused by environmental influences can be corrected and compensated, so that the TFBG can maintain a more stable and accurate state during the refractive index measurement process.

[0035] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Other modifications or equivalent replacements made by those of ordinary skill in the art to the technical solutions of the present invention should be covered within the scope of the claims of the present invention as long as they do not depart from the spirit and scope of the technical solutions of the present invention.

Claims

1. A demodulation method for tilted fiber Bragg gratings based on continuous wavelet transform, characterized in that It includes the following steps: S1. Hardware preprocessing, enhancing the reflection spectrum of TFBG cascaded with FBG; S2. Data preprocessing, reading the spectral data and screening the wavelength range to focus on the cladding mode region; S3. CWT, performing continuous wavelet transform to generate a coefficient matrix in the scale-wavelength domain; S4. Visualization processing, selecting the coefficient curve at a specific scale, which magnifies the peak characteristics of the cutoff mode; S5. AMPD peak searching, detecting the peak, and defining the wavelength corresponding to the first peak as the "cutoff wavelength"; S6. RMS, calculating the RMS of the wavelet transform coefficient matrix to prove the consistency between a single scale and the overall change; S7. Performing temperature stress compensation through TFBG cascaded with FBG to eliminate external interference; S8. Linearly fitting the compensated cutoff wavelength and refractive index to obtain the sensitivity and goodness-of-fit indicators.

2. The tilt fiber Bragg grating demodulation method based on continuous wavelet transform according to claim 1, characterized in that: In the S1, when the light intensity of the TFBG spectrum weakens during the detection process, the spectrum is enhanced by cascading FBG.

3. The tilt fiber Bragg grating demodulation method based on continuous wavelet transform according to claim 1, characterized in that: In the S2, according to the difference in the propagation mode in the TFBG, its spectrum can be divided into two regions: the core mode and the cladding mode; among them, only the cladding mode region is sensitive to the change of the external refractive index, and the core mode remains basically stable. Therefore, the analysis focus is concentrated on the cladding mode region.

4. The tilt fiber Bragg grating demodulation method based on continuous wavelet transform according to claim 1, characterized in that: The transformation formula of the continuous wavelet transform in the S3 is: wherein, is the mother wavelet function; a is the scale parameter, b is the translation parameter, is the corresponding wavelength of the spectrum, is the normalization factor, represents the complex conjugate of the mother wavelet function, is the corresponding intensity of the spectrum.

5. The tilt fiber Bragg grating demodulation method based on continuous wavelet transform according to claim 1, characterized in that: Each element of the coefficient matrix in the scale-wavelength domain in S4 corresponds to the signal energy intensity at a scale and wavelength. The visual heat map of the matrix can intuitively show the energy aggregation region, where the region with significant energy intensity changes corresponds to the position of the cutoff mode. To accurately determine the cutoff wavelength, a single scale of the coefficient curve .

6. The tilt fiber Bragg grating demodulation method based on continuous wavelet transform according to claim 1, characterized in that: In the S5, for the selected coefficient curve, the multi-scale automatic peak searching algorithm is used for automatic peak detection. The multi-scale automatic peak searching algorithm adaptively determines the optimal window length by calculating the number of local maxima of the signal under different window lengths, and then accurately locates the peak position and defines the wavelength of the first detected peak as the cutoff wavelength; the peak searching algorithms include the AMPD algorithm, the direct peak searching algorithm, the Gaussian fitting algorithm, and the power weighting algorithm.

7. The tilt fiber Bragg grating demodulation method based on continuous wavelet transform according to claim 1, characterized in that: In the S6, by calculating the RMS of the coefficient matrix after wavelet transform, the energy distribution characteristics of the signal at different scales corresponding to different wavelengths can be quantified, and further analysis shows that the energy mutation region at a single scale is consistent with the overall RMS trend, thereby verifying the accuracy of the extracted cutoff wavelength position.

8. The tilt fiber Bragg grating demodulation method based on continuous wavelet transform according to claim 1, characterized in that: In the above S7, when the spectral form is changed due to temperature and strain, the central wavelength of the FBG has good sensitivity to strain and temperature, and the FBG does not change with the change of the external refractive index. By virtue of this characteristic, the change of the TFBG spectrum caused by environmental influence is corrected and compensated, so that the TFBG demodulation is more stable and accurate.

9. The method for demodulating the tilted fiber Bragg grating based on continuous wavelet transform according to claim 1, wherein: In the above S8, the compensated cut-off wavelength and refractive index are linearly fitted to obtain the sensitivity and goodness-of-fit indexes.

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