Fast distributed brillouin sensing system and method based on low frequency chirped frequency

Abstract

The application discloses a kind of fast distributed Brillouin sensing system and method based on low frequency agile frequency, and the system includes: narrow linewidth laser, optical fiber coupler, polarization controller, low frequency agile frequency module, semiconductor optical amplifier, field programmable gate array, second doped fiber amplifier, second circulator, single sideband modulator, microwave source, disturbance polarizer, optical isolator, to be measured optical fiber, photodetector and data acquisition module.Low frequency agile frequency module utilizes direct digital frequency synthesizer to drive acoustooptic frequency shifter to carry out laser up-shift frequency modulation, reduce frequency conversion part from 10GHz magnitude to hundred MHz magnitude, greatly reduce the index requirement of equipment needed when system is built.The fast distributed Brillouin sensing system and method based on low frequency agile frequency provided by the application provide a kind of test method with high performance, low cost, flexible use for optical fiber distributed strain measurement.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of optical fiber sensing, and particularly relates to a fast distributed Brillouin sensing system and method based on low-frequency fast frequency agility. BACKGROUND

[0002] As a kind of distributed optical fiber sensing technology, Brillouin optical time domain analysis technology (BOTDA) utilizes optical fiber to perceive external strain and temperature change, and simultaneously measures strain and temperature, has the characteristics of high sensitivity, continuous uninterrupted, long sensing distance, anti-electromagnetic interference, high temperature resistance, corrosion resistance, etc., is suitable for various harsh environments, and has been widely applied in the fields of power cables, oil and gas pipelines, fire monitoring, etc.

[0003] In a traditional BOTDA system, pump light and probe light are input from two ends of an optical fiber, and the two beams of light meet in the optical fiber to cause Brillouin scattering. When the frequency difference between the pump light and the probe light is close to the Brillouin frequency shift of the optical fiber to be measured, the energy of the pump light is transferred to the probe light, which is called Brillouin amplification. By continuously adjusting the frequency of the probe light, the Brillouin gain spectrum of the optical fiber is obtained, and the center frequency of the Brillouin gain spectrum is the Brillouin frequency shift of the optical fiber to be measured. The strain change of the optical fiber changes the speed of the acoustic wave in the optical fiber, and further changes the size of the Brillouin frequency shift, so the strain change of the optical fiber to be measured can be obtained by measuring the Brillouin frequency shift. Therefore, by detecting the intensity of the probe light at different frequencies, the Brillouin gain spectrum of the whole optical fiber to be measured is reconstructed, and the strain information of each position of the optical fiber is obtained, so that the distributed measurement of the strain of the whole optical fiber can be realized.

[0004] In order to reconstruct the Brillouin gain spectrum of the whole optical fiber to be measured, the BOTDA system usually needs to change the frequency of the probe light by a certain frequency difference. The traditional BOTDA usually has an influence on the number of frequency sweeps of the probe light and the frequency switching speed when performing sensing measurement, and the measurement time is generally tens of seconds to several tens of minutes. Therefore, the system is only suitable for measuring strain and temperature under static or slow change conditions. In some special application occasions, such as dynamic environment monitoring of underwater towed cable, submarine cable laying, etc., the measurement speed of the system has higher requirements.

[0005] In order to improve the dynamic sensing and measuring capability of the system, many schemes for accelerating the scanning speed are proposed. For example, slope auxiliary technology, optical frequency comb technology, fast frequency agility technology, etc. Among them, the slope auxiliary technology regards the slope of the Brillouin gain spectrum as a linear interval, and detects the signal intensity when the slope changes to demodulate the Brillouin frequency shift, which only needs one sweep. However, the Brillouin gain spectrum bandwidth of a single-mode optical fiber is about 40MHz, which limits the dynamic range of the sensing test (strain about 600 ). The optical frequency comb technology is to replace the swept light of different time single frequency with the optical frequency comb of the same time multi-frequency, without scanning, but due to the limitation of the free spectral range between the frequency combs, it is difficult to improve the sensing measurement accuracy. The agile frequency technology is to write dozens of scanning waveforms into an arbitrary waveform generator (AWG) to replace the microwave source for frequency sweeping. The frequency switching time of the AWG is in the order of nanoseconds, compared with the frequency switching time of the microwave source in the order of milliseconds, which greatly reduces the frequency switching time of the frequency sweeping, and at the same time, it can also ensure a large dynamic range, but for a single-mode optical fiber, the Brillouin frequency shift is about 10GHz, and the bandwidth of the AWG should be higher than this number, for a GHz-level large-bandwidth device, the use cost is relatively expensive.

