CN102506915B - Three-order Raman amplification technology-based Brillouin optical time domain analysis system - Google Patents
Three-order Raman amplification technology-based Brillouin optical time domain analysis system Download PDFInfo
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- CN102506915B CN102506915B CN201110340681.2A CN201110340681A CN102506915B CN 102506915 B CN102506915 B CN 102506915B CN 201110340681 A CN201110340681 A CN 201110340681A CN 102506915 B CN102506915 B CN 102506915B
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Abstract
The invention discloses a three-order Raman amplification technology-based Brillouin optical time domain analysis system which comprises a laser, a coupler, a first electro-optic modulator, a second electro-optic modulator, a polarization scrambler, a first erbium-doped fiber amplifier (EDFA), a second EDFA, an optical circulator, a tunable filter, a detector, a data acquisition and processing system and a three-order Raman amplification system. According to the three-order Raman amplification technology-based Brillouin optical time domain analysis system, by applying the three-order Raman amplification, so that the scope of a low signal-to-noise ratio is greatly reduced (about 30 km), and the longer-distance high-quality transduction can be achieved.
Description
Technical field
The present invention relates to technical field of optical fiber sensing, relate in particular to a kind of Brillouin optical time domain analysis system.
Background technology
Distributed Optical Fiber Sensing Techniques based on Brillouin scattering reaches measuring accuracy, measurement range and spatial resolution all higher than other sensing technology in temperature, strain measurement, and therefore this technology has wide practical use at aspects such as buildings, petroleum pipe line, electric power facility health detection, fire alarms.Generally be divided into two kinds, Brillouin light domain reflectometer (BOTDR) and Brillouin optical time domain analysis instrument (BOTDA).The former utilizes spontaneous brillouin scattering phenomenon, can carry out single-ended measurement, but detectable signal a little less than, detection range is limited; The latter utilizes stimulated Brillouin scattering phenomenon, and detectable signal is stronger, and distance sensing is far away.
Traditional centralized amplifying technique of the general employing of BOTDA, be to apply erbium doped optical fibre light amplifier (EDFA) by Pulse Power Magnification to a certain extent before Brillouin's pumping wave enters optical fiber, because Brillouin's pumping light power is only stronger at optical fiber front end, and in optical fiber rear end, be subject to fibre loss and Brillouin to survey the consumption of light, intensity sharply declines, and has a strong impact on the Measurement Resolution of optical fiber rear end.Affected by this, the Brillouin optical time domain analysis system distance sensing <30km based on jumped amplifier technology.
On the other hand, along with the further raising to spatial resolution and measuring distance requirement, pulse width used will more and more narrow (sensing space resolution be directly proportional to pulse width), and the dutycycle of signal is also more and more less, thereby causes the sharply decline of signal to noise ratio (S/N ratio).That is to say, sending in the cycle of a pulse do not have the gross energy in the time slot of pulse very larger than the energy of relative pulse itself, its result is exactly the signal to noise ratio (S/N ratio) (particularly optical fiber connector) that has a strong impact on system, and sensing capabilities is seriously limited.
For non-linear, noise accumulation and the low problem of sensor fibre rear end resolution that overcomes that jumped amplifier causes, the centralized mode combining with single order distributed raman amplification of general employing, at front end, first utilize EDFA pulse signals tentatively to amplify, recycling single order distributed raman amplification technology paired pulses light amplifies.2005, the people such as Alahbabi M. N. reported the BOTDR that utilizes single order Raman amplifying technique, have reached 150km distance sensing.Weak point is that spatial resolution is only 50m.2010, we amplified single order bi-directional distributed formula Raman to be generalized to BOTDA sensor-based system first, and obtain 0.6 on 75km sensor fibre
oc temperature measurement accuracy and 10m spatial resolution.
Single order Raman amplifying technique has weakened signal power front and back end skewness to a certain extent.But in longer distance (>75km) Brillouin light sensor-based system, because Raman gain coefficienct is also exponential damping along optical fiber, make the method can not thoroughly eliminate power skewness phenomenon, and distance sensing is longer, fluctuates more serious.Consequently on transducing signal distributes, occurred one on a large scale, low signal-to-noise ratio region.Application second order Raman amplifying technique, can further improve the flatness that light signal distributes.J. the people such as D. Ania-Casta ó n has reported that a kind of FBG of utilization and second order Raman amplify the length that realizes apart from BOTDA.
