A Single-Mode Fiber-Based Large Dynamic Strain Distributed Brillouin Optical Time Domain Reflectometer

Abstract

The present invention discloses a single-mode fiber-based large-dynamic-strain distributed Brillouin optical time domain reflectometer, which includes a narrow linewidth laser, an optical amplification unit, a frequency shift unit, a pulse modulation unit, a first filtering unit, a second filtering unit, a photodetector, a third filtering unit, and an electrical signal modulation unit connected in sequence. The advantage of this sensing system is that it can achieve large-range dynamic strain monitoring at low cost. This optical time domain reflectometer has low cost, a simple structure, and can achieve large-range dynamic strain measurement.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber sensing, and specifically to a distributed Brillouin optical time domain reflectometer based on single-mode fiber with large dynamic strain. Background Technique

[0002] The Brillouin optical time domain reflectometer (BOTDR) is a distributed sensing technology based on the self-Brillouin scattering effect. By using the linear dependence of the frequency shift of Brillouin scattering in optical fiber on temperature and strain, continuous measurement of temperature and strain can be achieved. The BOTDR system has the advantages of single-end access and high sensitivity to temperature and strain, and has great application prospects in the structural health monitoring of large-scale infrastructure, such as the health monitoring systems of bridges and tunnels.

[0003] The ramp-assisted technique is a technique proposed to address the issue that the traditional Brillouin distributed sensing system cannot be used for dynamic strain measurement due to its long sampling time. This technique mainly utilizes the approximately linear region of the Brillouin spectral sideband to convert the change in Brillouin frequency shift into a change in Brillouin gain. This technique eliminates the frequency scanning process in the traditional Brillouin distributed sensing system, thereby increasing the sampling frequency of the system and enabling the system to achieve dynamic strain measurement. However, the approximately linear region of the Brillouin gain spectrum of ordinary single-mode fiber is usually only about 30 MHz, and the strain range that can be monitored is very limited, which has certain limitations for the measurement of large-range dynamic strain in real-life applications. Summary of the Invention

[0004] The purpose of the present invention is to provide a distributed Brillouin optical time domain reflectometer based on single-mode fiber with large dynamic strain to address the deficiencies of the prior art. This optical time domain reflectometer is low-cost, simple in structure, and capable of measuring large-range dynamic strain.

[0005] The technical solution for achieving the purpose of the present invention is as follows:

[0006] A distributed Brillouin optical time domain reflectometer based on single-mode fiber with large dynamic strain, comprising a narrowband laser, a first erbium-doped fiber amplifier, and a first fiber coupler connected in sequence, wherein:

[0007] The optical intensity of an output port of the first fiber coupler is 90% of the original laser intensity. This port is connected in sequence to an acousto-optic modulator connected to an arbitrary function signal generator, a second erbium-doped fiber amplifier, a first fiber circulator, and a second fiber circulator. The a end of the first fiber circulator is connected to the second erbium-doped fiber amplifier, the b end is connected to the first fiber Bragg grating, and the c end is connected to the a end of the second fiber circulator. The b end of the second fiber circulator is connected to the test fiber, and the c end of the second fiber circulator is connected to a second fiber coupler. The second fiber coupler is connected in sequence to a photodetector, a frequency equalizer, and an electrical signal modulation unit equipped with an acquisition card and a data processing program. The test fiber is also connected to a large strain application unit;

[0008] The optical intensity of the other output port of the first fiber coupler is 10% of the original laser intensity. This port is connected in sequence to a fiber three-ring polarization controller, an electro-optic modulator connected to a microwave signal generator, a third fiber circulator, a fiber polarization scrambler, and a second fiber coupler. The a end of the third fiber circulator is connected to the electro-optic modulator, the b end is connected to the second fiber Bragg grating, and the c end is connected to the fiber polarization scrambler. The second fiber Bragg grating is externally connected to a displacement platform that applies strain to the second fiber Bragg grating;

[0009] The polarization controller, electro-optic modulator, microwave signal generator that drives the electro-optic modulator, and fiber polarization scrambler constitute a frequency shift unit; the acousto-optic modulator that modulates pulses, the arbitrary function signal generator that drives the acousto-optic modulator, and the second erbium-doped fiber amplifier constitute a pulse modulation unit; the first fiber circulator and the first fiber Bragg grating constitute a first filtering unit; the third fiber circulator, the second fiber Bragg grating, and the displacement platform that applies strain to the second fiber Bragg grating constitute a second filtering unit, and the equalizer constitutes a third filtering unit.

