CN108426594B - Fiber grating reflection spectrum demodulation system of correlation algorithm - Google Patents
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- G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
- G01D5/40—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light specially adapted for use with infrared light
Abstract
The invention provides a fiber bragg grating reflection spectrum demodulation system of a correlation algorithm, which comprises a broadband light source, a 50:50 coupler, a sensing fiber and a fiber bragg grating demodulator, wherein a plurality of FBG sensors with different reflection center wavelengths are connected in series in the sensing fiber, light of the optical broadband light source is incident into the sensing fiber through the 50:59 coupler with a certain bandwidth, the FBG sensors with different reflection center wavelengths are reflected under the condition that the wavelengths meet the condition due to the effect of Bragg conditions, and the light of the wavelengths which do not meet the condition is transmitted; the external parameters are modulated into the reflection wavelength and enter the demodulator through the coupler for demodulation.
Description
The application is filed on 2016, 5, 24 and under the application number CN201610349040.6, and is named as a divisional application of a fiber grating reflection spectrum demodulation algorithm based on signal correlation matching.
Technical Field
The invention relates to the field of optical fiber sensing, in particular to a fiber grating reflection spectrum demodulation system of a correlation algorithm.
Background
Fiber Bragg Grating (FBG) sensors have rapidly become excellent sensor elements capable of measuring various physical quantities such as temperature, strain and pressure. It has the advantages of high sensitivity, no electromagnetic interference, good waterproof performance, small volume, light weight, high reliability, being capable of being embedded into composite materials, etc. The FBG modulates the reflection center wavelength through an external parameter to obtain sensing information. Therefore, the key to the demodulation of the FBG sensing system is to measure the change of the central wavelength of its reflection peak. Currently, FBG center wavelength demodulation methods can be divided into two categories: (1) based on grating diffraction spectral measurement or tunable laser scanning methods, such as CCD detection, tunable filter, etc.; (2) fixed filters, such as edge filtering, etc., are used. The problems of low detection speed, high system cost and the like exist when the tunable laser is used; the edge filtering method has the problems of inconvenience in multi-point multiplexing, contradiction between range and precision and the like. The CCD detection method using the plane grating or the body grating is widely applied because of the advantages of high spectrum detection speed, low system cost, simple multi-point multiplexing and the like.
However, the peak-seeking demodulation result obtained by the method has a direct relation with the resolution of the FBG reflection spectrum. The wavelength range of the common CCD with 256 pixels is 1524.5-1570 nm, and the optical resolution of the system is about 0.178 nm. And the calibrated FBG strain sensing system has the wavelength drift amount of about 1.14 pm/mu. At this point, the optical resolution of the CCD is much lower than the wavelength resolution required by the system. Therefore, to obtain such a small change, it is usually necessary to perform an algorithm processing on the data output by the CCD, and most commonly, a fitting algorithm such as a gaussian fitting algorithm is used. However, this algorithm has very significant disadvantages: (1) the noise immunity is poor due to the excessive dependence on the obtained data; (2) the difference between the FBG reflection spectrum and the standard function caused by packaging and the like is large, so that the fitting error is large.
Therefore, a demodulation system that improves the stability of wavelength measurement errors and demodulation accuracy at low signal-to-noise ratios is needed.
Disclosure of Invention
The invention aims to provide a fiber grating reflection spectrum demodulation system of a correlation algorithm, which comprises a broadband light source, a 50:50 coupler, a sensing fiber and a fiber grating demodulator, wherein the fiber grating reflection spectrum demodulation system comprises a broadband light source, a sensing fiber and a fiber grating demodulator
The FBG sensors with different reflection center wavelengths are connected in series in the sensing optical fiber, light of the optical broadband light source is incident into the sensing optical fiber through a 50:50 coupler with a certain bandwidth, the FBG sensors with different reflection center wavelengths are subjected to Bragg condition action, so that the wavelengths meeting the condition are reflected, and the light with the wavelengths not meeting the condition is transmitted;
The external parameters are modulated into the reflection wavelength and enter the demodulator through the coupler for demodulation.
The fiber grating demodulator is based on a multi-level diffraction grating and linear array infrared CCD principle, a light path adopts a transmission grating dispersion principle, and a reflection spectrum is subjected to dispersion treatment and then is projected onto a photosensitive surface of a linear array photoelectric detector;
And performing photoelectric conversion on the reflection spectrum on different pixels of the linear array photoelectric detector to convert the spectrum information into an electric signal.
