CN107907067B - Fiber bragg grating resonant wavelength determination method based on periodic modulation - Google Patents
Fiber bragg grating resonant wavelength determination method based on periodic modulation Download PDFInfo
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- 238000005305 interferometry Methods 0.000 description 2
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/16—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge
- G01B11/18—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge using photoelastic elements
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
- G01D5/00—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
- 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
- G01D5/32—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 with attenuation or whole or partial obturation of beams of light
- G01D5/34—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 with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
- G01D5/353—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 with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
- G01D5/35306—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 with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement
- G01D5/35309—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 with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement using multiple waves interferometer
- G01D5/35316—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 with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement using multiple waves interferometer using a Bragg gratings
Abstract
The invention discloses a fiber grating Bragg resonance wavelength determining method based on periodic modulation, which comprises the steps of obtaining a periodically changed reflection spectrum by applying periodic strain to a cantilever beam, carrying out Fourier transform on a change curve of an intensity value of each spectrum at each wavelength point along with modulation time, obtaining a ratio of a double-frequency component to a fundamental frequency component, reconstructing a spectrum with a narrowed half-width by using the ratio of each wavelength point, and thus obtaining the position of a peak point of the fiber grating reflection spectrum more accurately and realizing high-precision demodulation of a fiber grating. Compared with the prior art, the invention finally reconstructs the spectrum with narrower full width at half maximum, and further improves the demodulation precision.
Description
Technical Field
The invention relates to the field of optical fiber sensing, in particular to a method for accurately determining fiber bragg resonance wavelength based on periodic modulation.
Background
Fiber Bragg Grating (FBG) sensors are widely used in many fields due to their advantages of small size, light weight, strong anti-electromagnetic interference capability, and easy modulation. The demodulation of the fiber grating sensor adopts wavelength coding, so that the demodulation of the fiber grating is not influenced by the light intensity of a light source and the loss fluctuation caused by other appliances, and becomes the most widely applied demodulation technology. The FBG obtains the change of parameters such as temperature, vibration, dynamic and static strain and the like by demodulating the wavelength information. Common demodulation methods are: spectroscopy, edge filtering, interferometry, tunable narrowband light source, tunable F-P filtering. The spectrum method has ideal test results in the aspects of signal-to-noise ratio, optical power and the like, but the wavelength resolution ratio is lower, and some high-precision spectrum devices are expensive and large in size and cannot meet the requirements of actual engineering. Although the system structure and signal processing are simple, the edge filtering method is in the form of light intensity filtering, and the linear filtering characteristic of the filter is only approximate, so that errors are introduced by the fluctuation and other changes of the light source spectrum. The interferometry has high measurement resolution, good sensitivity and is suitable for measuring dynamic strain, but the demodulation range is smaller. The tunable narrow-band filtering method is not suitable for working in high and low temperature environments. The tunable F-P filtering method has wide application in various fields.
However, the improvement of the accuracy of these conventional demodulation methods is limited by the flatness of the peak position of the spectrum, so that it is important to improve the demodulation accuracy greatly whether a spectrum with a sharper peak position is reconstructed by some method.
Disclosure of Invention
In order to solve the problems encountered by the traditional demodulation method, the invention provides a fiber bragg resonance wavelength determination method based on periodic modulation, which is not used for directly demodulating the original spectrum, but is used for applying periodic vibration to a cantilever beam to obtain a periodically changed spectrum, reconstructing a spectrum with the same central wavelength as the previous reflection spectrum, and directly solving the position where the first derivative of the reconstructed spectrum curve is zero by utilizing the principle that the extreme point of the curve corresponds to the point where the first derivative is zero, so that the demodulation precision is finally improved.
The invention discloses a fiber bragg grating resonance wavelength determination method based on periodic modulation, which comprises the following steps:
Compared with the prior art, the invention has the following effects:
1. compared with a common demodulation method, the method has the advantages that the periodic movement of the FBG reflection spectrum is realized by applying periodic strain to the cantilever beam, and the spectrum with narrower half-height width is finally reconstructed by analyzing the change of the light intensity at each wavelength point along with the modulation spectrum, so that the demodulation precision can be further improved;
2. the derivation of the method of the invention is based on a broadband light source with Gaussian distribution, but is also applicable to broadband light sources with other distributions;
3. the invention can be used for a device which utilizes the extreme point of the curve to sense and the extreme point is modulated;
4. the demodulation method involved in the present invention can be used to measure the amplitude of low frequency vibrations.
Drawings
FIG. 1 is a flowchart illustrating an overall method for determining Bragg resonant wavelength of a fiber grating based on periodic modulation according to the present invention;
FIG. 2 is a schematic diagram of a demodulation apparatus for obtaining periodic FBG reflection spectra;
FIG. 3 is a spectrum of the periodic variation obtained by data acquisition and processing by the apparatus of FIG. 1;
FIG. 4 is a graph of light intensity at a portion of a location versus time;
FIG. 5 is a relatively narrow full width at half maximum spectrum of a reconstruction obtained by periodic modulation;
FIG. 6 is a graph of an unmodulated spectrum and a spectrum obtained after modulation;
FIG. 7 is a graph of the change in the extreme point position;
fig. 8 is a graph of the intensity change at the examined point.