But for the transmission range of >100km, flashlight distributes and also occurs obviously fluctuation, reappears low signal-to-noise ratio region on a large scale, has a strong impact on the omnidistance consistance of sensing quality.
Summary of the invention
For above-mentioned prior art, the technical problem to be solved in the present invention is to provide a kind of length that realizes apart from high-quality sensing, dwindles the Brillouin optical time domain analysis instrument based on three rank Raman amplifying techniques of low signal-to-noise ratio scope in transmission.
In order to solve the problems of the technologies described above, the present invention adopts following technical scheme: a kind of Brillouin optical time domain analysis system based on three rank Raman amplifying techniques, comprising: laser instrument, coupling mechanism, the first electrooptic modulator, the second electrooptic modulator, scrambler, the first Erbium-Doped Fiber Amplifier, the second Erbium-Doped Fiber Amplifier, optical circulator, tunable optic filter, detector, data acquisition processing system;
Described laser instrument is through the first isolator butt coupling device, the light beam that coupling mechanism produces laser instrument is divided into two bundles, the first light beam enters optical circulator through scrambler and the first Erbium-Doped Fiber Amplifier again through Polarization Controller and the first electrooptic modulator generation Brillouin pump light, described optical circulator connects tunable optic filter, tunable optic filter is by detector connection data acquisition processing system, and the first electrooptic modulator is through waveform generator connection data acquisition processing system; The second light beam produces Brillouin through the second Polarization Controller and the second electrooptic modulator and surveys light and through the second Erbium-Doped Fiber Amplifier, be connected the second isolator again, and described the second electrooptic modulator connects described data acquisition processing system by microwave generator; Also comprise three rank Raman amplification systems,
Described three rank Raman amplification systems comprise two WDM, two three rank raman pump sources, peak reflectivity >80%, the first fiber grating pair that centre wavelength is consistent and the second fiber grating pair, wherein, WDM is connected with optical circulator for by Brillouin's pump light and one of them three rank raman pump source is coupled and be connected sensor fibre through fiber grating of a fiber grating and the first fiber grating pair of the second fiber grating pair, another WDM is connected with the second optoisolator for Brillouin being surveyed to light and another three rank raman pump source is coupled and be connected sensor fibre through another fiber grating of the second fiber grating pair and another fiber grating of the first fiber grating pair, described the first fiber grating pair and the second fiber grating pair are used for respectively forming long apart from laserresonator, the laser of its generation is respectively as second order raman pump light and single order raman pump light, described three rank raman pump sources are used for second order pump light to produce and amplify.
Further, three rank raman pump source pumping wave wavelength are 12XXnm.
Further, the centre wavelength of described the first fiber grating pair is 13XXnm, and the centre wavelength of the second fiber grating pair (21) is 14XXnm, and the pumping wavelength spacing of adjacent order is positioned between 70 ~ 110nm.
Further, the light source live width <1MHz of described laser instrument, power is greater than 10dBm.
Further, the bandwidth of described Brillouin's pump light is 2.5GHz, and the frequency displacement that Brillouin surveys light is 10-11GHz, and bandwidth is 10GHz.
Further, described scrambler, for suppressing the polarization correlated of brillouin gain, improves Measurement Resolution, disturbs inclined to one side speed > 1KHz, output polarization degree < 5%.
Further, described data acquisition processing system comprises data collecting card and signal processing system, for data acquisition, processing and the control to described waveform generator, microwave generator.
Compared with prior art, the present invention has following beneficial effect: in the present invention, 12XXnm tri-rank pumpings laserresonator is produced one, second order pumping amplifies step by step, and pump light flatness improves step by step.With based on one, the second order Raman Brillouin sensing system of amplifying compares, transducing signal is more smooth along the distribution of optical fiber, has significantly improved the omnidistance consistance of sensing quality; For long, apart from temperature/strain sensing, can significantly improve spatial resolution, measuring accuracy and the sensitivity of monitoring system; With very little cost (, second order pump light source extra without increasing), obtain the obvious improvement of sensing capabilities and the significantly extension of distance sensing, possess certain practicality.
