CN116086506A - A large dynamic strain distributed Brillouin optical time domain reflectometer based on single-mode fiber - Google Patents

Abstract

The invention discloses a single-mode fiber-based large dynamic strain distributed Brillouin optical time domain reflectometer, which comprises a narrow linewidth laser, an optical amplifying unit, a frequency shifting unit, a pulse modulation unit, a first filtering unit, a second filtering unit, a photoelectric detector, a third filtering unit and an electric signal modulation unit which are sequentially connected. An advantage of such a sensing system is that a large range of dynamic strain monitoring can be achieved at low cost. The optical time domain reflectometer has low cost and simple structure, and can realize large-range dynamic strain measurement.

Description

A distributed Brillouin optical time-domain reflectometer based on large dynamic strain of single-mode optical fiber

Technical Field

The invention relates to the technical field of optical fiber sensing, in particular to a distributed Brillouin optical time domain reflectometer based on single-mode optical fiber large dynamic strain.

Background Art

Fiber Brillouin Optical Time Domain Reflectometer (BOTDR) is a distributed sensing technology based on the spontaneous Brillouin scattering effect. It uses the linear dependence of the frequency shift of Brillouin scattering in the optical fiber on temperature and strain to achieve continuous measurement of temperature and strain. The BOTDR system has the advantages of single-ended access and sensitive response to temperature and strain. It has great application prospects in the structural health monitoring of large-scale infrastructure, such as health monitoring systems for bridges and tunnels.

Slope-assisted technology is a technology proposed based on the fact that the traditional Brillouin distributed sensing system cannot be used for dynamic strain measurement due to the long sampling time of the system. This technology mainly uses the approximate linear region of the Brillouin spectrum sideband to convert the change of Brillouin frequency shift into the change of Brillouin gain. This technology eliminates the frequency sweep process in the traditional Brillouin distributed sensing system, thereby increasing the sampling frequency of the system and enabling the system to measure dynamic strain. However, the approximate linear region of the Brillouin gain spectrum of ordinary single-mode optical fiber is usually only about 30MHz, and the strain range that can be monitored is very limited, which has certain limitations for dynamic strain measurement in a large range of real-life applications.

Summary of the invention

The purpose of the present invention is to provide a large dynamic strain distributed Brillouin optical time domain reflectometer based on single-mode optical fiber to address the deficiencies of the prior art. The optical time domain reflectometer has low cost, simple structure and can realize large-scale dynamic strain measurement.

The technical solution for achieving the purpose of the present invention is:

A single-mode optical fiber large dynamic strain distributed Brillouin optical time domain reflectometer, comprising a narrow-band laser, a first erbium-doped fiber amplifier, and a first optical fiber coupler connected in sequence, wherein:

The light intensity of an output port of the first fiber coupler is 90% of the original laser intensity. This port is sequentially connected 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 a test fiber. The c end of the second fiber circulator is connected to a second fiber coupler. The second fiber coupler is sequentially connected to a photoelectric detector, a frequency equalizer, and an electrical signal modulation unit with an acquisition card and a data processing program. The test fiber is also connected to a large strain application unit.

The light intensity of another output port of the first fiber coupler is 10% of the original laser intensity. This port 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 polarization scrambler, and a second fiber coupler that are sequentially connected. 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.

The polarization controller, the electro-optic modulator, the microwave signal generator for driving the electro-optic modulator, and the optical fiber scrambler constitute a frequency shift unit; the acousto-optic modulator for modulating pulses, the arbitrary function signal generator for driving the acousto-optic modulator, and the second erbium-doped fiber amplifier constitute a pulse modulation unit; the first optical fiber circulator and the first optical fiber Bragg grating constitute a first filtering unit; the third optical fiber circulator, the second optical fiber Bragg grating, and the displacement platform for applying strain to the second optical fiber Bragg grating constitute a second filtering unit, and the equalizer constitutes a third filtering unit.

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

The splitting ratio of the second optical fiber coupler is 50%:50%.

The test optical fiber is a single-mode optical fiber.

