CN100552520C - A kind of method and apparatus of multiplexing and demodulating long period optical fiber optical grating array - Google Patents

Abstract

The present invention relates to a kind of live signal multiplexing and demodulation method and equipment of sensor of full optical fiber long period optical fiber optical grating array.The present invention select for use centre wavelength at the adjustable continuous wave laser of the single longitudinal mode of optical fiber communication wave band as light source, the frequency of the laser that light source is sent is modulated the back and by fibre optic isolater, fiber coupler laser is assigned to reference path and measures light path according to the light intensity fixed proportion.The laser that enters reference path is all reflected by FLM; Enter the laser of measuring light path and be assigned in the branch measurement light path on i road, reflected by FLM by the light intensity after the LPG introducing loss.The light intensity coupling back of each minute measurement light path reflected back and the light intensity of reference path reflected back produce beat signal once more through 2 * 2 fiber couplers, determine the wavelength shift of the centre wavelength of corresponding LPG by the analysis beat signal.Measuring accuracy height of the present invention has advantage with low cost, is fit to integrated and instrumentation.

Description

A kind of method and apparatus of multiplexing and demodulating long period optical fiber optical grating array

Technical field

The invention belongs to technical field of optical fiber sensing, related to a kind of multiplexing and demodulation method of live signal of sensor of full optical fiber long period optical fiber optical grating array, and the equipment of realizing this method.

Background technology

At sensory field of optic fibre, long period fiber grating (long-period grating, LPG) because it changes very responsively to little curved, temperature, stress, refractive index etc., and become crucial senser element in the industry, be particularly useful for the high-acruracy survey of liquid refractivity.

Optical frequency domain reflection technology (Optical Frequency-Domain Reflectometry, OFDR) be that the continuous light that LASER Light Source is sent is carried out frequency modulation (PFM), the beat frequency that produces based on the light that optical fiber or element under test are reflected detects then, so it only requires the demodulated equipment of low frequency, the beat frequency light frequency is less, has reduced cost and has kept higher detection susceptibility and spatial resolution simultaneously.If select suitable light source and scheme for use, it is very high that the range of dynamic measurement of OFDR and measuring accuracy can reach, and therefore can measure each reflection spot information in the light path easily.OFDR is widely used in the distributed sensing network, mainly is to be applied to reflective bragg grating (Fiber Bragg Grating) at present.

Bragg grating because its frequency domain reflection spectrum width is narrower, is generally less than 1nm, can realize wavelength-division easily, and the time grades multiplexing and demodulation.Long period fiber grating has the susceptibility higher than Bragg grating, but long period fiber grating, its spectrum signature are a plurality of transmission losss peak and deposit, and the frequency domain bandwidth broad of each loss peak, tens nanometers are generally arranged, on frequency domain, can't realize multiplexing and demodulation.Although long period fiber grating itself has the sensing characteristics to little curved, temperature, stress, refractive index sensitivity, it seldom is used to distributed measurement.As a same reason, so far, also nobody proposes based on the multiplexing real-time sensor-based system of the long period fiber grating of optical frequency territory reflecting system.

Summary of the invention

The present invention is exactly at the deficiencies in the prior art, has proposed a kind of multiplexed and demodulation scheme of the sensor based on long period fiber grating, and the equipment of realizing this method is provided simultaneously.

Method of the present invention may further comprise the steps:

1, select for use centre wavelength at the adjustable continuous wave laser of the single longitudinal mode of optical fiber communication wave band as light source, the frequency spectrum live width of light source is less than 1MHz; The frequency of the laser that light source is sent is modulated, and frequencies omega (t) becomes the cycle sawtooth wave function that t in time changes

ω(t)＝ω _c+2at -T/2＜t＜T/2 (1)

The time dependent function of the phase of light source (t) is

φ(t)＝ω _ct+at ²+φ ₀ -T/2＜t＜T/2 (2)

The light field of light source changes function in time and can be expressed as:

E(t)＝E ₀f(t)cos(ω _ct+at ²+φ ₀) -T/2＜t＜T/2 (3)

Wherein, ω _cBe the laser original frequency, 2a is the slope of the linear variation in the sawtooth wave, and T is the time period of change, E ₀Be light source initial light intensity, φ ₀Be initial phase; F (t) is a correction factor, and this coefficient is because the modulated function of introducing of variation in time that causes of light intensity.

