Dual-wavelength hybrid differential demodulation method for improving sensing precision of fiber bragg grating

Abstract

The invention discloses a dual-wavelength hybrid differential demodulation method for improving fiber bragg grating sensing precision, and belongs to the field of fiber bragg grating signal demodulation. In particular to a dual-wavelength differential demodulation method based on Gaussian and Lorentz hybrid fitting, which comprises the following steps of: according to the actual reflection spectrum characteristics of a fiber grating, in a dual-wavelength demodulation process, adopting a Gaussian and Lorentz hybrid fitting mode for the reflection spectrum, and finally performing unified correction in an algorithm. The demodulation result expands the dynamic range of the system and improves the wavelength demodulation precision, and the method can be effectively used in a high-precision fiber grating sensing system.

Description

Technical Field

[0001] The present invention belongs to the technical field of quasi - distributed fiber grating demodulation, and particularly relates to a dual - wavelength hybrid differential demodulation method for improving the sensing accuracy of fiber gratings. Background Art

[0002] Quasi - distributed fiber sensing technology has always been the most widely used long - distance and large - area sensing technology. It has advantages such as anti - interference and corrosion resistance that other sensing means do not have. In addition, in quasi - distributed fiber sensing technology, there are many fiber grating demodulation methods, such as spectrometer demodulation method, matching grating demodulation method, unbalanced Michelson interferometer demodulation method, etc. However, these demodulation methods all have disadvantages such as complex structure, high demodulation cost, low accuracy, and difficulty in adjusting the dynamic range and sensitivity. At present, the method of differential demodulation using dual - wavelength pulses has been realized. This method can connect more fiber grating sensors with the same wavelength in series. However, currently, the differential demodulation of dual - wavelength pulses is based on the Gaussian curve. Restricted by the fitting range of the Gaussian curve, generally, the Gaussian fitting formula has a better fitting effect at the point with the maximum slope, while the fitting effect is poor near the center position and at both ends. By using the Lorentz fitting formula for dual - wavelength differential demodulation, a better fitting effect can be achieved at the center or both ends.

[0003] Therefore, it is necessary to develop a dual - wavelength hybrid differential demodulation method for improving the sensing accuracy of fiber gratings to solve the above problems. Summary of the Invention

[0004] The purpose of the present invention is to provide a dual - wavelength hybrid differential demodulation method for improving the sensing accuracy of fiber gratings, which is used to achieve fiber grating demodulation with high spatial resolution and high wavelength resolution.

[0005] The technical solution adopted by the present invention is as follows:

[0006] A dual - wavelength hybrid differential demodulation method for improving the sensing accuracy of fiber gratings uses a dual - wavelength light source with appropriate wavelength spacing and consistent intensity fluctuations as the sensing light source, and inputs it into a quasi - distributed sensing unit composed of fiber gratings. The reflection spectra of the sensing units at different spatial positions will reflect the dual - wavelength signals. When the sensing unit is affected by environmental changes (such as temperature, strain, etc.), the central wavelength of its fiber grating will change accordingly. After being detected by the signal detection unit, the reflected dual - wavelength signals can be obtained and sent to the signal demodulation unit for signal demodulation, thereby improving the demodulation accuracy and dynamic range of the system.

[0007] A dual - wavelength hybrid differential demodulation method for improving the sensing accuracy of fiber gratings of the present invention includes the following steps:

[0008] Step 1: Select a dual-wavelength light source with the same intensity fluctuation magnitude and a wavelength near the maximum slope of the fiber grating reflection spectrum to be incident on the fiber grating sensing system. After reflection by the grating sensing system, it is received and demodulated by the signal detection and demodulation unit.

[0009] Step 2: Represent the dual-wavelength light intensity using the Gaussian fitting formula and determine the system constant a.

[0010] Step 3: Use the dual-wavelength difference demodulation algorithm with Gaussian fitting to measure the relationship between the sensing signal M and the central wavelength shift of the fiber grating.

