A Demodulation Method of Tilt Fiber Bragg Grating Based on Continuous Wavelet Transform

Abstract

This invention discloses a tilted fiber Bragg grating (FBG) demodulation method based on continuous wavelet transform (CWT). The method includes the following steps: reading spectral data and focusing on the cladding mode region; performing a CWT on the spectrum to generate a scale-wavelength domain coefficient matrix and visualizing it, locating the cutoff mode by the energy concentration region in the matrix; extracting the coefficient curve at a characteristic scale to amplify the peak characteristics; using a multi-scale automatic peak-finding algorithm to detect the first peak wavelength as the cutoff wavelength; calculating the RMS of the overall matrix to verify the accuracy of the cutoff wavelength at a single scale; calculating sensitivity and goodness of fit based on the linear relationship between the cutoff wavelength and the refractive index; and using a cascaded TFBG and FBG to compensate the spectrum and eliminate external environmental interference. This method eliminates complex preprocessing methods such as traditional denoising and envelope fitting, and directly achieves precise and rapid location of the cutoff mode through the CWT transform and the AMPD algorithm. This significantly improves demodulation efficiency and is suitable for high-precision real-time refractive index monitoring.

Description

Technical Field

[0001] The present invention relates to the field of fiber grating sensing technology, and in particular to a tilted fiber grating demodulation method based on continuous wavelet transform. Background Art

[0002] Over the past few decades, fiber Bragg grating (FBG) sensors have rapidly developed due to their unique advantages. Currently, fiber Bragg grating (FBG) sensing technology has successfully developed a series of sensors that can be used to measure various physical quantities, such as temperature, pressure, and refractive index. Among them, tilted fiber Bragg grating (TFBG), a fiber Bragg grating with a unique structure and multifunctional properties, has garnered widespread attention in recent years. Essentially, a TFBG is still a short-period fiber Bragg grating with uniform refractive index modulation. However, the axial angle introduced during its inscription disrupts the symmetric structure of the traditional fiber Bragg grating (FBG), resulting in a decrease in reflectivity. The incident light is then coupled in the grating region into backward-propagating cladding and core modes, forming a variety of coupling modes. This characteristic makes TFBGs extremely sensitive to refractive index changes 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 diverse. These include the conventional wavelength demodulation method and the intensity demodulation method. These two methods require the selection of specific peaks in the spectrum and the demodulation 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. Another method is to demodulate by detecting the wavelength shift of the cutoff mode of the cladding mode at different refractive indices. Existing cutoff mode detection methods include threshold method, envelope derivative method, cumulative length method, etc. However, these existing methods all rely on multi-level preprocessing steps such as spectral denoising, envelope fitting, interpolation smoothing, etc., making the preprocessing process too complicated. These complex preprocessing steps not only increase the time cost of data processing, but may also introduce additional errors, reducing the accuracy and stability of demodulation.

[0004] To this end, the present application designs a tilted fiber Bragg grating demodulation method based on continuous wavelet transform, which does not require complex preprocessing and can directly capture the cutoff mode characteristics to solve the above problems. Summary of the Invention

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

[0006] A tilted fiber Bragg grating demodulation method based on continuous wavelet transform is characterized by comprising the following steps:

[0007] S1, hardware preprocessing, TFBG cascade FBG enhanced reflective spectroscopy;

[0008] S2, data preprocessing, reads the spectral data and filters the wavelength range to focus on the cladding mode region;

[0009] S3, CWT, performs continuous wavelet transform to generate coefficient matrix in scale-wavelength domain;

[0010] S4, visualization processing, selects the coefficient curve of a specific scale, which amplifies the peak characteristics of the cutoff mode;

[0011] S5, AMPD peak search, detect the peak value, and define the wavelength corresponding to the first peak as the "cutoff wavelength";

[0012] S6, RMS, calculates the RMS of the wavelet transform coefficient matrix to prove the consistency of single scale and overall change;

[0013] S7, temperature stress compensation is performed by cascading TFBG and FBG to eliminate external interference;

[0014] S8, performing linear fitting on the compensated cutoff wavelength and the refractive index to obtain sensitivity and goodness of fit indicators.

[0015] Furthermore, in order to better implement the present invention, in said S1, if the light intensity of the TFBG reflection spectrum is weakened during the detection process, the spectrum is enhanced by cascading FBGs.

[0016] Furthermore, to better implement the present invention, the spectrum of S2 can be divided into two regions, core mode and cladding mode, based on the differences in propagation modes within the TFBG. Research has shown that only the cladding mode region is sensitive to changes in the external refractive index, while the core mode remains essentially stable. Therefore, the analysis focuses on the cladding mode region.