2, the laser after the modulation is assigned to laser reference path and measures light path according to the light intensity fixed proportion by 2 * 2 fiber couplers through behind the fibre optic isolater; The laser that enters reference path is L through length ₀(Fiber Loop Mirror FLM), and is all reflected by FLM directly to enter the optical fiber loop catoptron behind the compensated optical fiber; Enter the laser of measuring light path and by 1 * i fiber coupler the light intensity mean allocation is measured in the light path to the branch on i road, the laser that per minute is measured in the light path is respectively L through length ₁, L ₂L _iFibre delay line after directly enter in the middle of welding the optical fiber loop catoptron (FLM) of the long period fiber grating (LPG) of the sensing of being used as is arranged, the light intensity of being introduced after the loss by LPG is reflected by FLM.Wherein, L ₁, L ₂L _i＞L ₀, for fear of each interference that divides the signal of the mutual beat frequency of measurement light path that system is introduced, the length difference that any two branches are measured the fibre delay line in the light path is far smaller than the length difference of compensated optical fiber in wherein any fibre delay line and the reference path simultaneously.

When the centre wavelength of LPG transmission peaks owing to extraneously change (temperature change, bend, stress changes, environment variations in refractive index etc.) and after drifting about, the drift of the transmission spectrum of LPG has caused the light intensity transmission change of the wavelength at light source place, and then the signal light power that FLM reflects on this sensing light path can change thereupon.

3, each minute measured the light intensity of light path reflected back once more through 1 * i fiber coupler, the light intensity of coupling back and reference path reflected back is once more through 2 * 2 fiber couplers, in 2 * 2 fiber couplers, because the light path difference on each road, produce beat signal between each light signal

$I\left(t\right)=\frac{1}{4}<{|E_r\left(t\right)+E_s(t-\mathrm{\&tau;})|}^2>$

(4)

$=\frac{1}{4}{\left|E_r\right|}^2{\left|E_s\right|}^2{\left|f\left(t\right)\right|}^2(1+\cos \{{\mathrm{\&omega;}}_c\mathrm{\&tau;}-a{\mathrm{\&tau;}}^2+2\mathrm{a\&tau;t}\left\}\right),-T/2<t<T/2$

Wherein τ=2n Δ L/c is the time-delay that reference path and the optical path difference of measuring light path are introduced, and n is an optical fibre refractivity, and c is the light velocity in the vacuum, and Δ L is an optical path difference; E _r(t) and E _s(t-τ) is respectively reference path and divides the reflection light field of measuring light path;

The frequency of beat signal is

f _b＝2aτ＝4anΔL/c (5)

After this light signal is incorporated into photodiode and is converted into electric signal, collect by data collecting card.

4, after the data that data collecting card is collected are carried out Fourier transform by the fft analysis instrument, obtain the whole beat signals in the spectral domain of signal, remove each minute by high-pass filtering and measure beat signal between the light path, keep the beat signal that each minute measure light path and reference path, divide the length of the fibre delay line of measuring the light path setting simultaneously according to difference, divide the position of the beat signal of measuring the light path correspondence to make a distinction difference.

5, the transmission peaks centre wavelength that changes the LPG in each minute measurement light path is noted the relation of wavelength shift with the beat signal Strength Changes to the drift of long wave direction.

6, the LPG with each minute measurement light path correspondence is installed in the environment that needs sensing, when measured physical quantity changes in the environment, by measuring the variation that each minute measure the beat signal intensity of light path and reference path, determine the wavelength shift of the centre wavelength of corresponding LPG, thereby obtain the environmental physics amount of each LPG institute sensing.

