Multi-wavelength differential alignment distributed sensing device and method based on optical frequency comb

Abstract

The invention discloses a multi-wavelength differential alignment distributed sensing device and method based on an optical frequency comb, the sensing device comprises a quasi-distributed sensing unit, an optical frequency comb light source, a three-port circulator and a signal detection and demodulation unit, and the quasi-distributed sensing unit is composed of an optical fiber grating sensor array. The influence of light source intensity fluctuation and wavelength switching process in the traditional method is avoided, and the light frequency comb in the wide spectral range can cover the change of the reflection spectrum of the sensing unit caused by the environment, so that the sensing dynamic range of the system is expanded; and meanwhile, the distributed sensor has the characteristics of accurate spatial positioning and high wavelength resolution, and is convenient for implementing a time division multiplexing / wavelength division multiplexing 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 multi - wavelength differential demodulation quasi - distributed sensing device and method based on an optical frequency comb. 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 of the same wavelength in series. However, this method uses a tunable laser as a light source to demodulate the central wavelength of the fiber grating. The dual - wavelengths generated by its tuning have problems such as inconsistent intensity and complex wavelength switching process, which will introduce additional measurement errors and thus reduce the demodulation accuracy of the central wavelength of the fiber grating. The optical frequency comb has a wide spectral range, and the integrated degree of the micro - cavity optical frequency comb is higher, which can be applied to the demodulation of fiber gratings with miniaturized wide - wavelength demodulation. However, the interval between each wavelength comb tooth of the micro - cavity frequency comb is usually large. Using traditional fiber grating demodulation methods, it is impossible to obtain enough wavelength sampling points, and it is difficult to improve the demodulation accuracy.

[0003] Therefore, it is necessary to develop a multi - wavelength differential demodulation quasi - distributed sensing device and method based on an optical frequency comb to solve the above problems. Summary of the Invention

[0004] The purpose of the present invention is to provide a multi - wavelength differential demodulation quasi - distributed sensing device and method based on an optical frequency comb, which is used to realize the demodulation of fiber gratings with high spatial resolution and high wavelength resolution.

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

[0006] The multi - wavelength differential demodulation quasi - distributed sensing device based on an optical frequency comb includes a quasi - distributed sensing unit, an optical frequency comb light source, a three - port circulator, and a signal detection and demodulation unit. The quasi - distributed sensing unit consists of an array of fiber grating sensors; the signal detection and demodulation unit includes a signal detection module and a signal demodulation processing module. The first port of the three - port circulator is connected to the output end of the optical frequency comb light source, the second port of the three - port circulator is connected to the array of fiber grating sensors; the third port of the three - port circulator is connected to the input port of the signal detection module, and the output port of the signal detection module is connected to the signal demodulation processing module.

[0007] Specifically, the fiber Bragg grating sensor array 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.

[0008] Preferably, the signal detection module can be a spectrometer, or composed of a high-speed, broadband photodetector and a data conversion module; the signal demodulation and processing module includes at least one of a programmable logic device, a digital signal processing chip, and an embedded chip.

[0009] The sensing method of the multi-wavelength differential demodulation quasi-distributed sensing device based on an optical frequency comb includes: using an optical frequency comb with equal frequency intervals, a wide spectral range, and high flatness as the optical frequency comb light source, and inputting it into the quasi-distributed sensing unit composed of a fiber Bragg grating sensor array. The reflection spectra of the quasi-distributed sensing units at different spatial positions will contain multiple wide spectral signals with equal frequency intervals and different wavelength peak powers varying with the fiber Bragg grating reflection spectrum envelope. When the quasi-distributed sensing unit is affected by environmental changes, the central wavelength of the wide spectral signal reflected by the quasi-distributed sensing unit will change accordingly, where the environmental changes include temperature and strain changes; after the reflection spectrum is detected by the signal detection module, any different wavelength difference signals and their corresponding peak powers can be selected and combined multiple times, and sent to the signal demodulation and processing module for signal demodulation. The optical frequency comb with a wide spectral range can cover the changes in the reflection spectrum of the sensing unit caused by the environment.