[0017] Furthermore, in order to better implement the present invention, the transformation formula of the continuous wavelet transform in S3 is:

[0018]

[0019] in, 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 corresponding intensity of the spectrum.

[0020] Furthermore, in order to better implement the present invention, each element of the coefficient matrix of the scale-wavelength domain in S4 corresponds to the signal energy intensity at the scale and wavelength. The visualized heat map of the matrix can intuitively show the energy concentration area, where the area with significant energy intensity change corresponds to the position of the cutoff mode. In order to accurately determine the cutoff wavelength, the characteristic scale is selected. The coefficient curve of .

[0021] Furthermore, in order to better implement the present invention, the coefficient curve selected in S5 is automatically detected using a multi-scale automatic peak detection algorithm. The automatic multi-scale peak detection algorithm (AMPD) calculates the number of local maxima of the signal under different window lengths, adaptively determines the optimal window length, and then accurately locates the peak position and defines the wavelength of the first detected peak as the cutoff wavelength.

[0022] Furthermore, in order to better implement the present invention, in S6, by calculating the coefficient matrix after wavelet transformation The root mean square (RMS) of the signal energy distribution in the wavelength-scale two-dimensional space is intuitively displayed. This quantifies the energy distribution characteristics of the signal at different scales and wavelengths and helps identify correlations between specific wavelengths and scales. Comparing the coefficient for a single scale with the overall RMS trend verifies the rationality of the cutoff wavelength selection. If both show significant changes in the mutation region (cutoff wavelength), the cutoff wavelength at that location is accurate.

[0023] Furthermore, in order to better implement the present invention, in the S7, the central wavelength of the FBG has good sensitivity to strain and temperature and the FBG does not change due to external refractive index changes; with the help of this characteristic, the interference of the TFBG spectrum caused by temperature and strain is corrected and compensated, so that the TFBG maintains a more stable and accurate state during the refractive index measurement process.

[0024] Furthermore, in order to better implement the present invention, in said S8, a linear fit is performed on the compensated cutoff wavelength and the refractive index to obtain sensitivity and goodness of fit index;

[0025] A tilted fiber Bragg grating (FBG) demodulation system based on continuous wavelet transform (CWT) comprises a computer for executing the above-mentioned tilted fiber Bragg grating (FBG) refractive index sensing and demodulation method based on CWT; the computer is connected to a fiber Bragg grating (FBG) demodulator for acquiring a reflection spectrum of the tilted fiber Bragg grating (FBG); and the tilted fiber Bragg grating (FBG) is placed in a solution to be measured.

[0026] The beneficial effects of the present invention are:

[0027] In terms of demodulation efficiency, this invention dispenses with the cumbersome and complex preprocessing procedures of traditional methods and directly performs a continuous wavelet transform on the raw spectral data. This eliminates the need for time-consuming operations such as spectral denoising, envelope fitting, and interpolation smoothing, significantly shortening data processing time and significantly improving demodulation efficiency. This method can meet the demand for rapid measurement results in real-time monitoring scenarios, with outstanding performance indicators. The technical solution of the present invention has been proven to excel in sensitivity, resolution, and goodness of fit. The sensitivity can reach 544.34 nm / RIU, a significant improvement over traditional methods, capable of capturing minute changes in physical quantities. The goodness of fit R² is as high as 0.999, indicating that the demodulation results are highly consistent with the actual situation, effectively ensuring measurement accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a structural diagram of a tilted fiber Bragg grating demodulation system based on continuous wavelet transform according to the present invention;

[0029] Figure 2 is a graph showing the relationship between sucrose concentration and refractive index of the present invention;

[0030] Figure 3 Spectra of the tilted fiber Bragg grating of the present invention in solutions with different refractive indices;

[0031] Figure 4 Flowchart of the tilted fiber Bragg grating demodulation method based on continuous wavelet transform of the present invention;

[0032] Figure 5 is a heat map after continuous wavelet transformation of the present invention;

[0033] Figure 6 A coefficient curve diagram of the present invention for selecting a specific scale and automatically finding a peak using multiple scales;

[0034] Figure 7 A comparison chart of a single scale and overall RMS of the present invention;

[0035] Figure 8 The function model of the present invention after continuous wavelet transform demodulation;

[0036] Figure 9 This is a spectrum diagram of the tilted fiber Bragg grating cascaded fiber Bragg grating of the present invention.

[0037] In the figure,

[0038] 1. Computer, 2. Fiber Bragg grating (FBG) demodulator, 3. Tilt fiber Bragg grating (FBG), 4. Solution to be tested. DETAILED DESCRIPTION

[0039] The following will be combined with the accompanying drawings to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only some of the embodiments of the present invention, not all of them. Generally, the components of the embodiments of the present invention described and pointed out in the drawings herein can be arranged and designed in a variety of different configurations.