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 so that the test optical fiber is 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 an adjustable movement radius to drive the optical fiber to move parallel to the ground.

The second filtering unit applies strain to the Bragg grating to change the transmission spectrum of the Bragg grating, so that the Rayleigh scattered light is just outside the transmission spectrum and the Brillouin scattered light is just within the transmission spectrum for filtering.

The equalizer is a frequency equalizer whose filtering strength varies linearly at different frequencies, that is, the third filtering unit is a bandpass filter whose filtering strength varies linearly with frequency, so that there is a linear relationship between the Brillouin frequency shift at the strain occurrence location and the detection intensity.

The laser emitted by the narrow linewidth laser is amplified by the first erbium-doped fiber amplifier and then divided into two paths by the first fiber coupler. The downlink light (10%) is connected to the polarization controller in the frequency shift unit. The frequency spectrum in the output light of the frequency shift unit contains the frequency of the laser itself and two frequency sidebands. The polarization state of the incident light is changed by the three-ring polarization controller so that 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. The sidebands with relatively high frequency are filtered out by the second filter unit, and the sidebands with lower frequency are left as reference light. After entering the fiber polarization scrambler, the output light enters the second fiber coupler from one end of the input port; the uplink light (90%) is connected to the pulse modulation unit to be modulated into pulse light and amplified. Then it enters the second filtering unit for filtering operation and is connected through the a port of the first optical fiber circulator. The test optical fiber passes through the large strain application unit through a motor with adjustable movement frequency and adjustable movement amplitude to drive the optical fiber to produce an adjustable amplitude large strain movement. The Brillouin scattered light in the test optical fiber enters the b port of the first optical fiber circulator and is connected to the second optical fiber coupler from the c port of the first optical fiber circulator through the second optical fiber circulator. The two output ports of the second optical 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 so that the Brillouin frequency shift at the strain location and the detection intensity are linearly related. Then, it enters the electrical signal modulation unit from the output port of the third filtering unit for data acquisition and processing.

The Brillouin frequency shift induced by strain in optical fiber is shown in formula (1):

（1），

(1),

和

分别表示第i个入射光模式和第j个布里渊散射光模式的有效折射率，

表示光线内第k个声学模式的传播速度，

为入射光的波长。in

and

denote the effective refractive index of the i-th incident light mode and the j-th Brillouin scattered light mode, respectively.

represents the propagation speed of the kth acoustic mode in the light,

is the wavelength of the incident light.

The Brillouin gain is shown in formula (2):

（2），

(2),

为三波耦合的效率，

为布里渊增益谱的半高全宽。in

is the efficiency of three-wave coupling,

is the full width at half maximum of the Brillouin gain spectrum.

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.

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 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.

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.

This optical time domain reflectometer has low cost, simple structure and can realize large-scale dynamic strain measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic structural diagram of an embodiment.

FIG. 2 is a schematic diagram of the equalizer slope assist principle in an embodiment.

In the figure, 10. narrow-band 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 ring 24. second fiber Bragg grating 25. fiber polarization scrambler 26. second fiber coupler 27. photodetector 28. frequency equalizer 29. electrical signal modulation unit 30. large strain application unit.

DETAILED DESCRIPTION

The content of the present invention is further described below in conjunction with the drawings and embodiments, but the present invention is not limited thereto.

Example:

1 , a single-mode optical fiber large dynamic strain distributed Brillouin optical time domain reflectometer comprises a narrow-band laser 10, a first erbium-doped fiber amplifier 11, and a first optical fiber coupler 12 connected in sequence, wherein:

The light intensity of an output port of the first fiber coupler 12 is 90% of the original laser intensity. This port is sequentially connected to an acousto-optic modulator 13 connected to an arbitrary function signal generator 14, a second erbium-doped fiber amplifier 15, a first fiber circulator 16, and a second fiber circulator 18. The a end of the first fiber 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 a test fiber 19. The c end of the second fiber circulator 18 is connected to a second fiber coupler 26. The second fiber coupler 26 is sequentially connected to a photodetector 27, a frequency equalizer 28, and an electrical signal modulation unit 29 with a built-in acquisition card and a data processing program. The test fiber 19 is also connected to a large strain applying unit 30.