Accompanying drawing explanation
Fig. 1 is structured flowchart of the present invention;
Fig. 2 is under one, two, three rank Raman pump conditions, the comparison diagram that detection of optical power flatness changes with distance sensing.
Embodiment
Below in conjunction with the drawings and the specific embodiments, the invention will be further described.
Referring to Fig. 1, Brillouin optical time domain analysis system based on three rank Raman amplifying techniques, comprise laser instrument 1, the first isolator 2, coupling mechanism 3, the first Polarization Controller 4, the second Polarization Controller 16, the first electrooptic modulator 6, the second electrooptic modulator 17, scrambler 8, the first Erbium-Doped Fiber Amplifier (EDFA) 9, the second Erbium-Doped Fiber Amplifier (EDFA) 18, optical circulator 12, detector 10, tunable optic filter 11, data acquisition processing system 7, waveform generator 5, microwave generator 13, three rank Raman amplification systems, wherein, described laser instrument 1 involves probing wave for generation of Brillouin's pumping, light source live width <1MHz, power is greater than 10dBm, isolator 2 is connected with laser instrument 1, for the damage of avoiding reflected light to cause laser instrument, coupling mechanism 3 is connected with institute isolator 2, for the light beam that described laser instrument is produced, be divided into two bundles, the first Polarization Controller 4 is connected with coupling mechanism 3 with the second Polarization Controller 16, for overcoming the polarization correlated of described electrooptic modulator, totally two of described electrooptic modulators, the first electrooptic modulator 6 is used for producing Brillouin's pump light, bandwidth is 2.5GHz, the second electrooptic modulator 17 be used for producing frequency displacement approximately the Brillouin of 10-11GHz survey light, its bandwidth is 10GHz, scrambler 8 is connected with the first electrooptic modulator 6, be used for suppressing the polarization correlated of brillouin gain, improve Measurement Resolution, disturb inclined to one side speed >1KHz, output polarization degree <5%, totally two of described Erbium-Doped Fiber Amplifier (EDFA)s, the first Erbium-Doped Fiber Amplifier (EDFA) 9 is connected with scrambler 8, be used for amplifying Brillouin's pump light, the second Erbium-Doped Fiber Amplifier (EDFA) 18 is connected with the second electrooptic modulator 17 of 10GHz, be used for amplifying Brillouin and survey light, optical circulator 12 is used for Brillouin's pump light to be coupled into sensor fibre, Brillouin through amplifying is surveyed to optically-coupled simultaneously and enter described tunable optic filter 11, tunable optic filter 11 is connected with optical circulator 12, be used for filtering amplified spont-aneous emission noise, improve signal to noise ratio (S/N ratio), three dB bandwidth <0.1nm, described data acquisition processing system 7 is connected with tunable optic filter 11, it comprises data collecting card and signal processing system, be used for data acquisition, process and to described waveform generator, the control of microwave generator, tunable optic filter 11 is by detector 10 connection data acquisition processing systems 7, the first electrooptic modulator 6 is through waveform generator 5 connection data acquisition processing systems 7.
Described microwave generator 13 is connected with the second electrooptic modulator 17, is used for producing 10-11GHz microwave signal to drive the second electrooptic modulator 17, carrier frequency 10-11GHz.
Three rank Raman amplification systems comprise WDM15, 12XXnm tri-rank raman pump sources 14, the first fiber grating pair 20 of 13XXnm, and the second fiber grating pair 21 of 14XXnm, wherein, described WDM15 is used for Brillouin to survey light, Brillouin's pump light and described 12XXnm tri-rank raman pump sources 14 are coupled into sensor fibre, 12XXnm tri-rank raman pump sources are used for 13XXnm second order pump light to produce and amplify, wavelength is that the first fiber grating pair 20 of 13XXnm and the second fiber grating pair 21 of 14XXnm are used for respectively forming length apart from laserresonator, the laser of its generation is as second order raman pump light and single order raman pump light, improve spatial resolution and the precision of transducing signal flatness and long-distance sensing.
