CN102645268A - Optical frequency division multiplexing phase-sensitive optical time domain reflectometer - Google Patents

Abstract

The invention discloses an optical frequency division multiplexing phase-sensitive optical time domain reflectometer which comprises a narrow line width laser, an optical fiber coupler, a phase modulator, a scrambler, an erbium-doped optical fiber amplifier, an acoustic optical modulator, an optical fiber circulator, a signal generator module, a trigger source, a double balance detector, a low pass filter, a data acquisition card, a computer and a long-distance sensing optical fiber. According to the optical frequency division multiplexing phase-sensitive optical time domain reflectometer, the fundamental contradictions between the measuring distance and sampling rate in a distributed sensing system can be solved, and remoter or higher-frequency vibration is detected; and moreover, due to amplitude and phase information, the characteristics of a vibration source are judged at the first time, and the early warning can be provided immediately.

Description

Optical frequency division multiplex phase sensitive optical time domain reflectometer

Technical field

The present invention relates to optical time domain reflectometer, particularly a kind of optical frequency division multiplex phase sensitive optical time domain reflectometer.

Background technology

Phase sensitive optical time domain reflectometer (Phase sensitive optical time domain reflectometry; Hereinafter to be referred as Phase-OTDR) be a kind of new distribution type Fibre Optical Sensor meter based on Rayleigh scattering, can be to carrying out remote real time monitoring along the extraneous dynamic physical amount in the fibre circuit scope.Optical time domain reflectometer (optical time domain reflectometry with routine; Abbreviate OTDR as) different be; Phase-OTDR adopts the laser instrument of narrow linewidth (being about KHz) and minimum frequency drift as light source; Obtain the various characteristic physical quantitys (amplitude, frequency, phase place etc.) at place, disturbance location through back in the direct impulse width regions to the interference signal of Rayleigh scattering light, and incident is positioned through the echo time.Therefore; It is except the many characteristics of conventional distributing optical fiber sensing; Also have advantages such as disguise, bearing accuracy is high, data processing is simple, be particularly suitable for the security monitorings such as rock gas, petroleum pipe line and the health monitoring of civilian installation such as civil engineering works such as bridge, heavy construction.

Since H.F.Taylor in 1993 proposed Phase-OTDR, the appearance of this technology had greatly improved the sensitivity of distributing optical fiber sensing technology.But many employings was the method for direct detection at that time; Positional information that can only observational measurement generation disturbance; Amplitude, frequency and phase information that can't the quantitative measurment disturbing signal; Specifically referring to [H.F.Taylor and C.E.Lee.Apparatus and method for fiber optic intrusion sensing.U.S.Patent 5,1993:194847.].In order to improve the performance of Phase-OTDR, researchers have carried out extensive and deep research:

Formerly the technology one [referring to Lidong Lu, Yuejiang Song, Fan Zhu, Xuping Zhang; Dual frequency probe based coherent optical time reflectometry, Opt.Commun., 2012; In press] utilize phase-modulator near the laser instrument centre frequency, to produce many sidebands, control depth of modulation (1.44) simultaneously, make its 0 grade; ± 1 grade accounts for its principal ingredient, and the light pulse that will include three kinds of frequencies then is injected into optical fiber, adopts the method for Coherent Detection; Research shows that this technology can effectively reduce relevant rayleigh noise, has improved the dynamic range of 3dB; But receive the mutual restriction (pulse width is 1us) of BPF. and pulse width, therefore good bearing accuracy can not be arranged, only be suitable for the monitoring of the submarine optical fiber cable transmission line of long distance.The more important thing is that this technology only rests in the observational measurement, do not have to realize quantitative measurment, comprise amplitude, frequency and phase information disturbing signal.