[0006] Therefore, with the continuous improvement of the distributed optical fiber sensing system, in practical application, it is hoped that the use cost of the sensing system can be further reduced under the condition of meeting high resolution, high precision and dynamic range. SUMMARY

[0007] The application provides a fast distributed Brillouin sensing system and method based on low-frequency agile frequency, which solves the above-mentioned technical problems, and specifically adopts the following technical scheme:

[0008] A fast distributed Brillouin sensing system based on low-frequency agile frequency technology, comprising a narrow linewidth laser, an optical fiber coupler, a polarization controller, a low-frequency agile frequency module, a semiconductor optical amplifier, a field programmable gate array, a second doped fiber amplifier, a second circulator, a single sideband modulator, a microwave source, a polarization scrambler, an optical isolator, a fiber to be measured, a photodetector and a data acquisition module;

[0009] The laser emitted by the narrow linewidth laser is divided into two paths through the optical fiber coupler, the first path of light is used to generate pump light, and the second path of light is used to generate probe light;

[0010] The first path of light is output as horizontal polarization light P light through the polarization controller, and the horizontal polarization light P light is output after the low-frequency agile frequency module; the horizontal polarization light P light after the frequency upconversion is input into the semiconductor optical amplifier, and the semiconductor optical amplifier modulates the input horizontal polarization light P light after the frequency upconversion into upconversion pulse light under the control of the field programmable gate array, and the generated upconversion pulse light is amplified into pump light by the second doped fiber amplifier; the pump light is input through the 1 port of the second circulator and output through the 2 port into the fiber to be measured;

[0011] The second path of light enters the single sideband modulator, and the single sideband modulator modulates the input laser into probe light under the control of the microwave source; the probe light enters the fiber to be measured after passing through the polarization scrambler and the optical isolator in turn;

[0012] The pump light and the probe light meet in the optical fiber to be measured to cause Brillouin scattering, when the frequency difference between the pump light and the probe light satisfies the Brillouin amplification condition, the energy of the pump light is transferred to the probe light, the probe light intensity is output from the 3rd port of the second circulator after being amplified by Brillouin, is converted into an electric signal by a photoelectric detector, and is received by a data acquisition module.

[0013] In an embodiment, the low-frequency agile frequency module comprises a first circulator, a 0-degree polarization beam splitter, an acousto-optic frequency shifter, a direct digital frequency synthesizer, a Faraday rotator, a polarization maintaining reflector and a first doped fiber amplifier;

[0014] The horizontal polarization light P output by the polarization controller enters the low-frequency agile frequency module through the 1st port of the first circulator, and then sequentially passes through the 0-degree polarization beam splitter for the first time through the P' port input and the P port output, the acousto-optic frequency shifter, is reflected at the Faraday rotator and converted into vertical polarization light S; the reflected vertical polarization light S sequentially passes through the acousto-optic frequency shifter for the second time, passes through the 0-degree polarization beam splitter for the second time through the P port input and the S' port output, and is reflected at the polarization maintaining reflector without changing the polarization direction; the reflected vertical polarization light S sequentially passes through the 0-degree polarization beam splitter for the third time through the S' port input and the P port output, passes through the acousto-optic frequency shifter for the third time, is reflected at the Faraday rotator and converted into horizontal polarization light P; the reflected horizontal polarization light P sequentially passes through the acousto-optic frequency shifter for the fourth time, passes through the 0-degree polarization beam splitter for the fourth time through the P port input and the P' port output, and enters the first doped fiber amplifier through the 3rd port of the first circulator;

[0015] Under the modulation of the direct digital frequency synthesizer to the acousto-optic frequency shifter, the laser is up-converted once every time it passes through the acousto-optic frequency shifter, and the horizontal polarization light P which is up-converted four times is output from the low-frequency agile frequency module after being amplified by the first doped fiber amplifier.

[0016] The low-frequency agile frequency module of the application utilizes the digital frequency synthesizer DDS to drive the acousto-optic frequency shifter to perform laser up-conversion modulation, reduces the frequency conversion part from the order of 10GHz to the order of hundreds of MHz, and greatly reduces the index requirements of the equipment required when the system is built.

[0017] In an embodiment, the field programmable gate array outputs two trigger signals, one of which is received by the direct digital frequency synthesizer, and the other of which is received by the data acquisition module;

[0018] The direct digital frequency synthesizer outputs a trigger signal, which is received by the microwave source;

[0019] The rising edges of the respective trigger signals of the direct digital frequency synthesizer and the field programmable gate array are aligned with the starting ends of the respective modulation signal periods.

