CN100576049C - A kind of method and apparatus of optical fiber optical grating sensing network demodulation - Google Patents

Abstract

The present invention relates to a kind of method and apparatus of optical fiber Bragg grating sensing network fast demodulation.Use the continuous semiconductor laser instrument of narrow linewidth as light source among the present invention; Acousto-optic modulator is asymmetric to be placed in the Sarnia gram ring; The FBG sensing network is connected with Sarnia gram ring by one three port 3-dB fiber coupler; Each FBG that connects on the sensing network at room temperature bragg wavelength is consistent with the centre wavelength of LASER Light Source; But acousto-optic modulator is by the radio frequency signals drive of changeable frequency, when radio frequency signal frequency changes, Sarnia restrains interferometric transmissivity and changes, by fast fourier transform and related operation, obtain the drift value of the bragg wavelength of each sensing FBG on the sensing network, finally obtain the variation of the physical quantity of institute's sensing.The present invention is a kind of novel FBG sensing network, and being applicable to needs that multinode, high precision, anti-electromagnetic interference (EMI) are good, the field of the sensing network of real-time monitoring.

Description

A kind of method and apparatus of optical fiber optical grating sensing network demodulation

Technical field

The invention belongs to technical field of optical fiber sensing, being applicable to needs that multinode, high precision, anti-electromagnetic interference (EMI) are good, the field of the sensing network of real-time monitoring.Be particularly related to a kind of novel acousto-optic modulator change frequency of light wave that utilizes and caused that the effect of interferometer dissymmetrical structure and Fiber Bragg Grating FBG (FBG) hypotenuse detection technique realize the method for FBG sensing network demodulation, and the equipment of realizing this method.

Background technology

FBG owing to its distinctive inside of optical fibre sensitivity, Wavelength-encoding, be easy to a kind of important devices that advantages such as networking become Fibre Optical Sensor.Based on wavelength-division multiplex that has grown up (WDM) and Time Division Multiplexing technology, optical fiber optical grating array is widely used in the quasi-distributed sensing of optical fiber, as: the health monitoring of railway, bridge, dam etc., the monitoring temperature that the trunk power transmission line is along the line.Wherein, WDM requires each FBG sensor to have different bragg wavelengths, need a wideband light source (bandwidth is often greater than 40nm) as the input light source, also need the wavelength sensitive system of a cover relative complex to carry out the demultiplexing of multiplexed signals, as: adjustable Fabry Perot wave filter, Fourier spectrometer, wavelength sensitive coupling mechanism etc.And in TDM, generally adopt the input of burst pulse light source, the reflected light signal of each FBG sensor to utilize different time-delays to realize signal multiplexing, and by the high-speed door treatment circuit demultiplexing of demodulating end.This two covers multiplex technique all needs higher light source of cost and complicated demodulating system, has directly caused the cost of FBG sensing network high, has limited its a lot of practical applications.

Acousto-optic modulator utilizes Doppler effect, can accurately change the frequency through the light wave of modulator within the specific limits, is a kind of optical frequency translation device commonly used in the industry; Sarnia gram (Sagnac) ring is because the symmetry of its structure, temperature variation to external world, and mechanical vibration etc. are disturbed insensitive, and at Fibre Optical Sensor, fields such as optical fiber detection are used also very extensive; The quasi-distributed sensing network of FBG that the use Sagnac interferometer that change is changeed based on the acousto-optic modulator optical frequency is realized, simple in structure, strong interference immunity does not need photovalve at a high speed, and cost is very low, therefore is highly suitable for practical application.

Summary of the invention

Purpose of the present invention is exactly at the deficiencies in the prior art, uses acousto-optic modulator optical frequency translation technology, has proposed a kind of solution that realizes the quasi-distributed sensing network of novel FBG.Use (CW) semiconductor laser light resource continuously, low speed photo diodes, low speed data capture card and Fourier frequency meter, and the Sagnac interferometer is as the system core element, have low cost, anti-interference good, simple and practical, can realize the advantage that detects in real time.The equipment of realizing this method is provided simultaneously.