A fiber grating reflection spectrum demodulation algorithm based on a correlation algorithm comprises the following steps: a) obtaining high-precision and high-spectral-resolution fiber grating reflection spectrum from spectrometer as base sequence f in discrete autocorrelation function 1(n); b) then the fiber grating reflection spectrum acquired by the demodulator is subjected to peak pre-searching, and the fiber grating reflection spectrum acquired by the demodulator is subjected to zero filling to obtain a sequence f with the same length as the base sequence 2(n); c) base sequence f 1(n) and the zero-filled sequence f obtained by the demodulator 2(n), which can be considered as the result of the same sequence at different times, thus by performing an autocorrelation calculation on both sequences:
Preferably, wherein said sequence f 2(n) by the following steps The method comprises the following steps: a) selecting the sequence f 1Extreme point f in (n) 1(i) And 3 point pair sequences f are taken on the left and right sides of the sequence 1(n) intercepting to obtain a sequence f 2(n)'; b) to f 2(n)' zero filling, the zero filling is carried out to obtain a sequence f with the same length as the base sequence 2(n)。
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
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Further objects, features and advantages of the present invention will become apparent from the following description of embodiments of the invention, with reference to the accompanying drawings, in which:
Fig. 1 is a structural block diagram of a fiber grating reflection spectrum demodulation algorithm based on signal autocorrelation matching according to the present invention.
Fig. 2 is a schematic diagram of a sensing demodulation system of a multi-level diffraction grating and a fiber grating of a linear array infrared CCD.
Fig. 3(a) is a comparison graph of FBG peak wavelengths obtained by using a correlation matching method when the FBG reflection spectrum wavelengths are numerically simulated.
Fig. 3(b) is a comparison graph of FBG peak wavelength obtained by using gaussian fitting method when the FBG reflection spectrum wavelength is numerically simulated.
FIG. 3(c) shows the peak wavelength λ of the FBG from the correlated matched sampling points BWith the true value of wavelength λ TSchematic absolute error diagram of (1).
FIG. 3(d) is a graph of the peak wavelength λ of an FBG from a Gaussian fit to the sample points GWith the true value of wavelength λ TSchematic absolute error diagram of (1).
FIG. 4 is a comparison graph of peak wavelength errors of FBGs obtained by Gaussian fitting and correlation matching method.
FIG. 5 is a graph showing the actual reflection spectrum of FBG at 22 deg.C measured by a high-precision spectrometer.
FIG. 6(a) is a graph comparing the change in center wavelength from 32.2 deg.C to 23.5 deg.C measured by a high precision spectrometer and related matching methods.
FIG. 6(b) is a comparison of the change in center wavelength from 32.2 deg.C to 23.5 deg.C, measured by a high precision spectrometer, and Gaussian fit.
Detailed Description
Autocorrelation is a concept in signal analysis, which indicates the degree of correlation between values of the same time series at any two different times, and is a measure of the similarity between a signal and a delayed signal, and the maximum value is the time when the delay time is zero.
The continuous autocorrelation function is:
Rf(τ)＝f(τ)*f*(-τ)＝-∞f(t+τ)f*(t)dt＝-∞f(t)f*(t-τ)dt (1)
In the formula (1), f (t) is a time function, f (t) *Is the conjugate of a function of time. τ is the time delay.
The discrete autocorrelation function is:
In the formula (2), f (N) is a discrete sequence, and N is the length of the discrete sequence f (N).
Fig. 1 shows a block diagram of a fiber grating reflection spectrum demodulation algorithm based on signal autocorrelation matching according to the present invention. In the present invention, a discrete autocorrelation function is used, and as shown in fig. 1, first, step 101 is to obtain a high-precision and high-spectral-resolution fiber grating reflection spectrum from a spectrometer as a base sequence f in the discrete autocorrelation function 1(n), then step 102 is entered, and the fiber grating reflection spectrum acquired by the demodulator is subjected to step 103 to pre-peak searching. Selecting the sequence f 1Extreme point f in (n) 1(i) And 3 point pair sequences f are taken on the left and right sides of the sequence 1(n) intercepting to obtain a sequence f 2(n)'. Due to f 2The (n)' has less data, lower resolution and larger error when directly performing the correlation algorithm. Therefore, it is necessary to make f 2(n)' zero filling, the zero filling is carried out to obtain a sequence f with the same length as the base sequence 2(n) of (a). Since the shape of the reflection spectrum of the fiber grating is not easily changed in practical use, the fiber grating is considered to be So that the reflection spectrum shape of the fiber grating does not change under strain loading or temperature changes, the base sequence f 1(n) and the zero-filled sequence f obtained by the demodulator 2(n), which may be considered as the result of the same sequence at different times, then step 104 is entered by performing an autocorrelation calculation on both sequences:
Combining the autocorrelation function with f 2After (n) the sequences are aligned, the position of the maximum of the autocorrelation function R (n) is the position of the peak.
