CN104697557A - Novel circular frequency shifting based BOTDR (Brillouin Optical Time Domain Reflectometer) coherent detection device and method - Google Patents
Novel circular frequency shifting based BOTDR (Brillouin Optical Time Domain Reflectometer) coherent detection device and method Download PDFInfo
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Abstract
The invention discloses a novel circular frequency shifting based BOTDR (Brillouin Optical Time Domain Reflectometer) coherent detection device. The device comprises a laser, a first coupler, a controllable circular frequency shifting module, an electrooptical modulator, an optical fiber circulator, a second coupler, a sensing optical fiber, a balancing detector, a signal conditioning circuit, an acquiring card and a synchronizing device; the controllable circular frequency shifting module comprises a first optical switch, an acoustic optical modulator, an optical fiber amplifier, an loop optical fiber and a second optical switch. The invention further discloses a method of the novel circular frequency shifting based BOTDR coherent detection device. According to the device, the controllable circular frequency shifting module is introduced based on the existing BOTDR coherent detection sensing system, so that the frequency shifting of detecting light is achieved, the PD bandwidth requirement is reduced, the system cost is reduced, and the energy is greatly saved; in addition, the frequency shifting times can be controlled, and the BOTDR frequency sweeping function can be achieved.
Description
Technical field
The present invention relates to technical field of optical fiber sensing, particularly a kind of novel B OTDR coherent detection apparatus and method based on circulating frequency shift.
Background technology
Brillouin light Time Domain Reflectometry (BOTDR) technology is the Distributed Optical Fiber Sensing Techniques just grown up in recent years.It utilizes the Brillouin frequency shift dorsad of optical fiber to optical fiber institute is strained and temperature is very responsive, and there is the characteristic of better linear relationship, measured object is carried out to the sensing detection of distributed strain and temperature, have and can measure multiple physical parameter (as temperature, strain, line loss etc.), spatial resolution is high, distance sensing is long, measuring accuracy advantages of higher, particularly it can obtain distributed intelligence measured in whole fiber area by one-shot measurement, and the location presentation of measured point can be become measured and function that the is time, thus hidden danger and fault are accurately identified and located, can also be detected the continuous distribution formula that large complicated result carries out two and three dimensions by the reasonable erection of optical fiber, at infrastructure engineering facility as bridge, tunnel, dam, gymnasium, the safety and Health of powerline network and oil and gas pipes etc. detects and fault pre-alarming and demonstrate very tempting prospect in assessing.
Current BOTDR Detection Techniques mainly contain two large modes: direct detection mode and coherent detection mode.Wherein incoherent technique directly pulse signal is input to sensor fibre by circulator, and the Brillouin scattering then by returning to scattering in circulator directly detects.Shown in this method and the similar accompanying drawing 2 of device, laser instrument produces continuous print laser, by coupling mechanism 1 light splitting, one tunnel as local oscillator light to coupling mechanism 2, another road produces pulsed light through electrooptic modulator (ElectroopticalModulator, EOM) modulation, is injected into sensor fibre back reflection and returns Brillouin scattering to coupling mechanism 2, after being balanced detector detection, after signal conditioning circuit conditioning, collected card gathers, and then processes.This difference utilized between multimodal Brillouin spectrum, this just needs the photodetector (Photoelectric Detector, PD) of high bandwidth.In optical fiber, the luminous power of spontaneous brillouin scattering is very weak, and direct detection is more difficult, and this just causes direct detection mode measuring accuracy limited.