[0020] The trigger signal and the modulation signal of the direct digital frequency synthesizer and the field programmable gate array are clocked synchronously with the modulation signal of the microwave source.

[0021] In an embodiment, all the fiber devices in the low-frequency agile module are polarization-maintaining fiber devices.

[0022] In an embodiment, the direct digital frequency synthesizer outputs a sinusoidal modulation signal, whose modulation frequency is a wave train with a frequency that changes in steps over time, :

[0023] ;

[0024] wherein, is the initial frequency of the wave train, is the step size of the wave train, is the scanning period of the sinusoidal modulation signal;

[0025] The horizontal polarization light P light output by the polarization controller becomes a frequency-sweeping light with a period of after passing through the low-frequency agile module, and the frequency of the frequency-sweeping light is modulated as:

[0026] ;

[0027] wherein, is the frequency of the laser emitted by the narrow-linewidth laser (1).

[0028] In an embodiment, the field programmable gate array outputs a pulse modulation signal, and the pulse modulation repetition period is:

[0029] ;

[0030] wherein, represents the speed of light in the fiber, is the length of the fiber to be measured;

[0031] The relationship between the scanning period of the sinusoidal modulation signal and the pulse modulation repetition period is:

[0032] ;

[0033] wherein, is the average number of times of data collected by the data acquisition module. ​​​

[0034] In an embodiment, the microwave source outputs a radio frequency modulation signal, the frequency of which is a wave train with a frequency step change:

[0035] ;

[0036] ;

[0037] ;

[0038] wherein, is the Brillouin frequency shift of the optical fiber to be measured in a free state, is the maximum negative strain of the optical fiber to be measured, is the maximum positive strain of the optical fiber to be measured, is the period of the radio frequency modulation signal, is the number of radio frequency step changes, is the maximum value of the modulation frequency range interval of the direct digital frequency synthesizer, is the minimum value of the modulation frequency range interval of the direct digital frequency synthesizer;

[0039] The frequency of the probe light is modulated by a single sideband modulator to be:

[0040] ;

[0041] When , the microwave source (10) outputs a radio frequency modulation signal with a fixed frequency;

[0042] The period of the radio frequency modulation signal is related to the scanning period of the sinusoidal modulation signal as follows:

[0043] .

[0044] In an embodiment, the fast distributed Brillouin sensing system based on low-frequency agile frequency technology further comprises a third circulator and an optical fiber grating filter.

[0045] The probe light intensity is output from the 3-port of the second circulator after being Brillouin amplified, enters the optical fiber grating filter through the third circulator, and the lower sideband is filtered out, then enters the photodetector again through the third circulator to be converted into an electrical signal, which is received by the data acquisition module.

[0046] The application also provides the application of the fast distributed Brillouin sensing system based on low-frequency agile frequency technology in optical fiber strain measurement.

[0047] As a general inventive concept, the present application also provides a method for fiber strain measurement based on the above-mentioned fast distributed Brillouin sensing system based on low-frequency agile frequency technology, comprising:

[0048] The laser emitted by the narrow-linewidth laser is split into two paths by the optical coupler, the first path of light is used to generate pump light, and the second path of light is used to generate probe light;

[0049] The first path of light is output as horizontal polarization light P light by the polarization controller, and the horizontal polarization light P light is output after frequency upconversion by the low-frequency agile frequency module; the horizontal polarization light P light after frequency upconversion is input into a semiconductor optical amplifier, and the semiconductor optical amplifier modulates the input horizontal polarization light P light after frequency upconversion into upconverted pulse light under the control of a trigger signal output by a field programmable gate array; the generated upconverted pulse light is amplified into pump light by a second doped fiber amplifier; the pump light is input into the to-be-measured optical fiber through the 1 port of the second circulator.

[0050] The second path of light enters a single sideband modulator, and the single sideband modulator modulates the input laser into probe light under the control of a microwave source; the probe light enters the to-be-measured optical fiber after passing through a depolarizer and an optical isolator in sequence.

[0051] The pump light and the probe light meet in the to-be-measured optical fiber, and Brillouin scattering occurs; when the frequency difference between the pump light and the probe light satisfies the Brillouin amplification condition, the energy of the pump light is transferred to the probe light, and the probe light is output from the 3 port of the second circulator after being amplified by Brillouin, and is converted into an electrical signal by a photodetector and received by a data acquisition module.

