CN102937416B - A kind of fully distributed fiber switched based on orthogonal polarisation state strains and vibration sensing method and device - Google Patents
A kind of fully distributed fiber switched based on orthogonal polarisation state strains and vibration sensing method and device Download PDFInfo
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Abstract
A kind of fully distributed fiber switched based on orthogonal polarisation state strains and vibration sensor, comprise laser instrument (1), polarization-maintaining coupler (2), polarization maintaining optical fibre, pulse modulation module (3), image intensifer (4), circulator (5), sensor fibre (6), polarization state changeover apparatus (7), coupling mechanism (8), balance photodetector (9), microwave source (10), frequency mixer (11), bandpass filter (12), Signal acquiring and processing unit (13); The continuous light of the output of laser instrument is divided into two-way after polarization-maintaining coupler (2): wherein in the first via that exports of polarization-maintaining coupler (2), all connecting fibers are polarization maintaining optical fibre, after polarization state changeover apparatus, then be input to the first input end of coupling mechanism (8); Polarization state changeover apparatus has two states, makes the polarization direction of the linearly polarized light of output correspond to fast axle and the slow axis of polarization maintaining optical fibre respectively; Bandpass filter output terminal is connected to Signal acquiring and processing unit.
Description
Technical field
The present invention relates to a kind of fiber optic sensing devices strain and vibration being carried out to full distributed monitoring, especially one is in conjunction with Brillouin light Time Domain Reflectometry (BOTDR) technology and polarized light time domain reflection (POTDR) technology, and adopts fully distributed fiber strain and the vibrating sensing technology of polarization state changeover apparatus and relevant Heterodyne detect.
Background technology
Owing to being subject to external environment (as temperature when optical fiber, pressure, vibration etc.) when affecting, the intensity of transmission light in optical fiber, phase place, frequency, the parameters such as polarization state will change accordingly, just can obtain respective physical amount by these parameters measuring transmission light, this technology is called optical fiber sensing technology.
Relative to traditional electric quantity type sensor, Fibre Optical Sensor has highly sensitive, electromagnetism interference, volume is little, low price, the advantage of remote distributed measurement can be carried out, therefore since late 1970s, optical fiber sensing technology obtains and develops widely, has occurred based on Rayleigh scattering, Brillouin scattering, the distributed optical fiber sensing technology of Raman scattering etc.These optical fiber sensing technologies utilize the different physical parameters of light wave in optical fiber to carry out sensing, different sensing parameter that can be corresponding from physical quantity in measuring optical fiber.
1) optical fiber is strained when affecting, and the frequency of the Brillouin scattering that light wave produces wherein can offset, and is called Brillouin shift.The size that the size of Brillouin shift amount is strained to optical fiber is directly proportional.Brillouin light Time Domain Reflectometry (BOTDR) technology is by injected pulse light in optical fiber, and the Brillouin shift of Brillouin scattering that ranging pulse light produces in communication process in a fiber continuously, and then determine the strain information of optical fiber each position along the line.BOTDR technology is topmost a kind of distributed optical fiber sensing technology that can strain size in Measurement accuracy optical fiber so far.But the measurement capability of BOTDR technology to vibration event is more weak.Although because optical fiber can produce strain variation when being subject to extraneous vibration and affecting in theory simultaneously.But the impact that the faint strain caused due to microvibration on the one hand produces Brillouin shift is less, BOTDR technology is comparatively slow to the measuring speed of strain on the other hand, usually needs more than tens seconds.Therefore, BOTDR technology is difficult to use in measuring vibrations.
2) polarized light time domain reflection (POTDR) technology is injected pulse light in optical fiber equally.But the external event of optical fiber each position along the line is determined in the change of the polarization state of the scattered light that it is returned at optical fiber along the line by ranging pulse light, thus carries out full distributed measurement.Because the change of optical polarization in optical fiber is very sensitive to the response of external event, therefore can be used for measuring faint external event.Simultaneously because POTDR technology to judge the change of scatter light polarization state by light intensity signal, the response time is short, therefore can be used for measuring disturbance fast or vibration.For the distance sensing of 1km, the measurable maximum vibration frequency of POTDR is within 100KHz.But because the polarization state of light wave in general single mode fiber is difficult to keep constant in long-time, and the change of the strain that is subject to of optical fiber and polarization state is not one_to_one corresponding, and therefore POTDR technology is difficult to detect quasi-static strain and larger strain.