Coherent detection is detection mode relatively more conventional at present, detect by local oscillator light and flashlight are combined to deliver in detector by coupling mechanism, the evolution of the product of the midfrequent AC component that last detector can be made to detect and local oscillation optical power and signal light power is directly proportional, and is being not be only directly proportional to signal light power; Balanced detector and 50/50 coupling mechanism is adopted to realize heterodyne detection in addition, can be that detector output current amplitude doubles, the noise in circuit can be suppressed well, obtain high detection sensitivity and common-mode rejection ratio, " the Strain variationmeasurement with STFT based BOTDR sensing system " of the people such as domestic painting Guo Jie just employs system architecture as shown in Figure 2.For the detection of brillouin scattering signal, the difference on the frequency identical with Brillouin shift is there is (for 1550nm light source between scattered signal light with local oscillator light, the about 11GHz of Brillouin shift of general single mode fiber), or need the PD of a very large bandwidth, thus improve system cost at this moment.Solution conventional at present carries out shift frequency to local oscillator light or detection light, generally select microwave source to control electrooptic modulator (EOM) to carry out frequency upper shift or carry out frequency to local oscillator light moving down to detection light, finally make the frequency of coherent signal in 100,000,000 Hz magnitudes, thus the requirement reduced PD bandwidth, but the microwave source price of high frequency also costly and the shift frequency efficiency of EOM by operating point drift affect poor stability.
The people such as Japanese Tsuneo Horiguchi proposes a kind of optical fiber shift frequency BOTDR coherent detection structure as shown in Figure 3 in " Development of a Distributed Sensing TechniqueUsing Brillouin Scattering " in addition, this structure is Coherent Self-Heterodyne detection mode detection light being carried out to shift frequency, the continuous mode light that laser instrument is launched is divided into two bundles by coupling mechanism, one tunnel is as local oscillator light, and a road is as detection light.Detection light is through acousto-optic modulator 1 (Acoustic OpticalModulator, AOM) pulsed light is modulated into, then by coupling mechanism, most of optically-coupled is entered shift frequency loop, pulsed light frequency upper shift Δ f is made in AOM2, then by Erbium-Doped Fiber Amplifier (EDFA) (Erbium Doped Fiber Amplifier, EDFA) again through coupling mechanism after amplifying, fraction light directly outputs to EDFA and is amplified into detection optical fiber, remain most of light and again enter shift frequency loop, carry out next step shift frequency, this process repeats N time.Finally f can be reached in frequency
0at this moment be exactly meet the requirements of detection light, obtain after entering sensor fibre Brillouin signal light together with local oscillator light relevant then to after mixing signal process, obtain the Brillouin shift that optical fiber is along the line, this mode is conducive to reducing system cost while guarantee high detection sensitivity.But this mode also has a shortcoming, the detection exported only frequency interval is the optical comb of Δ f, in order to avoid influencing each other of the Brillouin scattering between two bundle different frequency light, just require that Δ f is greater than Brillouin scattering spectrum width (being about hundreds of MHz), the energy of this structure waste detection light.
Summary of the invention
Technical matters to be solved by this invention overcomes the deficiencies in the prior art, and a kind of novel B OTDR coherent detection apparatus and method based on circulating frequency shift are provided, this device, method are on existing BOTDR coherent detection sensor-based system basis, by introducing controlled circulating frequency shift module, thus the frequency displacement achieved detection light, reduce the bandwidth requirement of PD, reduce system cost, greatly saved energy and can realize the function of BOTDR frequency sweep by controlling the number of times of shift frequency.