Reference numerals:
1. the device comprises a signal generator, 2, a power amplifier, 3, a vibration exciter, 4, a cantilever frame adhered with an FBG, 5, an F-P etalon, 6, 980 a pumping source head, 7, FFP-TF, 8, a wavelength division multiplexer WDM, 9, an erbium-doped optical fiber, 10, a circulator, 11, a notch filter, 12, a photoelectric detector, 13, an optical fiber grating FBG, 14, a scanning driver, 15, a photoelectric detection circuit, 16, a data acquisition processor, 17 and a coupler.
Detailed Description
Embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.
Fig. 2 is a schematic diagram of a demodulation apparatus for obtaining periodic FBG reflection spectrum. The strain required to be obtained in the fiber grating bragg resonance wavelength determination method based on periodic modulation of the present invention may be provided by the apparatus or an apparatus having an equivalent function to this apparatus. The method is described in detail by taking the device shown in fig. 1 as an example, applying a periodically changing strain to the cantilever beam by using an exciter, and controlling the frequency and amplitude of vibration of the exciter through a signal generator and a power amplifier, wherein the frequency of the exciter is 1HZ, the amplitude of the vibration is 5V, and the frequency of a scanning light source is 400 HZ. The structure and the working process of the device are detailed as follows:
the fiber grating is fixed by epoxy glue along the axial direction of the cantilever beam (after curing for 24 hours, the experiment can be carried out). 980 pumping source head 1 forms narrowband scanning light after passing through tunable filter as light emitted by broadband light source, the frequency of the scanning light is set to be 400HZ, the number of sampling points of each channel is 25000, the output scanning light divides the light path into two paths through 1 x 2 coupler 17, one path is connected with F-P etalon 5, the other path enters through the port of circulator 10, the other port of circulator 10 is connected with fiber grating sensor 13, and finally the third port of circulator 10 and the output of F-P etalon 5 are connected with photoelectric detector 12. The data acquisition is controlled through upper computer software, and the FBG data information in the vibration process of the vibration exciter 3 is recorded.
In the following description, the output signal of the signal generator 1 is set to be sinusoidal (with a frequency of 1HZ and an amplitude of 5V), the exciter 3 is driven by the power amplifier 2 to generate mechanical vibration, so that the fiber grating 13 senses the periodically varying strain, and a periodically varying spectrum is obtained. Since the abscissa of the obtained original data is a sampling point, the sampling point is corresponded to the wavelength value by using the calibration of the F-P etalon to obtain a spectrogram having the abscissa as the wavelength value and the ordinate as the periodic variation of the light intensity, as shown in fig. 3.
The demodulation algorithm in this example will obtain a plot of the intensity at each wavelength point over time, and select the plots at some points, as shown in fig. 4. By performing Fourier transform on the graph at each sampling point in FIG. 3, the method
A reconstructed spectrogram is obtained as shown in fig. 4. In order to clearly observe that the original spectrum and the reconstructed spectrum have the same central wavelength and the full width at half maximum of the reconstructed spectrum is obviously narrowed, two spectrograms are drawn in one graph and normalized, as shown in fig. 6. It can be seen from fig. 5 that the full width at half maximum of the reconstructed spectrum is significantly narrower, and therefore the position of each extremum point is easy to read. By the foregoing analysis
The value at the extreme point tends to be positive infinity but in practice the peak is not infinite due to the resolution of the detector. As can be seen from FIG. 5, the full width at half maximum of the original curve is 0.26nm, while the full width at half maximum of the curve obtained by the method of the present invention is 3.4pm, so that the sensing precision is greatly improved.
Fig. 5 is a graph obtained by performing fourier transform on the graph of the light intensity with time at all the wavelength points in fig. 3 and using the ratio of the converted second harmonic component to the fundamental frequency component, that is, a reconstructed spectral diagram with a narrower full width at half maximum obtained by periodic modulation.
In the invention, FBG is adopted to sense the strain change on the cantilever beam, and the collection card is used for collecting the reflection spectrum of the fiber bragg grating in the vibration process of the cantilever beam; a method of reconstructing a spectrum having the same center wavelength as the original spectrum and demodulating the reconstructed spectrum (without directly demodulating the center wavelength of the spectrum) specifically operates as follows, as shown in fig. 1:
the half-height width of the reconstructed spectrum is narrowed, so that peak searching is facilitated, and the demodulation precision is improved.