Summary of the invention
The object of the invention is, provide a kind of not only can monitor strain but also the distributed optical fiber sensing device of fast vibration change can be measured.Especially the fully distributed fiber switched based on orthogonal polarisation state strains and vibration sensing method and device.Both can monitor strain event, again can measuring vibrations event, substantially increase measurement function and the range of application of distributed optical fiber sensing device, and ensure measuring accuracy.
Technical scheme of the present invention is, a kind of fully distributed fiber switched based on orthogonal polarisation state strains and vibration sensor, it is characterized in that comprising laser instrument (1), polarization-maintaining coupler (2), polarization maintaining optical fibre, pulse modulation module (3), image intensifer (4), circulator (5), sensor fibre (6), polarization state changeover apparatus (7), coupling mechanism (8), balance photodetector (9), microwave source (10), frequency mixer (11), bandpass filter (12), Signal acquiring and processing unit (13); The continuous light of the output of laser instrument (1) is divided into two-way after polarization-maintaining coupler (2): in the first via of wherein polarization-maintaining coupler output, all connecting fibers are polarization maintaining optical fibre, light wave is wherein as reference light, after polarization state changeover apparatus (7), then be input to the first input end of coupling mechanism (8); Polarization state changeover apparatus (7) has two states, makes the polarization direction of the linearly polarized light of output correspond to fast axle and the slow axis of polarization maintaining optical fibre respectively;
Sensor fibre (6) is injected as direct impulse light through circulator (5) behind the second tunnel pulse modulated module (3) that polarization-maintaining coupler exports, image intensifer (4); Brillouin scattering in sensor fibre enters the second input end of coupling mechanism (8) after circulator (5) returns; The mixed signal that Brillouin scattering and reference light are formed in coupling mechanism (8) is input to balance in photodetector (9) through two output terminals of coupling mechanism (8) and is converted into electric signal;
The output terminal of balance photodetector (9) is connected with the first input end of frequency mixer (11), microwave source (10) is connected with the second input end of frequency mixer (11), the output terminal of frequency mixer (11) is connected to bandpass filter (12), and the output terminal of bandpass filter (12) is connected to Signal acquiring and processing unit (13).
Polarization state changeover apparatus (7), when arbitrary a single state, carries out relevant Heterodyne detect by Brillouin scattering and reference light, obtains the change information of scatter light polarization state, determines external vibration event information with this.
Polarization state changeover apparatus (7) can be polarization switch, Polarization Controller or polarization analysis instrument etc. any the polarization state of linearly polarized light is switched mutually orthogonal two polarization directions device or instrument, linearly polarized light corresponds respectively to the first and second two kinds of duties of polarization state changeover apparatus (7) at the polarization of these two orthogonal directionss.
A kind ofly not only monitor strain but also the distributed optical fiber sensing device can measuring fast vibration change measuring optical fiber method for sensing can be carried out according to provided by the present invention, when polarization state changeover apparatus (7) is in any one duty, the signal that sensor fibre obtains is scattered light signal in sensor fibre and polarization direction along the linearly polarized light of the fast axle of polarization maintaining optical fibre or slow axis through coupling mechanism (8) and the result that balances after the relevant Heterodyne detect of photodetector (9); When sensor fibre (6) certain position be subject to strain affect time, the size of Brillouin shift of the Brillouin scattering of this position generation changes; When being subject to vibration effect, this position direct impulse polarisation of light state changes, then the polarization state of the Brillouin scattering in sensor fibre (6) changes; Therefore by measuring the centre frequency of Brillouin scattering, sensing being carried out to the strain of sensor fibre (6), by measuring the polarization state change of Brillouin scattering, sensing being carried out to the vibration of sensor fibre (6).
Further, the signal that sensor fibre (6) obtains includes the information of Brillouin scattering polarization state change, utilizes polarized light time domain reflection (POTDR) principle to measure, utilizes the change information of scatter light polarization state to carry out sensing to vibration.