The present invention is for solving the problems of the technologies described above by the following technical solutions:
According to a kind of novel B OTDR coherent detection device based on circulating frequency shift that the present invention proposes, comprise laser instrument, the first coupling mechanism, electrooptic modulator, optical fiber circulator, the second coupling mechanism, sensor fibre, balanced detector, signal conditioning circuit, capture card, synchronous device; Also comprise controlled circulating frequency shift module, described controlled circulating frequency shift module comprises the first photoswitch, acousto-optic modulator, Erbium-Doped Fiber Amplifier (EDFA), Fiber In The Loop FITE and the second photoswitch; Wherein,
Capture card, for exporting trigger pip to synchronous device;
Synchronous device, for when after receiving trigger pip in the different time sections preset, respectively to the first photoswitch, the second photoswitch, acousto-optic modulator, electrooptic modulator input pulse signal, thus control the first photoswitch, the second photoswitch, acousto-optic modulator, electrooptic modulator work;
Laser instrument, for exporting continuous print narrow-linewidth laser to the first coupling mechanism;
First coupling mechanism, for narrow-linewidth laser is divided into two-way: the first via is that initial probe light inputs to the first photoswitch, the second tunnel is that local oscillator light inputs to the second coupling mechanism;
First photoswitch, for when receiving the high level that synchronous device exports, being inputted initial probe light by its first input end, and exporting acousto-optic modulator to by its output terminal;
Acousto-optic modulator, for becoming pulsed light to export Erbium-Doped Fiber Amplifier (EDFA) to initial probe optical modulation;
Erbium-Doped Fiber Amplifier (EDFA), exports the second photoswitch to after being amplified by pulsed light;
Second photoswitch, for when receiving the low level that synchronous device exports, being inputted the pulsed light after amplifying by its input end, and exporting Fiber In The Loop FITE to by its first output terminal;
Fiber In The Loop FITE, for receive when the first photoswitch synchronous device export low level time, pulsed light after amplification is inputed to the second input end of the first photoswitch, so circulate in the circulation passage that is made up of the first photoswitch, acousto-optic modulator, Erbium-Doped Fiber Amplifier (EDFA), the second photoswitch, Fiber In The Loop FITE, when meeting the loop termination condition preset, synchronous device output low level to the second photoswitch, the second output terminal of the second photoswitch exports front end detection light to electrooptic modulator;
Electrooptic modulator, inputs to optical fiber circulator for the front end received detection optical modulation being become final detection light;
Optical fiber circulator, is inputted by its 1st port for finally detecting light, and is injected into sensor fibre by its 2nd port;
Sensor fibre, for when receiving final detection light, producing the 2nd port that Brillouin scattering inputs to optical fiber circulator, and exporting the second coupling mechanism to by the 3rd port of optical fiber circulator;
Second coupling mechanism, after the Brillouin scattering that will receive and local oscillator optical mixing, exports coherent light to balanced detector;
Balanced detector, exports signal conditioning circuit to after converting coherent light to electric signal;
Signal conditioning circuit, for nursing one's health electric signal, exporting conditioned signal and obtaining spectrum information to capture card is treated.
As a kind of further prioritization scheme of novel B OTDR coherent detection device based on circulating frequency shift of the present invention, described in the loop termination condition that presets be cycle index.
As a kind of further prioritization scheme of novel B OTDR coherent detection device based on circulating frequency shift of the present invention, described first coupling mechanism is 90:10 coupling mechanism.
As a kind of further prioritization scheme of novel B OTDR coherent detection device based on circulating frequency shift of the present invention, described second coupling mechanism is 50:50 coupling mechanism.
Based on a kind of method of the novel B OTDR coherent detection device based on circulating frequency shift, comprise the following steps,
Step one, be set in advance in different time sections input the first switch pulse respectively to the first photoswitch, the second photoswitch inputs the second switch pulse, acousto-optic modulator inputs the first modulating pulse, electrooptic modulator inputs the second modulating pulse;
Step 2, employing laser instrument export continuous print narrow-linewidth laser, and this narrow-linewidth laser is divided into two-way through the first coupling mechanism: the first via inputs to the first photoswitch as initial probe light, and the second tunnel is that local oscillator light inputs to the second coupling mechanism;
Step 3, when the first switch pulse is high level, the first input end of the first photoswitch is in time period t
1conducting, generation pulse width is t
1laser pulse input to acousto-optic modulator; In time period t
1after, the second input end conducting of the first photoswitch;
Step 4, control the pulse width t of the first modulating pulse
2, acousto-optic modulator output frequency moves the pulsed light of Δ f to Erbium-Doped Fiber Amplifier (EDFA), the pulsed light after amplification inputs to the second photoswitch; Wherein, t
2>t
1;
Step 5, when the second switch pulse be low level, pulsed light after amplification is exported by the first output terminal of the second photoswitch, after Fiber In The Loop FITE, input to again the second input end of the first photoswitch, so circulate in the circulation passage that is made up of the first photoswitch, acousto-optic modulator, Erbium-Doped Fiber Amplifier (EDFA), the second photoswitch, Fiber In The Loop FITE; When meeting the loop termination condition preset, the second switch pulse is low level, then the second output terminal of the second photoswitch exports front end detection light to electrooptic modulator, through being modulated into pulse width t
0final detection light input to optical fiber circulator by the 1st port of optical fiber circulator, and be injected into sensor fibre by the 2nd port of optical fiber circulator; Wherein, t
2>t
1>t
0;
Step 6, finally detect light when sensor fibre receives, produce the 2nd port that Brillouin scattering inputs to optical fiber circulator, and exported by the 3rd port of optical fiber circulator and after mixing, exported coherent light with local oscillator light, obtain spectrum information through conditioning, process after this coherent light converts electric signal to.