Formerly the technology two [referring to Y.L.Lu, T.Zhu, L.A.Chen; And X.Y.Bao, Distributed vibration sensor based on coherent detection of phase-OTDR, J.Lightw.Technol.; 2010; 28 (22), 3243-3249.] propose to utilize the method for Phase-OTDR Coherent Detection to go to measure the forcing frequency of extraneous sound field first, it is expanded to distributed vibrating sensing field.But also only be confined to the amplitude and the frequency information of observational measurement disturbing signal, do not have quantitatively to obtain the phase information of optical fiber link and disturbance location.On the other hand, owing to receive the restriction of sampling rate, the highest detectable forcing frequency is less than 1KHz, and spatial resolution is about 5m, then can't measure for the disturbance information of higher frequency.This just big limitations the range of application of this novel sensing technology, be the monitoring ultrasonic more than the MHz such as forcing frequency, thereby also can't embody the advance and the superiority of this technology to a certain extent.

Summary of the invention

In order to overcome the above-mentioned shortcoming of technology formerly; The present invention proposes a kind of optical frequency division multiplex phase sensitive optical time domain reflectometer, develops some bottleneck problems such as the low signal-to-noise ratio that is faced, low sampling rate, non-quantitation detection in the hope of breaking through at present relevant distributing optical fiber sensing field.

Technical solution of the present invention is following:

A kind of optical frequency division multiplex phase sensitive optical time domain reflectometer; Its characteristics are to comprise narrow linewidth laser, fiber coupler, phase-modulator, scrambler, Erbium-Doped Fiber Amplifier (EDFA), acousto-optic modulator, optical fiber circulator, signal generator module, triggering source, two balance detection device, low-pass filter, data collecting card, computing machine and long-distance sensing optical fiber, and the position of said elements concerns as follows:

The light that described narrow linewidth laser sends is divided into flashlight and this flash of light preceding an earthquake through described fiber coupler; Connect described phase-modulator, scrambler, Erbium-Doped Fiber Amplifier (EDFA), acousto-optic modulator and optical fiber circulator along the flashlight direction successively through optical fiber; Second port of this optical fiber circulator connects an end of described long-distance sensing optical fiber; The 3rd port of this optical fiber circulator connects the input end of described pair of balance detection device, the control end of the described phase-modulator of output termination of described signal generator module through optical fiber;

Described the flash of light preceding an earthquake imported another input end of described pair of balance detection device through optical fiber; The output terminal of this pair balance detection device is through described low-pass filter, the described computing machine of data acquisition clamping; This computing machine has the data processing module of Wiener filtering and quadrature phase demodulation thereof; The output terminal in described triggering source links to each other with the control end of described acousto-optic modulator, signal generator module and data collecting card respectively, carries out synchronous triggering control.

The light that narrow linewidth laser sends at first is divided into two-way through fiber coupler, and one the tunnel is flashlight, another Lu Weiben flash of light preceding an earthquake.Flashlight at first gets into phase-modulator, and said signal generator module removes to drive phase-modulator, thereby realizes the step-by-step movement frequency sweep of flashlight frequency.Then the light of phase-modulator output enters into scrambler, thereby eliminates the influence of polarization to system.And then be amplified into acousto-optic modulator through Erbium-Doped Fiber Amplifier (EDFA).Acousto-optic modulator is modulated into pulsed light with continuous light, on optical frequency, has introduced a constant frequency displacement simultaneously.Second port through three port circulators is injected into long-distance sensing optical fiber then.Backward scattered Reyleith scanttering light returns the output of process circulator the 3rd port and enters into two balance detection devices.This flash of light preceding an earthquake directly enters into two balance detection devices in addition.Since phase-modulator can near laser instrument output center frequency, produce many sidebands (0 grade, ± 1 grade, ± 2 grades ...), the beat signal of therefore two balance detection devices outputs is the effect through low-pass filter at first, other sidebands of filtering.Enter into data collecting card then, be converted into digital signal after, carry out real time data processing and demonstration at last on computers.In this system, the triggering source is connecting acousto-optic modulator, signal generator module and data collecting card simultaneously in addition, makes its three export signal Synchronization.