[0010] Furthermore, the output light of the optical frequency comb appears as picosecond pulsed light in the time domain and as an optical frequency comb with equal frequency intervals and high flatness in the frequency domain. Its comb tooth frequency is expressed as:

[0011] (1)

[0012] Where, represents the optical frequency comb tooth frequency, n represents the nth optical frequency comb tooth starting from the initial frequency, represents the repetition frequency, represents the carrier envelope phase shift frequency or the initial frequency;

[0013] The wavelength corresponding to the frequency can be expressed as:

[0014] (2)

[0015] Where, represents the central wavelength corresponding to the nth optical frequency comb tooth, c represents the speed of light, with a magnitude of approximately 3×10 8 m / s, represents the carrier envelope phase shift frequency or the initial frequency, and n represents the nth optical comb tooth starting from the initial frequency. represents the repetition frequency;

[0016] The interval between the central wavelengths corresponding to two adjacent comb teeth of the optical frequency comb can be expressed as:

[0017] (3).

[0018] Further, looking at the signal reflected back by the fiber grating in the frequency domain, the peak powers corresponding to different wavelengths are I1, I2... In n , where n depends on the number of optical comb teeth; among them, the peak power I of a certain wavelength k can be expressed as:

[0019] 4

[0020] where represents the initial power when the optical frequency comb tooth is incident, k is a natural number, and b is a system constant. represents the central wavelength of the fiber grating, is the full width at half maximum of the grating;

[0021] When demodulating the signal, select a pair of peak powers I m , I n of the reflected optical frequency comb teeth within the spectrum range of the fiber grating, which can be expressed as:

[0022] (5)

[0023] (6)

[0024] From formulas (5) and (6), the sensing signal M can be obtained and expressed as:

[0025] (7)

[0026] where , is the central reflection wavelength of the quasi-distributed sensing unit, I m and I n are the peak powers of the optical frequency teeth. The drift amount demodulated from the reflection center of the fiber grating under external action and the change amount of the sensing signal are linearly related;

[0027] The sensitivity s can be expressed as:

[0028] (8)

[0029] Among them, is the wavelength difference between and Therefore, the larger

[0030] is, the higher the sensitivity is, but the corresponding dynamic range is smaller; First, select a pair of optical frequency comb teeth with a wavelength difference of and send them into the signal demodulation processing module for processing. Then, select another adjacent pair of optical frequency comb teeth where = =

[0031] (9)

[0032] (10)

[0033] where M1 and M2 are the sensing signals when selecting and respectively. , , are the reflected peak powers of the (m + k)-th, m-th, and (m - k)-th optical frequency comb teeth respectively; represents the full width at half maximum of the grating; represents the wavelength difference of the selected dual-wavelength optical frequency comb teeth; ; ; represents the central wavelength of the fiber grating;

[0034] By making a unified correction of the constant terms for formulas (9) and (10) to make them on a straight line, we can obtain:

[0035] (11)

[0036] This can improve the dynamic range and demodulation accuracy of the wavelength demodulation of the device.

[0037] Furthermore, the optical frequency comb signal output is a short pulse in the time domain. The time t for the optical pulse to reach the signal detection module after reflecting back and forth through the optical path at different positions of the fiber grating can be used to locate the fiber grating, and its position z can be expressed as:

[0038] (12)

[0039] where c is the speed of light in a vacuum and n is the refractive index of the optical fiber.

[0040] In summary, due to the adoption of the above technical solutions, the beneficial effects of the present invention are as follows:

[0041] 1. Compared with traditional detection and reflection, an optical frequency comb is used as the light source, which has a lower cost. Moreover, the differential method is adopted for demodulation, which can effectively resist interference, improve the accuracy of the system, and the sensitivity and dynamic range are adjustable, making this solution more flexible.