The theoretical basis of the invention is as follows:
generally, in a common broadband light source, the spectrum of the light source is gaussian or gaussian-like, and therefore the reflection spectrum of the light grating is also gaussian or gaussian-like, which can be expressed by the following formula:
I(x)＝exp(-((x-b).\c).^2) (1)
the parameter b in the formula represents the position of the center wavelength of the reflection spectrum, the parameter c reflects the full width at half maximum of the reflection spectrum to a certain extent, and the larger the value of c, the larger the full width at half maximum of the spectrum.
When the cantilever beam senses the periodically changing strain, the grating region of the fiber grating is stretched or compressed to a certain degree, which causes the reflection spectrum of the fiber grating to move, if the strain sensed by the cantilever beam is in sine or cosine form, a periodically changing spectrum is obtained, that is, the change relationship of the extreme point of the spectrum with time is as follows:
x(t)＝x1+Δx·sin(2πft) (2)
wherein x is1The position of the spectrum extreme point when no modulation is added, delta x is the amplitude of the added modulation, and f is the frequency of the spectrum extreme point shift when the added modulation is added;
for a certain position of the wavelength point, the change of the intensity of the reflection spectrum with time is examined. When the point of investigation is exactly located at x1Then, the extreme point position x (t) shows the variation curve of the formula (2), which is shown in fig. 7 as the variation curve of the extreme point position x (t).
After fourier transformation is performed on such a graph, the frequency doubling component is dominant, and the frequency doubling component is basically close to 0, so that the ratio of the two components theoretically approaches infinity, and the point is exactly the position of the extreme point of the original spectrum, and the extreme value of the point after transformation is found to approach infinity.
When a point is considered to be far from the position of the extreme point of the original spectrum, the intensity change of the point at this time is as shown in fig. 8.
After fourier transformation of such a pattern, a frequency doubling component thereof is dominant and a frequency doubling component is substantially close to 0, and thus the ratio of the frequency doubling component to the frequency doubling component tends to be 0 for points located far from the extreme point.
For other points, the intensity profile over time should be a superposition of the fundamental frequency component and the double frequency and high frequency components.
After performing fourier expansion on the curve of the light intensity variation with time at each wavelength point, the first term corresponds to the fundamental frequency component, the intensity of which is proportional to the first derivative of the curve of the light intensity variation with time at each sampling point, and the second term corresponds to the second derivative of the frequency doubling component, the intensity of which is proportional to the second derivative of the curve, as shown in formula (3):
wherein, E (f) and E (2f) represent the intensity values of the first frequency multiplication component and the second frequency multiplication component after Fourier transformation, respectively, and Y ' (t) and Y ' ' (t) represent the first derivative and the second derivative of the curve, respectively.
For the extreme points of the original spectrum: e < u > A >Extreme value(f)≈0，
And a position E far away from the extreme pointLinearity(2f)≈0，
Wherein E < u > C >Extreme value(f) And E-Extreme value(2f) Respectively representing the intensity value of a frequency doubling component and the intensity value of a frequency doubling component of the extreme point position. E < u > A >Linearity(f) AndE|linearity(2f) The intensity values of a frequency doubling component and a frequency doubling component which are far away from the position of the extreme point on the curve are respectively represented.
Based on the above analysis, it can be seen that if we do Fourier expansion on the time-varying light intensity curve at each wavelength point, and do it
Wherein, E (f) and E (2f) represent the intensity values of the frequency doubling component and the frequency doubling component after Fourier transform respectively. And according to the curve changing along with the wavelength, the maximum point of the curve corresponds to the maximum point of the original reflection spectrum.
By using the method, the half-height width of the reconstructed spectrum is much narrower than that of the original reflection spectrum, so that the position of an extreme point is more favorably found, and the demodulation precision is greatly improved.
Claims (1)
1. A fiber bragg grating resonance wavelength determination method based on periodic modulation is characterized by comprising the following steps:
the method comprises the following steps that (1) a cantilever beam is excited by periodic vibration, so that a fiber grating fixed on the cantilever beam senses periodically changed strain, a high-speed acquisition card acquires original data information of a spectrum in the vibration process, and a sampling point corresponds to a wavelength value through calibration of an etalon to obtain a periodically changed spectrogram with the abscissa as the wavelength value and the ordinate as the light intensity;
step (2), recording the variation curve of the light intensity value of each point at each wavelength point along with the modulation time, and carrying out Fourier transformation on the curve to obtain the ratio of the intensity of the double frequency component to the intensity of the fundamental frequency component
Wherein E (f) and E (2f) represent the intensity values of the frequency-doubled component and the frequency-doubled component after Fourier transform, respectively;
step (3), on the same curve, reconstructing a spectrum with narrowed half-height width by utilizing the ratio of the intensity of the double frequency component to the intensity of the fundamental frequency component at each wavelength point in the step (2), making an original reflection spectrum, and ensuring that the two curves have the same maximum value;
directly demodulating the reconstructed spectrum to obtain the position of an extreme point of the original spectrum; and obtaining the position of the peak point of the fiber grating reflection spectrum, and realizing high-precision demodulation of the fiber grating.
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