CN101319920A - Method and equipment for optical fibre optical grating sensing array demodulation - Google Patents

Abstract

The invention relates to a method and a device for demodulating a fiber bragg grating sensor array. In the invention, a continuous semiconductor laser with narrow line width is used as an optical source; an electro-optic modulator is connected with a Sagnac loop; an FBG sensing array is connected with the Sagnac loop by a three-port 3-dB fiber coupler; the bragg wavelengths of each sensing FBG connected in series on the sensing array under the room temperature are consistent with the central wavelength of the laser optical source; besides, the reflection rates are all less than 3 percent; the electro-optic modulator is driven by a RF signal the frequencies of which can be modulated; the change of the transmittance is detected by a low-speed photodiode; the drift amount of the bragg wavelengths of each sensing FBG on the sensing array is obtained by carrying out fast Fourier transformation and relevant operations on the data collected by a collecting card, and finally the change of the sensed physical quantity is obtained. The invention can have an extremely high response speed for meeting the requirements of real time sensing and has a relative lower cost; besides, as the sensing array is connected with the Sagnac loop, the performances of anti-external temperature fluctuation and anti-mechanical disturbance of the device are high.

Description

A kind of method and apparatus of optical fibre optical grating sensing array demodulation

Technical field

The invention belongs to technical field of optical fiber sensing, be applicable to the field of the real-time sensing network that needs quasi-distributed, high precision, anti-electromagnetic interference (EMI).Be particularly related to a kind of novel electrooptic modulator change frequency of light wave that utilizes and caused that Sarnia gram asymmetric effect of (Sagnac) ring structure and Fiber Bragg Grating FBG (FBG) hypotenuse detection technique realize the method for FBG sensing array 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 burst pulse light source input, the reflected light signal of each FBG sensor to utilize different time-delays to realize signal multiplexing, receive the pulse of reflection with the high speed optoelectronic probe, 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.

Electrooptic modulator utilizes nonlinear effect, can accurately change the frequency through the light wave of modulator within the specific limits; 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 very extensive; The quasi-distributed sensor array of FBG that use realizes based on the Sagnac ring of electrooptic modulator optical frequency translation, 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 electrooptic modulator optical frequency translation technology, has proposed a kind of solution that realizes the quasi-distributed sensor array of novel FBG.Use the continuous semiconductor LASER Light Source, low speed photo diodes, low speed data capture card and Fourier frequency meter, and the Sagnac ring is as the system core element have the advantage that low cost, anti-interference are good, simple and practical, can realize real-time detection.The equipment of realizing this method is provided simultaneously.

Method of the present invention may further comprise the steps:

1, centre wavelength enters in Sarnia gram (Sagnac) ring after laser that the continuous semiconductor LASER Light Source of communication band is sent is by fibre optic isolater and four port 3-dB fiber couplers.

2, laser is divided into two-way after entering Sarnia gram ring, wherein one the tunnel enter the known measuring section single-mode fiber of length, incide the optical fiber Bragg grating sensing array that is connected into by a plurality of Fiber Bragg Grating FBGs consistent with LASER Light Source centre wavelength by the three port 3-dB fiber couplers that insert in the measuring section single-mode fiber again, laser is by each Fiber Bragg Grating FBG reflection on the sensor array; Laser after the reflection is got back to the measuring section single-mode fiber through three port 3-dB fiber couplers, produces frequency conversion by electrooptic modulator, and the sinusoidal signal generator that described electrooptic modulator is modulated by frequency drives; Laser after the frequency conversion is got back to four port 3-dB fiber couplers through the known linkage section single-mode fiber of length; Got back to four port 3-dB fiber couplers, its electric field strength E by i Fiber Bragg Grating FBG beam reflected on the sensor array _1-iFor:

$E_{1-i}=-J_1\left(\mathrm{\α}\frac{\mathrm{\π}}{2}\right)E_i\left\{\begin{array}{c}\exp (i(\mathrm{\ω}-\mathrm{\Ω})t+\mathrm{ni}[\frac{L_a+2L_i}{C}\mathrm{\ω}+\frac{L_b}{C}(\mathrm{\ω}-\mathrm{\Ω})])\\ +\exp (i(\mathrm{\ω}+\mathrm{\Ω})t+\mathrm{ni}[\frac{L_a+2L_i}{C}\mathrm{\ω}+\frac{L_b}{C}(\mathrm{\ω}+\mathrm{\Ω})])\end{array}\right\}---\left(1\right)$

