CN218724246U - Distributed Brillouin optical time domain reflectometer based on large dynamic strain of single-mode optical fiber - Google Patents
Distributed Brillouin optical time domain reflectometer based on large dynamic strain of single-mode optical fiber Download PDFInfo
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- CN218724246U CN218724246U CN202222987557.1U CN202222987557U CN218724246U CN 218724246 U CN218724246 U CN 218724246U CN 202222987557 U CN202222987557 U CN 202222987557U CN 218724246 U CN218724246 U CN 218724246U
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Abstract
The utility model discloses a based on big dynamic strain distributing type brillouin optical time domain reflectometer of single mode fiber, including narrow line width laser instrument, light amplification unit, the unit of shifting frequency that connects in proper order, pulse modulation unit, first filter unit, second filter unit, photoelectric detector, third filter unit, signal of telecommunication modulation unit. The sensing system has the advantage of realizing wide-range dynamic strain monitoring at low cost. The optical time domain reflectometer has low cost and simple structure and can realize large-range dynamic strain measurement.
Description
Technical Field
The utility model relates to an optical fiber sensing technology field specifically is a based on big dynamic strain distribution formula brillouin optical time domain reflectometer of single mode fiber.
Background
The BOTDR is a distributed sensing technology based on spontaneous Brillouin scattering effect, and can realize continuous measurement of temperature and strain by utilizing the linear dependence of the frequency shift of Brillouin scattering in optical fiber on temperature and strain. The BOTDR system has the advantages of single-ended access, sensitive response to temperature and strain and the like, and has a huge application prospect in the aspect of structural health monitoring of large-scale infrastructure, such as health monitoring systems of bridges and tunnels.
The slope auxiliary technology is a technology which is provided based on the fact that a traditional Brillouin distributed sensing system cannot be used for dynamic strain measurement due to the fact that the sampling time of the system is too long, the technology mainly utilizes an approximate linear region of a Brillouin spectrum sideband to convert the variable quantity of Brillouin frequency shift into the change of Brillouin gain, and the technology omits a frequency sweeping process in the traditional Brillouin distributed sensing system, so that the sampling frequency of the system is improved, and the system can achieve dynamic strain measurement. However, the approximate linear region of the brillouin gain spectrum of the common single-mode fiber is only about 30MHz generally, and the strain range capable of being monitored is very limited, so that the measurement of dynamic strain with a large range in real life application has certain limitation.
SUMMERY OF THE UTILITY MODEL
The utility model aims at the not enough of prior art, and provide a based on big dynamic strain distribution formula brillouin optical time domain reflectometer of single mode fiber. The optical time domain reflectometer has low cost and simple structure and can realize large-range dynamic strain measurement.
Realize the utility model discloses the technical scheme of purpose is:
a distributed Brillouin optical time domain reflectometer based on large dynamic strain of a single mode fiber comprises a narrow band laser, a first erbium-doped fiber amplifier and a first fiber coupler which are sequentially connected, wherein:
the light intensity of one output port of the first optical fiber coupler is 90% of the original laser intensity, the port is connected with an acousto-optic modulator, a second erbium-doped optical fiber amplifier, a first optical fiber circulator and a second optical fiber circulator which are connected with an arbitrary function signal generator in sequence, the end a of the first optical fiber circulator is connected with the second erbium-doped optical fiber amplifier, the end b of the first optical fiber circulator is connected with a first optical fiber Bragg grating, the end c of the first optical fiber circulator is connected with the end a of the second optical fiber circulator, the end b of the second optical fiber circulator is connected with a second optical fiber coupler, the second optical fiber coupler is connected with a photoelectric detector, a frequency equalizer and an electric signal modulation unit internally provided with an acquisition card and a data processing program in sequence, and the test optical fiber is also connected with a large strain applying unit;
the light intensity of the other output port of the first optical fiber coupler is 10% of the original laser intensity, the port is connected with an optical fiber three-ring polarization controller, an electro-optical modulator connected with a microwave signal generator, a third optical fiber circulator, an optical fiber polarization scrambler and a second optical fiber coupler which are sequentially connected, the end a of the third optical fiber circulator is connected with the electro-optical modulator, the end b is connected with a second optical fiber Bragg grating, the end c 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 polarization scrambler form a frequency shift unit; the pulse modulation unit consists of an acousto-optic modulator for modulating pulses, an arbitrary function signal generator for driving the acousto-optic modulator and a second erbium-doped fiber amplifier; the first optical fiber circulator and the first optical fiber Bragg grating form 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 form a second filtering unit, and the equalizer forms a third filtering unit.