[0042] 2. Compared with traditional dual-wavelength demodulation, the optical frequency comb can correct and unify signals with the same wavelength difference, which can improve the dynamic range of the system.

[0043] 3. The spectral range of the frequency comb is wide, and the integration level of the microcavity optical frequency comb is higher, which can be applied to the demodulation of fiber Bragg gratings for miniaturized wide-wavelength demodulation. However, the interval between each wavelength comb tooth of the microcavity frequency comb is usually large, and it is difficult to obtain enough wavelength sampling points using traditional fiber Bragg grating demodulation methods, making it difficult to improve the demodulation accuracy. By using the dual-wavelength demodulation method, high-precision wavelength demodulation of fiber Bragg gratings can be achieved using two comb teeth in the microcavity frequency comb, with a small amount of calculation, which can promote the development of integrated large-capacity fiber Bragg grating demodulators. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the 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 therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings, where:

[0045] Figure 1 is a schematic structural diagram of a multi-wavelength differential demodulation quasi-distributed sensing device based on an optical frequency comb in the present invention;

[0046] Figure 2 In it, a is a schematic diagram of the reflectivity changing with wavelength under multi-wavelength reflection, and b is a schematic diagram of the demodulation signal changing with the offset of the central wavelength.

[0047] The labels in the figure are: 1 - optical frequency comb light source, 2 - three-port circulator, 3 - fiber Bragg grating sensor array, 4 - signal detection module, 5 - signal demodulation processing module. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0048] 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 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.

[0049] 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, such that a process, method, article or device comprising a series of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article or device. Without further limitation, an element defined by the phrase "comprising an..." does not exclude the presence of additional identical elements in the process, method, article or device comprising the element.

[0050] The features and performance of the present invention will be further described in detail below in conjunction with embodiments.

[0051] As Figure 1 shown, a multi-wavelength differential demodulation calibration distributed sensing device based on an optical frequency comb includes a quasi-distributed sensing unit, an optical frequency comb light source 1, a three-port circulator 2, and a signal detection and demodulation unit. The quasi-distributed sensing unit is composed of an optical fiber grating sensor array 3; the signal detection and demodulation unit includes a signal detection module 4 and a signal demodulation processing module 5. The first port of the three-port circulator 2 is connected to the output end of the optical frequency comb light source 1, and the second port of the three-port circulator 2 is connected to the optical fiber grating sensor array 3; the third port of the three-port circulator 2 is connected to the input port of the signal detection module 4, and the output port of the signal detection module 4 is connected to the signal demodulation processing module 5. The optical frequency comb light source is incident on the optical fiber grating sensor array, and after being reflected by the optical fiber grating array, it is input into the signal detection module. When incident, the peak power of each optical frequency comb tooth is I0; the quasi-distributed sensing unit converts the change in the environmental physical quantity at different spatial positions into the change in the reflection peak power of the optical fiber grating; the signal detection and demodulation unit detects and demodulates the output signal of the quasi-distributed sensing unit to obtain the change in the environmental physical quantity.

[0052] The optical fiber grating sensor array 3 is a single Gaussian optical fiber grating, or a series structure composed of multiple Gaussian optical fiber gratings with the same central wavelength, or a parallel structure composed of multiple Gaussian optical fiber gratings with the same central wavelength, or a series-parallel structure composed of multiple Gaussian optical fiber gratings with the same central wavelength.

[0053] The signal detection module 4 can be a spectrometer, or a combination of a high-speed, broadband photodetector and a data conversion module; the signal demodulation processing module 5 includes at least one of a programmable logic device, a digital signal processing chip, and an embedded chip.

