CN101319919A - Method and device for frequency domain optical fiber optical grating sensing network demodulation - Google Patents

Abstract

The invention relates to a method and a device for demodulating a frequency domain fiber grating sensing network. In the invention, an electro-optic modulator is connected with a Sagnac loop; an FBG sensing network formed by connecting a plurality of sensing FBGs in series is connected with the Sagnac loop by a three-port 3-dB fiber coupler; the bragg wavelengths of each reference FBG on a reference FBG array are the same as the sensing FBGs with same sequence numbers. The electro-optic modulator is driven by a RF signal with variable frequencies. When the frequencies of the RF signal are changed, the change of the transmittance is detected by a photodiode; the drift amount of the bragg wavelengths of each sensing FBG on the sensing network is obtained by carrying out fast Fourier transformation and relevant operations on the output data of the photodiode 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; besides, as the sensing network 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 frequency domain optical fiber optical grating sensing network 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 multinode, high precision, anti-electromagnetic interference (EMI), particularly a kind ofly utilize electrooptic modulator to change frequency of light wave to cause that Sarnia gram asymmetric effect of (Sagnac) ring 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 need to require a wavelength sensitive system of overlapping 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.

Electrooptic modulator 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 sensing network of FBG that the use Sagnac structure that change is changeed based on the electrooptic modulator optical frequency realizes, 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 sensing network of novel FBG.Use wideband light source, low speed photo diodes, low speed data capture card and Fourier frequency meter, and asymmetric Sagnac structure 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 the Sarnia gram ring after light beam that the wideband light source of optical communicating waveband sends is by fibre optic isolater and four port 3-dB fiber couplers.

2, light beam 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 network that the sensor fibre Bragg grating by a plurality of different bragg wavelengths is connected into by the three port 3-dB fiber couplers that insert in the measuring section single-mode fiber again; Light beam is by each sensor fibre Bragg grating reflection on the sensing network, and reflected light has comprised the reflectance spectrum of all sensor fibre Bragg gratings; Reflection back light beam 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; Light beam after the frequency conversion is got back to four port 3-dB fiber couplers through the known linkage section single-mode fiber of length; Electric field strength E when i sensor fibre Bragg grating beam reflected got back to four port 3-dB fiber couplers on the sensing network _1-iFor:

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

E wherein _iBe the electric field intensity of i sensor fibre Bragg grating beam reflected, J ₁Be first-order bessel function, α is the normalized amplitude of electrooptic modulator drive signal, ω _iBe the angular frequency of the bragg wavelength correspondence of i sensor fibre Bragg grating, 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 in the vacuum, L _iBe the fiber lengths between i sensor fibre 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 light beam at first passes through the known linkage section single-mode fiber of length, produces frequency conversion by electrooptic modulator then, and the light beam after the frequency conversion enters the known measuring section single-mode fiber of length; Light beam incides the optical fiber Bragg grating sensing network by the three port 3-dB fiber couplers that insert in the measuring section single-mode fiber, light beam is by each sensor fibre Bragg grating reflection on the sensing network, reflection back light beam 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 sensor fibre Bragg grating beam reflected got back to four port 3-dB fiber couplers on the sensing network _2-iFor:

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

The two-beam of i sensor fibre Bragg grating correspondence interferes in four port 3-dB fiber couplers, and the transmission light intensity is:

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

3, the light beam through Sarnia gram ring incides the Fiber Bragg Grating FBG referential array that is connected into by a plurality of reference optical fiber Bragg gratings by optical fiber circulator, each reference optical fiber Bragg grating in the Fiber Bragg Grating FBG referential array is corresponding one by one with each sensor fibre Bragg grating in the optical fiber Bragg grating sensing network, and corresponding reference optical fiber Bragg grating is identical with the bragg wavelength of sensor fibre Bragg grating.

4, photodiode is surveyed the intensity by Fiber Bragg Grating FBG referential array beam reflected, photodiode be f by frequency _b, f _b＜＜Ω, the light intensity that is received by photodiode is:

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

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.

Photodiode is converted into electric signal with light intensity signal, is gathered by data collecting card, and carries out fast Fourier transform (FFT), obtains each corresponding peak of formula (5) on frequency spectrum, by measuring the frequency f at each peak _iThe position L of the sensor fibre Bragg grating that obtains each peak correspondence on the optical fiber Bragg grating sensing network _i

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

5, on the frequency spectrum intensity at each peak by the position extent decision of each sensing FBG centre wavelength of correspondence with the centre wavelength of the reference FBG identical with its sequence number.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.