Fig. 2 is a schematic diagram of a sensing demodulation system of a multi-level diffraction grating and a fiber grating of a linear array infrared CCD. As shown in fig. 2, the broad spectrum light source is ASE (amplified spontaneous emission light source). The light of the broadband light source passes through a 50: the 50 coupler is incident into the sensing fiber. The FBG sensors with different reflection center wavelengths are connected in series in the sensing optical fiber, and the wavelengths meeting the conditions are reflected and the light with the wavelengths not meeting the conditions is transmitted under the action of Bragg conditions. At this time, the external parameter is modulated into the reflection wavelength, and enters the demodulator via the coupler for demodulation. The demodulator is based on the multi-level diffraction grating and linear array infrared CCD principle, the light path adopts the transmission grating dispersion principle, and the reflected spectrum is subjected to dispersion treatment and then projected onto the photosensitive surface of the linear array photoelectric detector, so that the reflected spectrum is subjected to photoelectric conversion on different pixels of the linear array photoelectric detector, and the spectrum information is converted into an electric signal for subsequent demodulation.
The invention is based on the multilevel diffraction grating and the linear array infrared CCD fiber grating sensing demodulation system shown in figure 2. Suppose that the demodulation wavelength range is 1524.5-1570 nm, and the CCD has 256 pixels. In order to make the simulation closer to the practical engineering application of FBG, combining the long-term field test experience of this subject group, we use a biased peak formed by two close gaussian peaks superimposed as the simulation spectrum, and the center wavelength of this spectrum is: 1543.933 nm.
Numerical simulation FBG reflection spectrum The wavelength direction is translated and the translated FBG reflection spectrum is sampled at 256 points at a fixed wavelength to simulate the loading process. Fig. 3(a) is a comparison graph of FBG peak wavelengths obtained by using a correlation matching method when the FBG reflection spectrum wavelengths are numerically simulated. Fig. 3(b) is a comparison graph of FBG peak wavelength obtained by using gaussian fitting method when the FBG reflection spectrum wavelength is numerically simulated. FIGS. 3(a) and 3(b) are comparison graphs of FBG peak wavelengths obtained by applying Gaussian fitting and correlation matching methods respectively when the wavelengths of the reflection spectrums of the numerical simulation FBGs are shifted by 0.001-0.312 nm, namely, the corresponding loading is 1-300 mu. FIG. 3(c) shows the peak wavelength λ of the FBG from the correlated matched sampling points BWith the true value of wavelength λ TSchematic absolute error diagram of (1). FIG. 3(d) is a graph of the peak wavelength λ of an FBG from a Gaussian fit to the sample points GWith the true value of wavelength λ TSchematic absolute error diagram of (1). FBG peak wavelength lambda obtained from correlated matching sampling points BWith the true value of wavelength λ TAbsolute error of ae＝λB-λTIn the range of-0.010 to 0.005nm, as shown in FIG. 3 (c). Corresponding FBG peak wavelength lambda obtained by Gaussian fitting of sampling points GWith the true value of wavelength λ TAbsolute error of bs＝λB-λTIn the range of-0.043 to 0.112nm, as shown in FIG. 3 (d).
Under the condition that the reflection spectrum of the numerical simulation FBG is kept still, the FBG is continuously sampled 1000 times in the same sampling mode. FIG. 4 is a comparison graph of peak wavelength errors of FBGs obtained by Gaussian fitting and correlation matching method. The average absolute error of the correlation matching can be seen after averaging the two asLess than the average absolute error obtained by using the traditional Gaussian algorithm bs. Simulation results show that compared with a traditional method for reconstructing an FBG reflection spectrum to obtain the FBG peak wavelength offset by Gaussian fitting of sampling points, the error stability of the FBG peak wavelength offset obtained by adopting a new method for relevant matching of the sampling points is greatly increased, and the absolute error is greatly reduced.
The fiber grating reflection spectrum demodulation algorithm based on the correlation algorithm according to the invention is verified and analyzed through the following experiments:
The actual reflection spectrum of the FBG at 22 ℃ at room temperature was measured using a high precision spectrometer (model AQ6370D) with a peak wavelength of 1556.849nm and stored after power normalization, as shown in fig. 5. FIG. 5 is a graph showing the actual reflection spectrum of FBG at 22 deg.C measured by a high-precision spectrometer.