The splitting ratio of the first optical fiber coupler is 90% to 10%, one path of the first optical fiber coupler is optically connected to the optical path of the acousto-optic modulator connected with the arbitrary function signal generator, and the other path of the first optical fiber coupler is optically connected to the optical path of the acousto-optic modulator connected with the arbitrary function signal generator.
The splitting ratio of the second optical fiber coupler is 50% to 50%.
The test fiber is a single mode fiber.
The large strain applying unit is provided with a motor with an adjustable motion 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 so as to ensure that the optical fiber is strained in the large strain stretching process, and the other end of the motor drives the optical fiber to move parallel to the ground in an adjustable motion radius state.
The second filtering unit is used for filtering the Rayleigh scattering light just outside the transmission spectrum and the Brillouin scattering light just inside the transmission spectrum by applying strain to the Bragg grating to change the transmission spectrum of the Bragg grating.
The equalizer is a frequency equalizer with filtering strength linearly changing under different frequencies, namely the third filtering unit is a band-pass filter with filtering strength linearly changing along with the frequency, so that the Brillouin frequency shift quantity at a strain position and the detection strength are in a linear relation.
After laser emitted by a narrow-linewidth laser is subjected to light amplification through a first erbium-doped fiber amplifier, a first fiber coupler is divided into two paths of light, the next path of light (10%) is connected with a polarization controller in a frequency shift unit, the frequency spectrum in output light of the frequency shift unit comprises the frequency of the laser and two frequency sidebands, the polarization state of incident light is changed through a three-ring polarization controller, so that the energy at the frequency point of the laser is transferred to the two frequency sidebands to increase the energy of the frequency sidebands, then the sidebands with higher relative frequency are filtered out through a second filtering unit, the remaining sidebands with lower frequency are called reference light, and the output light enters a second fiber coupler from an input port at one end of the second fiber coupler; the upper path light (90%) is connected with the pulse modulation unit to be modulated into pulse light and amplified, then enters the second filtering unit to be subjected to filtering operation, is connected with the port a of the first optical fiber circulator, the test optical fiber is driven by the large-strain applying unit through the motor with adjustable motion frequency and adjustable motion amplitude to generate large-strain motion with adjustable amplitude, brillouin scattered light in the test optical fiber enters the port b of the first optical fiber circulator, is connected with the second optical fiber coupler through the port c of the first optical fiber circulator and is output to the input port of the third filtering unit from the output end of the photoelectric detector after being connected with the input end of the photoelectric detector, so that the Brillouin frequency shift quantity at the position where strain occurs and the detection intensity form a linear relation, and then enters the electric signal modulation unit from the output port of the third filtering unit to be subjected to data acquisition processing.
The strain-induced brillouin frequency shift in an optical fiber is shown in equation (1):
wherein
And &>
Respectively represents the effective refractive indexes of the ith incident light mode and the jth Brillouin scattering light mode, and->
Represents the propagation speed of the kth acoustic mode in the light ray, is greater than>
Is the wavelength of the incident light.
The brillouin gain is shown in equation (2):
wherein
For the efficiency of the three-wave coupling>
The full width at half maximum of the brillouin gain spectrum.
The technical scheme adopts a slope auxiliary technology of data processing, and is a technology provided for solving the problem that the traditional Brillouin distributed sensing system cannot be used for dynamic strain measurement due to the fact that the sampling time of the system is too long, the technical scheme utilizes an approximate linear region of a Brillouin gain spectrum sideband and Brillouin frequency shift to form approximate linear correspondence, changes of the Brillouin frequency shift are converted into changes of Brillouin gain, the process that the Brillouin frequency shift at the current moment corresponding to a peak point of a gain spectrum is found after frequency sweeping is carried out by an electric modulator in the traditional Brillouin distributed sensing system is omitted, the sensing time of the system is saved, the sampling frequency of the system is improved, and the system can realize the dynamic strain measurement.