Provided by the invention based on the long method apart from Brillouin optical time domain analysis instrument sensing capabilities of three rank Raman amplifying techniques improvement, the method comprises the following steps:
A builds a Brillouin optical time domain analysis sensor-based system;
B makes two couples of peak reflectivity >80%, the fiber grating that centre wavelength is consistent;
C is fused to sensor fibre both sides by fiber grating pair, and it swashs penetrates light formation one, second order raman pump source;
D is coupled into sensor fibre by three rank Raman pump lasers by a pair of wavelength division multiplexer (WDM);
E inject to survey light and Brillouin's pump light to optical fiber, to microwave generator frequency sweep, draws power-Brillouin shift-apart from three-dimensional plot, through Lorentz curve matching, draw the temperature/Strain Distribution of sensor fibre.
Fig. 2 is under one, two, three rank Raman pump conditions, the comparison diagram that detection of optical power flatness changes with distance sensing, in this figure, three rank raman pumping wavelengths are 1288nm, and the fiber bragg grating center wavelength of generation one, second order Raman pump laser is respectively 1455nm and 1365nm.Centre wavelength lays respectively near the one-level Ji Er class stokes wave length of three rank raman pumping wavelengths, and the pumping wavelength spacing of adjacent order is positioned between 70 ~ 110nm.
As seen from the figure, fiber lengths one regularly, utilize three rank Ramans to amplify to make signal distributions flatness be improved significantly; When power swing is 4dB, the transmission range that corresponding one, two, three rank Ramans amplify is respectively 82km, 110km and 140km.Visible, application three rank Ramans amplify, and have significantly dwindled low signal-to-noise ratio scope (about 30km), can realize the high-quality sensing of longer distance.
Claims (5)
1. the Brillouin optical time domain analysis system based on three rank Raman amplifying techniques, comprising: laser instrument (1), coupling mechanism (3), the first electrooptic modulator (6), the second electrooptic modulator (17), scrambler (8), the first Erbium-Doped Fiber Amplifier (9), the second Erbium-Doped Fiber Amplifier (18), optical circulator (12), tunable optic filter (11), detector (10), data acquisition processing system (7);
Described laser instrument (1) is through the first isolator (2) butt coupling device (3), the light beam that coupling mechanism (3) produces laser instrument (1) is divided into two bundles, the first light beam enters optical circulator (12) through scrambler (8) and the first Erbium-Doped Fiber Amplifier (9) again through Polarization Controller (4) and the first electrooptic modulator (6) generation Brillouin pump light, described optical circulator (12) connects tunable optic filter (11), tunable optic filter (11) is by detector (10) connection data acquisition processing system (7), the first electrooptic modulator (6) is through waveform generator (5) connection data acquisition processing system (7), the second light beam produces Brillouin through the second Polarization Controller (16) and the second electrooptic modulator (17) and surveys light and through the second Erbium-Doped Fiber Amplifier (18), be connected the second isolator (19) again, described the second electrooptic modulator (17) connects described data acquisition processing system (7) by microwave generator (13), it is characterized in that: also comprise three rank Raman amplification systems, described three rank Raman amplification systems comprise two WDM(15), two three rank raman pump sources (14), peak reflectivity >80%, the first fiber grating pair (20) that centre wavelength is consistent and the second fiber grating pair (21), wherein, a WDM(15) be connected with optical circulator (12) for by Brillouin's pump light and one of them three rank raman pump source (14) is coupled and be connected sensor fibre through (20) fiber gratings of a fiber grating and the first fiber grating pair of the second fiber grating pair (21), another WDM(15) be connected with the second optoisolator (19) for Brillouin being surveyed to light and another three rank raman pump sources (14) are coupled and be connected sensor fibre through another fiber grating of the second fiber grating pair (21) with another fiber grating of the first fiber grating pair (20), described the first fiber grating pair (20) and the second fiber grating pair (21) are used for respectively forming long apart from laserresonator, the laser of its generation is respectively as second order raman pump light and single order raman pump light, described three rank raman pump sources (14) are used for second order pump light to produce and amplify, three rank raman pump source (14) pumping wave wavelength are 12XXnm, the centre wavelength of described the first fiber grating pair (20) is 13XXnm, the centre wavelength of the second fiber grating pair (21) is 14XXnm, and the pumping wavelength spacing of adjacent order is positioned between 70 ~ 110nm, described WDM(15) be used for that Brillouin is surveyed to light, Brillouin's pump light and described 12XXnm tri-rank raman pump sources (14) and be coupled into sensor fibre, 12XXnm tri-rank raman pump sources are used for 13XXnm second order pump light to produce and amplify.