[0054] The sensing method of a multi-wavelength differential demodulation calibrated distributed sensing device based on an optical frequency comb includes: using an optical frequency comb with equal frequency intervals, a wide spectral range, and high flatness as the optical frequency comb light source 1 and inputting it into a quasi-distributed sensing unit composed of a fiber grating sensor array 3. The reflection spectra of the quasi-distributed sensing units at different spatial positions will contain multiple wide spectral signals with equal frequency intervals and different peak powers of different wavelengths varying with the envelope of the fiber grating reflection spectrum. When the quasi-distributed sensing unit is affected by environmental changes, the central wavelength of the wide spectral signal reflected by the quasi-distributed sensing unit will change accordingly, where the environmental changes include temperature and strain changes. After the reflection spectrum is detected by the signal detection module 4, any different wavelength difference signals and their corresponding peak powers can be selected and combined multiple times and sent to the signal demodulation processing module 5 for signal demodulation. The optical frequency comb with a wide spectral range can cover the changes in the reflection spectrum of the sensing unit caused by the environment.

[0055] The output light of the optical frequency comb appears as picosecond pulsed light in the time domain and as an optical frequency comb tooth with equal frequency intervals and high flatness in the frequency domain. Its comb tooth frequency is expressed as:

[0056] (1)

[0057] Where represents the optical comb tooth frequency, n represents the nth optical comb tooth starting from the initial frequency, represents the repetition frequency, represents the carrier envelope phase shift frequency or the initial frequency;

[0058] The wavelength corresponding to the frequency can be expressed as:

[0059] (2)

[0060] Where represents the central wavelength corresponding to the nth optical frequency comb tooth, c represents the speed of light, with a magnitude of approximately 3×10 8 m / s, represents the carrier envelope phase shift frequency or the initial frequency, n represents the nth optical comb tooth starting from the initial frequency, represents the repetition frequency;

[0061] The interval between the central wavelengths corresponding to two adjacent comb teeth of the optical frequency comb can be expressed as:

[0062] (3).

[0063] Looking at the signal reflected back by the fiber grating in the frequency domain, the peak powers corresponding to different wavelengths are I1, I2... I n , n depends on the number of optical comb teeth; where, a certain wavelength The peak power I k can be expressed as:

[0064] (4)

[0065] Wherein, represents the initial power when the optical frequency comb teeth are incident, k is a natural number, and b is a system constant, represents the central wavelength of the fiber grating, is the full width at half maximum of the grating;

[0066] When signal demodulation is performed, a pair of peak powers I m , I n of the reflected optical frequency comb teeth within the fiber grating spectrum range are selected, and they can be respectively expressed as:

[0067] (5)

[0068] (6)

[0069] From formula (5) and formula (6), the sensing signal M can be obtained and expressed as:

[0070] (7)

[0071] Wherein, , is the central reflection wavelength of the quasi-distributed sensing unit, I m and I n are the peak powers of the optical frequency teeth, and the reflection center of the fiber grating has a linear relationship with the change amount of the sensing signal ;

[0072] The sensitivity s can be expressed as:

[0073] (8)

[0074] Wherein, is and 's wavelength difference; therefore, the larger it is, the higher the sensitivity, but the corresponding dynamic range is smaller;

[0075] can be shown as in Figure 2 a. First, a pair of optical frequency comb teeth with a wavelength difference of are selected and sent to the signal demodulation processing module 5 for processing, and then another adjacent pair of optical frequency comb teeth is selected, where = = After taking the logarithmic differences of them respectively, the obtained results can be expressed as follows:

[0076] (9)

[0077] (10)

[0078] where M1 and M2 are the sensing signals when selecting and respectively, , , are the reflection peak powers of the m + k-th, m-th, and m - k-th optical frequency comb teeth respectively; represents the full width at half maximum of the grating; represents the wavelength difference of the selected dual-wavelength optical frequency comb teeth; ; ; represents the central wavelength of the fiber grating;

[0079] By making a unified correction to the constant terms of formulas (9) and (10) to make them on a straight line, we can obtain:

[0080] (11)

[0081] As shown in Figure 2 b, this method broadens the dynamic range of the system, increases the allowable drift of the fiber grating, and increases the range of environmental variables that can be measured, thereby improving the dynamic range and demodulation accuracy of the wavelength demodulation of the device.