Further, when polarization state changeover apparatus (7) to work etc. long-time respectively under the first and second two kinds of duties, then the heterodyne signal power obtained under these two kinds of duties is added up, then can eliminate the impact of optical polarization change in sensor fibre.Brillouin light Time Domain Reflectometry (BOTDR) principle now can be utilized to measure, utilize the frequency information of Brillouin scattering to carry out sensing to the strain in optical fiber.
When polarization state changeover apparatus (7) is in any one duty, because the polarization state of pulsed light diverse location is in a fiber different, there is corresponding undulation by the pulsed light obtained after the relevant Heterodyne detect of balance photodetector (9) at the Brillouin signal that optical fiber produces along the line.After Brillouin signal in different time repetitive measurement optical fiber, by comparing the difference of undulation between them or utilizing the frequency spectrum of methods analyst optical fiber position undulation change of time-frequency conversion, pulsed light can be obtained in optical fiber in the situation of change of the polarization state of certain position.
Constantly can produce Brillouin scattering along the line at optical fiber when pulsed light transmits in sensor fibre, its centre frequency has certain skew relative to the centre frequency of detection light, and this skew is called as Brillouin shift v
b; Be v by the set of frequency of microwave source (10)
m(v
mwith v
bcomparatively close), then this high-frequency signal is after frequency mixer (11), and its centre frequency can be switched to | v
b-v
m|; Adjusted the frequency of the output signal of frequency mixer (11) by the frequency of scanning microwave source (10), when the signal power by bandpass filter (12) reaches maximum, can obtain the Brillouin shift of Brillouin scattering, its size is v
b=v
m-f
bPF, wherein f
bPFfor the centre frequency of bandpass filter (12).The scattered light that sensing device utilizes when measuring vibrations and strain is Brillouin scattering.
Beneficial effect of the present invention: present invention incorporates BOTDR technology and POTDR technology, both can monitor strain event, again can measuring vibrations event, substantially increase measurement function and the range of application of distributed optical fiber sensing device, significantly can reduce the rate of failing to report of system simultaneously.The present invention only utilizes Brillouin scattering light signal to carry out the sensing straining and vibrate, and system and device is simple, holistic cost and number of devices much less than the independent superposition of two systems.While utilizing POTDR technology to carry out vibration survey, by switching the duty of polarization state changeover apparatus, realize the measurement utilizing BOTDR technology to strain, therefore can carry out the measurement of vibration and strain simultaneously, in Measuring Time, ratio utilizes BOTDR technology and the Measuring Time of POTDR technology to strain and vibration to shorten a lot respectively, and can realize the uninterrupted measurement of strain and vibration.
Accompanying drawing explanation
Fig. 1 is that a kind of fully distributed fiber switched based on polarization state provided by the invention strains and vibration sensor structural drawing.
Embodiment
As shown in Figure 1, a kind of fully distributed fiber switched based on orthogonal polarisation state strains the structure with vibration sensor, comprises laser instrument (1), polarization-maintaining coupler (2), polarization maintaining optical fibre (2-1), pulse modulation module (3), image intensifer (4), circulator (5), sensor fibre (6), polarization state changeover apparatus (7), coupling mechanism (8), balance photodetector (9), microwave source (10), frequency mixer (11), bandpass filter (12), Signal acquiring and processing unit (13).The continuous light of the output of laser instrument (1) is divided into two-way after polarization-maintaining coupler (2): in the first via of wherein polarization-maintaining coupler output, all connecting fibers are polarization maintaining optical fibre, light wave is wherein as reference light, after polarization state changeover apparatus (7), then be input to the first input end of coupling mechanism (8); Polarization state changeover apparatus (7) has two states, makes the polarization direction of the linearly polarized light of output correspond to fast axle and the slow axis of polarization maintaining optical fibre respectively; Sensor fibre (6) is injected as direct impulse light through circulator (5) behind the second tunnel pulse modulated module (3) that polarization-maintaining coupler exports, image intensifer (4); Brillouin scattering in sensor fibre enters the second input end of coupling mechanism (8) after circulator (5) returns; The mixed signal that Brillouin scattering and reference light are formed in coupling mechanism (8) is input to balance in photodetector (9) through two output terminals of coupling mechanism (8) and is converted into electric signal.