J wherein ₁Be first-order bessel function, α is the normalized amplitude of electrooptic modulator drive signal, E _iBe the electric field intensity of i Fiber Bragg Grating FBG laser light reflected, ω is the angular frequency of laser, and Ω is the angular frequency of electrooptic modulator drive signal.N is the refractive index of single-mode fiber, and C is the light velocity, L _iBe the fiber lengths between i Fiber Bragg Grating FBG 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, produces frequency conversion by electrooptic modulator then, and the laser after the frequency conversion enters the known measuring section single-mode fiber of length; Laser incides the optical fiber Bragg grating sensing array by the three port 3-dB fiber couplers that insert in the measuring section single-mode fiber, laser is by each Fiber Bragg Grating FBG reflection on the optical fiber Bragg grating sensing array, laser after the reflection is got back to the measuring section single-mode fiber through three port 3-dB fiber couplers, returns four port 3-dB fiber couplers; Electric field strength E when i Fiber Bragg Grating FBG laser light reflected got back to four port 3-dB fiber couplers on the sensor array _2-iFor:

$E_{2-i}={-J}_1\left(\mathrm{\α}\frac{\mathrm{\π}}{2}\right)E_i\left\{\begin{array}{c}\exp (i(\mathrm{\ω}-\mathrm{\Ω})t+\mathrm{ni}[\frac{L_b}{C}\mathrm{\ω}+\frac{L_a+2L_i}{C}(\mathrm{\ω}-\mathrm{\Ω})])\\ +\exp (i(\mathrm{\ω}+\mathrm{\Ω})t+\mathrm{ni}[\frac{L_b}{C}\mathrm{\ω}+\frac{L_a+2L_i}{C}(\mathrm{\ω}+\mathrm{\Ω})])\end{array}\right\}---\left(2\right)$

Two bundle laser of i Fiber Bragg Grating FBG correspondence interfere the intensity I of the laser of transmission in four port 3-dB fiber couplers _T-iFor:

$I_{T-i}={J_1}^2\left(\mathrm{\α}\frac{\mathrm{\π}}{2}\right){\left|E_i\right|}^2\left\{\begin{array}{c}4+2\cos \left[2(\mathrm{\Ωt}+\frac{\mathrm{n\Ω}}{C}{L}_b)\right]+2\cos \left[2(\mathrm{\Ωt}+\frac{\mathrm{n\Ω}}{C}(L_a+2L_i))\right]\\ +4\cos \left[(2\mathrm{\Ωt}+\frac{\mathrm{n\Ω}}{C}(L_a+L_b+2L_i))\right]+4\cos \left[\frac{\mathrm{n\Ω}}{C}(L_a+2L_i-L_b)\right)]\end{array}\right\}---\left(3\right)$

3, photodiode is surveyed the intensity of the laser that sees through Sarnia gram ring of i Fiber Bragg Grating FBG correspondence, and light intensity signal is converted into electric signal simultaneously, photodiode be f by frequency _b, f _b＜＜Ω, the light intensity I that receives by photodiode _TFor:

$I_T={J_1}^2\left(\mathrm{\α}\frac{\mathrm{\π}}{2}\right)\{4+4\cos \left[\frac{\mathrm{n\Ω}}{C}(L_a+2L_i+L_b)\right]\}---\left(4\right)$

The driving frequency Ω of electrooptic modulator makes linear change according to 2 π wt, and by linear sweep, the transmitted light intensity of each sensing FBG laser light reflected is pressed cos (f respectively _iT) change

$f_{i}t=2\mathrm{\πn}\frac{L_a+2L_i-L_b}{C}\mathrm{wt}---\left(5\right)$

F wherein _iFrequency for the light intensity variation.