The present invention adopts above technical scheme compared with prior art, has following technique effect:
(1) the present invention adopts controlled circulating frequency shift module that detection light is carried out to shift frequency and carries out coherent detection, the bandwidth requirement to PD can be reduced, the high frequency modulated microwave source of EOM can be saved, reduce system cost, the accurate control of capture card pulse simultaneously to system and device can put forward high-octane utilization factor;
(2) by changing the times N of controlled circulating frequency shift module shift frequency, the pulsed light centre frequency be finally input in sensor fibre can be changed, thus with f
0centered by travel through whole Brillouin spectrum (spectrum width is about hundreds of MHz), its frequency difference is Δ f (Δ f scope is at 5-10MHz), just achieves the function of BOTDR frequency sweep.
Accompanying drawing explanation
Fig. 1 is system construction drawing of the present invention.
Fig. 2 is BOTDR coherent detection system construction drawing.
Fig. 3 is optical fiber shift frequency BOTDR coherent detection system construction drawing.
Fig. 4 is the frequency diagram of BOTDR frequency sweep.
Fig. 5 is the four tunnel gating pulse synchronized relation figure that synchronous device produces.
Embodiment
Below in conjunction with accompanying drawing, technical scheme of the present invention is described in further detail:
As shown in Figure 1, based on a novel B OTDR coherent detection device for circulating frequency shift, comprise laser instrument, the first coupling mechanism, controlled circulating frequency shift module, electrooptic modulator, optical fiber circulator, the second coupling mechanism, sensor fibre, balanced detector, signal conditioning circuit, capture card, synchronous device; Described controlled circulating frequency shift module comprises the first photoswitch, acousto-optic modulator, Erbium-Doped Fiber Amplifier (EDFA), Fiber In The Loop FITE, the second photoswitch; Wherein,
Capture card, for exporting trigger pip to synchronous device;
Synchronous device, for when after receiving trigger pip in the different time sections preset, respectively to the first photoswitch, the second photoswitch, acousto-optic modulator, electrooptic modulator input pulse signal, thus control the first photoswitch, the second photoswitch, acousto-optic modulator, electrooptic modulator work;
Laser instrument, for exporting continuous print narrow-linewidth laser to the first coupling mechanism;
First coupling mechanism, for narrow-linewidth laser is divided into two-way: the first via is that initial probe light inputs to the first photoswitch, the second tunnel is that local oscillator light inputs to the second coupling mechanism;
First photoswitch, for when receiving the high level that synchronous device exports, initial probe light is inputted by the first input end of the first photoswitch, and exports acousto-optic modulator to by the output terminal of the first photoswitch;
Acousto-optic modulator, for becoming pulsed light to export Erbium-Doped Fiber Amplifier (EDFA) to initial probe optical modulation;
Erbium-Doped Fiber Amplifier (EDFA), exports the second photoswitch to after being amplified by pulsed light;
Second photoswitch, for when receiving the low level that synchronous device exports, the pulsed light after amplification is inputted by the input end of the second photoswitch, and exports Fiber In The Loop FITE to by the first output terminal of the second photoswitch;
Fiber In The Loop FITE, for receive when the first photoswitch synchronous device export low level time, pulsed light after amplification is inputed to the second input end of the first photoswitch, so circulate in the circulation passage that is made up of the first photoswitch, acousto-optic modulator, Erbium-Doped Fiber Amplifier (EDFA), the second photoswitch, Fiber In The Loop FITE, when meeting the loop termination condition preset, synchronous device output low level to the second photoswitch, the second output terminal of the second photoswitch exports front end detection light to electrooptic modulator;