Method of the present invention may further comprise the steps:

1, as light source, laser instrument 3dB live width is smaller or equal to 0.1nm at the single-frequency continuous semiconductor laser instrument of optical communicating waveband for use centre wavelength.The laser that light source sends by fibre optic isolater and four port 3-dB fiber couplers after, enter in the Sagnac ring.

2, laser is divided into two-way after entering the Sagnac ring, wherein one the tunnel at first passes through the known measuring section single-mode fiber of length, incides the FBG sensing network through three port 3-dB fiber couplers again; Incident light is by each FBG reflection of the series connection on the sensing network, and reflected light is got back in the Sagnac ring through three port 3-dB fiber couplers; After the laser of getting back to Sagnac ring passes through acousto-optic modulator again, the frequency blue shift.Crystalline material is TeO in the described acousto-optic modulator ₂, sinusoidal signal generator produces drive acousto-optic modulator, TeO ₂Crystal forms Bragg grating under ultrasound wave drives, Doppler effect makes the one-level light generation frequency displacement of diffraction; Laser after the frequency conversion is got back to four port 3-dB fiber couplers at last through the known linkage section single-mode fiber of length; The phase place recruitment has nothing in common with each other when each sensing FBG laser light reflected is got back to four port 3-dB fiber couplers on the sensing network, wherein by i sensing FBG laser light reflected phase place recruitment is:

$\mathrm{\Δ}{\mathrm{\φ}}_{1-i}=2\mathrm{\πn}[\frac{L_a+2L_i}{C}v+\frac{L_b}{C}(v+\mathrm{\Δv})]---\left(1\right)$

Wherein subscript i represents i sensing FBG on the sensing network, Δ φ _1-iBe the phase place recruitment of i sensing FBG laser light reflected on the sensing network, n is the refractive index of single-mode fiber when frequency is v, and C is the light velocity, and v is the centre frequency of semiconductor laser, and Δ v is the driving frequency of acousto-optic modulator, L _iBe the fiber lengths between i sensing FBG on the sensing network and the three port 3-dB fiber couplers, L _aBe the length of measuring section single-mode fiber, L _bBe the length of linkage section single-mode fiber, L _a＞＞L _b

Another road laser at first passes through the known linkage section single-mode fiber of length, pass through acousto-optic modulator again after, the laser frequency blue shift; Laser after the frequency conversion incides the FBG sensing network through three port 3-dB fiber couplers; Incident light is by each FBG reflection of the series connection on the sensing network, and reflected light is got back in the Sagnac ring through three port 3-dB fiber couplers; The laser of getting back to the Sagnac ring passes through the known measuring section single-mode fiber of length again, gets back to four port 3-dB fiber couplers; The phase place recruitment has nothing in common with each other when each sensing FBG laser light reflected is got back to four port 3-dB fiber couplers on the sensing network, wherein by i sensing FBG laser light reflected phase place recruitment is:

${\mathrm{\Δ\φ}}_{2-i}=2\mathrm{\πn}[\frac{L_b}{C}v+\frac{L_a+2L_i}{C}(v+\mathrm{\Δv})]+\mathrm{\π}---\left(2\right)$

Δ φ wherein _2-iPhase place recruitment for i sensing FBG laser light reflected on another road sensing network.The phase place that π introduces for the Sagnac interferometer increases.

Two bundle laser of i FBG correspondence interfere in four port 3-dB fiber couplers, and the phase place of the laser of transmission is respectively

$\mathrm{\Δ}{\mathrm{\φ}}_{\mathrm{Ti}}=2\mathrm{\πn}\frac{L_a+2L_i-L_b}{C}\mathrm{\Δv}+\mathrm{\π}---\left(3\right)$

The driving frequency Δ v of acousto-optic modulator makes linear change according to ω t, and by linear sweep, the transmitted light intensity of each sensing FBG laser light reflected is pressed cos (f respectively _iT) change

$f_{i}t=2\mathrm{\πn}\frac{L_a+2L_i-L_b}{C}\mathrm{\ωt}---\left(4\right)$

F wherein _iFrequency for the light intensity variation.