The optical signal of the single mode sensing optical fiber is converted into an electrical signal, a better linear relation is formed between intensity and frequency signals which are artificially changed on a Brillouin gain spectrum through the characteristic that a frequency equalizer matched with the sensing optical fiber linearly changes along with the change of frequency of light intensity set in the frequency equalizer, a frequency shift amount is determined by an electro-optical modulator, the Brillouin frequency shift amount in the optical fiber is changed due to the occurrence of external strain of the optical fiber to be detected, at the moment, a Brillouin scattering light intensity signal detected at the frequency shift position selected by the electro-optical modulator can linearly correspond to the frequency shift amount of the Brillouin gain spectrum which changes due to the external strain, so that the linear change from the Brillouin scattering light intensity to the Brillouin gain spectrum frequency shift amount is realized, and the relationship of reflecting strain through monitoring the Brillouin frequency shift amount in a BOTDR system is skillfully converted into the relationship of reacting strain through monitoring the change of the Brillouin scattering light intensity by using a slope auxiliary method in the data processing process.
According to the technical scheme, a slope auxiliary method is combined with a Brillouin optical time domain reflectometer to measure large-range strain in the single-mode fiber, large-range strain sensing on the single-mode fiber is achieved through low cost, and the problems in practical application such as engineering monitoring are effectively solved.
The optical time domain reflectometer has low cost and simple structure and can realize large-range dynamic strain measurement.
Drawings
Fig. 1 is a schematic structural diagram of the embodiment.
Fig. 2 is a diagram of an embodiment equalizer ramp assist concept.
In the figure, 10, a narrow-band laser 11, a first erbium-doped fiber amplifier 12, a first fiber coupler 13, an acoustic-optical modulator 14, an arbitrary function signal generator 15, a second erbium-doped fiber amplifier 16, a first circulator 17, a first fiber Bragg grating 18, a second fiber circulator 19, a test fiber 20, a polarization controller 21, an electro-optical modulator 22, a microwave signal generator 23, a third fiber ring 24, a second fiber Bragg grating 25, a fiber polarizer 26, a second fiber coupler 27, a photoelectric detector 28, a frequency equalizer 29, an electric signal modulation unit 30 and a large strain applying unit are arranged.
Detailed Description
The contents of the present invention will be further described with reference to the accompanying drawings and examples, but the present invention is not limited thereto.
Example (b):
referring to fig. 1, a distributed brillouin optical time domain reflectometer based on single-mode fiber large dynamic strain comprises a narrow-band laser 10, a first erbium-doped fiber amplifier 11, and a first fiber coupler 12, which are connected in sequence, wherein:
the light intensity of one output port of the first optical fiber coupler 12 is 90% of the original laser intensity, the port is connected with an acousto-optic modulator 13, a second erbium-doped optical fiber amplifier 15, a first optical fiber circulator 16 and a second optical fiber circulator 18 which are connected with an arbitrary function signal generator 14 in sequence, the end a of the first optical fiber circulator 16 is connected with the second erbium-doped optical fiber amplifier 15, the end b is connected with a first optical fiber Bragg grating 17, the end c is connected with the end a of the second optical fiber circulator 18, the end b of the second optical fiber circulator 18 is connected with a test optical fiber 19, the end c of the second optical fiber circulator 18 is connected with a second optical fiber coupler 26, the second optical fiber coupler 26 is connected with a photoelectric detector 27, a frequency equalizer 28 and an electric signal modulation unit 29 internally provided with an acquisition card and a data processing program in sequence, and the test optical fiber 19 is also connected with a large strain applying unit 30;
the light intensity of the other output port of the first optical fiber coupler 12 is 10% of the original laser intensity, the port is connected with an optical fiber three-ring polarization controller 20, an electro-optical modulator 21 connected with a microwave signal generator 22, a third optical fiber circulator 23, an optical fiber polarization scrambler 25 and a second optical fiber coupler 26 which are connected in sequence, the end a of the third optical fiber circulator 23 is connected with the electro-optical modulator 21, the end b is connected with a second optical fiber Bragg grating 24, the end c is connected with the optical fiber polarization scrambler 25, and the second optical fiber Bragg grating 24 is externally connected with a displacement platform for applying strain to the second optical fiber Bragg grating;
in this example, the polarization controller 20, the electro-optical modulator 21, the microwave signal generator 22 for driving the electro-optical modulator 21, and the optical fiber scrambler 25 form a frequency shift unit; an acousto-optic modulator 13 for modulating pulses, an arbitrary function signal generator 14 for driving the acousto-optic modulator 13 and a second erbium-doped fiber amplifier 15 form a pulse modulation unit; the first optical fiber circulator 16 and the first fiber bragg grating 17 form a first filtering unit; the third fiber circulator 23, the second fiber bragg grating 24, and the displacement stage for applying strain to the second fiber bragg grating constitute a second filter unit, and the equalizer 28 constitutes a third filter unit.