Electrooptic modulator, inputs to optical fiber circulator for the front end received detection optical modulation being become final detection light;
Optical fiber circulator, for inputing to optical fiber circulator by finally detecting the 1st port of light by optical fiber circulator, and is injected into sensor fibre by the 2nd port of optical fiber circulator;
Sensor fibre, for when receiving final detection light, producing the 2nd port that Brillouin scattering inputs to optical fiber circulator, and exporting the second coupling mechanism to by the 3rd port of optical fiber circulator;
Second coupling mechanism, after the Brillouin scattering that will receive and local oscillator optical mixing, exports coherent light to balanced detector;
Balanced detector, exports signal conditioning circuit to after converting coherent light to electric signal;
Signal conditioning circuit, for nursing one's health electric signal, exporting conditioned signal and obtaining spectrum information to capture card is treated.
The described loop termination condition preset is cycle index.
Described first coupling mechanism is 90:10 coupling mechanism.
Described second coupling mechanism is 50:50 coupling mechanism.
The model of laser instrument of the present invention is Agilent N7714, and this laser wavelength is 1550nm, and live width is 100kHz, and Output optical power is 16dBm; The switching rate of the first photoswitch, the second photoswitch is 100kHz; The model of acousto-optic modulator is Gooch & Housego, can realize the frequency upper shift of 200MHz; Erbium-Doped Fiber Amplifier (EDFA) (EDFA) model is Amonics, and the highest gain amplifier is 23dBm; EOM model is Photline 20GHz intensity modulator, and output extinction ratio is 40dB; Balanced detector bandwidth 1GHz, enlargement factor 40dB.
Specific experiment condition: the length of sensor fibre is 25km, under normal temperature, the Brillouin shift of general single mode fiber is 10.85Ghz; The pulse width entering sensor fibre is 50ns; Because this experiment AOM single shift frequency is 200MHz, so the frequency displacement of about 11GHz will be realized, need 55 times; Fiber lengths in shift frequency loop is 500m, to add on loop that the time delay on two photoswitches, AOM and EDFA is that (the optical patchcord length of each optical device is 2m to 120ns, ignoring the delay of device inside structure), then whole optical fiber circuit will produce the delay of about 5.12us, being then 281.6us completing the whole cyclic process time, being equivalent to the time of propagating in the optical fiber that a 28.16km is long.
The concrete steps of Binding experiment parameter are as follows:
Step one: laser instrument produces CW Mode for Laser by 90/10 coupling mechanism, the laser of 10% is as the local oscillator light of coherent detection, and original frequency is f
z, the photoswitch 1 that at this moment in capture card, the first switch pulse controls makes the conducting of A1 and K1 road, and Shi90%Na road laser enters closed circuit, at time t
1b1 and K1 conducting after=1us, block the light path that continuous light enters, generation pulse width is t
1the laser pulse of=1us.
Step 2: at this moment t
1=1us pulsed light arrives AOM, by controlling the pulse width t of the first modulating pulse
2=2us (wherein t
2>t
1) working time of control AOM covers light pulse time of passing through, obtain the pulsed light of frequency upper shift Δ f, then amplified by EDFA and make up pulse in whole closed circuit through loss that the first photoswitch, AOM, Fiber In The Loop FITE bring, then through the second photoswitch, at this moment the second photoswitch opens K2 and B2 path under the control of the second switch pulse, make the pulsed light through amplifying again enter closed circuit by the first photoswitch through Fiber In The Loop FITE, wherein circulation required time is Δ t=5.12us at every turn.