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

In this example, the polarization controller 20, the electro-optic modulator 21, the microwave signal generator 22 driving the electro-optic modulator 21, and the optical fiber scrambler 25 constitute a frequency shift unit; the acousto-optic modulator 13 for modulating pulses, the arbitrary function signal generator 14 driving 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 optical fiber Bragg grating 17 constitute a first filtering unit; the third optical fiber circulator 23, the second optical fiber Bragg grating 24, and the displacement platform for applying strain to the second optical fiber Bragg grating constitute a second filtering unit, and the equalizer 28 constitutes a third filtering unit.

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

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

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

In this example, the large strain applying 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 so that the test optical fiber is 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 an adjustable movement radius to drive the optical fiber to move parallel to the ground.

In this example, the second filtering unit applies strain to the Bragg grating 24 to change the transmission spectrum of the Bragg grating 24, so that the Rayleigh scattered light is just outside the transmission spectrum and the Brillouin scattered light is just within the transmission spectrum for filtering.

In this example, the equalizer 28 is a frequency equalizer whose filtering strength varies linearly at different frequencies, that is, the third filtering unit is a bandpass filter whose filtering strength varies linearly with frequency, so that there is a linear relationship between the Brillouin frequency shift at the strain location and the detection intensity.

The laser light emitted by the narrow linewidth laser 10 is amplified by the first erbium-doped fiber amplifier 11 and then divided into two paths of light by the first fiber coupler 12. The lower path 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. The polarization state of the incident light is changed by the three-ring polarization controller 20 so that 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. The sidebands with relatively higher frequencies are filtered out by the second filtering unit, and the sidebands with lower frequencies are left, which are called reference light. After entering the fiber polarization scrambler 25, the output light enters the second fiber coupler 26 from one end of the input port of the second fiber coupler 26; the upper path light (90%) is connected to the pulse modulation unit, modulated into pulse light and amplified, and then Enter the second filtering unit for filtering operation, and be connected through the a port of the first optical fiber circulator 16. The test optical fiber 19 drives the optical fiber to generate a large strain movement with an adjustable amplitude through the large strain applying unit 30 through a motor with adjustable movement frequency and adjustable movement amplitude. The Brillouin scattered light in the test optical fiber 19 enters the b port of the first optical fiber circulator 16, and is connected to the second optical fiber coupler 26 from the c port of the first optical fiber circulator 16 through the second optical fiber circulator 18. The two output ports of the second optical 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, so that the Brillouin frequency shift at the strain location and the detection intensity are linearly related, and then enters the electrical signal modulation unit 29 from the output port of the third filtering unit for data acquisition and processing.

The Brillouin frequency shift induced by strain in optical fiber is shown in formula (1):

（1），

(1),

和

分别表示第i个入射光模式和第j个布里渊散射光模式的有效折射率，

表示光线内第k个声学模式的传播速度，

为入射光的波长。in

and

denote the effective refractive index of the i-th incident light mode and the j-th Brillouin scattered light mode, respectively.

represents the propagation speed of the kth acoustic mode in the light,

is the wavelength of the incident light.

The Brillouin gain is shown in formula (2):

（2），

(2),

为三波耦合的效率，

为布里渊增益谱的半高全宽。in

is the efficiency of three-wave coupling,

is the full width at half maximum of the Brillouin gain spectrum.

2 , this example utilizes the approximate linear region of the Brillouin gain spectrum sideband and the Brillouin frequency shift to form an approximate linear correspondence, converting the change in the Brillouin frequency shift into a change in the Brillouin gain, thereby eliminating 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 finding the Brillouin frequency shift at the current moment corresponding to the peak point of the gain spectrum, thereby saving the system sensing time and thus increasing 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.