[0011] Step 4: Adjust the incident wavelength, repeat Step 1, and use the Gaussian fitting formula to find the wavelength threshold value at which the Gaussian formula fitting fails.

[0012] Step 5: Select another dual-wavelength light source with the same intensity fluctuation magnitude and a wavelength near the center of the fiber grating spectrum to be incident on the fiber grating sensing system. After reflection by the grating sensing system, it is received and demodulated by the signal detection and demodulation unit.

[0013] Step 6: Represent the dual-wavelength light intensity using the Lorentz fitting formula and determine the system constants k, b, c, and d.

[0014] Step 7: Use the dual-wavelength difference demodulation algorithm with Lorentz fitting to measure the sensing signal M ’ and the relationship between the central wavelength shift of the fiber grating.

[0015] Step 8: Make a unified correction to the dual-wavelength difference demodulation formula with Lorentz fitting and the dual-wavelength difference demodulation formula with Gaussian fitting.

[0016] Step 9: Use the unified corrected formula for demodulation of the fiber optic sensing system to demodulate the change amount of the external environment.

[0017] In summary, due to the adoption of the above technical solutions, the beneficial effects of the present invention are as follows:

[0018] 1. Compared with the traditional spectral demodulation scheme, the present invention adopts a dual-wavelength difference demodulation algorithm based on mixed fitting of Gaussian and Lorentz, making the system sensitivity and dynamic range adjustable, and this scheme has higher flexibility. And the calculation amount of this demodulation method is small, which can promote the development of integrated large-capacity fiber grating demodulators.

[0019] 2. Compared with the traditional dual-wavelength demodulation, Lorentz fitting formula is used for mixed difference in some places where Gaussian fitting is not accurate enough. The present invention expands the dynamic range of the system, enabling a larger range of environmental changes to be measured, and can effectively improve the accuracy of the system. Description of the Drawings

[0020] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other relevant drawings can also be obtained based on these drawings, where:

[0021] Figure 1 is the flowchart of the present invention;

[0022] Figure 2 is a schematic diagram of data fitting using the solution of the present invention; where, a represents a schematic diagram of fitting using a Gaussian and Lorentz mixture, and b is a schematic diagram of differential demodulation after fitting using a Gaussian and Lorentz mixture;

[0023] Figure 3 is a schematic diagram of the fiber Bragg grating sensing system used in the present invention.

[0024] The markings in the figure are: 1 - dual-wavelength light source, 2 - three-port circulator, 3 - fiber Bragg grating sensor array, 4 - information detection module, 5 - information demodulation and processing module. Detailed implementation manners

[0025] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0026] It should be noted that relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device. Without further limitation, an element defined by the phrase "including one..." does not exclude the existence of additional identical elements in the process, method, article or device including the said element.

[0027] The features and performance of the present invention will be further described in detail below with reference to the embodiments.

[0028] Embodiment

[0029] The present invention is a dual-wavelength hybrid differential demodulation method for improving the sensing accuracy of fiber Bragg gratings, and its process can be as Figure 1As shown

[0030] In step one, a dual-wavelength light source with the same intensity fluctuation magnitude and a wavelength near the maximum slope of the fiber Bragg grating reflection spectrum is selected to be incident on the fiber Bragg grating sensing system. After being reflected by the grating sensing system, the signal is detected by the signal detection and demodulation signal receiving signal, and then sent to the demodulation system for demodulation.

[0031] The signal detection and demodulation unit detects the peak power reflected from the distributed sensing unit, and the reflected peak powers are respectively , , which can be shown by Figure 2 a Gaussian fitting curve. In step two, the dual-wavelength light intensity is expressed by the Gaussian fitting formula, and the system constant a can be expressed as:

[0032] (1)

[0033] (2)

[0034] Among them, represents the initial power when the optical frequency comb teeth are incident, a is the system constant, represents the central wavelength of the fiber Bragg grating, is the full width at half maximum of the grating. and respectively represent the central wavelengths of the incident dual-wavelength light source.