[0040] Therefore, 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 invention as claimed, but is merely intended to represent 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 work are within the scope of protection of the present invention.

[0041] Figures 1-9 This is a specific embodiment of the present invention, which is a tilted fiber Bragg grating demodulation method and system based on continuous wavelet transform.

[0042] like Figure 1 As shown, this embodiment constructs an experimental system to collect spectral data of TFBG at different refractive indices. An SM125 fiber optic sensor interrogator is used to acquire and record TFBG spectra. This device features a scanning range of 1510-1590nm, a wavelength resolution of 0.1pm, a wavelength accuracy of ±1pm, and a full-spectrum sampling rate of 2Hz, enabling high-precision, continuous monitoring of rapid dynamic processes. A reflective tilted fiber Bragg grating structure was employed in the experiment, where the optical fiber end was cut to form a vertical end face, enabling the acquisition of reflective spectra. During the experiment, the prepared reflective TFBG probe was vertically inserted into sucrose solutions of varying mass fractions, ensuring that the TFBG sensitive area was completely immersed and the insertion depth was consistent each time. This eliminated the influence of positional factors on the experimental results and ensured that the ambient temperature was stably controlled at 25±1°C to prevent interference with the spectral data due to temperature fluctuations.

[0043] like Figure 2 As shown, the test solutions were prepared with sucrose solution at mass fractions of 6%, 10%, 14%, 18%, 22%, 26%, 30%, 34%, 38%, 42%, 46% and 50%, respectively, and the corresponding refractive indices were tested and were 1.3417RIU, 1.3478RIU, 1.3541RIU, 1.3606RIU, 1.3674RIU, 1.3743RIU, 1.3814RIU, 1.3887RIU, 1.3962RIU, 1.4039RIU, 1.4118RIU and 1.4199RIU, and a linear relationship model between concentration and refractive index was constructed.

[0044] like Figure 3As shown, after the acquisition is completed, the spectral data of each group are fitted, and it can be seen that the cutoff mode shifts to a longer wavelength as the concentration increases.

[0045] like Figure 4 As shown in FIG, the flowchart of the tilted fiber Bragg grating refractive index sensing demodulation method based on continuous wavelet transform of this embodiment mainly realizes the precise positioning of the cutoff mode through multi-scale feature extraction and automatic peak search algorithm. The specific process is as follows:

[0046] S1, hardware preprocessing, TFBG cascade FBG enhanced reflective spectroscopy;

[0047] S2, data preprocessing, reads the spectral data and filters the wavelength range to focus on the cladding mode region;

[0048] S3, CWT, performs continuous wavelet transform to generate coefficient matrix in scale-wavelength domain;

[0049] S4, visualization processing, selects the coefficient curve of a specific scale, which amplifies the peak characteristics of the cutoff mode;

[0050] S5, AMPD peak search, detect the peak value, and define the wavelength corresponding to the first peak as the "cutoff wavelength";

[0051] S6, RMS, calculates the RMS of the wavelet transform coefficient matrix to prove the consistency of single scale and overall change;

[0052] S7, temperature stress compensation is performed by cascading TFBG and FBG to eliminate external interference;

[0053] S8, performing linear fitting on the compensated cutoff wavelength and the refractive index to obtain sensitivity and goodness of fit indicators.

[0054] like Figure 5 As shown in the figure, before performing continuous wavelet transform, it is necessary to 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, so only the spectrum within the cladding mode range is selected for analysis. The original spectral data is decomposed using continuous wavelet transform to generate a two-dimensional scale-wavelength domain wavelet coefficient matrix. Each element of the matrix represents the signal energy intensity at the corresponding scale and wavelength. The visualized heat map of the matrix can intuitively show the energy concentration area, where the boundary between strong and weak energy corresponds to the position of the cutoff mode.

[0055]

[0056] in, is the mother wavelet function; is the scale parameter, is the translation parameter and its 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.

[0057] like Figure 6 As shown, in order to accurately determine the cutoff wavelength, a single scale is selected (such as Scale5) coefficient curve , for the selected coefficient curve, the AMPD algorithm is used to automatically detect the peak value. Defined as a length signal and define the window length as ,in By calculating each scale and Local maximum of :

[0058]

[0059] Will Form a Matrix Then, by Sum each row of to get the total number of local maxima for each window length:

[0060]

[0061] Maximum scale Depend on The maximum value of . Then, remove the matrix middle elements and form a new matrix .like A column in the matrix If all elements of are 1, then the sampling points corresponding to this column The position of the first effective peak is the cut-off wavelength.