[0052] The time when the signal is received by the data acquisition module is used to determine the specific position of the to-be-measured optical fiber where the stimulated Brillouin effect acts; after all the scanning periods at one frequency are completed, the Brillouin gain value of the to-be-measured optical fiber at each spatial position at the frequency is obtained; after all the frequencies are scanned, the received data is averaged multiple times, wavelet transformed to reduce noise, and the highest peak is found through Lorentz fitting to obtain the Brillouin frequency shift of the to-be-measured optical fiber at each spatial position.

[0053] According to the relationship between the Brillouin frequency shift and the strain, the strain of the to-be-measured optical fiber at each spatial position under the current state is measured.

[0054] ;

[0055] wherein, represents the strain change sensitivity coefficient, is the Brillouin frequency shift of the to-be-measured optical fiber in a free state.

[0056] ​​The application has the advantages that the provided fast distributed Brillouin sensing system based on low-frequency agile frequency conversion technology provides a test method with high performance, low cost and flexible use for optical fiber distributed strain measurement.

[0057] The application adopts low-frequency agile frequency conversion technology, reduces the frequency conversion part from 10GHz to hundreds of MHz, greatly reduces the index requirements of the equipment needed when building the system, and reduces the system cost.

[0058] Unlike the traditional Mach-Zehnder modulator (MZM) after frequency modulation, the carrier and upper and lower sidebands are difficult to separate, which causes the problem of signal-to-noise ratio reduction. The application uses an acousto-optic frequency shifter as an optical frequency up-conversion device in low-frequency agile frequency conversion, and uses a direct digital frequency synthesizer as a sine signal generation device. The signal is converted into an analog waveform output through analog-to-digital conversion and a low-pass filter. Due to the open-loop structure, the frequency switching time is extremely short. The low-frequency agile frequency conversion module built in this way has the advantages of high frequency resolution, short frequency switching time, flexible modulation, low cost, strong stability and the like. BRIEF DESCRIPTION OF DRAWINGS

[0059] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the embodiment or prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0060] Figure 1 The structure diagram of a fast distributed Brillouin sensing system based on low-frequency agile frequency conversion technology of the present application;

[0061] Figure 2 The structure and optical propagation path diagram of the low-frequency agile frequency conversion module of the present application;

[0062] Figure 3 The frequency diagram of the trigger signal, pump light and probe light of the present application;

[0063] Figure 4 The frequency relationship diagram of the pump light and the probe light of the present application;

[0064] Narrow linewidth laser 1, fiber coupler 2, polarization controller 3, low frequency agile frequency module 4, semiconductor optical amplifier 5, field programmable gate array 6, second doped fiber amplifier 7, second circulator 8, single sideband modulator 9, microwave source 10, polarization scrambler 11, optical isolator 12, optical fiber to be measured 13, third circulator 14, fiber grating filter 15, photodetector 16, data acquisition module 17. DETAILED DESCRIPTION

[0065] The embodiments of the present application are described in detail below with reference to the accompanying drawings. The embodiments described below are examples for explaining the present application and should not be construed as limiting the present application.

[0066] Referring to Figure 1 The fast distributed Brillouin sensing system based on low frequency agile frequency technology of the present embodiment includes a narrow linewidth laser 1 (DFB in the figure), a fiber coupler 2, a polarization controller 3 (PC in the figure), a low frequency agile frequency module 4, a semiconductor optical amplifier 5 (SOA in the figure), a field programmable gate array 6 (FPGA in the figure), a second doped fiber amplifier 7 (EDFA2 in the figure), a second circulator 8 (CIR2 in the figure), a single sideband modulator 9 (SSBM in the figure), a microwave source 10, a polarization scrambler 11 (PS in the figure), an optical isolator 12 (ISO in the figure), an optical fiber to be measured 13 (FUT in the figure), a third circulator 14 (CIR3 in the figure), a fiber grating filter 15 (FBG in the figure), a photodetector 16 (PD in the figure) and a data acquisition module 17.

[0067] The laser emitted by the narrow linewidth laser 1 is divided into two paths with the same power after passing through the 50:50 fiber coupler 2, the first path of light is used to generate pump light, and the second path of light is used to generate probe light.

[0068] The first path of light is output as horizontal polarization light P light by the polarization controller 3, and the horizontal polarization light P light is output after the low frequency agile frequency module 4 is up-converted; the up-converted horizontal polarization light P light passes through the semiconductor optical amplifier 5, and the semiconductor optical amplifier 5 is controlled by the trigger signal output by the field programmable gate array 6 to modulate the input up-converted horizontal polarization light P light into up-converted pulsed light, and the generated up-converted pulsed light is amplified by the second doped fiber amplifier 7, and the amplified up-converted pulsed light is pump light; the pump light is input through the 1 port of the second circulator 8 and output through the 2 port into the optical fiber to be measured 13.