[0035] In step three, the dual-wavelength difference demodulation algorithm with Gaussian fitting is used to measure the relationship between the sensing signal M and the offset of the fiber Bragg grating center wavelength. The relationship between the measured sensing signal M and the offset of the fiber Bragg grating can be expressed as:

[0036] (3)

[0037] Among them, , is the central reflection wavelength of the sensing unit, and are respectively the peak powers of the dual-wavelength reflected light, and the reflection center of the fiber Bragg grating is demodulated to obtain the drift amount under the external action, which has a linear relationship with the change amount of the sensing signal.

[0038] Furthermore, the sensitivity s can be expressed as:

[0039] (4)

[0040] Among them, is and is the wavelength difference between them. Therefore, The larger it is, the higher the sensitivity, but the corresponding dynamic range is smaller.

[0041] In step four, adjust the incident wavelength, repeat step one, and use the Gaussian fitting formula to find the wavelength threshold at which the Gaussian formula fitting fails. When the difference between the actual optical power and the optical power obtained by Gaussian fitting just reaches a certain degree, the corresponding wavelength at this time is the threshold at which the Gaussian fitting fails. It can be expressed as:

[0042] (5)

[0043] Where represents the actual optical intensity after the dual-wavelength is reflected by the fiber grating, represents the initial power when the optical frequency comb teeth are incident, a is a system constant, represents the central wavelength of the fiber grating, is the full width at half maximum of the grating, represents the set optical intensity difference at which the Gaussian fitting fails, represents the threshold at which the Gaussian fitting fails.

[0044] In step five, select a dual-wavelength light source with the same intensity fluctuation and wavelengths near the center of the fiber grating spectrum to be incident on the fiber grating sensing system. After being reflected by the grating sensing system, the signal is detected and the demodulation signal is received, and then sent to the demodulation system for demodulation.

[0045] In step six, as shown in the Lorentz fitting curve of Figure 2 a, represent the dual-wavelength optical intensity with the Lorentz fitting formula and determine the system constants k, b, c, d. Its Lorentz fitting formula can be expressed as:

[0046] (6)

[0047] (7)

[0048] Where , respectively represent the optical power magnitudes of the dual-wavelengths after being reflected by the grating system, represents the initial power when the optical frequency comb teeth are incident, b, c, d, k are system constants, represents the central wavelength of the fiber grating, and respectively represent the central wavelengths of the incident dual-wavelength light source.

[0049] In step seven, use the Lorentz fitting dual-wavelength difference demodulation algorithm to measure the relationship between the sensing signal M ’ and the offset of the fiber grating central wavelength. The sensing signal based on Lorentz fitting The relationship with the central wavelength shift can be expressed as:

[0050] (8)

[0051] Where, , is the central reflection wavelength of the sensing unit, and are respectively the peak powers of the dual-wavelength reflected light, d is a system constant, and the reflection center of the fiber grating demodulates the drift amount under the external action, which has a linear relationship with the change amount

[0052] of the sensing signal. Where, the sensitivity

[0053] (9)

[0054] Where, is and is the wavelength difference between k and b are system constants. Therefore,

[0055] In step eight, the demodulation formulas of the dual-wavelength difference decomposition by Lorentz fitting and Gaussian fitting are corrected and unified. By correcting and unifying formulas (3) and (8) so that they are on a straight line, we can obtain:

[0056] (10)

[0057] In step nine, the unified corrected formula is used for demodulation of the fiber optic sensing system, and its formula can be obtained from Figure 2 as shown in b: The demodulation curve of the dual-wavelength difference decomposition after Gaussian fitting correction and the demodulation curve of the dual-wavelength difference decomposition after Lorentz fitting correction are on a straight line. Therefore, the demodulation signal quantity can be directly substituted into the formula to demodulate the change quantity of the external environment, realizing high dynamic range and high-precision measurement.