As sensing element, the light intensity that the skew of the centre wavelength of each LPG transmission peaks that environmental change causes makes each minute measure light path changes with the LPG in each minute measurement light path.Because the light signal strength of light source and reference path all remains unchanged the intensity of each beat signal

The light intensity of measuring light path with each minute is directly proportional.

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

The equipment of realizing such scheme is: the digital signal generator output terminal is electrically connected with the current controling end of semiconductor laser; The output terminal of semiconductor laser is connected with an input port light of 2 * 2 fiber couplers by fibre optic isolater, another input port of 2 * 2 fiber couplers is connected with the input end 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 fft analysis instrument; An output port of 2 * 2 fiber couplers is connected with optical fiber loop catoptron light by compensated optical fiber; Another output port of 2 * 2 fiber couplers is connected with the input port light of 1 * i fiber coupler, and the i of a 1 * i fiber coupler output port has the optical fiber loop catoptron light of the long period fiber grating of the sensing of being used as to be connected by i bar fiber delay line with i middle welding respectively.

Described semiconductor laser adopts the adjustable continuous semiconductor laser instrument of single longitudinal mode of optical fiber communication wave band.

The present invention mainly is applicable to the multiplexing and demodulation of multichannel LPG sensor signal, spatial resolution is up to several millimeters, the maximum demodulation number of channel reaches hundreds of, can reach very high measuring accuracy by the processing to interference signal, can carry out synchronously monitoring in real time to each sensing point.Measure surveying instruments such as comparing spectrometer simultaneously in real time, have advantage with low cost, be fit to integrated and instrumentation.

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, the output terminal of digital signal generator 1 is electrically connected with the current drives end of semiconductor laser 2.Semiconductor laser 2 is connected with an input port light of 2 * 2 fiber couplers 4 by optoisolator 3.An output port of fiber coupler 4 is connected with an input end light of fiber annular catoptron 5 through behind the compensated optical fiber of certain-length.Another output port of fiber coupler 4 is connected with the input port light of the coupling mechanism 8 of 1 * n.Each output port of coupling mechanism 8 is with after delay line 8 light of different length are connected, and light has connected the fiber annular catoptron that welding has long period fiber grating 6 respectively.Another input port of fiber coupler is connected with the input end light of photodiode 9, and the output terminal of photodiode 9 is electrically connected with data collecting card 10.The output terminal of data collecting card 10 is electrically connected with the fft analysis instrument.Above-mentioned a plurality of LPG sensors 6 are embedded in the monitored buildings as microbend sensor, the Lights section and signal demodulation part are all in monitoring center, laser enters a plurality of LPG sensors 6 by single-mode fiber, passes flashlight back monitoring center by single-mode fiber again and carries out signal demodulation and processing.

Concrete detection method may further comprise the steps:

The current drives frequency of setting signal generator output is f.Select for use centre wavelength at the single longitudinal mode continuous semiconductor laser instrument of communication band as light source, the wavelength frequency range of laser instrument is less than 1MHz, having exported the frequency sweep frequency through the laser instrument after the frequency modulation (PFM) is f, swept frequency range is the continuous light of B.The light signal of light source output is assigned to reference path by fiber coupler with light intensity and measures in the light path.Each LPG in the measurement light path is as sensor, and each LPG sensor place divides the light signal of measuring light path to have different light paths, through fiber annular mirror reflects back into optical fibers coupling mechanism; Carry the flashlight of each road LPG heat transfer agent and reference light by the fiber coupler single-mode fiber that is coupled back, finish the multiplexed of transducing signal.Light signal carries out record by data collecting card after carrying out opto-electronic conversion by photodiode.The data of noting are carried out frequency-domain analysis through the fft analysis instrument.Has different optical path differences between light signal in every branch road LPG sensor and the reference path.These different optical path differences cause having occurred on the frequency domain different beat signals.