The output terminal of balance photodetector (9) is connected with the first input end of frequency mixer (11), microwave source (10) is connected with the second input end of frequency mixer (11), the output terminal of frequency mixer (11) is connected to bandpass filter (12), and the output terminal of bandpass filter (12) is connected to Signal acquiring and processing unit (13).
The connecting fiber of reference path is polarization maintaining optical fibre, and the optical fiber of detection light path can be single-mode fiber, also can be polarization maintaining optical fibre, the better effects if of polarization maintaining optical fibre.
Polarization state changeover apparatus (7) can be polarization switch, Polarization Controller or polarization analysis instrument etc. any the polarization state of linearly polarized light is switched mutually orthogonal two polarization directions device or instrument, linearly polarized light corresponds respectively to the first and second two kinds of duties of polarization state changeover apparatus (7) at the polarization of these two orthogonal directionss.
Pulse modulation module (3) is positioned at detection light path, and polarization state changeover apparatus (7) is positioned at reference path.Pulse modulation module is used for Light Modulation to become pulsed light.The width of pulsed light is determined by spatial resolution required when measuring, and is generally between the μ s of 10ns ~ 10.
The scattered light that sensing device utilizes when measuring vibrations and strain is Brillouin scattering.
As one specifically to the example that strain and vibration are measured, its concrete implementation step is as follows:
The continuous light that laser instrument (1) sends is linearly polarized light, and its centre wavelength is 1550nm, and live width is 2MHz, and power is 10dBm.This light wave is divided into two-way after polarization-maintaining coupler (2), wherein the first via is as reference light, after polarization state changeover apparatus (7) the optical device analyser N7788B of Agilent company of the U.S. (used herein be), enter the first input end of the coupling mechanism (8) that coupling ratio is 50:50.Second road light pulse modulated module (3) (electrooptic modulator) is modulated into the pulsed light that pulsewidth is 20ns, then after image intensifer (4) (Erbium-Doped Fiber Amplifier (EDFA)) amplifies, enters sensor fibre (6) as direct impulse light through circulator (5).Sensor fibre (6) is common single-mode fiber, its refractive index n=1.46, the velocity of sound v in optical fiber
a=5945m/s.Constantly can produce Brillouin scattering along the line at optical fiber when pulsed light transmits in sensor fibre, its centre frequency has certain skew relative to the centre frequency of detection light, and this skew is called as Brillouin shift v
b, its size v
b=2nv
a/ λ=11.2GHz.When certain position of sensor fibre be subject to strain affect time, the size of the Brillouin shift of the Brillouin scattering that this position can be made to produce changes, when being subject to vibration effect, this position direct impulse polarisation of light state can be made to change, and then the polarization state of the Brillouin scattering causing it to produce in a fiber change.Therefore by measuring the centre frequency of Brillouin scattering, sensing can being carried out to the strain of optical fiber, by measuring the polarization state change of Brillouin scattering, sensing can be carried out to the vibration of optical fiber.
Until after Brillouin scattering returns along optical fiber, enter coupling mechanism (8) together with the reference light in the first via.Their mixed signal is converted to electric signal after balance photodetector (9) reception that response frequency can cover about 11.2GHz.The frequency of this electric signal is the difference on the frequency of Brillouin scattering and reference light, and its centre frequency is v
b.Be v by the set of frequency of microwave source (10)
b(v
mwith v
bcomparatively close), then this high-frequency signal is after frequency mixer (11), and its centre frequency can be switched to | v
b-v
m|.Adjusted the frequency of the output signal of frequency mixer (11) by the frequency of scanning microwave source (10), when the signal power by bandpass filter (12) reaches maximum, can obtain the Brillouin shift of Brillouin scattering, its size is v
b=v
m-f
bPF, wherein f
bPFfor the centre frequency of bandpass filter (12).