[0069] Referring to Figure 2The low-frequency agile frequency conversion module 4 includes a first circulator 401 (CIR1 in the figure), a 0° polarization beam splitter 402 (PBS in the figure), an acousto-optic frequency shifter 403 (AOF in the figure), a direct digital frequency synthesizer 404 (DDS in the figure), a Faraday rotator 405 (FRM in the figure), a polarization maintaining reflector 406 (PRM in the figure), and a first doped fiber amplifier 407 (EDFA1 in the figure). The fiber devices in the low-frequency agile frequency conversion module 4 are all polarization maintaining fiber devices.

[0070] The propagation path of the laser in the low-frequency agile frequency conversion module 4 is as follows:

[0071] 1) The horizontal polarization light P output by the polarization controller 3 enters the low-frequency agile frequency conversion module 4 through the 1 port of the first circulator 401, and then sequentially passes through the 0° polarization beam splitter 402 for the first time through the P' port input and the P port output, passes through the acousto-optic frequency shifter 403 for the first time, is reflected at the Faraday rotator 405, and converts the horizontal polarization light P into vertical polarization light S;

[0072] 2) The reflected vertical polarization light S sequentially passes through the acousto-optic frequency shifter 403 for the second time, passes through the 0° polarization beam splitter 402 for the second time through the P port input and the S' port output, and is reflected at the polarization maintaining reflector 406 without changing the polarization direction;

[0073] 3) The reflected vertical polarization light S sequentially passes through the 0° polarization beam splitter 402 for the third time through the S' port input and the P port output, passes through the acousto-optic frequency shifter 403 for the third time, is reflected at the Faraday rotator 405, and converts the vertical polarization light S into horizontal polarization light P;

[0074] 4) The reflected horizontal polarization light P sequentially passes through the acousto-optic frequency shifter 403 for the fourth time, passes through the 0° polarization beam splitter 402 for the fourth time through the P port input and the P' port output, and enters the first doped fiber amplifier 407 through the 3 port of the first circulator 401.

[0075] Under the modulation of the direct digital frequency synthesizer 404 to the acousto-optic frequency shifter 403, the laser is up-converted once every time it passes through the acousto-optic frequency shifter 403, and the horizontal polarization light P that is up-converted four times is amplified by the first doped fiber amplifier 407 and then output from the low-frequency agile frequency conversion module 4.

[0076] The direct digital frequency synthesizer 404 outputs a sinusoidal modulation signal, and the modulation frequency is a wave train with a frequency that changes in steps over time, : :

[0077] ;

[0078] wherein, is the initial frequency of the wave train, is the step size of the wave train, is the scanning period of the sinusoidal modulation signal;

[0079] The horizontal polarized light P light output by the polarization controller 3 becomes the sweep frequency light with the period of the frequency step changes over time The frequency

[0080] ;

[0081] wherein, is the frequency of the laser light emitted by the narrow linewidth laser 1.

[0082] The field programmable gate array 6 outputs the pulse modulation signal, and the pulse modulation repetition period is:

[0083] ;

[0084] wherein, represents the speed of light in the optical fiber, which is about , is the length of the optical fiber to be measured 13.

[0085] The second light enters the single sideband modulator 9, which modulates the input laser light under the control of the microwave source 10 to obtain the probe light. The probe light enters the optical fiber to be measured 13 after passing through the polarization scrambler 11 and the optical isolator 12 in turn.

[0086] The microwave source 10 outputs the radio frequency modulation signal, and the frequency is a wave train with frequency step changes:

[0087] ;

[0088] ;

[0089] ;

[0090] wherein, is the Brillouin frequency shift of the optical fiber to be measured 13 in the free state, is the maximum negative strain of the optical fiber to be measured 13, is the maximum positive strain of the optical fiber to be measured 13, is the period of the radio frequency modulation signal, is the number of radio frequency step changes, The maximum value of the modulation frequency range interval of the direct digital frequency synthesizer 404, The minimum value of the modulation frequency range interval of the direct digital frequency synthesizer 404.

[0091] The frequency of the probe light Is modulated by the single sideband modulator 9 to be:

[0092] ;

[0093] When The microwave source 10 outputs the radio frequency modulation signal frequency as a fixed value.

[0094] The field programmable gate array 6 outputs two trigger signals in addition to the output pulse modulation signal, one of which is received by the direct digital frequency synthesizer 404 to control when to switch to the next scanning frequency; the other is received by the data acquisition module 17 to trigger the start time of the signal acquisition in each pulse modulation repetition period. The direct digital frequency synthesizer 404 outputs a trigger signal to the microwave source 10 in addition to the sinusoidal modulation signal to expand the sensing dynamic range. The rising edge of the trigger signal of the direct digital frequency synthesizer 404 and the field programmable gate array 6 is aligned with the start of the corresponding modulation signal period. The trigger signal and the frequencies of the pump light and the probe light are shown in Figure 3 .