[0010] The splitting ratio of the first fiber coupler is 90%:10%. One path of the 90% light is connected to the optical path of the acousto-optic modulator connected to an arbitrary function signal generator, and one path of the 10% light is connected to the optical path of the acousto-optic modulator connected to an arbitrary function signal generator.

[0011] The splitting ratio of the second fiber coupler is 50%:50%.

[0012] The test fiber is a single-mode fiber.

[0013] The large strain application unit is equipped with a motor with an adjustable movement radius of 0 - 4 cm. The motor is placed in the middle of the test fiber. One end of the motor is fixed to the test fiber to keep the test fiber in a taut state to ensure that the fiber is strained during the large strain stretching process. The other end of the motor is in a state of adjustable movement radius to drive the fiber to move parallel to the ground.

[0014] The second filtering unit filters by applying strain to the fiber Bragg grating to change the transmission spectrum of the fiber Bragg grating, so that the Rayleigh scattered light is exactly outside the transmission spectrum and the Brillouin scattered light is exactly inside the transmission spectrum.

[0015] The equalizer is a frequency equalizer with a linearly varying filtering intensity at different frequencies, that is, the third filtering unit is a band-pass filter with a filtering intensity linearly varying with frequency, so that there is a linear relationship between the Brillouin frequency shift amount and the detection intensity at the location where strain occurs.

[0016] The laser emitted by the narrow linewidth laser is optically amplified by the first erbium-doped fiber amplifier and then divided into two beams of light by the first fiber coupler. The lower beam of light (10%) is connected to the polarization controller in the frequency shifting unit. The frequency spectrum of the output light of the frequency shifting unit contains the frequency of the laser itself and two frequency sidebands. By changing the polarization state of the incident light through the three-ring polarization controller, the energy at the frequency point of the laser itself is transferred to the two frequency sidebands, increasing the energy of the frequency sidebands. Then, the relatively higher-frequency sideband is filtered out by the second filtering unit, and the lower-frequency sideband is left and called the reference light. After entering the fiber polarization scrambler, the output light enters the second fiber coupler from one input port of the second fiber coupler; the upper beam of light (90%) is connected to the pulse modulation unit, modulated into pulsed light and amplified, and then enters the second filtering unit for filtering operation. It is connected through port a of the first fiber circulator. The test fiber is driven by a motor with an adjustable movement frequency and an adjustable movement amplitude through the large strain application unit to make the fiber undergo a large strain movement with an adjustable amplitude. The Brillouin scattered light in the test fiber enters port b of the first fiber circulator and passes through port c of the first fiber circulator and is connected to the second fiber circulator, and then to the second fiber coupler. The two output ports of the second fiber coupler are connected to the input end of the photodetector, and then output from the output end of the photodetector to the input port of the third filtering unit to make the Brillouin frequency shift amount and the detection intensity at the location where strain occurs have a linear relationship, and then enter the electrical signal modulation unit for data acquisition and processing from the output port of the third filtering unit.

[0017] The Brillouin frequency shift caused by strain in the optical fiber is shown in formula (1):

[0018] (1),

[0019] where and respectively represent the effective refractive indices of the i-th incident light mode and the j-th Brillouin scattered light mode, represents the propagation speed of the k-th acoustic mode in the light ray, is the wavelength of the incident light.

[0020] The Brillouin gain is shown in formula (2):

[0021] (2),

[0022] in is the efficiency of three-wave coupling, is the full width at half maximum of the Brillouin gain spectrum.

[0023] The slope-assisted technology of data processing adopted in this technical solution is a technology proposed to solve the problem that the traditional Brillouin distributed sensing system cannot be used for dynamic strain measurement due to the long sampling time of the system. This technical solution uses the approximate linear region of the sideband of the Brillouin gain spectrum and the Brillouin frequency shift to form an approximate linear correspondence, and converts the change of the Brillouin frequency shift into the change of the Brillouin gain. This eliminates the process of using an electro-optical modulator to scan the frequency in the traditional Brillouin distributed sensing system to obtain the Brillouin gain spectrum at the current moment and then find the Brillouin frequency shift at the current moment corresponding to the peak point of the gain spectrum, thereby saving the system sensing time and improving the sampling frequency of the system, so that the system can realize the measurement of dynamic strain.

[0024] The technical solution converts the optical signal of the single-mode sensing optical fiber into an electrical signal and uses a frequency equalizer matched with the sensing optical fiber to artificially change a section of the intensity on the Brillouin gain spectrum through the characteristic that the light intensity changes linearly with the frequency. The electro-optic modulator determines the frequency shift. Due to the external strain of the optical fiber to be measured, the Brillouin frequency shift in the optical fiber changes. At this time, the Brillouin scattered light intensity signal detected at the frequency shift selected by the electro-optic modulator can linearly correspond to the frequency shift of the Brillouin gain spectrum that changes due to the external strain, thereby realizing the linear change between the Brillouin scattered light signal intensity and the Brillouin gain spectrum frequency shift. By using the slope-assisted method in the data processing process, the strain relationship reflected by monitoring the Brillouin frequency shift in the BOTDR system is cleverly converted to the strain relationship reflected by monitoring the change in the intensity of the Brillouin scattered light signal.