Electric signal carries out fast Fourier transform (FFT) through data collecting card, obtains each corresponding peak of formula (5) on frequency spectrum.By measuring the frequency f at each peak _iObtain the corresponding position L of sensor fibre Bragg grating on the optical fiber Bragg grating sensing network _i:

$L_i=(\frac{{\mathrm{Cf}}_i}{2\mathrm{\πn\ω}}-L_a+L_b)/2---\left(6\right)$

4, on the frequency spectrum intensity at each peak by the position extent decision of the centre wavelength of each sensing Bragg grating centre wavelength of correspondence and light source.Stress application on each sensor fibre Bragg grating successively makes centre wavelength 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 Fiber Bragg Grating FBG is installed in the environment that needs sensing, when measured physical quantity changed in the environment, the centre wavelength of each sensor fibre Bragg grating was moved, and caused that the intensity of peak value corresponding on the frequency spectrum changes; The relation of the intensity change at each corresponding peak on amount of movement and the frequency spectrum according to the centre wavelength of record obtains the amount of movement of each Fiber Bragg Grating FBG centre wavelength, finally obtains the environmental physics amount of each Fiber Bragg Grating FBG institute sensing.

The wavelength shift of the centre wavelength of Fiber Bragg Grating FBG and be prior art to the pass between the environmental physics quantitative changeization 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, the output port of four port 3-dB fiber couplers is connected with the input end light of photodiode, 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 fast fourier transform analyser.

The centre wavelength of described LASER Light Source is at communication band, and its 3dB live width is smaller or equal to 0.1nm.

Two other port of four port 3-dB fiber couplers connects by single-mode fiber, and single-mode fiber is divided into two sections by electrooptic modulator, is respectively measuring section single-mode fiber and linkage section single-mode fiber, and length is respectively L _aAnd L _b, L _a＞＞L _b, the electric drive signal mouth of electrooptic 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 the measuring section single-mode fiber respectively, and another port is connected with optical fiber Bragg grating sensing array light.

Described optical fiber Bragg grating sensing array is in series by a plurality of Fiber Bragg Grating FBGs consistent with LASER Light Source centre wavelength, and the reflectivity of Fiber Bragg Grating FBG is less than 3%.

Among the present invention, electrooptic modulator utilizes nonlinear effect, can accurately change the frequency through the light wave of modulator within the specific limits.Simultaneously, it is insensitive that the Sarnia gram encircles the interference of temperature variation, mechanical vibration etc. to external world.The present invention is applicable to the quasi-distributed sensor array of general FBG, compares with traditional scheme, has adopted electronic frequency scanning, rather than uses the Wavelength scanning device of the machinery control of low speed, can accomplish high response speed, satisfies 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 sensor array is connected in the Sarnia gram 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 electrooptic 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 electrooptic 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 measuring section single-mode fiber 11 respectively, and another output port is connected with sensor array single-mode fiber 13.A plurality of sensing FBG 12 are connected on the sensor array single-mode fiber 13 successively.

Concrete detection method may further comprise the steps:

Centre wavelength enters in Sarnia gram (Sagnac) ring after laser that the continuous semiconductor LASER Light Source of communication band is sent is by fibre optic isolater and four port 3-dB fiber couplers.

Laser is divided into two-way after entering Sarnia gram ring, wherein one the tunnel enter the known measuring section single-mode fiber of length, incide the optical fiber Bragg grating sensing array that is connected into by a plurality of Fiber Bragg Grating FBGs consistent with LASER Light Source centre wavelength by the three port 3-dB fiber couplers that insert in the measuring section single-mode fiber again, laser is by each Fiber Bragg Grating FBG reflection on the sensor array; Laser after the reflection is got back to the measuring section single-mode fiber through three port 3-dB fiber couplers, produces frequency conversion by electrooptic modulator, and the sinusoidal signal generator that described electrooptic modulator is modulated by frequency drives; Laser after the frequency conversion is got back to four port 3-dB fiber couplers through the known linkage section single-mode fiber of length; Got back to four port 3-dB fiber couplers, its electric field strength E by i Fiber Bragg Grating FBG beam reflected on the sensor array _1-iFor:

$E_{1-i}=-J_1\left(\mathrm{\α}\frac{\mathrm{\π}}{2}\right)E_i\left\{\begin{array}{c}\exp (i(\mathrm{\ω}-\mathrm{\Ω})t+\mathrm{ni}[\frac{L_a+2L_i}{C}\mathrm{\ω}+\frac{L_b}{C}(\mathrm{\ω}-\mathrm{\Ω})])\\ +\exp (i(\mathrm{\ω}+\mathrm{\Ω})t+\mathrm{ni}[\frac{L_a+2L_i}{C}\mathrm{\ω}+\frac{L_b}{C}(\mathrm{\ω}+\mathrm{\Ω})])\end{array}\right\}---\left(1\right)$