The frequency of beat signal and the pass of optical path difference are

f _b＝2aτ＝4anΔL/c (5)

Wherein a=Bf/2 is half of slope of light source frequency sawtooth wave center line deformationization, and n is an optical fibre refractivity, and c is the light velocity in the vacuum, and Δ L is an optical path difference.Though each light intensity of measuring light path is low than reference path, the beat frequency that their optical path differences each other cause also can be present in the data of being obtained.Measure the interference of signal of the mutual beat frequency of light path for fear of each to system's introducing, need to set the fibre delay line length of each branch road, the length difference that makes any two branches measure the fibre delay line in the light path is far smaller than the length difference of compensated optical fiber in wherein any fibre delay line and the reference path.Each signal frequency of measuring the mutual beat frequency of light path is well below measuring-signal like this, and these interfering frequencies can be removed by modes such as high-pass filterings.

After the data that data collecting card is collected are carried out Fourier transform by the fft analysis instrument, obtain the whole beat signals in the spectral domain of signal, remove each minute by high-pass filtering and measure beat signal between the light path, keep the beat signal that each minute measure light path and reference path, divide the length of the fibre delay line of measuring the light path setting simultaneously according to difference, divide the position of the beat signal of measuring the light path correspondence to make a distinction difference.

The light signal that photodiode detects is:

$I\left(t\right)=\frac{1}{4}<{|E_r\left(t\right)+E_s(t-\mathrm{\&tau;})|}^2>$

(4)

$=\frac{1}{4}{\left|E_r\right|}^2{\left|E_s\right|}^2{\left|f\left(t\right)\right|}^2(1+\cos \{{\mathrm{\&omega;}}_c\mathrm{\&tau;}-a{\mathrm{\&tau;}}^2+2\mathrm{a\&tau;t}\left\}\right),-T/2<t<T/2$

By (4) as can be seen, the intensity of each beat signal

The amplitude of measuring the beat signal of light path with each minute is directly proportional.For LPG, if extraneous environment (temperature, stress, little curved, refractive index etc.) change, the transmission peaks of LPG can produce drift, the relative position of LPG transmission peaks centre wavelength and light source place wavelength changes, and therefore light intensity changes in the transmissivity at optical source wavelength place.Change each sensing LPG surrounding environment one by one, its centre wavelength is moved to the long wave direction, amount of movement that writes down its transmission peaks centre wavelength and the relation that the intensity of corresponding beat signal changes are calibrated, as Fig. 2.Each sensing LPG is installed in the environment that needs sensing, when measured physical quantity changes in the environment, use the calibration data, relatively each sensing LPG is in the beat signal intensity of frequency spectrum correspondence, can obtain the amount of movement of each sensing LPG centre wavelength, further can obtain the physical quantity of each sensing LPG institute sensing, thereby realize multiplexing and demodulation the LPG sensor.

Claims (2)

1, a kind of method of multiplexing and demodulating long period optical fiber optical grating array is characterized in that this method may further comprise the steps:

(1) select for use centre wavelength at the adjustable continuous wave laser of the single longitudinal mode of optical fiber communication wave band as light source, the frequency spectrum live width of light source is less than 1MHz; The frequency of the laser that light source is sent is modulated, and frequencies omega (t) becomes the cycle sawtooth wave function that t in time changes

ω(t)＝ω _c+2at -T/2＜t＜T/2 (1)

The time dependent function of the phase of light source (t) is

φ(t)＝ω _ct+at ²+φ ₀ -T/2＜t＜T/2 (2)

The light field of light source changes function representation in time:

E(t)＝E ₀f(t)cos(ω _ct+at ²+φ ₀) -T/2＜t＜T/2 (3)

Wherein, ω _cBe the laser original frequency, 2a is the slope of the linear variation in the sawtooth wave, and T is the time period of change, E ₀Be light source initial light intensity, φ ₀Be initial phase; F (t) is a correction factor, and this coefficient is because the modulated function of introducing of variation in time that causes of light intensity;