[0025] This technical solution measures the large-scale strain in the single-mode optical fiber through the slope-assisted method combined with the Brillouin optical time-domain reflectometer, realizes the large-scale strain sensing on the single-mode optical fiber at a relatively low cost, and effectively solves the difficulties in practical applications such as engineering monitoring. The system realizes the large-scale strain sensing on the single-mode optical fiber at a relatively low cost, and effectively solves the difficulties in practical applications such as engineering monitoring, and effectively solves the difficulties in applying optical fiber sensing in large-scale strain engineering in real life.

[0026] This optical time domain reflectometer has low cost, simple structure and can realize large-scale dynamic strain measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a structural schematic diagram of an embodiment.

[0028] Figure 2 Schematic diagram of the equalizer ramp assist in the embodiment.

[0029] In the figure, 10. Narrowband laser; 11. First erbium-doped fiber amplifier; 12. First fiber coupler; 13. Acousto-optic modulator; 14. Arbitrary function signal generator; 15. Second erbium-doped fiber amplifier; 16. First circulator; 17. First fiber Bragg grating; 18. Second fiber circulator; 19. Test fiber; 20. Polarization controller; 21. Electro-optic modulator; 22. Microwave signal generator; 23. Third fiber circulator; 24. Second fiber Bragg grating; 25. Fiber depolarizer; 26. Second fiber coupler; 27. Photoelectric detector; 28. Frequency equalizer; 29. Electrical signal modulation unit; 30. Large strain application unit. Specific implementation mode

[0030] The content of the present invention will be further described below in conjunction with the accompanying drawings and embodiments, but it is not a limitation to the present invention.

[0031] Embodiment:

[0032] Refer to Figure 1 , a large dynamic strain distributed Brillouin optical time domain reflectometer based on a single-mode fiber, including a narrowband laser 10, a first erbium-doped fiber amplifier 11, and a first fiber coupler 12 connected in sequence, wherein:

[0033] The light intensity of an output port of the first fiber coupler 12 is 90% of the original laser intensity. This port is connected to an acousto-optic modulator 13 connected to an arbitrary function signal generator 14 in sequence, a second erbium-doped fiber amplifier 15, a first circulator 16, and a second fiber circulator 18. The a end of the first circulator 16 is connected to the second erbium-doped fiber amplifier 15, the b end is connected to the first fiber Bragg grating 17, and the c end is connected to the a end of the second fiber circulator 18. The b end of the second fiber circulator 18 is connected to the test fiber 19, and the c end of the second fiber circulator 18 is connected to the second fiber coupler 26. The second fiber coupler 26 is connected to a photoelectric detector 27, a frequency equalizer 28, and an electrical signal modulation unit 29 with an internal acquisition card and data processing program in sequence. The test fiber 19 is also connected to a large strain application unit 30;

[0034] The optical intensity of another output port of the first optical fiber coupler 12 is 10% of the original laser intensity. This port is connected in sequence to an optical fiber three-ring polarization controller 20, an electro-optic modulator 21 connected to a microwave signal generator 22, a third optical fiber circulator 23, an optical fiber depolarizer 25, and a second optical fiber coupler 26. The a end of the third optical fiber circulator 23 is connected to the electro-optic modulator 21, the b end is connected to a second fiber Bragg grating 24, and the c end is connected to the optical fiber depolarizer 25. The second fiber Bragg grating 24 is externally connected to a displacement platform that applies strain to the second fiber Bragg grating.

[0035] In this example, the polarization controller 20, the electro-optic modulator 21, the microwave signal generator 22 that drives the electro-optic modulator 21, and the optical fiber depolarizer 25 constitute a frequency shift unit; the acousto-optic modulator 13 for modulating pulses, any function signal generator 14 that drives the acousto-optic modulator 13, and the second erbium-doped fiber amplifier 15 constitute a pulse modulation unit; the first optical fiber circulator 16 and the first fiber Bragg grating 17 constitute a first filtering unit; the third optical fiber circulator 23, the second fiber Bragg grating 24, and the displacement platform that applies strain to the second fiber Bragg grating constitute a second filtering unit, and the equalizer 28 constitutes a third filtering unit.