[0082] The output of the optical frequency comb signal is a short pulse in the time domain. The time t for the optical pulse to reach the signal detection module after reflecting back and forth through the optical path from different-position fiber gratings can be used to locate the fiber grating, and its position z can be expressed as:

[0083] (12)

[0084] where c is the speed of light in a vacuum and n is the refractive index of the optical fiber.

[0085] According to the change amount of the measurement signal, the drift amount of the reflection center wavelength of the fiber grating sensor array 3 is demodulated, and based on the demodulated drift amount of the reflection center wavelength of the fiber grating sensor array 3, the physical quantity of the fiber grating sensor array 3 is accurately measured.

[0086] Through the above steps, due to the characteristics of the optical frequency comb with equal spacing and equal intensity, the influence of the wavelength switching process and the intensity fluctuation of the light source is avoided, making this method have a high demodulation accuracy, and enabling the dynamic range and sensitivity of the system to be freely adjusted, improving the flexibility and precision of the system. Moreover, the dynamic range of the system can be improved by correcting and unifying the dual-wavelength signals with equal wavelength differences. The miniaturization and integration of the system are realized.

[0087] The above are only the preferred embodiments of the present invention and are 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 principles of the present invention shall be included in the protection scope of the present invention.

Claims

1. A multi-wavelength differential demodulation and calibration distributed sensing device based on an optical frequency comb, characterized in that, It includes a quasi-distributed sensing unit, an optical frequency comb light source (1), a three-port circulator (2), and a signal detection and demodulation unit. The quasi-distributed sensing unit consists of an optical fiber grating sensor array (3); the signal detection and demodulation unit includes a signal detection module (4) and a signal demodulation and processing module (5). The first port of the three-port circulator (2) is connected to the output end of the optical frequency comb light source (1), the second port of the three-port circulator (2) is connected to the optical fiber grating sensor array (3); the third port of the three-port circulator (2) is connected to the input port of the signal detection module (4), and the output port of the signal detection module (4) is connected to the signal demodulation and processing module (5).

2. The multi-wavelength differential demodulation calibration distributed sensing device based on an optical frequency comb according to claim 1, wherein The optical fiber grating sensor array (3) is a single Gaussian optical fiber grating, or a series structure composed of multiple Gaussian optical fiber gratings with the same central wavelength, or a parallel structure composed of multiple Gaussian optical fiber gratings with the same central wavelength, or a series-parallel structure composed of multiple Gaussian optical fiber gratings with the same central wavelength.

3. The multi-wavelength differential demodulation and calibration distributed sensing device based on an optical frequency comb according to claim 1, characterized in that: The signal detection module (4) can be a spectrometer, or composed of a high-speed, broadband photodetector and a data conversion module; the signal demodulation and processing module (5) includes at least one of a programmable logic device, a digital signal processing chip, and an embedded chip.

4. The sensing method of the multi-wavelength differential demodulation and calibration distributed sensing device based on an optical frequency comb according to any one of claims 1-3, characterized in that, It includes: using an optical frequency comb with equal frequency intervals, a wide spectral range, and high flatness as the optical frequency comb light source (1), and inputting it into the quasi-distributed sensing unit composed of the optical fiber grating sensor array (3). The reflection spectra of the quasi-distributed sensing unit at different spatial positions will contain multiple wide spectral signals with equal frequency intervals and different wavelength peak powers varying with the envelope of the optical fiber grating reflection spectrum. When the quasi-distributed sensing unit is affected by environmental changes, the central wavelength of the wide spectral signal reflected by the quasi-distributed sensing unit will change accordingly, where the environmental changes include temperature and strain changes; after the reflection spectrum is detected by the signal detection module (4), any different wavelength difference signals and their corresponding peak powers can be selected and combined multiple times, and sent to the signal demodulation and processing module (5) for signal demodulation. The optical frequency comb with a wide spectral range can cover the changes in the reflection spectrum of the sensing unit caused by the environment.