[0058] The unified corrected formula used can be expressed as:

[0059] (11)

[0060] Where s is the sensitivity of the dual-wavelength difference decomposition demodulation based on the Gaussian fitting formula, is the sensitivity of the dual-wavelength difference decomposition demodulation based on the Lorentz fitting curve, , is the central wavelength of the fiber Bragg grating. , , , are the threshold wavelengths corresponding to the failure of Gaussian fitting.

[0061] When the external environment of the fiber Bragg grating changes, the change amount of the center of the fiber Bragg grating spectrum can be demodulated according to the change amount of M, so as to obtain the change amount of the external environment.

[0062] Such as Figure 3 shown is a grating sensing system for implementing the present invention:

[0063] The dual-wavelength light source 1 can be generated by a dual-wavelength laser, a tunable laser, an optical frequency comb, etc.

[0064] The quasi-distributed sensing unit includes a three-port circulator 2 and a fiber Bragg grating sensor array 3. The first port of the three-port circulator 2 is connected to the output end of the optical frequency comb light source. The fiber Bragg grating sensor array 3 is a single Gaussian fiber Bragg grating or a series structure composed of multiple Gaussian fiber Bragg gratings with the same central wavelength or a parallel structure composed of multiple Gaussian fiber Bragg gratings with the same central wavelength or a series-parallel structure composed of multiple Gaussian fiber Bragg gratings with the same central wavelength.

[0065] The signal detection and demodulation unit includes an information detection module 4 and an information demodulation processing module 5. The information detection module 4 can be a photodetector, or composed of a photodetector and a data conversion module. The information demodulation processing module 5 is implemented by one or more of a single-chip microcomputer, a programmable logic device, a digital signal processing chip, and an embedded chip.

[0066] Through the above steps 1 to 9, due to the complementarity of the Gaussian fitting formula and the Lorentz fitting formula, the dynamic range of the system is improved, the accuracy of the system is further improved, and the structure is simple and the calculation amount is small, which is beneficial to the miniaturization and integration of the system.

[0067] The above is only the preferred embodiment of the present invention and is not intended to limit the protection scope of the present invention. Any modifications, equivalent replacements, and improvements made by those skilled in the art within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A dual-wavelength hybrid differential demodulation method for improving the sensing accuracy of fiber Bragg gratings, characterized in that It includes the following steps: Step 1: Select a dual-wavelength light source with the same intensity fluctuation magnitude and a wavelength near the maximum slope of the fiber grating reflection spectrum to be incident on the fiber grating sensing system. After being reflected by the grating sensing system, it is received and demodulated by the signal detection and demodulation unit; Step 2: Represent the dual-wavelength light intensity with the Gaussian fitting formula and determine the system constant a; Step 3: Use the dual-wavelength difference demodulation algorithm with Gaussian fitting to measure the relationship between the sensing signal M and the central wavelength shift of the fiber grating; Step 4: Adjust the incident wavelength, repeat Step 1, and use the Gaussian fitting formula to find the wavelength threshold at which the Gaussian formula fitting fails; Step 5: Select another dual-wavelength light source with the same intensity fluctuation magnitude and a wavelength near the center of the fiber grating spectrum to be incident on the fiber grating sensing system. After being reflected by the grating sensing system, it is received and demodulated by the signal detection and demodulation unit; Step 6: Represent the dual-wavelength light intensity with the Lorentz fitting formula and determine the system constants k, b, c, d; Step 7: Measure the relationship between the sensing signal M ’ and the central wavelength shift of the fiber Bragg grating by using the dual-wavelength difference demodulation algorithm with Lorentz fitting. Step 8: Make a unified correction to the dual-wavelength difference demodulation formula with Lorentz fitting and the dual-wavelength difference demodulation formula with Gaussian fitting; Step 9: Use the unified corrected formula to demodulate the fiber optic sensing system, thereby demodulating the change amount of the external environment.