[0036] In this example, the splitting ratio of the first optical fiber coupler is 90%:10%. One path of the 90% light is connected to the optical path of the acousto-optic modulator connected to the arbitrary function signal generator, and one path of the 10% light is connected to the optical path of the acousto-optic modulator connected to the arbitrary function signal generator.

[0037] In this example, the splitting ratio of the second optical fiber coupler is 50%:50%.

[0038] In this example, the test optical fiber 19 is a single-mode optical fiber.

[0039] In this example, the large strain application unit 30 is provided with a motor with an adjustable movement radius of 0 - 4 cm. The motor is placed in the middle of the test optical fiber 19. One end of the motor is fixed to the test optical fiber to keep the test optical fiber in a taut state to ensure that the optical fiber is strained during the large strain stretching process. The other end of the motor is in a state of adjustable movement radius to drive the optical fiber to move parallel to the ground.

[0040] In this example, the second filtering unit changes the transmission spectrum of the Bragg grating 24 by applying strain to the Bragg grating 24, so that the Rayleigh scattered light is exactly outside the transmission spectrum and the Brillouin scattered light is exactly inside the transmission spectrum for filtering.

[0041] In this example, the equalizer 28 is a frequency equalizer with a linearly varying filtering intensity at different frequencies, that is, the third filtering unit is a band-pass filter with a filtering intensity linearly varying with frequency, so that there is a linear relationship between the Brillouin frequency shift amount and the detection intensity at the strained location.

[0042] The laser emitted by the narrow linewidth laser 10 is optically amplified by the first erbium-doped fiber amplifier 11 and then divided into two beams of light by the first fiber coupler 12. The lower beam of light (10%) is connected to the polarization controller 20 in the frequency shift unit. The output light of the frequency shift unit contains a frequency spectrum including the frequency of the laser itself and two frequency sidebands. By changing the polarization state of the incident light through the three-ring polarization controller 20, the energy at the frequency point of the laser itself is transferred to the two frequency sidebands to increase the energy of the frequency sidebands. Then, the relatively higher-frequency sideband is filtered out by the second filtering unit, and the lower-frequency sideband, called the reference light, enters the fiber polarization scrambler 25, and the output light enters the second fiber coupler 26 from one input port of the second fiber coupler 26; the upper beam of light (90%) is connected to the pulse modulation unit, modulated into pulsed light and amplified, and then enters the second filtering unit for filtering operation. It is connected through port a of the first fiber circulator 16. The test fiber 19 undergoes a large-strain motion with an adjustable amplitude through the large-strain application unit 30 driven by a motor with an adjustable motion frequency and an adjustable motion amplitude. The Brillouin scattered light in the test fiber 19 enters port b of the first fiber circulator 16 and passes through port c of the first fiber circulator 16 to be connected to the second fiber coupler 26 through the second fiber circulator 18. The two output ports of the second fiber coupler 26 are connected to the input end of the photodetector 27, and then output from the output end of the photodetector 27 to the input port of the third filtering unit. After making the Brillouin frequency shift amount and the detection intensity at the strain location show a linear relationship, it enters the electrical signal modulation unit 29 from the output port of the third filtering unit for data acquisition and processing.

[0043] The Brillouin frequency shift induced by strain in the optical fiber is shown in Equation (1):

[0044] (1),

[0045] where and represent the effective refractive indices of the i-th incident light mode and the j-th Brillouin scattered light mode respectively, represents the propagation speed of the k-th acoustic mode in the optical fiber, is the wavelength of the incident light.

[0046] The Brillouin gain is shown in Equation (2):

[0047] (2),

[0048] where is the efficiency of the three-wave coupling, is the full width at half maximum of the Brillouin gain spectrum.

[0049] Refer to Figure 2In this example, the approximate linear region of the Brillouin gain spectrum sideband and the Brillouin frequency shift are used to form an approximate linear correspondence, and the change in the Brillouin frequency shift is converted into a change in the Brillouin gain. This eliminates the process of using the electro-optic modulator 21 to scan the frequency in the traditional Brillouin distributed sensing system to obtain the Brillouin gain spectrum at the current moment and then find the Brillouin frequency shift at the current moment corresponding to the peak point of the gain spectrum. This saves the system sensing time and thus increases the sampling frequency of the system, enabling the system to measure dynamic strain, thereby reducing costs and simplifying the system structure, and realizing large-scale strain relationship sensing using single-mode optical fibers in various application projects in life.