2. the Brillouin optical time domain analysis system based on three rank Raman amplifying techniques according to claim 1, is characterized in that: the light source live width <1MHz of described laser instrument, power is greater than 10dBm.
3. the Brillouin optical time domain analysis system based on three rank Raman amplifying techniques according to claim 1, is characterized in that: the bandwidth of described Brillouin's pump light is 2.5GHz, and the frequency displacement that Brillouin surveys light is 10-11GHz, and bandwidth is 10GHz.
4. the Brillouin optical time domain analysis system based on three rank Raman amplifying techniques according to claim 1, it is characterized in that: described scrambler (8) is for suppressing the polarization correlated of brillouin gain, improve Measurement Resolution, disturb inclined to one side speed > 1KHz, output polarization degree < 5%.
5. the Brillouin optical time domain analysis system based on three rank Raman amplifying techniques according to claim 1, it is characterized in that: described data acquisition processing system (7) comprises data collecting card and signal processing system, for data acquisition, processing and the control to described waveform generator, microwave generator.
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| CN103557877B (en) * | 2013-11-01 | 2016-01-06 | 南昌航空大学 | Based on the long distance BOTDR system that multiple EDFA amplifies online |
| CN108240874B (en) * | 2018-01-15 | 2019-12-17 | 武汉工程大学 | Gain competition temperature measuring device |
| CN111969406A (en) * | 2020-07-08 | 2020-11-20 | 南京邮电大学 | Brillouin optical frequency comb generation device and method based on Raman effect |
Citations (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5265106A (en) * | 1991-06-10 | 1993-11-23 | Compagnie Industrielle Des Lasers Cilas | Raman Laser |
| CN1528035A (en) * | 2001-03-14 | 2004-09-08 | ŷ���ϰ��������о���֯ | Narrow bard high power fibre lasers with wider wavelength covering range |
| CN101162158A (en) * | 2007-11-15 | 2008-04-16 | 中国计量学院 | Ultra-remote distributed fiber raman and brillouin photons sensor |
| CN101762290A (en) * | 2010-02-03 | 2010-06-30 | 电子科技大学 | Distributed Raman amplification-based Brillouin optical time domain analysis system |
| US20110044371A1 (en) * | 2007-06-29 | 2011-02-24 | Uto International Corporation | Distributed optical fiber sensor system |
-
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- 2011-11-02 CN CN201110340681.2A patent/CN102506915B/en active Active
Patent Citations (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5265106A (en) * | 1991-06-10 | 1993-11-23 | Compagnie Industrielle Des Lasers Cilas | Raman Laser |
| CN1528035A (en) * | 2001-03-14 | 2004-09-08 | ŷ���ϰ��������о���֯ | Narrow bard high power fibre lasers with wider wavelength covering range |
| US20110044371A1 (en) * | 2007-06-29 | 2011-02-24 | Uto International Corporation | Distributed optical fiber sensor system |
| CN101162158A (en) * | 2007-11-15 | 2008-04-16 | 中国计量学院 | Ultra-remote distributed fiber raman and brillouin photons sensor |
| CN101762290A (en) * | 2010-02-03 | 2010-06-30 | 电子科技大学 | Distributed Raman amplification-based Brillouin optical time domain analysis system |
Non-Patent Citations (1)
| Title |
|---|
| 基于双向拉曼放大的布里渊光时域分析***;张超，等;《物理学报》;20100831;第59卷(第8期);5523-5527 * |
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