The water bath experiment measures the shift of FBG peak wavelength along with the change of strain, the temperature is from 32.2 ℃ to 23.5 ℃, a high-precision thermometer is used for measuring and recording the deformation process, a linear array CCD fiber grating demodulation system with 256 pixels in the wavelength range of 1524.5 nm to 1570nm is used for measuring the FBG reflection spectrum with the shift of wavelength along with the change of strain, and the wavelength interval of the CCD to the measuring point of the FBG reflection spectrum is about 0.178 nm. FIG. 6(a) is a graph comparing the change in center wavelength from 32.2 deg.C to 23.5 deg.C measured by a high precision spectrometer and related matching methods. The peak offset obtained by the correlation matching method and the peak offset of the high-precision spectrometer are in the range of-0.0331-0.055 nm, and the standard deviation of the error is about 0.0266. FIG. 6(b) is a comparison of the change in center wavelength from 32.2 deg.C to 23.5 deg.C, measured by a high precision spectrometer, and Gaussian fit. The peak offset obtained by the Gaussian fitting method and the peak offset of the high-precision spectrometer are in the range of-0.088-0.2844 nm, and the standard deviation of errors is about 0.1001.
The actual measurement result of the embodiment shows that compared with the traditional method for obtaining the peak offset by Gaussian fitting the FBG reflection spectrum, the FBG peak offset error obtained by adopting the new correlation matching method is more stable and the accuracy is greatly improved.
The invention provides a novel method for demodulating an FBG peak value based on coherent matching, which is used for measuring the wavelength offset of a reflection peak of a special-shaped FBG with high precision. Compared with the traditional Gaussian fitting FBG peak demodulation method, the method has the following advantages: 1) the original spectrum data is collected through the high-precision spectrometer, the actual shape of the FBG reflection spectrum can be adapted, the influence of the reflection peak shape on a demodulation algorithm is reduced, and the accuracy of FBG peak demodulation is effectively improved. 2) The insensitivity of the demodulation algorithm to the relative position of the peak value and the actual sampling point is obviously enhanced, and the stability of errors is improved. The test result proves that compared with the traditional Gaussian algorithm, the error is reduced by half by using the correlation matching algorithm, and the error stability is greatly improved.
Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.
Claims (3)
1. A fiber grating reflection spectrum demodulation algorithm based on a correlation algorithm and suitable for a demodulation system, wherein the demodulation system comprises a broadband light source, a 50:50 coupler, a sensing fiber and a fiber grating demodulator, and the fiber grating demodulation algorithm
The FBG sensors with different reflection center wavelengths are connected in series in the sensing optical fiber, light of the optical broadband light source is incident into the sensing optical fiber through a 50:50 coupler with a certain bandwidth, the FBG sensors with different reflection center wavelengths are subjected to Bragg condition action, so that the wavelengths meeting the condition are reflected, and the light with the wavelengths not meeting the condition is transmitted;
The external parameters are modulated into the reflection wavelength and enter a demodulator through the coupler for demodulation;
The fiber grating reflection spectrum demodulation algorithm comprises the following steps:
a) Obtaining high-precision and high-spectral-resolution fiber grating reflection spectrum from spectrometer as base sequence f in discrete autocorrelation function 1(n)；
b) Then the fiber grating reflection spectrum acquired by the demodulator is subjected to peak pre-searching, and the fiber grating reflection spectrum acquired by the demodulator is subjected to zero filling to obtain a sequence f with the same length as the base sequence 2(n)；
c) Base sequence f 1(n) and the zero-filled sequence f obtained by the demodulator 2(n), which can be considered as the result of the same sequence at different times, thus by performing an autocorrelation calculation on both sequences:
d) Combining the autocorrelation function with f 2(n) after alignment of the sequences The position of the maximum of the autocorrelation function R (n) is the position of the peak;
e) And outputting the result.
2. The demodulation algorithm of the fiber grating reflection spectrum according to claim 1, wherein the fiber grating demodulator is based on the multi-level diffraction grating and the line array infrared CCD principle, the light path adopts the transmission grating dispersion principle, and the reflection spectrum is subjected to dispersion treatment and then is projected onto the light sensing surface of the line array photoelectric detector;
And performing photoelectric conversion on the reflection spectrum on different pixels of the linear array photoelectric detector to convert the spectrum information into an electric signal.
3. The fiber grating reflectance spectrum demodulation algorithm of claim 1, wherein the sequence f 2(n) is obtained by the following steps:
a) Selecting the sequence f 1Extreme point f in (n) 1(i) And 3 point pair sequences f are taken on the left and right sides of the sequence 1(n) intercepting to obtain a sequence f 2(n)′；
b) To f 2(n)' zero filling, the zero filling is carried out to obtain a sequence f with the same length as the base sequence 2(n)。
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| CN105823497B (en) | 2017-12-19 |
| CN108426594A (en) | 2018-08-21 |
| CN105823497A (en) | 2016-08-03 |
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