[0095] The relationship between the sinusoidal modulation signal scanning period And the pulse modulation repetition period Is:

[0096] ;

[0097] Wherein, The average number of data collected by the data acquisition module 17.

[0098] The relationship between the radio frequency modulation signal period And the sinusoidal modulation signal scanning period Is:

[0099] .

[0100] The trigger signal and the modulation signal of the direct digital frequency synthesizer 404 and the field programmable gate array 6 and the modulation signal clock of the microwave source 10 are synchronized.

[0101] The relationship between the frequencies of the pump light and the probe light is as follows: Figure 4As shown, the pump light and the probe light meet in the fiber under test 13 and Brillouin scattering occurs. When the frequency difference between the pump light and the probe light satisfies the Brillouin amplification condition (i.e., the frequency difference between the pump light and the probe light is close to the Brillouin frequency shift of the fiber under test 13), the energy of the pump light is transferred to the probe light. The intensity of the probe light is amplified by Brillouin and output from port 3 of the second circulator 8. After passing through the third circulator 14 and entering the fiber grating filter 15, the lower sideband filtered out is passed through the third circulator 14 again and enters the photodetector 16 to be converted into an electrical signal, which is received by the data acquisition module 17.

[0102] As an example, a 2km single-mode fiber with a Brillouin frequency shift around 10.86GHz was used as the fiber under test (13) for testing. In the pump optical path, a field-programmable gate array (FPGA) 6 outputs a periodicity. 25 A pulse signal with a pulse width of 20ns; the acousto-optic frequency shifter 403 has a center frequency of 200MHz, and the direct digital frequency synthesizer 404... =175MHz~ =225MHz range Frequency sweeping is performed with a step size of 1MHz; therefore, the pump light is modulated to a repetition frequency of 40kHz and a pulse width of... The pulsed light is 20 ns in duration. The theoretical spatial resolution of the system is 2 m, and the frequency resolution is 4 MHz. In the continuous optical path, the output frequency of microwave source 10 remains constant at 10.66 GHz. What is the theoretical dynamic range of the system? It is 200MHz.

[0103] The data acquisition module 17 acquires the output sensor signals, and its trigger signal is provided by the field-programmable gate array 6. The sampling rate of the acquisition card follows the Nyquist law. For example, the sampling rate of the acquisition card is 200 MSa / s.

[0104] A segment of the fiber 13 under test is placed tightly against the fiber stretcher, and the system's rapid measurement capability is measured by applying a sinusoidal stretch to the fiber. As an example, a 1Hz sinusoidal signal is applied to the fiber stretcher to test the system's rapid measurement capability.

[0105] The specific location of the stimulated Brillouin effect in the fiber under test 13 is determined based on the time of signal reception received by the data acquisition module 17. The data acquired by the data acquisition module 17 is segmented. After completing all scan cycles at a frequency, the Brillouin gain value at each spatial location of the fiber under test 13 at that frequency is obtained. The above operation is repeated for each frequency. After completing the scan of all frequencies, the received data is denoised by multiple averaging and wavelet transforms, and the highest peak is found by Lorentz fitting to obtain the Brillouin frequency shift at each spatial location of the fiber under test 13. .

[0106] According to the relationship between the Brillouin frequency shift and the strain, the strain of the to-be-measured optical fiber 13 at each spatial position in the current state is completed measurement:

[0107] ;

[0108] wherein, represents the strain change sensitivity coefficient.

[0109] That is, the numerical value of the system at the position where the dynamic signal is applied to the optical fiber is obtained.

[0110] The basic principles, main features and advantages of the present application are shown and described above. Those skilled in the art should understand that the above examples do not limit the present application in any form, and any technical solutions obtained by equivalent replacement or equivalent transformation fall within the protection scope of the present application.