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 (7)

1. The utility model provides a large dynamic strain distributed Brillouin optical time domain reflectometer based on single mode fiber, which is characterized in that, including narrowband laser, first erbium-doped fiber amplifier, the first fiber coupler of order connection, wherein:

an output port of the first optical fiber coupler is connected with an acousto-optic modulator connected with an arbitrary function signal generator, a second erbium-doped optical fiber amplifier, a first optical fiber circulator and a second optical fiber circulator which are sequentially connected, an a end of the first optical fiber circulator is connected with the second erbium-doped optical fiber amplifier, a b end of the first optical fiber circulator is connected with the first optical fiber Bragg grating, a c end of the first optical fiber circulator is connected with an a end of the second optical fiber circulator, a b end of the second optical fiber circulator is connected with a test optical fiber, a c end of the second optical fiber circulator is connected with the second optical fiber coupler, the second optical fiber coupler is sequentially connected with a photoelectric detector, a frequency equalizer and an electric signal modulation unit with an acquisition card and a data processing program, and the test optical fiber is also connected with a large strain applying unit;

the other output port of the first optical fiber coupler is connected with an optical fiber tricyclic polarization controller, an electro-optical modulator, a third optical fiber circulator, an optical fiber polarization scrambler and a second optical fiber coupler which are sequentially connected, wherein the a end of the third optical fiber circulator is connected with the electro-optical modulator, the b end of the third optical fiber circulator is connected with the second optical fiber Bragg grating, the c end of the third optical fiber circulator is connected with the optical fiber polarization scrambler, and the second optical fiber Bragg grating is externally connected with a displacement platform for applying strain to the second optical fiber Bragg grating;

the polarization controller, the electro-optical modulator, a microwave signal generator for driving the electro-optical modulator and the optical fiber scrambler form a frequency shift unit; the acousto-optic modulator for modulating the pulse, the arbitrary function signal generator for driving the acousto-optic modulator and the second erbium-doped fiber amplifier form a pulse modulation unit; the first optical fiber circulator and the first optical fiber Bragg grating form a first filtering unit; the third fiber circulator, the second fiber Bragg grating and a displacement platform for applying strain to the second fiber Bragg grating form a second filtering unit, and the equalizer forms a third filtering unit.

2. The single-mode fiber-based large dynamic strain distributed brillouin optical time domain reflectometer according to claim 1, wherein the split ratio of the first optical fiber coupler is 90%:10%,90% of one path of light is connected to the optical path of the acousto-optic modulator connected to the arbitrary function signal generator, and 10% of one path of light is connected to the optical path of the acousto-optic modulator connected to the arbitrary function signal generator.

3. The single-mode fiber-based large dynamic strain distributed brillouin optical time domain reflectometer according to claim 1, wherein the split ratio of the second optical fiber coupler is 50%:50%.

4. The single mode fiber based large dynamic strain distributed brillouin optical time domain reflectometer according to claim 1, wherein the test optical fiber is a single mode fiber.

5. The large dynamic strain distributed brillouin optical time domain reflectometer based on single-mode fiber according to claim 1, wherein the large strain applying unit is provided with a motor with an adjustable movement radius of 0-4cm, the motor is arranged in the middle of the test optical fiber, one end of the motor is fixed with the test optical fiber to enable the test optical fiber to be in a tight state, and the other end of the motor drives the optical fiber to move parallel to the ground in a state of adjustable movement radius.

6. The single-mode fiber-based large dynamic strain distributed brillouin optical time domain reflectometer according to claim 1, wherein the second filtering unit changes the transmission spectrum of the bragg grating by applying strain to the bragg grating, so that the rayleigh scattered light is just outside the transmission spectrum and the brillouin scattered light is just inside the transmission spectrum for filtering.

7. The single-mode fiber-based large dynamic strain distributed brillouin optical time domain reflectometer according to claim 1, wherein the equalizer is a frequency equalizer with linearly changing filtering intensity at different frequencies, that is, the third filtering unit is a band-pass filter with linearly changing filtering intensity with frequency, so that the brillouin frequency shift amount at the strain position and the detection intensity have a linear relationship.

Images