Direct impulse light constantly can produce backward Brillouin scattering during transmission in sensor fibre (6), for the Brillouin scattering avoiding different direct impulse light to produce in sensor fibre (6) disturbs mutually, can only allow to there is a direct impulse light in sensor fibre (6) when gathering signal simultaneously.Therefore pulse modulation module (3) is synchronous with Signal acquiring and processing unit (13).Timing when just to have incided from direct impulse light when receiving Brillouin scattering light in sensor fibre (6), until just can make pulse modulation module (3) produce next direct impulse light after receiving the Brillouin scattering that direct impulse light produces at sensor fibre (6) end.The position l producing Brillouin scattering in sensor fibre (6) obtains by the light velocity in optical fiber and mistiming, i.e. l=c Δ t/2n, wherein c ≈ 3 × 10
8m/s is the light velocity in vacuum, Δ t for from timing to the Brillouin scattering institute elapsed time receiving required location.Be L sensor fibre (6) for length, the time interval Δ T of two direct impulse light should be greater than 2nL/c, i.e. pulsed light time of needing of round trip in a fiber, if sensor fibre (6) length is 10km, then and 2nL/c ≈ 100 μ s.
When each microwave source changes to new frequency, polarization state changeover apparatus (7) is first operated in the first duty, makes the polarization direction of reference light along the slow axis of polarization maintaining optical fibre in the first via.Because the polarization state of the Brillouin scattering produced from optical fiber diverse location is different, so mix with reference light the signal obtained after balancing the relevant Heterodyne detect of photodetector (9) from the Brillouin scattering of optical fiber diverse location generation can produce corresponding undulation, this can affect the accuracy to brillouin frequency shift measurement, but this contains the information of optical fiber optical polarization along the line simultaneously.Because the energy of Brillouin scattering is very little, for obtaining higher signal to noise ratio (S/N ratio), when each microwave source changes to new frequency, the signal to obtaining all is needed to carry out N progressive mean.When polarization state changeover apparatus (7) is in the first duty, to signal repeated measurement N/2 time, the time therefore needed is N Δ T/2.If sensor fibre (6) length is time interval Δ T=120 μ s between 10km, two pulsed lights, average time N=16384 time, then the time N Δ T/2 ≈ 1s that needs under this duty of polarization state changeover apparatus (7).Then measure the change at optical fiber diverse location place of the signal that obtains for this N/2 time by contrast or utilize these signals of the methods analyst of time-frequency conversion can obtain the situation of change of optical fiber optical polarization along the line at the frequency spectrum at optical fiber diverse location place, and then realization is to the sensing of optical fiber along linearly coupled.Then make polarization state changeover apparatus (7) be operated in the second duty, make the polarization direction of reference light along the fast axle of polarization maintaining optical fibre in the first via.The signal now obtained includes the information of optical fiber optical polarization along the line equally.Under this state to signal repeated measurement N/2 time, by analyzing the situation of change that can obtain optical fiber optical polarization along the line equally, and then can ensure that the Vibration Condition to optical fiber is along the line carries out continual sensing measurement.The signal that polarization state changeover apparatus (7) measures N/2 time respectively under two kinds of duties is carried out progressive mean, the polarization state can eliminated due to Brillouin scattering changes the undulation of the relevant heterodyne signal caused along the line at optical fiber, also be equivalent to carry out N time on average to Brillouin signal simultaneously, thus improve the accuracy to Brillouin signal power measurement.And then change the frequency of microwave source, then repeat above-mentioned measuring process, finally can obtain Brillouin shift, and then realize the measurement strained in optical fiber.
Claims (5)
1. the fully distributed fiber switched based on orthogonal polarisation state strains and vibration sensor, it is characterized in that comprising laser instrument (1), polarization-maintaining coupler (2), polarization maintaining optical fibre (2-1), pulse modulation module (3), image intensifer (4), circulator (5), sensor fibre (6), polarization state changeover apparatus (7), coupling mechanism (8), balance photodetector (9), microwave source (10), frequency mixer (11), bandpass filter (12), Signal acquiring and processing unit (13); The continuous light of the output of laser instrument (1) is divided into two-way after polarization-maintaining coupler (2): wherein in the first via that exports of polarization-maintaining coupler (2), all connecting fibers are polarization maintaining optical fibre, light wave is wherein as reference light, after polarization state changeover apparatus (7), then be input to the first input end of coupling mechanism (8); Polarization state changeover apparatus (7) has two states, makes the polarization direction of the linearly polarized light of output correspond to fast axle and the slow axis of polarization maintaining optical fibre respectively;
Sensor fibre (6) is injected as direct impulse light through circulator (5) behind the second tunnel pulse modulated module (3) that polarization-maintaining coupler (2) exports, image intensifer (4); Brillouin scattering in sensor fibre (6) enters the second input end of coupling mechanism (8) after circulator (5) returns; The mixed signal that Brillouin scattering and reference light are formed in coupling mechanism (8) is input to balance in photodetector (9) through two output terminals of coupling mechanism (8) and is converted into electric signal;
The output terminal of balance photodetector (9) is connected with the first input end of frequency mixer (11), microwave source (10) is connected with the second input end of frequency mixer (11), the output terminal of frequency mixer (11) is connected to bandpass filter (12), and the output terminal of bandpass filter (12) is connected to Signal acquiring and processing unit (13); Make the polarization state of linearly polarized light be polarization switch, Polarization Controller or polarization analysis instrument at the polarization state changeover apparatus (7) that mutually orthogonal two polarization directions switch, linearly polarized light corresponds respectively to the first and second two kinds of duties of polarization state changeover apparatus (7) at the polarization of mutually orthogonal two orthogonal directionss; Pulse modulation module is positioned at detection light path, and polarization state changeover apparatus is positioned at reference path.