6, 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: centre wavelength is connected by the input port light of fibre optic isolater with four port 3-dB fiber couplers at the wideband light source of optical communicating waveband, the output port of four port 3-dB fiber couplers is connected with the input port of optical fiber circulator, the output port of optical fiber circulator is connected with the input end light of photodiode, photodiode output is electrically connected with the input end of data collecting card, the output terminal of data collecting card is electrically connected with the fast fourier transform analyser, and the Centronics port of optical fiber circulator is connected with Fiber Bragg Grating FBG referential array light;

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 network light;

Described optical fiber Bragg grating sensing network is in series by the sensor fibre Bragg grating of a plurality of different bragg wavelengths, and the bragg wavelength of each sensor fibre Bragg grating is 0.6～1.2nm at interval; Each reference optical fiber Bragg grating in the Fiber Bragg Grating FBG referential array is corresponding one by one with each sensor fibre Bragg grating in the optical fiber Bragg grating sensing network, and corresponding reference optical fiber Bragg grating is identical with the bragg wavelength of sensor fibre Bragg grating.

Among the present invention, electrooptic modulator can accurately make the light wave generation frequency displacement through modulator within the specific limits.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, wideband light source 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 passes through optical fiber circulator 4 and is connected with reference to FBG array 16 light, be in series with reference to FBG 15 by a plurality of with reference to FBG array 16, the output port of optical fiber circulator 4 is connected with the input end light of photodiode 5, the output terminal of photodiode 5 is electrically connected with the input end of data collecting card 6, and the output terminal of data collecting card 6 is electrically connected with fft analysis instrument 7.

Two two other 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 9, are respectively measuring section single-mode fiber 12 and linkage section single-mode fiber 8, and length is respectively L _aAnd L _b, L _a＞＞L _bThe electric drive signal mouth of electrooptic modulator 9 is electrically connected with sinusoidal signal generator 10.The two-port of inserting the input end of three port 3-dB fiber couplers, 11, three port 3-dB fiber couplers 11 in the measuring section single-mode fiber 12 is connected with measuring section single-mode fiber 12 respectively, and another port is connected with sensing network single-mode fiber 13.A plurality of sensing FBG 14 are connected on the sensing network single-mode fiber 13 successively.

Concrete detection method may further comprise the steps:

Centre wavelength enters in the Sarnia gram ring after light beam that the wideband light source of optical communicating waveband sends is by fibre optic isolater and four port 3-dB fiber couplers.

Light beam 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 network that the sensor fibre Bragg grating by a plurality of different bragg wavelengths is connected into by the three port 3-dB fiber couplers that insert in the measuring section single-mode fiber again; Light beam is by each sensor fibre Bragg grating reflection on the sensing network, and reflected light has comprised the reflectance spectrum of all sensor fibre Bragg gratings; Reflection back light beam 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; Light beam after the frequency conversion is got back to four port 3-dB fiber couplers through the known linkage section single-mode fiber of length; Electric field strength E when i sensor fibre Bragg grating beam reflected got back to four port 3-dB fiber couplers on the sensing network ^1-iFor:

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

E wherein _iBe the electric field intensity of i sensor fibre Bragg grating beam reflected, J ₁Be first-order bessel function, α is the normalized amplitude of electrooptic modulator drive signal, ω _iBe the angular frequency of the bragg wavelength correspondence of i sensor fibre Bragg grating, 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 in the vacuum, L _iBe the fiber lengths between i sensor fibre 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 light beam at first passes through the known linkage section single-mode fiber of length, produces frequency conversion by electrooptic modulator then, and the light beam after the frequency conversion enters the known measuring section single-mode fiber of length; Light beam incides the optical fiber Bragg grating sensing network by the three port 3-dB fiber couplers that insert in the measuring section single-mode fiber, light beam is by each sensor fibre Bragg grating reflection on the sensing network, reflection back light beam 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 sensor fibre Bragg grating beam reflected got back to four port 3-dB fiber couplers on the sensing network _2-iFor:

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

The two-beam of i sensor fibre Bragg grating correspondence interferes in four port 3-dB fiber couplers, and the transmission light intensity is:

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

The light beam that sees through Sarnia gram ring incides the Fiber Bragg Grating FBG referential array that is connected into by a plurality of reference optical fiber Bragg gratings by optical fiber circulator, each reference optical fiber Bragg grating in the Fiber Bragg Grating FBG referential array is corresponding one by one with each sensor fibre Bragg grating in the optical fiber Bragg grating sensing network, and corresponding reference optical fiber Bragg grating is identical with the bragg wavelength of sensor fibre Bragg grating.

Photodiode is surveyed the intensity by Fiber Bragg Grating FBG referential array beam reflected, photodiode be f by frequency _b, f _b＜＜Ω, the light intensity that is received by photodiode is:

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

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.