J wherein ₁Be first-order bessel function, α is the normalized amplitude of electrooptic modulator drive signal, E _iBe the electric field intensity of i Fiber Bragg Grating FBG laser light reflected, ω is the angular frequency of laser, and Ω is the angular frequency of electrooptic modulator drive signal.N is the refractive index of single-mode fiber, and C is the light velocity, L _iBe the fiber lengths between i Fiber Bragg Grating FBG 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, produces frequency conversion by electrooptic modulator then, and the laser after the frequency conversion enters the known measuring section single-mode fiber of length; Laser incides the optical fiber Bragg grating sensing array by the three port 3-dB fiber couplers that insert in the measuring section single-mode fiber, laser is by each Fiber Bragg Grating FBG reflection on the optical fiber Bragg grating sensing array, laser after the reflection is got back to the measuring section single-mode fiber through three port 3-dB fiber couplers, returns four port 3-dB fiber couplers; Electric field strength E when i Fiber Bragg Grating FBG laser light reflected got back to four port 3-dB fiber couplers on the sensor array _2-iFor:

$E_{2-i}={-J}_1\left(\mathrm{\α}\frac{\mathrm{\π}}{2}\right)E_i\left\{\begin{array}{c}\exp (i(\mathrm{\ω}-\mathrm{\Ω})t+\mathrm{ni}[\frac{L_b}{C}\mathrm{\ω}+\frac{L_a+2L_i}{C}(\mathrm{\ω}-\mathrm{\Ω})])\\ +\exp (i(\mathrm{\ω}+\mathrm{\Ω})t+\mathrm{ni}[\frac{L_b}{C}\mathrm{\ω}+\frac{L_a+2L_i}{C}(\mathrm{\ω}+\mathrm{\Ω})])\end{array}\right\}---\left(2\right)$

Two bundle laser of i Fiber Bragg Grating FBG correspondence interfere the intensity I of the laser of transmission in four port 3-dB fiber couplers _T-iFor:

$I_{T-i}={J_1}^2\left(\mathrm{\α}\frac{\mathrm{\π}}{2}\right){\left|E_i\right|}^2\left\{\begin{array}{c}4+2\cos \left[2(\mathrm{\Ωt}+\frac{\mathrm{n\Ω}}{C}{L}_b)\right]+2\cos \left[2(\mathrm{\Ωt}+\frac{\mathrm{n\Ω}}{C}(L_a+2L_i))\right]\\ +4\cos \left[(2\mathrm{\Ωt}+\frac{\mathrm{n\Ω}}{C}(L_a+L_b+2L_i))\right]+4\cos \left[\frac{\mathrm{n\Ω}}{C}(L_a+2L_i-L_b)\right)]\end{array}\right\}---\left(3\right)$

Photodiode is surveyed the intensity of the laser that sees through Sarnia gram ring of i Fiber Bragg Grating FBG correspondence, and light intensity signal is converted into electric signal simultaneously, photodiode be f by frequency _b, f _b＜＜Ω, the light intensity I that receives by photodiode _TFor:

$I_T={J_1}^2\left(\mathrm{\α}\frac{\mathrm{\π}}{2}\right)\{4+4\cos \left[\frac{\mathrm{n\Ω}}{C}(L_a+2L_i+L_b)\right]\}---\left(4\right)$

The driving frequency Ω of electrooptic modulator makes linear change according to 2 π wt, and by linear sweep, the transmitted light intensity of each sensing FBG laser light reflected is pressed cos (f respectively _iT) change

$f_{i}t=2\mathrm{\πn}\frac{L_a+2L_i-L_b}{C}\mathrm{wt}---\left(5\right)$

F wherein _iFrequency for the light intensity variation.

Electric signal carries out fast Fourier transform (FFT) through data collecting card, obtains each corresponding peak of formula (5) on frequency spectrum.By measuring the frequency f at each peak _iObtain the corresponding position L of sensor fibre Bragg grating on the optical fiber Bragg grating sensing network _i:

$L_i=(\frac{{\mathrm{Cf}}_i}{2\mathrm{\πn\ω}}-L_a+L_b)/2---\left(6\right)$

The intensity at each peak is by the position extent decision of the centre wavelength of each sensing Bragg grating centre wavelength of correspondence and light source on the frequency spectrum.As shown in Figure 2, stress application on each sensor fibre Bragg grating successively makes centre wavelength to the drift of long wave direction, the calibration data of the intensity at each corresponding peak on the amount of movement of records center wavelength and the frequency spectrum.