3, the intensity that sees through the light wave of Sagnac ring is surveyed by photodiode, after light intensity signal is converted into electric signal, through data collecting card, carries out fast Fourier transform (FFT), can obtain each corresponding peak of formula (4) on frequency spectrum.By measuring the frequency f at each peak _iSize can obtain the position L of sensing FBG on the FBG sensing network of each peak correspondence _i

$L_i=(\frac{C{f}_i}{2\mathrm{\πn\ω}}-L_a+L_b)/2---\left(5\right)$

4, on the frequency spectrum intensity at each peak by the position extent decision of the centre wavelength of each sensing FBG centre wavelength of correspondence and light source.Change centre wavelength that each minute measure the FBG in the light path to the drift of long wave direction, the relation of the intensity change at each corresponding peak on the amount of movement of records center wavelength and the frequency spectrum.

5, each FBG is installed in the environment that needs sensing, when measured physical quantity changes in the environment, use the data of step 4 record, by the intensity change amount of each sensing FBG at the peak value of frequency spectrum correspondence, obtain the amount of movement of each FBG centre wavelength, thereby obtain the environmental physics amount of each FBG institute sensing.

The wavelength shift of the centre wavelength of FBG and be prior art to the pass between the variation of the environment of inductive sensing (variations such as little curved, temperature, stress).

The equipment of realizing such scheme is: semiconductor laser is connected by the input port light of fibre optic isolater with four port 3-dB fiber couplers; Another input port of four port 3-dB fiber couplers is connected with the input end light of photodiode, and photodiode output is electrically connected with the input end of data collecting card, and the output terminal of data collecting card is electrically connected with the fft analysis instrument.

Two output ports of four port 3-dB fiber couplers connect by single-mode fiber, and single-mode fiber is divided into two sections by acousto-optic modulator, are respectively measuring section single-mode fiber and linkage section single-mode fiber, and length is respectively L _aAnd L _b, L _a＞＞L _bThe electric drive signal mouth of acousto-optic modulator is electrically connected with sinusoidal signal generator.Insert three port 3-dB fiber couplers in the measuring section single-mode fiber, the two-port of the input end of three port 3-dB fiber couplers is connected with two sections measuring section single-mode fibers respectively, and another port is connected with the sensing network single-mode fiber.A plurality of sensing FBG are connected on the sensing network single-mode fiber successively.

Among the present invention, acousto-optic modulator utilizes Doppler effect, can accurately change the frequency through the light wave of modulator within the specific limits, is a kind of optical frequency translation device commonly used in the industry.Simultaneously, it is insensitive that Sagnac encircles the interference of temperature variation, mechanical vibration etc. to external world.

The present invention is applicable to the quasi-distributed sensing network of general FBG, compare with traditional FBG sensing network scheme, adopted electronic frequency scanning, rather than use the Wavelength scanning device of the machinery control of low speed, can accomplish high response speed, satisfy the requirement of real-time sensing; And owing to do not need short-pulse laser, high-speed photodiode and high-speed data acquisition card, so cost is relatively low; Because sensing network is connected in the Sagnac ring, anti-ambient temperature fluctuation of equipment and mechanical disturbance performance are strong in addition.

Description of drawings

Fig. 1 is an one-piece construction synoptic diagram of the present invention;

Fig. 2 is the calibration data of the present invention's one specific embodiment.

Embodiment

As shown in Figure 1, semiconductor laser 1 is connected with an input port light of four port 3-dB fiber couplers 3 by fibre optic isolater 2; Another input port of four port 3-dB fiber couplers 3 is connected with the input end light of photodiode 4, and the output terminal of photodiode 4 is electrically connected with the input end of data collecting card 5, and the output terminal of data collecting card 5 is electrically connected with fft analysis instrument 6.Two output ports of four port 3-dB fiber couplers 3 connect by single-mode fiber, and single-mode fiber is divided into two sections by acousto-optic modulator 8, are respectively measuring section single-mode fiber 11 and linkage section single-mode fiber 7, and length is respectively L _aAnd L _b, L _a＞＞L _bThe electric drive signal mouth of acousto-optic modulator 8 is electrically connected with sinusoidal signal generator 9.The two-port of inserting the input end of three port 3-dB fiber couplers, 10, three port 3-dB fiber couplers 10 in the measuring section single-mode fiber 11 is connected with two sections measuring section single-mode fibers 11 respectively, and another port is connected with sensing network single-mode fiber 13.A plurality of sensing FBG 12 are connected on the sensing network single-mode fiber 13 successively.