In this example, the first optical fiber coupler has a splitting ratio of 90% to 10%,90% of one path is optically connected to the optical path of the acousto-optic modulator connected to the arbitrary function signal generator, and 10% of one path is optically connected to the optical path of the acousto-optic modulator connected to the arbitrary function signal generator.
The splitting ratio of the second fiber coupler in this example is 50% to 50%.
The test fiber 19 in this example is a single mode fiber.
In this example, the large strain applying unit 30 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 19, one end of the motor is fixed with the test optical fiber to ensure that the test optical fiber is in a tight state in order to ensure that the optical fiber is strained during the large strain stretching process, and the other end of the motor is in an adjustable movement radius state to drive the optical fiber to move parallel to the ground.
In this example, the second filtering means performs filtering by applying 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 inside the transmission spectrum.
In this example, the equalizer 28 is a frequency equalizer in which the filtering strength linearly changes at different frequencies, that is, the third filtering unit is a band-pass filter in which the filtering strength linearly changes with the frequency, so that the brillouin frequency shift amount at the point where strain occurs and the detection strength have a linear relationship.
After laser emitted by a narrow-linewidth laser 10 is subjected to light amplification through a first erbium-doped fiber amplifier 11, a first fiber coupler 12 is divided into two paths of light, the next path of light (10%) is connected with a polarization controller 20 in a frequency shift unit, output light of the frequency shift unit comprises frequency spectrum including the frequency of the laser and two frequency sidebands, the polarization state of incident light is changed through a three-ring polarization controller 20, so that the energy at the frequency point of the laser is transferred to the two frequency sidebands to increase the energy of the frequency sidebands, the sidebands with higher relative frequencies are filtered through a second filtering unit, the remaining sidebands with lower frequencies are called reference light, and the reference light enters a fiber polarization scrambler 25, and then the output light enters a second fiber coupler 26 from an input port at one end of the second fiber coupler 26; the upper path light (90%) is connected with the pulse modulation unit to be modulated into pulse light and amplified, and then enters the second filtering unit to be filtered, and is connected with the port a of the first optical fiber circulator 16, the test optical fiber 19 is driven by a motor with adjustable motion frequency and adjustable motion amplitude through a large strain applying unit 30 to generate large strain motion with adjustable amplitude, brillouin scattering light in the test optical fiber 19 enters the port b of the first optical fiber circulator 16, is connected with the second optical fiber coupler 26 through the port c of the first optical fiber circulator 16 and the second optical fiber circulator 18, two output ports of the second optical fiber coupler 26 are connected with the input port of the photoelectric detector 27 and then are output to the input port of the third filtering unit from the output port of the photoelectric detector 27, so that the brillouin frequency shift quantity and the detection intensity at the strain position form a linear relation, and then enter the electric signal modulation unit 29 from the output port of the third filtering unit to perform data acquisition processing.
The strain-induced brillouin frequency shift in an optical fiber is shown in equation (1):
wherein
And &>
Respectively represents the effective refractive indexes of the ith incident light mode and the jth Brillouin scattering light mode, and->
Represents the propagation speed of the kth acoustic mode in the light ray, is greater than>
Is the wavelength of the incident light.
The brillouin gain is shown in equation (2):
wherein
For the efficiency of the three-wave coupling>
The full width at half maximum of the brillouin gain spectrum.
Referring to fig. 2, in the present embodiment, an approximately linear region of a brillouin gain spectrum sideband and brillouin frequency shift form an approximately linear correspondence, and a variation amount of brillouin frequency shift is converted into a variation of brillouin gain, so that a process of finding the brillouin frequency shift at a current moment corresponding to a gain spectrum peak point after obtaining a brillouin gain spectrum at the current moment by using an electric modulator 21 in a conventional brillouin distributed sensing system is omitted, a system sensing time is saved, a sampling frequency of the system is increased, the system can realize measurement of dynamic strain, cost is reduced, a system structure is simplified, and large-scale strain relation sensing of a single mode fiber in various application projects in life is realized.