[0050] In this example, the optical signal of the single-mode sensing optical fiber 19 is converted into an electrical signal, and the frequency equalizer 28 matched with the sensing optical fiber is used to artificially change a section of the intensity on the Brillouin gain spectrum through the characteristic that the light intensity changes linearly with the frequency. The frequency equalizer 28 has the characteristic that the light intensity changes linearly with the frequency, so that a better linear relationship is formed between the intensity and the frequency signal. The electro-optic modulator 21 determines the frequency shift. Due to the external strain of the optical fiber 19 to be tested, the Brillouin frequency shift in the optical fiber changes. At this time, the Brillouin scattered light intensity signal detected at the frequency shift selected by the electro-optic modulator 21 can linearly correspond to the frequency shift of the Brillouin gain spectrum caused by the external strain, thereby realizing the linear change between the Brillouin scattered light signal intensity and the Brillouin gain spectrum frequency shift. By using the slope-assisted method in the data processing process, the strain relationship reflected by monitoring the Brillouin frequency shift in the BOTDR system is cleverly converted to the strain relationship reflected by monitoring the change in the intensity of the Brillouin scattered light signal.

Claims

1. A distributed Brillouin optical time domain reflectometer based on single-mode fiber with large dynamic strain, characterized in that, it includes a narrowband laser, a first erbium-doped fiber amplifier, and a first fiber coupler connected in sequence, where: One output port of the first fiber coupler is connected to an acousto-optic modulator connected in sequence to an arbitrary function signal generator, a second erbium-doped fiber amplifier, a first fiber circulator, and a second fiber circulator. The a end of the first fiber circulator is connected to the second erbium-doped fiber amplifier, the b end is connected to the first fiber Bragg grating, and the c end is connected to the a end of the second fiber circulator. The b end of the second fiber circulator is connected to the test fiber, and the c end of the second fiber circulator is connected to a second fiber coupler. The second fiber coupler is connected in sequence to a photodetector, a frequency equalizer, and an electrical signal modulation unit with an internal acquisition card and data processing program. The test fiber is also connected to a large strain application unit; The other output port of the first fiber coupler is connected to a fiber three-ring polarization controller, an electro-optic modulator connected to a microwave signal generator, a third fiber circulator, a fiber depolarizer, and a second fiber coupler. The a end of the third fiber circulator is connected to the electro-optic modulator, the b end is connected to the second fiber Bragg grating, and the c end is connected to the fiber depolarizer. The second fiber Bragg grating is externally connected to a displacement platform that applies strain to the second fiber Bragg grating; The polarization controller, electro-optic modulator, microwave signal generator that drives the electro-optic modulator, and fiber depolarizer constitute a frequency shift unit; the acousto-optic modulator for modulating pulses, the arbitrary function signal generator that drives the acousto-optic modulator, and the second erbium-doped fiber amplifier constitute a pulse modulation unit; the first fiber circulator and the first fiber Bragg grating constitute a first filtering unit; the third fiber circulator, the second fiber Bragg grating, and the displacement platform that applies strain to the second fiber Bragg grating constitute a second filtering unit, and the equalizer constitutes a third filtering unit; The second filtering unit filters by applying strain to the Bragg grating to change the transmission spectrum of the Bragg grating, so that the Rayleigh scattered light is exactly outside the transmission spectrum and the Brillouin scattered light is exactly inside the transmission spectrum; The equalizer is a frequency equalizer with a linearly varying filtering intensity at different frequencies, that is, the third filtering unit is a band-pass filter with a filtering intensity linearly varying with frequency, so that there is a linear relationship between the Brillouin frequency shift amount and the detection intensity at the location where strain occurs.

2. The distributed Brillouin optical time domain reflectometer based on single-mode fiber with large dynamic strain according to claim 1, characterized in that, The splitting ratio of the first fiber coupler is 90%:10%. One path of the 90% light is connected to the optical path of the acousto-optic modulator connected to an arbitrary function signal generator, and one path of the 10% light is connected to the optical path of the acousto-optic modulator connected to an arbitrary function signal generator.

3. The distributed Brillouin optical time domain reflectometer based on single-mode fiber with large dynamic strain according to claim 1, characterized in that, The splitting ratio of the second fiber coupler is 50%:50%.

4. The distributed Brillouin optical time domain reflectometer based on single-mode fiber with large dynamic strain according to claim 1, characterized in that, The test fiber is a single-mode fiber.

5. The distributed Brillouin optical time domain reflectometer based on single-mode fiber with large dynamic strain according to claim 1, characterized in that, the large strain application unit is provided with a motor with an adjustable movement radius of 0-4 cm. The motor is placed in the middle of the test optical fiber. One end of the motor is fixed to the test optical fiber to keep the test optical fiber in a taut state, and the other end of the motor is in a state of adjustable movement radius to drive the optical fiber to move parallel to the ground.

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