Claims

1. A fast distributed Brillouin sensing system based on low-frequency agile frequency conversion, characterized in that, include: Narrow linewidth laser (1), fiber coupler (2), polarization controller (3), low frequency agile conversion module (4), semiconductor optical amplifier (5), field programmable gate array (6), second erbium-doped fiber amplifier (7), second circulator (8), single sideband modulator (9), microwave source (10), polarization scrambler (11), optical isolator (12), fiber under test (13), photodetector (16), and data acquisition module (17). The laser emitted by the narrow linewidth laser (1) is split into two paths by the fiber coupler (2). The first path is used to generate pump light, and the second path is used to generate probe light. The first light is output as horizontally polarized light P by the polarization controller (3). After passing through the low-frequency fast conversion module (4), it outputs horizontally polarized light P that has undergone up-conversion. The up-converted horizontally polarized light P passes through the semiconductor optical amplifier (5). Under the control of the trigger signal output by the field programmable gate array (6), the semiconductor optical amplifier (5) modulates the input up-converted horizontally polarized light P into up-converted pulse light. The generated up-converted pulse light is amplified into pump light by the second erbium-doped fiber amplifier (7). The pump light is input through port 1 and output through port 2 of the second circulator (8) and enters the fiber under test (13). The second light enters the single-sideband modulator (9), which, under the control of the microwave source (10), down-converts the input laser into a probe light; the probe light passes through the polarizer (11) and the optical isolator (12) in sequence before entering the optical fiber under test (13). When the pump light and the probe light meet in the fiber under test (13), Brillouin scattering occurs. When the frequency difference between the pump light and the probe light satisfies the Brillouin amplification condition, the energy of the pump light is transferred to the probe light. The intensity of the probe light is amplified by Brillouin and output from port 3 of the second circulator (8). It is converted into an electrical signal by the photodetector (16) and received by the data acquisition module (17). The low-frequency agile frequency conversion module (4) includes a first circulator (401), a 0° polarization beam splitter (402), an acousto-optic frequency shifter (403), a direct digital frequency synthesizer (404), a Faraday rotator (405), a polarization-maintaining reflector (406), and a first erbium-doped fiber amplifier (407). The horizontally polarized light P-beam output by the polarization controller (3) enters the low-frequency agile conversion module (4) through port 1 of the first circulator (401), and then passes through the 0° polarization beam splitter (402) for the first time through port 2 of the first circulator (401), with input at port P' and output at port P. It then passes through the acousto-optic frequency shifter (403) for the first time, and is reflected at the Faraday rotator (405), converting the horizontally polarized light P-beam into vertically polarized light S-beam. The reflected vertically polarized light S-beam then passes through the acousto-optic frequency shifter (403) for the second time, and then through the 0° polarization beam splitter (402) for the second time, with input at port P and output at port S'. The polarization direction is reflected at the deflector (406) but not changed; the reflected vertically polarized light S passes through the 0° polarization beam splitter (402) for the third time, with the input at port S' and the output at port P. It passes through the acousto-optic frequency shifter (403) for the third time, and is reflected at the Faraday rotator (405), converting the vertically polarized light S into the horizontally polarized light P. The reflected horizontally polarized light P passes through the acousto-optic frequency shifter (403) for the fourth time, and then through the 0° polarization beam splitter (402) for the fourth time, with the input at port P and the output at port P'. It then enters the first erbium-doped fiber amplifier (407) through port 3 of the first circulator (401). Under the modulation of the acousto-optic frequency shifter (403) by the direct digital frequency synthesizer (404), the laser undergoes an up-conversion once every time it passes through the acousto-optic frequency shifter (403). The horizontally polarized P-light, which undergoes four up-conversions, is amplified by the first erbium-doped fiber amplifier (407) and output to the low-frequency agile frequency conversion module (4).

2. The fast distributed Brillouin sensing system based on low-frequency agile frequency conversion according to claim 1, characterized in that, The field programmable gate array (6) outputs two trigger signals, one of which is received by the direct digital frequency synthesizer (404), and the other is received by the data acquisition module (17); The direct digital frequency synthesizer (404) outputs a trigger signal, which is received by the microwave source (10); The rising edge of the trigger signal of the direct digital frequency synthesizer (404) and the field programmable gate array (6) is aligned with the beginning of the corresponding modulation signal period; The trigger and modulation signals of the direct digital frequency synthesizer (404) and the field programmable gate array (6) are clocked together with the modulation signal of the microwave source (10).

3. The fast distributed Brillouin sensing system based on low-frequency agile frequency conversion according to claim 1, characterized in that, The fiber optic devices in the low-frequency agile frequency conversion module (4) are all polarization-maintaining fiber optic devices.

4. The fast distributed Brillouin sensing system based on low-frequency agile frequency conversion according to claim 1, characterized in that, The direct digital frequency synthesizer (404) outputs a sinusoidal modulated signal with a modulation frequency of for A frequency over time Step-change wave train : ； in, The initial frequency of the wave train. For the step size of the wave train, The scanning period of the sinusoidal modulation signal; The horizontally polarized light P-beam output by the polarization controller (3) is converted into a period of after passing through the low-frequency agile frequency conversion module (4). of A frequency over time The frequency of the sweeping light with a step change Modulated as: ； in, The frequency at which the laser is emitted by the narrow linewidth laser (1).