Photodiode is converted into electric signal with light intensity signal, is gathered by data collecting card, and carries out fast Fourier transform (FFT), obtains each corresponding peak of formula (5) on frequency spectrum, by measuring the frequency f at each peak _iThe position L of the sensor fibre Bragg grating that obtains each peak correspondence 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 with the centre wavelength of the reference FBG identical with its sequence number of each sensing FBG centre wavelength of correspondence on the frequency spectrum.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.

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.As shown in Figure 2, the calibration data of the intensity at each corresponding peak on amount of movement and the frequency spectrum according to the centre wavelength of record obtain the amount of movement of each Fiber Bragg Grating FBG centre wavelength, finally obtain 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 frequency domain optical fiber optical grating sensing network demodulation is characterized in that the concrete steps of this method are:

A. centre wavelength enters in the Sarnia gram ring after light beam that the wideband light source of optical communicating waveband sends is by fibre optic isolater and four port 3-dB fiber couplers;

B. light beam 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 network that the sensor fibre Bragg grating by a plurality of different bragg wavelengths is connected into by the three port 3-dB fiber couplers that insert in the measuring section single-mode fiber again; Light beam is by each sensor fibre Bragg grating reflection on the sensing network, and reflected light has comprised the reflectance spectrum of all sensor fibre Bragg gratings; Reflection back light beam 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; Light beam after the frequency conversion is got back to four port 3-dB fiber couplers through the known linkage section single-mode fiber of length; Electric field strength E when i sensor fibre Bragg grating beam reflected got back to four port 3-dB fiber couplers on the sensing network _1-iFor:

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

E wherein _iBe the electric field intensity of i sensor fibre Bragg grating beam reflected, J ₁Be first-order bessel function, α is the normalized amplitude of electrooptic modulator drive signal, ω _iBe the angular frequency of the bragg wavelength correspondence of i sensor fibre Bragg grating, 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 in the vacuum, L _iBe the fiber lengths between i sensor fibre 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 light beam at first passes through the known linkage section single-mode fiber of length, produces frequency conversion by electrooptic modulator then, and the light beam after the frequency conversion enters the known measuring section single-mode fiber of length; Light beam incides the optical fiber Bragg grating sensing network by the three port 3-dB fiber couplers that insert in the measuring section single-mode fiber, light beam is by each sensor fibre Bragg grating reflection on the sensing network, reflection back light beam 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 sensor fibre Bragg grating beam reflected got back to four port 3-dB fiber couplers on the sensing network _2-iFor:

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

The two-beam of i sensor fibre Bragg grating correspondence interferes in four port 3-dB fiber couplers, and the transmission light intensity is:

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

C. the light beam that sees through Sarnia gram ring incides the Fiber Bragg Grating FBG referential array that is connected into by a plurality of reference optical fiber Bragg gratings by optical fiber circulator, each reference optical fiber Bragg grating in the Fiber Bragg Grating FBG referential array is corresponding one by one with each sensor fibre Bragg grating in the optical fiber Bragg grating sensing network, and corresponding reference optical fiber Bragg grating is identical with the bragg wavelength of sensor fibre Bragg grating;

D. photodiode is surveyed the intensity by Fiber Bragg Grating FBG referential array beam reflected, photodiode be f by frequency _b, f _b＜＜Ω, the light intensity that is received by photodiode is:

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

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;

Photodiode is converted into electric signal with light intensity signal, is gathered by data collecting card, and carries out fast fourier transform; , on frequency spectrum, obtain each corresponding peak of formula (5), by measuring the frequency f at each peak _iThe position L of the sensor fibre Bragg grating that obtains each peak correspondence on the optical fiber Bragg grating sensing network _i

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

E. 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;

F. 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: centre wavelength is connected by the input port light of fibre optic isolater with four port 3-dB fiber couplers at the wideband light source of optical communicating waveband, the output port of four port 3-dB fiber couplers is connected with the input port of optical fiber circulator, the output port of optical fiber circulator is connected with the input end light of photodiode, photodiode output is electrically connected with the input end of data collecting card, the output terminal of data collecting card is electrically connected with the fast fourier transform analyser, and the Centronics port of optical fiber circulator is connected with Fiber Bragg Grating FBG referential array light;

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 network light;

Described optical fiber Bragg grating sensing network is in series by the sensor fibre Bragg grating of a plurality of different bragg wavelengths, and the bragg wavelength of each sensor fibre Bragg grating is 0.6～1.2nm at interval; Each reference optical fiber Bragg grating in the Fiber Bragg Grating FBG referential array is corresponding one by one with each sensor fibre Bragg grating in the optical fiber Bragg grating sensing network, and corresponding reference optical fiber Bragg grating is identical with the bragg wavelength of sensor fibre Bragg grating.

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