Characteristics of the present invention and advantage are:

(1) the present invention adopts a kind of Phase-OTDR based on digital Coherent Detection; Not only can improve the dynamic range of total system; Solve the low problem of sensitivity that the Rayleigh rear orientation light causes a little less than too; And whole follow-up signal Processing does not need loaded down with trivial details hardware device all with the computer software realization.Simultaneously in signal processing; In order to reduce various random noises, comprise the phase noise that laser instrument is introduced, ASE noise and thermonoise that amplifier EDFA and detector are introduced; We have adopted digital filtering (Wiener filtering) technology, thereby the phase place that is implemented under the low signal-to-noise ratio is accurately estimated.

(2) the present invention has adopted the quadrature phase demodulation technology, for existing P hase-OTDR has increased the phase demodulating function, has accomplished the breakthrough from the observational measurement to the quantitative measurment.Because the scattered light phase place is corresponding one by one with external disturbance, can obtain extraneous faint disturbance information, the sensitivity that has improved total system greatly through the demodulation phase variation.Simultaneously in order to overcome the mutual restriction relation of sampling rate and measuring distance in the system, the employing Optical Frequency Multiplexing Technology improves the sampling rate of system, thereby realizes the detection of farther distance or higher forcing frequency.

Description of drawings

Fig. 1 is the structural representation of optical frequency division multiplex phase sensitive optical time domain reflectometer embodiment of the present invention;

Fig. 2 is based on the modulation system structural representation of optical frequency division multiplex signal;

Fig. 3 is the process flow diagram of employed Wiener filtering and quadrature phase demodulation thereof among the present invention;

Embodiment

Below in conjunction with embodiment and accompanying drawing the present invention is described further.

See also Fig. 1 earlier; Fig. 1 is the structural representation of optical frequency division multiplex phase sensitive optical time domain reflectometer embodiment of the present invention; Visible by figure, the structure of optical frequency division multiplex phase sensitive optical time domain reflectometer of the present invention comprises narrow linewidth laser 1, fiber coupler 2, phase-modulator 3, scrambler 4, Erbium-Doped Fiber Amplifier (EDFA) 5, acousto-optic modulator 6, optical fiber circulator 7, signal generator module 8, triggering source 9, two balance detection device 10, low-pass filter 11, data collecting card 12, computing machine 13 and long-distance sensing optical fiber 14.

The light that narrow linewidth laser 1 sends at first is divided into two-way through fiber coupler 2, and one the tunnel is flashlight, another Lu Weiben flash of light preceding an earthquake.Flashlight gets into phase-modulator 3 along optical fiber 201, and the electric signal of signal generator module 8 outputs removes phase modulation modulator 3 through 801, thereby realizes the step-by-step movement frequency sweep of flashlight frequency, specifically can be referring to Fig. 2 (a).The continuous light of phase-modulator 3 outputs enters into scrambler 4 along optical fiber 301, thereby eliminates the influence of polarization to system.After then getting into Erbium-Doped Fiber Amplifier (EDFA) 5 amplifications, enter into acousto-optic modulator 6 along optical fiber 501 along optical fiber 401.Acousto-optic modulator 6 is modulated into pulsed light with continuous light, on optical frequency, has introduced a constant frequency displacement simultaneously, specifically can be referring to Fig. 2 (b).Enter into circulator 7 along optical fiber 601 then, second port 701 of process circulator 7 is injected into described long-distance sensing optical fiber 14.The 3rd port 702 outputs that the backward scattered Reyleith scanttering light of this long-distance sensing optical fiber 14 returns through described circulator 7 enter into described pair of balance detection device 10.This flash of light preceding an earthquake directly enters into two balance detection devices 10 through optical fiber 202 in addition.Since phase-modulator can near laser instrument output center frequency, produce many sidebands (0 grade, ± 1 grade, ± 2 grades ...).The beat signal of therefore two balance detection device 10 outputs at first enters into low-pass filter 11 through 1001, other sidebands of filtering.Enter into data collecting card 12 along 1101 then, be converted into digital signal after, get into computing machines 13 through 1201, this computing machine has the data processing module of Wiener filtering and quadrature phase demodulation thereof, thereby carries out follow-up digital signal processing.In this system, triggering source 9 is connecting acousto-optic modulator 6 through 901 in addition, is connecting the signal generator module through 902, is connecting data collecting card through 903, makes its three's signal output synchronously.