5. The fast distributed Brillouin sensing system based on low-frequency agile frequency conversion according to claim 4, characterized in that, The field-programmable gate array (6) outputs a pulse modulation signal, and its pulse modulation repetition period is... for: ； in, This represents the speed of light in an optical fiber. The length of the optical fiber (13) to be tested; Sine modulation signal scan period With pulse modulation repetition period The relationship is: ； in, The average number of times the data is collected by the data acquisition module (17).

6. The fast distributed Brillouin sensing system based on low-frequency agile frequency conversion according to claim 4, characterized in that, The microwave source (10) outputs a radio frequency modulated signal, the frequency of which is... for A wave train with a step frequency change: ； ； ； in, The Brillouin frequency shift of the fiber under test (13) in its free state. The maximum negative strain of the fiber under test (13) is given. The maximum normal strain of the fiber under test (13) is given. The period of the radio frequency modulation signal. This represents the number of radio frequency step changes. The maximum value of the modulation frequency range of the direct digital frequency synthesizer (404) is given. This is the minimum value of the modulation frequency range of the direct digital frequency synthesizer (404); Detecting the frequency of light After being modulated by the single-sideband modulator (9), it becomes: ； when At that time, the frequency of the radio frequency modulation signal output by the microwave source (10) is a fixed value; Radio frequency modulation signal period With the scanning period of the sinusoidal modulation signal The relationship is: 。 7. The fast distributed Brillouin sensing system based on low-frequency agile frequency conversion according to claim 1, characterized in that, The low-frequency agile frequency fast distributed Brillouin sensing system also includes a third circulator (14) and a fiber optic grating filter (15). The probe light intensity is amplified by Brillouin and output from port 3 of the second circulator (8). After passing through the third circulator (14) and entering the fiber optic grating filter (15), the lower sideband is filtered out and then passes through the third circulator (14) again. It is then converted into an electrical signal by the photodetector (16) and received by the data acquisition module (17).

8. The application of the low-frequency agile frequency conversion-based fast distributed Brillouin sensing system according to any one of claims 1 to 7 in fiber optic strain measurement.

9. A method for fiber optic strain measurement based on a low-frequency agile, high-speed distributed Brillouin sensing system as described in any one of claims 1 to 7, characterized in that, include: The laser emitted by the narrow linewidth laser (1) is split into two paths by the fiber coupler (2). The first path is used to generate pump light, and the second path is used to generate probe light. The first light is output as horizontally polarized light P by the polarization controller (3). After passing through the low-frequency fast conversion module (4), it outputs horizontally polarized light P that has undergone up-conversion. The up-converted horizontally polarized light P passes through the semiconductor optical amplifier (5). Under the control of the trigger signal output by the field programmable gate array (6), the semiconductor optical amplifier (5) modulates the input up-converted horizontally polarized light P into up-converted pulse light. The generated up-converted pulse light is amplified into pump light by the second erbium-doped fiber amplifier (7). The pump light is input through port 1 and output through port 2 of the second circulator (8) and enters the fiber under test (13). The second light enters the single-sideband modulator (9), which, under the control of the microwave source (10), down-converts the input laser into a probe light; the probe light passes through the polarizer (11) and the optical isolator (12) in sequence before entering the optical fiber under test (13). When the pump light and the probe light meet in the fiber under test (13), Brillouin scattering occurs. When the frequency difference between the pump light and the probe light satisfies the Brillouin amplification condition, the energy of the pump light is transferred to the probe light. The intensity of the probe light is amplified by Brillouin and output from port 3 of the second circulator (8). It is converted into an electrical signal by the photodetector (16) and received by the data acquisition module (17). The specific location of the stimulated Brillouin effect in the fiber under test (13) is determined according to the time when the signal is received by the data acquisition module (17). After all scanning cycles at a frequency are completed, the Brillouin gain value at each spatial location of the fiber under test (13) at that frequency is obtained. After scanning all frequencies, the received data is denoised by multiple averaging and wavelet transforms, and the highest peak is found by Lorentz fitting to obtain the Brillouin frequency shift at each spatial location of the fiber under test (13). ; Based on the relationship between Brillouin frequency shift and strain, the strain of the fiber under test (13) at each spatial location in its current state was determined. Measurement: ； in, Represents the sensitivity coefficient to strain changes. The Brillouin frequency shift of the fiber under test (13) in its free state.

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