2. the fully distributed fiber switched based on orthogonal polarisation state according to claim 1 strains and vibration sensor, it is characterized in that when polarization state changeover apparatus (7) is in any one duty, the signal that sensor fibre (6) obtains is scattered light signal in sensor fibre (6) and polarization direction along the linearly polarized light of the fast axle of polarization maintaining optical fibre or slow axis through coupling mechanism (8) and the result that balances after the relevant Heterodyne detect of photodetector (9); When sensor fibre (6) certain position be subject to strain affect time, the size of Brillouin shift of the Brillouin scattering of this position generation changes; When being subject to vibration effect, this position direct impulse polarisation of light state changes, and then the polarization state of the Brillouin scattering in sensor fibre (6) is changed; Therefore by measuring the centre frequency of Brillouin scattering, sensing being carried out to the strain of sensor fibre (6), by measuring the polarization state change of Brillouin scattering, sensing being carried out to the vibration of sensor fibre (6);
The signal that sensor fibre (6) obtains includes the information of Brillouin scattering polarization state change, utilizes polarized light time domain reflection method to measure, utilizes the change information of scatter light polarization state to carry out sensing to vibration.
3. the fully distributed fiber switched based on orthogonal polarisation state according to claim 2 strains and vibration sensor, it is characterized in that waiting for a long time when polarization state changeover apparatus (7) works respectively under the first and second two kinds of duties, the heterodyne signal power obtained under these two kinds of duties is added up, then can eliminate the impact of optical polarization change in sensor fibre; Brillouin light time domain reflection method now can be utilized to measure, utilize the frequency information of Brillouin scattering to carry out sensing to the strain in optical fiber.
4. the fully distributed fiber switched based on orthogonal polarisation state according to claim 3 strains and vibration sensor, it is characterized in that when polarization state changeover apparatus (7) is in any one duty, because the polarization state of pulsed light diverse location is in a fiber different, there is corresponding undulation by the pulsed light obtained after the relevant Heterodyne detect of balance photodetector (9) at the Brillouin signal that optical fiber produces along the line; After Brillouin signal in different time repetitive measurement optical fiber, by comparing the difference of undulation between them or utilizing the frequency spectrum of methods analyst optical fiber position undulation change of time-frequency conversion, pulsed light can be obtained in optical fiber in the situation of change of the polarization state of certain position.
5. the fully distributed fiber switched based on orthogonal polarisation state according to claim 3 strains and vibration sensor, constantly Brillouin scattering can be produced along the line at optical fiber when it is characterized in that pulsed light transmits in sensor fibre, its centre frequency has certain skew relative to the centre frequency of detection light, and this skew is called as Brillouin shift
ν b ; By the set of frequency of microwave source (10) be
ν m ,
ν m with
ν b close, then this high-frequency signal is after frequency mixer (11), and its centre frequency can be turned changes ∣
ν b -ν m ∣; Adjusted the frequency of the output signal of frequency mixer (11) by the frequency of scanning microwave source (10), when the signal power by bandpass filter (12) reaches maximum, can obtain the Brillouin shift of Brillouin scattering, its size is
ν b =
ν m -f bPF , wherein
f bPF for the centre frequency of bandpass filter (12).
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