Step 3: pulsed light has circulated N=55 time altogether in the loop, moves f in light frequency
0=11GHZ, required time is t
b=281.6us.Then control to make K2 and A2 path by the second switch pulse, frequency displacement light injects to EOM from closed circuit, and under the control of the second modulating pulse, pulsed light being modulated into width is t
0pulsed light (the t of=50ns
2>t
1>t
0), inject after in circulator and enter sensor fibre, simultaneously due to t
1>t
0, the halfway part of pulsed light edge shift frequency can be avoided, ensure the consistance of the direct impulse light center frequency that EOM exports.
Step 4: the actual frequency shift due to Brillouin scattering is f
b=10.85GHZ, therefore by frequency upper shift f
0the Stokes light frequency that scattering is returned is f
z+ 150MHZ, this signal and frequency are f
zlocal oscillator light relevant after, balanced detector will obtain the intermediate-freuqncy signal of 150MHz, this signal is after amplifying, with the sampling rate collection of 2GHz, at numeric field, signal is processed, obtain the Brillouin shift of optical fiber along road, through relevant theory calculate process thus the sensing achieved temperature/strain in sensor fibre.
The difference of each change cycle index N, makes the last pulsed frequency produced at f
0along with frequency difference Δ f travels through (Δ f scope is at 5-10MHz) in neighbouring certain limit, thus achieve the function of frequency sweep, ergodic process as shown in Figure 4.
Being the four tunnel gating pulse synchronized relation figure that synchronous device produces as shown in Figure 5, is the graph of a relation of the first switch pulse, the first modulating pulse, the second switch pulse, the second modulating pulse respectively in figure.
Above content is in conjunction with concrete preferred implementation further description made for the present invention, can not assert that specific embodiment of the invention is confined to these explanations.For general technical staff of the technical field of the invention, without departing from the inventive concept of the premise, some simple deductions can also be made or substitute, all should be considered as belonging to protection scope of the present invention.
Claims (5)
1., based on a novel B OTDR coherent detection device for circulating frequency shift, comprise laser instrument, the first coupling mechanism, electrooptic modulator, optical fiber circulator, the second coupling mechanism, sensor fibre, balanced detector, signal conditioning circuit, capture card, synchronous device; It is characterized in that, also comprise controlled circulating frequency shift module, described controlled circulating frequency shift module comprises the first photoswitch, acousto-optic modulator, Erbium-Doped Fiber Amplifier (EDFA), Fiber In The Loop FITE and the second photoswitch; Wherein,
Capture card, for exporting trigger pip to synchronous device;
Synchronous device, for when after receiving trigger pip in the different time sections preset, respectively to the first photoswitch, the second photoswitch, acousto-optic modulator, electrooptic modulator input pulse signal, thus control the first photoswitch, the second photoswitch, acousto-optic modulator, electrooptic modulator work;
Laser instrument, for exporting continuous print narrow-linewidth laser to the first coupling mechanism;
First coupling mechanism, for narrow-linewidth laser is divided into two-way: the first via is that initial probe light inputs to the first photoswitch, the second tunnel is that local oscillator light inputs to the second coupling mechanism;
First photoswitch, for when receiving the high level that synchronous device exports, being inputted initial probe light by its first input end, and exporting acousto-optic modulator to by its output terminal;
Acousto-optic modulator, for becoming pulsed light to export Erbium-Doped Fiber Amplifier (EDFA) to initial probe optical modulation;
Erbium-Doped Fiber Amplifier (EDFA), exports the second photoswitch to after being amplified by pulsed light;
Second photoswitch, for when receiving the low level that synchronous device exports, being inputted the pulsed light after amplifying by its input end, and exporting Fiber In The Loop FITE to by its first output terminal;