2. A dual-wavelength hybrid differential demodulation method for improving the sensing accuracy of fiber Bragg gratings according to claim 1, characterized in that: The Gaussian fitting formula in Step 2 is: (1)， (2)， Among them, and represent the light intensities after the double wavelengths are reflected by the fiber grating respectively, represents the initial power when the optical frequency comb teeth are incident, a is a system constant, represents the central wavelength of the fiber grating, is the full width at half maximum of the grating; and represent the central wavelengths of the incident double-wavelength light source respectively.

3. A dual-wavelength hybrid differential demodulation method for improving the sensing accuracy of fiber Bragg gratings according to claim 2, characterized in that, In Step 3, the relationship between measuring the sensing signal M and the fiber grating offset can be expressed as: (3)， Among them, , is the central reflection wavelength of the sensing unit, and are respectively the peak powers of the dual-wavelength reflected light. The central reflection drift amount demodulated under the external action has a linear relationship with the change amount of the sensing signal ; The sensitivity s is expressed as: (4)， Among them, is and the wavelength difference; therefore, the larger it is, the higher the sensitivity, but the corresponding dynamic range is smaller.

4. A dual-wavelength hybrid differential demodulation method for improving the sensing accuracy of fiber Bragg gratings according to claim 3, characterized in that, In Step 4, when the difference between the actual optical power and the optical power obtained by Gaussian fitting just reaches a certain level, the corresponding wavelength at this time is the threshold at which Gaussian fitting fails, which can be expressed as: (5)， where I represents the actual optical intensity after the double-wavelength is reflected by the fiber grating, represents the initial power when the optical frequency comb teeth are incident, and a is a system constant, represents the central wavelength of the fiber grating, is the full width at half maximum of the grating, represents the optical intensity difference at which the set Gaussian fitting fails, represents the threshold when the Gaussian fitting fails.

5. A dual-wavelength hybrid differential demodulation method for improving the sensing accuracy of fiber Bragg gratings according to claim 4, characterized in that, In Steps 5 and 6, the Lorentz fitting formula can be expressed as: (6)， (7)， Among them, , respectively represent the optical power magnitudes after the double-wavelength light is reflected by the grating system, represents the initial power when the optical frequency comb teeth are incident, and b, c, d, k are system constants, represents the central wavelength of the fiber grating, and respectively represent the central wavelengths of the incident double-wavelength light source.

6. A dual-wavelength hybrid differential demodulation method for improving the sensing accuracy of fiber Bragg gratings according to claim 5, characterized in that, In step seven, the relationship between the sensing signal based on Lorentz fitting and the central wavelength shift is expressed as: (8)， Among them, , is the central reflection wavelength of the sensing unit, and are the peak powers of the double-wavelength reflected light respectively, d is a system constant, and the central reflection of the fiber grating demodulates the drift amount under the external action, which has a linear relationship with the change amount of the sensing signal; Among them, the sensitivity can be expressed as: (9)， Among them, is and 's wavelength difference, and k and b are system constants; therefore, The larger it is, the higher the sensitivity, but the corresponding dynamic range is smaller.

7. A method for a dual-wavelength hybrid differential demodulation algorithm for improving the sensing accuracy of fiber Bragg gratings, characterized in that, In Step 8, make a unified correction to Formula (3) and Formula (8) so that they are on a straight line, and we can get: (10)。 8. A method for a dual-wavelength hybrid differential demodulation algorithm for improving the sensing accuracy of fiber Bragg gratings, characterized in that, In Step 9, the unified corrected formula used can be expressed as: (11)， where s is the sensitivity of the dual-wavelength difference demodulation based on the Gaussian fitting formula, is the sensitivity of the dual-wavelength difference demodulation based on the Lorentz fitting curve, , is the central wavelength of the fiber Bragg grating; , , , are the threshold wavelengths corresponding to the failure of the Gaussian fitting; When the external environment of the fiber grating changes, the change amount of the fiber grating spectrum center can be demodulated according to the change amount of M, thereby obtaining the change amount of the external environment.

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