Each Fiber Bragg Grating FBG is installed in the environment that needs sensing, and when measured physical quantity changed in the environment, the centre wavelength of each sensor fibre Bragg grating was moved, and caused that the intensity of peak value corresponding on the frequency spectrum changes; The relation of the intensity change at each corresponding peak on amount of movement and the frequency spectrum according to the centre wavelength of record obtains the amount of movement of each Fiber Bragg Grating FBG centre wavelength, finally obtains the environmental physics amount of each Fiber Bragg Grating FBG institute sensing.

The wavelength shift of the centre wavelength of Fiber Bragg Grating FBG and be prior art to the pass between the environmental physics quantitative changeization of inductive sensing (variations such as little curved, temperature, stress).

Claims (2)

1, a kind of method of optical fibre optical grating sensing array demodulation is characterized in that the concrete steps of this method are:

A. centre wavelength enters in the Sarnia gram ring after laser that the continuous semiconductor LASER Light Source of communication band is sent is by fibre optic isolater and four port 3-dB fiber couplers;

B. laser is divided into two-way after entering Sarnia gram ring, wherein one the tunnel enter the known measuring section single-mode fiber of length, incide the optical fiber Bragg grating sensing array that is connected into by a plurality of Fiber Bragg Grating FBGs consistent with LASER Light Source centre wavelength by the three port 3-dB fiber couplers that insert in the measuring section single-mode fiber again, laser is by each Fiber Bragg Grating FBG reflection on the sensor array; Laser after the reflection is got back to the measuring section single-mode fiber through three port 3-dB fiber couplers, produces frequency conversion by electrooptic modulator, and the sinusoidal signal generator that described electrooptic modulator is modulated by frequency drives; Laser after the frequency conversion is got back to four port 3-dB fiber couplers through the known linkage section single-mode fiber of length; Got back to four port 3-dB fiber couplers, its electric field strength E by i Fiber Bragg Grating FBG beam reflected on the sensor array _1-iFor:

$E_{1-i}=-J_1\left(\mathrm{\α}\frac{\mathrm{\π}}{2}\right)E_i\left\{\begin{array}{c}\exp (i(\mathrm{\ω}-\mathrm{\Ω})t+\mathrm{ni}[\frac{L_a+{2L}_i}{C}\mathrm{\ω}+\frac{L_b}{C}(\mathrm{\ω}-\mathrm{\Ω})\left]\right)\\ +\exp (i(\mathrm{\ω}+\mathrm{\Ω})t+\mathrm{ni}[\frac{L_a+{2L}_i}{C}\mathrm{\ω}+\frac{L_b}{C}(\mathrm{\ω}+\mathrm{\Ω})\left]\right)\end{array}\right\}---\left(1\right)$

J wherein ₁Be first-order bessel function, α is the normalized amplitude of electrooptic modulator drive signal, E _iBe the electric field intensity of i Fiber Bragg Grating FBG laser light reflected, ω is the angular frequency of laser, and Ω is the angular frequency of electrooptic modulator drive signal, and n is the refractive index of single-mode fiber, and C is the light velocity, L _iBe the fiber lengths between i Fiber Bragg Grating FBG 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, produces frequency conversion by electrooptic modulator then, and the laser after the frequency conversion enters the known measuring section single-mode fiber of length; Laser incides the optical fiber Bragg grating sensing array by the three port 3-dB fiber couplers that insert in the measuring section single-mode fiber, laser is by each Fiber Bragg Grating FBG reflection on the optical fiber Bragg grating sensing array, laser after the reflection is got back to the measuring section single-mode fiber through three port 3-dB fiber couplers, returns four port 3-dB fiber couplers; Electric field strength E when i Fiber Bragg Grating FBG laser light reflected got back to four port 3-dB fiber couplers on the sensor array _2-iFor:

$E_{2-i}=-J_1\left(\mathrm{\α}\frac{\mathrm{\π}}{2}\right)E_i\left\{\begin{array}{c}\exp (i(\mathrm{\ω}-\mathrm{\Ω})t+\mathrm{ni}[\frac{L_b}{C}\mathrm{\ω}+\frac{L_a+{2L}_i}{C}(\mathrm{\ω}-\mathrm{\Ω})\left]\right)\\ +\exp (i(\mathrm{\ω}+\mathrm{\Ω})t+\mathrm{ni}[\frac{L_b}{C}\mathrm{\ω}+\frac{L_a+{2L}_i}{C}(\mathrm{\ω}+\mathrm{\Ω})\left]\right)\end{array}\right\}---\left(2\right)$