The explanation of each device is following:

Narrow linewidth laser 1 is the light source of optical frequency division multiplex phase sensitive optical time domain reflectometer of the present invention; The light source that adopts in the instance of the present invention is the single frequency optical fiber laser of short cavity phosphate dbr structure, also can adopt the narrow linewidth of other types, the laser instrument of minimum frequency drift.

Fiber coupler 2 is general single mode fibers, the 1550nm wave band, and port 2 * 2, splitting ratio are 1: 9.

Phase-modulator 3 is mainly used in the step-by-step movement frequency sweep that realizes the flashlight frequency, and the phase-modulator that adopts in the instance of the present invention is the polarization irrelevant phase-modulator, also can adopt other frequency shifter, electrooptic modulator for example, acousto-optic frequency shifters etc.

Scrambler 4 is mainly used in and eliminates the influence of polarization noise to system.

Erbium-Doped Fiber Amplifier (EDFA) 5 adopts commercial photoelectricity one EDFA module or the EDFA that oneself builds all can.

Acousto-optic modulator 6 is used for the continuous light of laser instrument output is modulated into pulsed light, and the acousto-optic modulator that adopts in the instance of the present invention is the acousto-optic modulator that AA company produces, and also can select the acousto-optic modulator or the electrooptic modulator of other types for use.

Fiber optical circulator 7 is one three fiber port circulators, also can adopt the way of incoming fiber optic coupling mechanism and isolator, plays the scheme that is equal to the fiber optical circulator effect.

Signal generator module 8; The rf modulated signal that generation is used for applying on the phase-modulator; What adopt in the instance of the present invention is the signal generator module that AD company produces, and also can select signal generator or signal generating circuit that other can the step-by-step movement frequency sweep for use.

Triggering source 9 is used to realize that signal generator module, acousto-optic modulator and data collecting card make its three synchronous.

Two balance detection devices 10; It is the high speed detector that light signal is converted into electric signal; The photodetector that adopts in the instance of the present invention is the detector that Newport company produces, and can realize the high-speed optical pulse acquisition of signal, can certainly select the high-speed photodetector of other types for use.

Low-pass filter 11, the radio-frequency component of main filtering electric signal.

Data collecting card 12 mainly carries out the data-triggered collection.In order to obtain high time resolution, need to adopt high-speed data acquisition card.The data collecting card that adopts among the present invention is the digitizer that Gage company produces, and also can adopt the high-speed collection card of other companies.

Computing machine 13 mainly is that the data that data capture card 12 collects are handled in real time and shown.This computing machine has the data processing module of Wiener filtering and quadrature phase demodulation thereof, and its process flow diagram is referring to Fig. 3.The system handles software of writing in the instance of the present invention is based on data collecting card and LabVIEW programmes, also can be based on the realization of programming of other platform such as Matlab or C language.

Long-distance sensing optical fiber 14 is single-mode fibers of common communications, and wavelength is 1550nm.

Ultimate principle of the present invention is following:

1, the sideband effect of phase-modulator

When angular frequency is that the light wave of ω incides phase-modulator, it will produce many equally spaced sidebands near optical frequencies, can be expressed as:

$E=\sqrt{P_0}\exp \left(\mathrm{jwt}\right)\underset{q=-\∞}{\overset{\∞}{\mathrm{\Σ}}}{J}_q\left(A_m\right)\exp \left(\mathrm{jq}{\mathrm{\ω}}_{m}t\right)$

The amplitude of representing phase-modulator output light when unmodulated, ω _mAnd A _mThe angular frequency and the depth of modulation of modulation signal of expression phase-modulator, q representes sideband frequency progression at different levels, and is desirable 0, ± 1, ± 2 ...In the instance of the present invention, we only select wherein-1 grade of sideband for use, through control depth of modulation A _m=1.8, make it reach maximal value, and other sidebands are less.