In the embodiment, an optical signal of a single mode sensing optical fiber 19 is converted into an electrical signal, a frequency equalizer 28 matched with the sensing optical fiber enables a section of intensity and a frequency signal to be artificially changed on a Brillouin gain spectrum to form a better linear relation through the characteristic that the set light intensity in the frequency equalizer 28 linearly changes along with the change of frequency, an electro-optical modulator 21 determines a frequency shift amount, the Brillouin frequency shift amount in the optical fiber changes due to the occurrence of external strain of the optical fiber 19 to be measured, at the moment, a Brillouin scattering light intensity signal detected at the frequency shift position selected by the electro-optical modulator 21 can linearly correspond to the frequency shift amount of the changed Brillouin gain spectrum due to the external strain, so that the linear change from the intensity of the Brillouin scattering light signal to the frequency shift amount of the Brillouin gain spectrum is realized, and the strain relation reflected by monitoring the Brillouin frequency shift amount in a BOTDR system is skillfully converted into the strain relation through monitoring the intensity change reaction of the Brillouin scattering light signal by using a slope auxiliary method in a data processing process.
Claims (7)
1. The distributed Brillouin optical time domain reflectometer based on the large dynamic strain of the single-mode optical fiber is characterized by comprising a narrow-band laser, a first erbium-doped optical fiber amplifier and a first optical fiber coupler which are sequentially connected, wherein:
one output port of the first optical fiber coupler is connected with an acousto-optic modulator, a second erbium-doped optical fiber amplifier, a first optical fiber circulator and a second optical fiber circulator which are sequentially connected and connected with an arbitrary function signal generator, wherein the a end of the first optical fiber circulator is connected with the second erbium-doped optical fiber amplifier, the b end of the first optical fiber circulator is connected with the first optical fiber Bragg grating, the c end of the first optical fiber circulator is connected with the a end of the second optical fiber circulator, the b end of the second optical fiber circulator is connected with a test optical fiber, the 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 internally provided 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 three-ring 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 electro-optical modulator is connected with a microwave signal generator at the end a of the third optical fiber circulator, the second optical fiber Bragg grating is connected at the end b, the optical fiber polarization scrambler is connected at the end c, 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 an optical fiber polarization scrambler form a frequency shift unit; the pulse modulation unit consists of an acousto-optic modulator for modulating pulses, an arbitrary function signal generator for driving the acousto-optic modulator and a second erbium-doped fiber amplifier; the first optical fiber circulator and the first optical fiber Bragg grating form 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 form a second filtering unit, and the equalizer forms a third filtering unit.
2. The distributed brillouin optical time domain reflectometer based on single mode fiber large dynamic strain according to claim 1, wherein the splitting ratio of the first optical fiber coupler is 90%:10%,90% of one path is optically connected to the optical path of the acousto-optic modulator connected to the arbitrary function signal generator, and 10% of one path is optically connected to the optical path of the acousto-optic modulator connected to the arbitrary function signal generator.
3. The distributed Brillouin optical time domain reflectometer based on single mode fiber large dynamic strain according to claim 1, wherein the splitting ratio of the second fiber coupler is 50%:50%.
4. The distributed brillouin optical time domain reflectometer based on single mode fiber large dynamic strain according to claim 1, wherein the test fiber is a single mode fiber.
5. The distributed Brillouin optical time domain reflectometer based on single-mode optical fiber large dynamic strain according to claim 1, wherein the large strain applying unit is provided with a motor with an adjustable motion 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 is in the shape of the adjustable motion radius to drive the optical fiber to move parallel to the ground.
6. The distributed brillouin optical time domain reflectometer based on single-mode optical fiber large dynamic strain according to claim 1, wherein the second filtering unit is used for filtering the rayleigh scattered light just outside the transmission spectrum and the brillouin scattered light just inside the transmission spectrum by applying strain to the bragg grating to change the transmission spectrum of the bragg grating.
7. The distributed brillouin optical time domain reflectometer based on single mode fiber large dynamic strain according to claim 1, wherein the equalizer is a frequency equalizer with filter strength linearly changing at different frequencies, that is, the third filtering unit is a band-pass filter with filter strength linearly changing with frequency, so that the brillouin frequency shift amount at the strain position and the detection strength are in a linear relationship.
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