Concrete detection method may further comprise the steps:

Use centre wavelength at the single-frequency continuous semiconductor laser instrument of optical communicating waveband as light source, laser instrument 3dB live width is smaller or equal to 0.1nm.The laser that light source sends by fibre optic isolater and four port 3-dB fiber couplers after, enter in the Sagnac ring.Laser is divided into two-way after entering the Sagnac ring, wherein one the tunnel at first passes through the known measuring section single-mode fiber of length, incides the FBG sensing network through three port 3-dB fiber couplers again; Incident light is by each FBG reflection of the series connection on the sensing network, and reflected light is got back in the Sagnac ring through three port 3-dB fiber couplers; After the laser of getting back to Sagnac ring passes through acousto-optic modulator again, the frequency blue shift.Crystalline material is TeO in the described acousto-optic modulator ₂, sinusoidal signal generator produces drive acousto-optic modulator, TeO ₂Crystal forms Bragg grating under ultrasound wave drives, Doppler effect makes the one-level light generation frequency displacement of diffraction; Laser after the frequency conversion is got back to four port 3-dB fiber couplers at last through the known linkage section single-mode fiber of length; The phase place recruitment has nothing in common with each other when each sensing FBG laser light reflected is got back to four port 3-dB fiber couplers on the sensing network, wherein by i sensing FBG laser light reflected phase place recruitment is:

$\mathrm{\Δ}{\mathrm{\φ}}_{1-i}=2\mathrm{\πn}[\frac{L_a+2L_i}{C}v+\frac{L_b}{C}(v+\mathrm{\Δv})]---\left(1\right)$

Wherein subscript i represents i sensing FBG on the sensing network, Δ φ _1-iBe the phase place recruitment of i sensing FBG laser light reflected on the sensing network, n is the refractive index of single-mode fiber when frequency is v, and C is the light velocity, and v is the centre frequency of semiconductor laser, and Δ v is the driving frequency of acousto-optic modulator, L _iBe the fiber lengths between i sensing FBG on the sensing network and the three port 3-dB fiber couplers, L _aBe the length of measuring section single-mode fiber, L _bBe the length of linkage section single-mode fiber, L _a＞＞L _b

Another road laser at first passes through the known linkage section single-mode fiber of length, pass through acousto-optic modulator again after, the laser frequency blue shift; Laser after the frequency conversion incides the FBG sensing network through three port 3-dB fiber couplers; Incident light is by each FBG reflection of the series connection on the sensing network, and reflected light is got back in the Sagnac ring through three port 3-dB fiber couplers; The laser of getting back to the Sagnac ring passes through the known measuring section single-mode fiber of length again, gets back to four port 3-dB fiber couplers; The phase place recruitment has nothing in common with each other when each sensing FBG laser light reflected is got back to four port 3-dB fiber couplers on the sensing network, wherein by i sensing FBG laser light reflected phase place recruitment is:

$\mathrm{\Δ}{\mathrm{\φ}}_{2-i}=2\mathrm{\πn}[\frac{L_b}{C}v+\frac{L_a+2L_i}{C}(v+\mathrm{\Δv})]+\mathrm{\π}---\left(2\right)$

Δ φ wherein _2-iPhase place recruitment for i sensing FBG laser light reflected on another road sensing network.The phase place that π introduces for the Sagnac interferometer increases.