(2) laser after the modulation is assigned to laser reference path and measures light path according to the light intensity fixed proportion by 2 * 2 fiber couplers by behind the fibre optic isolater; The laser that enters reference path is L through length ₀Directly enter the optical fiber loop catoptron behind the compensated optical fiber, and all reflected by the optical fiber loop catoptron; Enter the laser of measuring light path and by 1 * i fiber coupler the light intensity mean allocation is measured in the light path to the branch on i road, the laser that per minute is measured in the light path is respectively L through length ₁, L ₂L _iFibre delay line directly enter in the middle of welding the optical fiber loop catoptron of the long period fiber grating of the sensing of being used as is arranged, introduce light intensity after the loss by the optical fiber loop mirror reflects by long period fiber grating; Wherein, L ₁, L ₂L _i＞L ₀, the length difference that any two branches are measured the fibre delay line in the light path is far smaller than the length difference of compensated optical fiber in wherein any fibre delay line and the reference path;

(3) light intensity of each minute measurement light path reflected back is once more through 1 * i fiber coupler, and the light intensity of coupling back and reference path reflected back in 2 * 2 fiber couplers, produces beat signal between each light signal once more through 2 * 2 fiber couplers,

$I\left(t\right)=\frac{1}{4}<{|E_r\left(t\right)+E_s(t-\mathrm{\&tau;})|}^2>$ (4)

$=\frac{1}{4}{\left|E_r\right|}^2{\left|E_s\right|}^2{\left|f\left(t\right)\right|}^2(1+\cos \{{\mathrm{\&omega;}}_c\mathrm{\&tau;}-{\mathrm{a\&tau;}}^2+2\mathrm{a\&tau;t}\left\}\right),-T/2<t<T/2$

Wherein τ=2n Δ L/c is the time-delay that reference path and the optical path difference of measuring light path are introduced, and n is an optical fibre refractivity, and c is the light velocity in the vacuum, and Δ L is an optical path difference; E _r(t) and E _s(t-τ) is respectively reference path and divides the reflection light field of measuring light path;

The frequency of beat signal is

f _b＝2aτ＝4anΔL/c (5)

After this light signal is incorporated into photodiode and is converted into electric signal, collect by data collecting card;

(4) after the data that data collecting card is collected are carried out Fourier transform by the fft analysis instrument, obtain the whole beat signals in the spectral domain of signal, remove each minute by high-pass filtering and measure beat signal between the light path, keep the beat signal that each minute measure light path and reference path, divide the length of the fibre delay line of measuring the light path setting simultaneously according to difference, divide the position of the beat signal of measuring the light path correspondence to make a distinction difference;

(5) the transmission peaks centre wavelength that changes the long period fiber grating in each minute measurement light path is noted the relation of wavelength shift with the beat signal Strength Changes to the drift of long wave direction;

(6) long period fiber grating with each minute measurement light path correspondence is installed in the environment that needs sensing, when measured physical quantity changes in the environment, by measuring the variation that each minute measure light path and the beat signal intensity of reference path, determine the wavelength shift of centre wavelength of the long period fiber grating of correspondence.

2, realize the equipment of method according to claim 1, it is characterized in that: the digital signal generator output terminal is electrically connected with the current controling end of semiconductor laser; The output terminal of semiconductor laser is connected with an input port light of 2 * 2 fiber couplers by fibre optic isolater, another input port of 2 * 2 fiber couplers is connected with the input end 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 fft analysis instrument; An output port of 2 * 2 fiber couplers is connected with optical fiber loop catoptron light by compensated optical fiber; Another output port of 2 * 2 fiber couplers is connected with the input port light of 1 * i fiber coupler, and the i of a 1 * i fiber coupler output port has the optical fiber loop catoptron light of the long period fiber grating of the sensing of being used as to be connected by i bar fiber delay line with i middle welding respectively; Described semiconductor laser is the adjustable continuous semiconductor laser instrument of the single longitudinal mode of optical fiber communication wave band.

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