Fiber In The Loop FITE, for receive when the first photoswitch synchronous device export low level time, pulsed light after amplification is inputed to the second input end of the first photoswitch, so circulate in the circulation passage that is made up of the first photoswitch, acousto-optic modulator, Erbium-Doped Fiber Amplifier (EDFA), the second photoswitch, Fiber In The Loop FITE, when meeting the loop termination condition preset, synchronous device output low level to the second photoswitch, the second output terminal of the second photoswitch exports front end detection light to electrooptic modulator;
Electrooptic modulator, inputs to optical fiber circulator for the front end received detection optical modulation being become final detection light;
Optical fiber circulator, is inputted by its 1st port for finally detecting light, and is injected into sensor fibre by its 2nd port;
Sensor fibre, for when receiving final detection light, producing the 2nd port that Brillouin scattering inputs to optical fiber circulator, and exporting the second coupling mechanism to by the 3rd port of optical fiber circulator;
Second coupling mechanism, after the Brillouin scattering that will receive and local oscillator optical mixing, exports coherent light to balanced detector;
Balanced detector, exports signal conditioning circuit to after converting coherent light to electric signal;
Signal conditioning circuit, for nursing one's health electric signal, exporting conditioned signal and obtaining spectrum information to capture card is treated.
2. a kind of novel B OTDR coherent detection device based on circulating frequency shift according to claim 1, is characterized in that, described in the loop termination condition that presets be cycle index.
3. a kind of novel B OTDR coherent detection device based on circulating frequency shift according to claim 1, it is characterized in that, described first coupling mechanism is 90:10 coupling mechanism.
4. a kind of novel B OTDR coherent detection device based on circulating frequency shift according to claim 1, it is characterized in that, described second coupling mechanism is 50:50 coupling mechanism.
5., based on the method for a kind of novel B OTDR coherent detection device based on circulating frequency shift according to claim 1, it is characterized in that, comprise the following steps,
Step one, be set in advance in different time sections input the first switch pulse respectively to the first photoswitch, the second photoswitch inputs the second switch pulse, acousto-optic modulator inputs the first modulating pulse, electrooptic modulator inputs the second modulating pulse;
Step 2, employing laser instrument export continuous print narrow-linewidth laser, and this narrow-linewidth laser is divided into two-way through the first coupling mechanism: the first via inputs to the first photoswitch as initial probe light, and the second tunnel is that local oscillator light inputs to the second coupling mechanism;
Step 3, when the first switch pulse is high level, the first input end of the first photoswitch is in time period t
1conducting, generation pulse width is t
1laser pulse input to acousto-optic modulator; In time period t
1after, the second input end conducting of the first photoswitch;
Step 4, control the pulse width t of the first modulating pulse
2, acousto-optic modulator output frequency moves the pulsed light of Δ f to Erbium-Doped Fiber Amplifier (EDFA), the pulsed light after amplification inputs to the second photoswitch; Wherein, t
2>t
1;
Step 5, when the second switch pulse be low level, pulsed light after amplification is exported by the first output terminal of the second photoswitch, after Fiber In The Loop FITE, input to again the second input end of the first photoswitch, so circulate in the circulation passage that is made up of the first photoswitch, acousto-optic modulator, Erbium-Doped Fiber Amplifier (EDFA), the second photoswitch, Fiber In The Loop FITE; When meeting the loop termination condition preset, the second switch pulse is low level, then the second output terminal of the second photoswitch exports front end detection light to electrooptic modulator, through being modulated into pulse width t
0final detection light input to optical fiber circulator by the 1st port of optical fiber circulator, and be injected into sensor fibre by the 2nd port of optical fiber circulator; Wherein, t
2>t
1>t
0;
Step 6, finally detect light when sensor fibre receives, produce the 2nd port that Brillouin scattering inputs to optical fiber circulator, and exported by the 3rd port of optical fiber circulator and after mixing, exported coherent light with local oscillator light, obtain spectrum information through conditioning, process after this coherent light converts electric signal to.
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