Two bundle laser of i FBG correspondence interfere in four port 3-dB fiber couplers, and the phase place of the laser of transmission is respectively

$\mathrm{\Δ}{\mathrm{\φ}}_{\mathrm{Ti}}=2\mathrm{\πn}\frac{L_a+2L_i-L_b}{C}\mathrm{\Δv}+\mathrm{\π}---\left(3\right)$

The driving frequency Δ v of acousto-optic modulator makes linear change according to ω t, and by linear sweep, the transmitted light intensity of each sensing FBG laser light reflected is pressed cos (f respectively _iT) change

$f_{i}t=2\mathrm{\πn}\frac{L_a+2L_i-L_b}{C}\mathrm{\ωt}---\left(4\right)$

F wherein _iFrequency for the light intensity variation.

The intensity that sees through the light wave of Sagnac ring is surveyed by photodiode, after light intensity signal is converted into electric signal, through data collecting card, carries out fast Fourier transform (FFT), can obtain each corresponding peak of formula (4) on frequency spectrum.By measuring the frequency f at each peak _iSize can obtain the position L of sensing FBG on the FBG sensing network of each peak correspondence _i

$L_i=(\frac{C{f}_i}{2\mathrm{\πn\ω}}-L_a+L_b)/2---\left(5\right)$

The intensity at each peak is by the position extent decision of the centre wavelength of each sensing FBG centre wavelength of correspondence and light source on the frequency spectrum.Change centre wavelength that each minute measure the FBG in the light path to the drift of long wave direction, the relation of the intensity change at each corresponding peak on the amount of movement of records center wavelength and the frequency spectrum.

Each FBG is installed in the environment that needs sensing, when measured physical quantity changes in the environment, use the data of calibration, by the intensity change amount of each sensing FBG at the peak value of frequency spectrum correspondence, obtain the amount of movement of each FBG centre wavelength, thereby obtain the environmental physics amount of each FBG institute sensing.

Claims (2)

1, a kind of method of optical fiber optical grating sensing network demodulation is characterized in that this method may further comprise the steps:

(1) as light source, laser instrument 3dB live width is smaller or equal to 0.1nm at the single-frequency continuous semiconductor laser instrument of optical communicating waveband for use centre wavelength; The laser that light source sends by fibre optic isolater and four port 3-dB fiber couplers after, enter in the Sarnia gram ring;

(2) laser is divided into two-way after entering Sarnia gram ring, wherein one the tunnel at first passes through the known measuring section single-mode fiber of length, incides the bragg grating sensing network through three port 3-dB fiber couplers again; Incident light is by each bragg grating reflection of the series connection on the sensing network, and reflected light is got back in the Sarnia gram ring through three port 3-dB fiber couplers; After the laser of getting back to Sarnia gram ring passes through acousto-optic modulator again, the frequency blue shift;

Crystalline material is TeO in the described acousto-optic modulator ₂, sinusoidal signal generator produces drive acousto-optic modulator, TeO ₂Crystal forms Bragg grating under ultrasound wave drives, Doppler effect makes the one-level light generation frequency displacement of diffraction; Laser after the frequency conversion is got back to four port 3-dB fiber couplers at last through the known linkage section single-mode fiber of length; The phase place recruitment has nothing in common with each other when being got back to four port 3-dB fiber couplers by each bragg grating laser light reflected on the sensing network, wherein by i bragg grating laser light reflected phase place recruitment is:

$\mathrm{\Δ}{\mathrm{\φ}}_{1-i}=2\mathrm{\πn}[\frac{L_a+2L_i}{\text{C}}v+\frac{L_b}{C}(v+\mathrm{\Δv})---\left(1\right)$

Wherein subscript i represents i bragg grating on the sensing network, Δ φ _1-iBe the phase place recruitment of i bragg grating laser light reflected on the sensing network, n is the refractive index of single-mode fiber when frequency is v, and C is the light velocity, and v is the centre frequency of semiconductor laser, and Δ v is the driving frequency of acousto-optic modulator, L _iBe the fiber lengths between i bragg grating on the sensing network and the three port 3-dB fiber couplers, L _aBe the length of measuring section single-mode fiber, L _bBe the length of linkage section single-mode fiber, L _a＞＞L _b