[0062] like Figure 7 As shown, by calculating the coefficient matrix after wavelet transformation The root mean square (RMS) of the signal can intuitively show the changing trend of energy distribution in the wavelength-scale two-dimensional space. It can quantify the energy distribution characteristics of the signal at different scales and wavelengths and help to discover the correlation characteristics of specific wavelengths and scales. Further analysis of the RMS values ​​corresponding to all scales can help to highlight the global characteristics of the signal. Comparing the coefficient of a single scale with the overall RMS trend can verify the rationality of the cutoff wavelength selection. If both show obvious changes in the mutation area (cutoff wavelength) position, it indicates the accuracy of the cutoff wavelength position at that position. Figure 7 The RMS and single-scale figures of the medium refractive index of 1.3417 show that the energy mutation area is around the wavelength of 1524nm, which proves that the cutoff wavelength at a single scale is consistent with that under the overall trend.

[0063] like Figure 8 As shown, based on the determined cutoff wavelength and the pre-established functional relationship model between the cutoff wavelength and the refractive index, the sensitivity is 544.34nm / RIU and the R² is 0.999.

[0064] like Figure 9 As shown, if the TFBG experiences light intensity attenuation during measurement, the spectrum can be compensated by cascading FBGs. Furthermore, when temperature and strain cause spectral changes, the FBG's central wavelength is highly sensitive to these conditions and remains unchanged by external changes in the refractive index. This characteristic allows correction and compensation for environmental variations in the TFBG spectrum, ensuring a more stable and accurate TFBG during refractive index measurements.

[0065] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention and are not limiting. Other modifications or equivalent substitutions made to the technical solution of the present invention by ordinary technicians in this field should be included in the scope of the claims of the present invention as long as they do not depart from the spirit and scope of the technical solution of the present invention.

Claims

1. A tilted fiber Bragg grating demodulation method based on continuous wavelet transform, characterized in that: The following steps are involved: S1, hardware preprocessing, TFBG cascade FBG enhanced reflective spectroscopy; S2, data preprocessing, reads the spectral data and filters the wavelength range to focus on the cladding mode region; S3, CWT, performs continuous wavelet transform to generate a coefficient matrix in the scale-wavelength domain ; S4, visualization processing, selects the coefficient curve of a specific scale, which amplifies the peak characteristics of the cutoff mode; Each element of the coefficient matrix in the scale-wavelength domain corresponds to the signal energy intensity at the scale and wavelength; the visualized heat map of the matrix can intuitively show the energy concentration area, where the area with significant energy intensity changes corresponds to the position of the cutoff mode; in order to accurately determine the cutoff wavelength, a single scale is selected The coefficient curve of ; S5, AMPD peak search, detects peaks and defines the wavelength corresponding to the first peak as the "cutoff wavelength." Based on the selected coefficient curve, a multi-scale automatic peak search algorithm is used for automatic peak detection. The multi-scale automatic peak search algorithm calculates the number of local maxima of the signal under different window lengths, adaptively determines the optimal window length, and then accurately locates the peak position and defines the wavelength of the first detected peak as the cutoff wavelength. Peak search algorithms include AMPD, direct peak search, Gaussian fitting, and power weighting. S6, calculate the root mean square (RMS) of the wavelet transform coefficient matrix to prove the consistency of the single scale and the overall change; 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; Further analysis of the energy mutation region at a single scale is consistent with the overall RMS trend, thus verifying the accuracy of the extracted cutoff wavelength position; S7, temperature stress compensation is performed by cascading TFBG and FBG to eliminate external interference; S8, performing linear fitting on the compensated cutoff wavelength and the refractive index to obtain sensitivity and goodness of fit indicators.

2. The tilted fiber Bragg grating demodulation method based on continuous wavelet transform according to claim 1, characterized in that: In the above-mentioned S1, the light intensity of the TFBG spectrum is weakened during the detection process, and the spectrum is enhanced by cascading FBGs.

3. The tilted fiber Bragg grating demodulation method based on continuous wavelet transform according to claim 1, characterized in that: Based on the differences in the propagation modes in the TFBG, the spectrum of S2 can be divided into two regions: the core mode and the cladding mode. Among them, only the cladding mode region will respond sensitively to changes in the external refractive index, and the core mode remains basically stable. Therefore, the analysis focuses on the cladding mode region.

4. The tilted 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 S3 is: in, 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 corresponding intensity of the spectrum.

5. The tilted fiber Bragg grating demodulation method based on continuous wavelet transform according to claim 1, characterized in that: In S7, when temperature and strain cause the spectral morphology to change, the central wavelength of the FBG is highly sensitive to strain and temperature, and the FBG remains unchanged by external changes in the refractive index. This characteristic allows correction and compensation for changes in the TFBG spectrum caused by environmental influences, making TFBG demodulation more stable and accurate.

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