5. The sensing method of the multi-wavelength differential demodulation and calibration distributed sensing device based on an optical frequency comb according to claim 4, wherein The output light of the optical frequency comb appears as picosecond pulsed light in the time domain and as an optical frequency comb tooth with equal frequency intervals and high flatness in the frequency domain. Its comb tooth frequency is expressed as: （1） Among them, represents the frequency of the optical comb teeth, and n represents the nth optical comb tooth starting from the initial frequency, represents the repetition frequency, represents the carrier envelope phase shift frequency or the initial frequency; The wavelength corresponding to the frequency can be expressed as: （2） Among them, represents the central wavelength corresponding to the nth optical frequency comb tooth, c represents the speed of light, with a magnitude of approximately 3×10 8 m / s, represents the carrier-envelope phase shift frequency or the initial frequency, n represents the nth optical comb tooth starting from the initial frequency, represents the repetition frequency; The interval between the central wavelengths corresponding to two adjacent comb teeth of the optical frequency comb can be expressed as: （3）。 6. The sensing method of the multi-wavelength differential demodulation calibration distributed sensing device based on an optical frequency comb according to claim 5, characterized in that, From the perspective of the frequency domain, the peak powers corresponding to different wavelengths of the signals reflected by the fiber grating are I1, I2... In respectively, where n depends on the number of teeth of the optical frequency comb. Among them, the peak power I n of a certain wavelength can be expressed as: k ​ （4） Among them, represents the initial power when the optical frequency comb is incident, k is a natural number, and b is a system constant, represents the central wavelength of the fiber grating, is the full width at half maximum of the grating; When demodulating the signal, select a pair of peak powers I of the reflected optical frequency comb teeth within the fiber grating spectrum range m , I n , which can be respectively expressed as: （5） （6） From formula (5) and formula (6), the sensing signal M can be obtained and expressed as: （7） Among them, , is the central reflection wavelength of the quasi-distributed sensing unit, I m and I n are the peak powers of the optical frequency teeth, and the reflection center of the fiber grating The drift amount demodulated under the external action and the change amount of the sensing signal are linearly related; The sensitivity s can be expressed as: （8） Among them, is and the wavelength difference; therefore, the larger it is, the higher the sensitivity, but the corresponding dynamic range is smaller; First, select a pair of optical frequency comb teeth with a wavelength difference of and send them into the signal demodulation processing module (5) for processing. Then, select another adjacent pair of optical frequency comb teeth , where = = . The results obtained after respectively taking the logarithmic difference of them can be expressed as follows: （9） （10） Among them, M1 and M2 are the sensing signals when selecting and respectively, , , are the reflected peak powers of the (m + k)-th, m-th, and (m - k)-th optical frequency comb teeth respectively; represents the full width at half maximum of the grating; represents the wavelength difference of the selected dual-wavelength optical frequency comb teeth; ; ; represents the central wavelength of the fiber grating; By making a unified correction of the constant terms for formula (9) and formula (10) to make them on a straight line, we can get: （11） Thereby, the dynamic range and demodulation accuracy of the wavelength demodulation of the device can be improved.

7. The sensing method of the multi-wavelength differential demodulation and calibration distributed sensing device based on an optical frequency comb according to claim 6, characterized in that, The output of the optical frequency comb signal is a short pulse in the time domain. The time t for the optical pulse to travel back and forth through the optical path and be reflected by the optical fiber gratings at different positions and reach the signal detection module can be used to locate the optical fiber grating, and its position z can be expressed as: （12） Where, c is the speed of light in vacuum, and n is the refractive index of the optical fiber.

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