Another road laser at first passes through the known linkage section single-mode fiber of length, pass through acousto-optic modulator again after, the laser frequency blue shift; Laser after the frequency conversion incides the bragg grating sensing network through three port 3-dB fiber couplers; Incident light is by each bragg grating reflection of the series connection on the sensing network, and reflected light is got back in the Sarnia gram ring through three port 3-dB fiber couplers; The laser of getting back to Sarnia gram ring passes through the known measuring section single-mode fiber of length again, gets back to four port 3-dB fiber couplers; The phase place recruitment has nothing in common with each other when being got back to four port 3-dB fiber couplers by each bragg grating laser light reflected on the sensing network, wherein by i bragg grating laser light reflected phase place recruitment is:

$\mathrm{\Δ}{\mathrm{\φ}}_{2-i}=2\mathrm{\πn}[\frac{L_b}{C}v+\frac{L_a+2L_i}{C}(v+\mathrm{\Δv})]+\mathrm{\π}---\left(2\right)$

Δ φ wherein _2-iBe the phase place recruitment of i bragg grating laser light reflected on another road sensing network, π is that the phase place that Sarnia gram interferometer is introduced increases;

Two bundle laser of i bragg grating correspondence interfere in four port 3-dB fiber couplers, and the phase place of the laser of transmission is respectively

$\mathrm{\Δ}{\mathrm{\φ}}_{\mathrm{Ti}}=2\mathrm{\πn}\frac{L_a+2L_i-L_b}{C}\mathrm{\Δv}+\mathrm{\π}---\left(3\right)$

The driving frequency Δ v of acousto-optic modulator makes linear change according to ω t, and by linear sweep, the transmitted light intensity of each bragg grating laser light reflected is pressed cos (f respectively _iT) change

$f_{i}t=2\mathrm{\πn}\frac{L_a+2L_i-L_b}{C}\mathrm{\ωt}---\left(4\right)$

F wherein _iFrequency for the light intensity variation;

(3) intensity that sees through the light wave of Sarnia gram ring is surveyed by photodiode, after light intensity signal is converted into electric signal, through data collecting card, carries out fast Fourier transform (FFT), can obtain each corresponding peak of formula (4) on frequency spectrum; By measuring the frequency f at each peak _iSize obtain the position L of bragg grating on the bragg grating sensing network of each peak correspondence _i

$L_i=(\frac{{\mathrm{Cf}}_i}{2\mathrm{\πn\ω}}-L_a+L_b)/2---\left(5\right)$

(4) on the frequency spectrum intensity at each peak by the position extent decision of the centre wavelength of each bragg grating centre wavelength of correspondence and light source, change centre wavelength that each minute measure the bragg grating in the light path to the drift of long wave direction, the relation of the intensity change at each corresponding peak on the amount of movement of records center wavelength and the frequency spectrum;

(5) each bragg grating is installed in the environment that needs sensing, when measured physical quantity changes in the environment, use the data of step 4 record, by the intensity change amount of each bragg grating at the peak value of frequency spectrum correspondence, obtain the amount of movement of each bragg grating centre wavelength, thereby obtain the environmental physics amount of each bragg grating institute sensing.

2, the employed equipment of the method for optical fiber optical grating sensing network demodulation as claimed in claim 1 is characterized in that: semiconductor laser is connected by the input port light of fibre optic isolater with four port 3-dB fiber couplers; Another input port of four port 3-dB fiber couplers is connected with the input end light of photodiode, and photodiode output is electrically connected with the input end of data collecting card, and the output terminal of data collecting card is electrically connected with the fft analysis instrument;

Two output ports of four port 3-dB fiber couplers connect by single-mode fiber, and single-mode fiber is divided into two sections by acousto-optic modulator, are respectively measuring section single-mode fiber and linkage section single-mode fiber, and length is respectively L _aAnd L _b, L _a＞＞L _bThe electric drive signal mouth of acousto-optic modulator is electrically connected with sinusoidal signal generator; Insert three port 3-dB fiber couplers in the measuring section single-mode fiber, the two-port of the input end of three port 3-dB fiber couplers is connected with two sections measuring section single-mode fibers respectively, and another port is connected with the sensing network single-mode fiber; A plurality of bragg gratings are connected on the sensing network single-mode fiber successively.

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