2, choosing of optical frequency division multiplex modulation parameter:

Because we only are to use-1 grade of sideband of phase-modulator, thus should manage to consider the swept frequency range of signal generator module, thus avoid the light generation aliasing of sidebands at different levels, finally pass through the light of other sidebands of low pass filter filters out.In Fig. 2, three physical parameters have been provided in addition: step-by-step movement frequency interval (Δ ω), time cycle (T) and pulse width (τ).τ has determined spatial resolution, and τ is more little, and spatial resolution is high more; T has determined temporal resolution, and T is more little, represents detectable frequency high more.In theory, under the certain situation of swept frequency range, (Δ ω) is more little for the step-by-step movement frequency interval, represents step-by-step movement frequency sweep number n big more.But seek out high spatial resolution, τ is more little, finally obtains the power spectrum of beat signal through Fast Fourier Transform (FFT), and its side frequency can be overlapped.Therefore, we must select reasonable modulation parameter, make it reach optimization.Normally, side frequency interval delta ω must be greater than the inverse that incides pulse width τ in the optical fiber.

3, follow-up digital signal processing

Long-distance sensing optical fiber 14 is incided in the different light pulses of different frequency constantly; The Rayleigh scattering light that returns at described pair of balance detection device 10 with after this flash of light preceding an earthquake mixes; The beat signal that obtains is through the data collecting card collection; Obtain the beat frequency digital signal of different frequency:

isolates the beat frequency digital signal of each frequency then through Fast Fourier Transform (FFT) (FFT) and window function (Blackman-Harris).

For the beat frequency digital signal of certain frequency (E for example _LOE _k(t) cos (Δ ω _kT+ φ (t))), Fig. 3 has provided the process flow diagram based on Wiener filtering and quadrature phase demodulation thereof.Its concrete steps are following:

At first the beat frequency digital signal is through digital smoothness filtering, the burr information that filtering is unnecessary;

And then computing machine 13 provides exp (the i Δ ω of a standard _kT) signal multiplies each other itself and beat frequency digital signal:

$E_{\mathrm{LO}}{E}_k\left(t\right)\cos (\mathrm{\Δ}{\mathrm{\ω}}_{k}t+\mathrm{\φ}\left(t\right))\×\exp \left(\mathrm{i\Δ}{\mathrm{\ω}}_{k}t\right)$

$=\frac{E_{\mathrm{LO}}{E}_k\left(t\right)}{2}[\cos \left(\mathrm{\φ}\left(t\right)\right)-i\sin \left(\mathrm{\φ}\left(t\right)\right)]+\frac{E_{\mathrm{Lo}}{E}_k\left(t\right)}{2}[\cos (2\mathrm{\Δ}{\mathrm{\ω}}_{k}t+\mathrm{\φ}\left(t\right))+i\sin (2\mathrm{\Δ}{\mathrm{\ω}}_{k}t+\mathrm{\φ}\left(t\right))$

Through wave digital lowpass filter, the high frequency item above the filtering in the expression formula obtains then

$\frac{E_{\mathrm{LO}}{E}_k\left(t\right)}{2}\cos \mathrm{\φ}\left(t\right)-i\frac{E_{\mathrm{LO}}{E}_k\left(t\right)}{2}\sin \mathrm{\φ}\left(t\right)---\left(1\right)$

Divided by the real part item, negate tangent then just can be in the hope of its phase information φ (t) with imaginary part item in the formula (1).