Two bundle laser of i Fiber Bragg Grating FBG correspondence interfere the intensity I of the laser of transmission in four port 3-dB fiber couplers _T-iFor:

$I_{T-i}={J_1}^2\left(\mathrm{\α}\frac{\mathrm{\π}}{2}\right){\left|E_i\right|}^2\left\{\begin{array}{c}4+2\cos \left[2(\mathrm{\Ωt}+\frac{\mathrm{n\Ω}}{C}{L}_b)\right]+2\cos \left[2(\mathrm{\Ωt}+\frac{\mathrm{n\Ω}}{C}(L_a+2L_i))\right]\\ +4\cos \left[(2\mathrm{\Ωt}+\frac{\mathrm{n\Ω}}{C}(L_a+L_b+{2L}_i))\right]+4\cos \left[\frac{\mathrm{n\Ω}}{C}(L_a+{2L}_i-L_b)\right]\end{array}\right\}---\left(3\right)$

C. photodiode is surveyed the intensity of the laser that sees through Sarnia gram ring of i Fiber Bragg Grating FBG correspondence, and light intensity signal is converted into electric signal simultaneously, photodiode be f by frequency _b, f _b＜＜Ω, the light intensity I that receives by photodiode _TFor:

$I_T={J_1}^2\left(\mathrm{\α}\frac{\mathrm{\π}}{2}\right)\{4+4\cos [\frac{\mathrm{n\Ω}}{C}(L_a+{2L}_i+L_b)\left]\right\}---\left(4\right)$

The driving frequency Ω of electrooptic modulator makes linear change according to 2 π wt, and by linear sweep, the transmitted light intensity of each sensing FBG laser light reflected is pressed cos (f respectively _iT) change

$f_{i}t=2\mathrm{\πn}\frac{L_a+{2L}_i-L_b}{C}\mathrm{wt}---\left(5\right)$

F wherein _iFrequency for the light intensity variation;

Electric signal carries out fast fourier transform through data collecting card, obtains each corresponding peak of formula (5) on frequency spectrum, by measuring the frequency f at each peak _iObtain the corresponding position L of sensor fibre Bragg grating on the optical fiber Bragg grating sensing network _i:

$L_i=(\frac{{\mathrm{Cf}}_i}{2\mathrm{\πn\ω}}-L_a+L_b)/2---\left(6\right)$

D. stress application on each sensor fibre Bragg grating successively makes centre wavelength 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;

E. each Fiber Bragg Grating FBG is installed in the environment that needs sensing, when measured physical quantity changed in the environment, the centre wavelength of each sensor fibre Bragg grating was moved, and caused that the intensity of peak value corresponding on the frequency spectrum changes; The relation of the intensity change at each corresponding peak on amount of movement and the frequency spectrum according to the centre wavelength of record obtains the amount of movement of each Fiber Bragg Grating FBG centre wavelength, finally obtains the environmental physics amount of each Fiber Bragg Grating FBG institute sensing.

2, the employed equipment of measuring method according to claim 1, it is characterized in that: semiconductor laser is connected by the input port light of fibre optic isolater with four port 3-dB fiber couplers, the output port of four port 3-dB fiber couplers is connected with the input end light of photodiode, 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 fast fourier transform analyser;

Two other port of four port 3-dB fiber couplers connects by single-mode fiber, and single-mode fiber is divided into two sections by electrooptic modulator, is respectively measuring section single-mode fiber and linkage section single-mode fiber, and length is respectively L _aAnd L _b, L _a＞＞L _b, the electric drive signal mouth of electrooptic 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 the measuring section single-mode fiber respectively, and another port is connected with optical fiber Bragg grating sensing array light;

The centre wavelength of described LASER Light Source is at communication band, and its 3dB live width is smaller or equal to 0.1nm; Described optical fiber Bragg grating sensing array is in series by a plurality of Fiber Bragg Grating FBGs consistent with LASER Light Source centre wavelength, and the reflectivity of Fiber Bragg Grating FBG is less than 3%.

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