$\tan \left(\mathrm{\φ}\right)t=-\frac{\frac{E_{\mathrm{LO}}{E}_k\left(t\right)}{2}\sin \mathrm{\φ}\left(t\right)}{\frac{E_{\mathrm{LO}}{E}_k\left(t\right)}{2}\cos \mathrm{\φ}\left(t\right)}---\left(2\right)$

$\mathrm{\φ}\left(t\right)=-\mathrm{arcatn}-\frac{\frac{E_{\mathrm{LO}}{E}_k\left(t\right)}{2}\sin \mathrm{\φ}\left(t\right)}{\frac{E_{\mathrm{LO}}{E}_k\left(t\right)}{2}\cos \mathrm{\φ}\left(t\right)}---\left(3\right)$

In instance of the present invention, we adopt the Angle function in the MATLAB tool box, and the phase of trying to achieve (t) through phase unwrapping Unwrap function, is launched it between [π, π] at [∞ ,+∞], just can be in the hope of real phase information φ (t).

Certainly, in this system, exist various random noises, comprise the phase noise that laser instrument is introduced, ASE noise that amplifier EDFA and detector are introduced and thermonoise etc.Therefore we adopt Wiener filtering (Wiener) technology that described φ (t) is carried out filtering; Reduce various noises; Thereby obtain in accurate phase information

instance of the present invention, what we adopted is undelayed Wiener filtering transport function:

Wherein, φ (z) is for comprising the transform of various random noise φ (t);

is the transform through

after the Wiener filtering; α is by additive noise variance

and the common decision of phase noise variance

, and specifically expression is as follows:

$\mathrm{\α}=\frac{{\mathrm{\σ}}_w^2+{2\mathrm{\σ}}_n^2-{\mathrm{\σ}}_w\sqrt{{\mathrm{\σ}}_w^2+{4\mathrm{\σ}}_n^2}}{{2\mathrm{\σ}}_n^2}---\left(5\right)$

The phase noise variance of laser instrument can be expressed as

${\mathrm{\σ}}_w^2=2\mathrm{\πT\Δv}---\left(6\right)$

Wherein the time cycle of T indicating impulse, Δ v representes the live width of laser instrument.

For additive noise, we only consider the noise of two balance detection devices, adopt given noise power spectrum in the instructions, confirm its variance according to following formula:

${\mathrm{\σ}}_n^2=\frac{3B}{T^2}{S}_n\left(f\right)---\left(7\right)$

Wherein: B representes the bandwidth of system, the time cycle of T indicating impulse, S _n(f) be the noise power spectrum that provides in the instructions.

After (6) formula and (7) formula compute phase noise variance

and additive noise variance ; Carry it into (5) formula; Thereby calculate the α value; Bring undelayed Wiener filtering transport function (4) formula then into; Can obtain

and pass through the Z inverse transformation at last, obtain accurate phase information

Claims (1)

1. optical frequency division multiplex phase sensitive optical time domain reflectometer; It is characterized in that comprising narrow linewidth laser (1), fiber coupler (2), phase-modulator (3), scrambler (4), Erbium-Doped Fiber Amplifier (EDFA) (5), acousto-optic modulator (6), optical fiber circulator (7), signal generator module (8), triggering source (9), two balance detection device (10), low-pass filter (11), data collecting card (12), computing machine (13) and long-distance sensing optical fiber (14), the position of said elements concerns as follows:

The light that described narrow linewidth laser (1) sends is divided into flashlight and this flash of light preceding an earthquake through described fiber coupler (2); Connect described phase-modulator (3), scrambler (4), Erbium-Doped Fiber Amplifier (EDFA) (5), acousto-optic modulator (6) and optical fiber circulator (7) along the flashlight direction successively through optical fiber; Second port of this optical fiber circulator (7) connects an end of described long-distance sensing optical fiber (14); The 3rd port of this optical fiber circulator (7) connects the input end of described pair of balance detection device (10), the control end of the described phase-modulator of output termination (3) of described signal generator module (8) through optical fiber;

Described the flash of light preceding an earthquake imported another input end of described pair of balance detection device (10) through optical fiber; The output terminal of this pair balance detection device (10) connects described computing machine (13) through described low-pass filter (11), data collecting card (12); This computing machine has the data processing module of Wiener filtering and quadrature phase demodulation thereof; The output terminal in described triggering source (9) links to each other with described acousto-optic modulator (6), signal generator module (8) and the control end of data collecting card (12) respectively, carries out synchronous triggering control.

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