CN201716502U - Bragg grating high-speed demodulating system based on cascading long-period fiber bragg grating - Google Patents

Abstract

The utility model discloses a fiber bragg grating (FBG) high-speed demodulating system based on a cascading long-period fiber bragg grating (LPFG), belonging to the health monitoring field of an intelligent structure. In the system, a broadband light source (1) is connected with one output end of a Y-shaped fiber coupler (3) by a single-mode optical fiber (2); one input end of the Y-shaped optical fiber coupler (3) is connected with a fiber Bragg grating (4); the other output end of the Y-shaped optical fiber coupler (3) is connected with a cascading long-period grating (6); and the other end of the cascading long-period grating is connected with a high-speed photoelectric detector (7); an output end of the high-speed photoelectric detector is connected with a conditioning circuit (8) and is connected with a computer (10) connected with a data acquisition card (9). In the demodulating method, the characteristic of the cascading long-period grating is effectively combined with the centre wavelength monitoring technology of a fiber grating sensor, therefore, the system has the advantages of high sensitivity, high demodulating speed, low cost and the like and can be used for impact and vibration monitoring in an aerospacecraft, a bridge, a dam, an underwater construction and the like.

Description

Bragg grating high-speed demodulating system based on cascade-connection long period fiber grating

Technical field

The utility model relates to a kind of Bragg grating high-speed demodulating system based on cascade-connection long period fiber grating, belongs to optical-fiber intelligent material structure health monitoring field.

Background technology

The application of fiber-optic grating sensor is a field in the ascendant, and boundless development prospect is arranged, and the topmost obstacle of a large amount of practical applications of limit fibre grating sensor is the demodulation of transducing signal at present.Demodulation is to utilize fiber grating to have the characteristics of Wavelength-encoding, can obtain the variable condition of transducing signal by Wavelength demodulation.Research and development are applicable to and reduce cost the demodulating system of practical engineering application, are that fiber-optic grating sensor can obtain the vital problem promoted in practical engineering application.

At present, the practicability demodulating system of fiber grating sensing signal has following several scheme: (1) adopts wideband light source and tunable fiber F-P wave filter that sensor fibre grating reflection spectrum is carried out filter scan, this demodulation method exists high-precision tunable Fabry Perot chamber price higher, the problem that the filtering loss is big; (2) the light spectrum image-forming technology that adopts dispersion element and ccd array to combine is carried out wavelength analysis, the (FBG) demodulator body is long-pending big, not portable, unsuitable on-the-spot the use, the more important thing is that it can not export electric signal, this record, storage for measurement result, show and the electric signal that offers control loop necessity with reach industrial processes automatically the purpose of control all be totally unfavorable; (3) adopt coupling grating filter method that the sensor fibre grating is carried out Wavelength demodulation, this demodulation method is simple in structure, cost is low but precision is not high.The researcher of various countries has proposed other demodulation methods in succession, as tunable narrow-band lasing light emitter method, non-equilibrium Mach-Zehnder interferometer mode, Fourier transform demodulation method, and the optical fiber grating sensing element is realized sensing measurement etc. as the fiber laser cavity mirror.

The application form of state's sensors such as America and Europe mainly is with the optical fiber Bragg raster Information Monitoring, to extract metrical information in optical fiber Bragg grating sensor reflection wavelength introducing spectrometer or the interferometer, the optical fiber BRAGG grating-IS wavelength demodulation device of U.S. MICRONOPTICS company is the representative of this field advanced technology.And domestic sensing technology is compared with states such as America and Europes, also there is a certain distance, particularly in the general character basic technology of Fibre Optical Sensor, semipilot technology, produce aspect the equipment technology particularly outstandingly, but domesticly in recent years successively obtaining a collection of application prospect extensive studies achievement aspect fiber-optic grating sensor and the demodulation thereof.

Department of electrical engineering of The Hong Kong Polytechnic University and contemporary optics research institute of Nankai University cooperate, coupling grating tunable fiber laser wavelength scanning addressing demodulation scheme was once proposed, promptly adopt the wavelength tuning element of fiber grating as fiber laser, the step-scan of system controlled by computer fiber laser, and the output signal of synchronous acquisition photodetector, according to the foveal reflex wavelength value of determining sensing grating corresponding to the tuning step number of the maximum output of photodetector.Because the scanning step of fiber laser is bigger, the wavelength shift resolution that the method obtained is not high enough.In order to improve wavelength shift resolution, they have done improvement to it.In the improvement project, after the system controlled by computer fiber laser is finished the single pass process, the relation data between photodetector output voltage and the optical maser wavelength is carried out curve fitting, determine the foveal reflex wavelength value of sensing grating by match with Gauss-Newton method.Like this, improve demodulation resolution, thereby realized high-resolution temperature and strain measurement.

Zhejiang University's information and electronic engineering system are studied the phase carrier modulation-demodulation technology of interferometric optical fiber sensor, analysis has drawn new phase carrier detection bandwidth requirement, and in the laboratory, detected comparatively stable analog sensed signal with the phase carrier detection mode, use piezoelectric ceramics to transfer device and resetting system to introduce bigger additional noise problem mutually in the active tracking homodyne detection thereby overcome.Therefore, it is multiplexing not to be only applicable to passive detection, remote measurement and array, and in remote, rugged surroundings, passive detection and multiplexing sensor-based system very big application prospect is arranged.

The smart instrument of Tsing-Hua University is that precision measurement technology and instrument National Key Laboratory are studied the phase generated carrier demodulation techniques of interferometric optical fiber sensor, and the improvement that the phase generated carrier demodulation method has been carried out, propose to adopt a reference sensor to monitor the influence of light source frequency shake or environmental drift, eliminate the influence of system's thermal perturbation by measuring method that the sense light fiber Bragg grating compares with the reference light fiber Bragg grating.

Beijing Institute of Technology's photoelectric project ties up to carries out in the research of Mach-Zehnder interference type optical fiber hydrophone, primary study the demodulation principle of the interference type optical fiber hydrophone formed with 3 * 3 coupling mechanisms, and to point out to adopt 3 * 3 coupler demodulation techniques are the preferred versions that constitute the full optical fiber interference type hydrophone system.This be because adopt 3 * 3 coupling mechanisms make interferometer be output into 120 ° more reliable and stable, thereby simple and practical.

199～202 pages of optics journal 2004 the 24th the 2nd phases of volume have reported that contemporary optics research institute of Nankai University studies long period fiber grating edge filter linear demodulation method.Demodulation scheme is that the light that wideband light source sends enters sensing grating through three-dB coupler.Form narrow-band spectrum by sensing Bragg grating reflection back, be divided into the two-way light beam through coupling mechanism again.Wherein a branch of through linear wave filter arrival photoelectric detector.Another bundle directly detects, to compensate the influence that experiment is caused owing to intensity of light source fluctuation.This scheme is applicable to dynamically, static measurement, and it is lower to have a cost, advantage such as easy to use, but demodulation accuracy is lower, but and the wavelength coverage of demodulation little.

The utility model content

The purpose of this utility model is to provide a kind of monitoring accuracy height, speed fast, the optical fiber Bragg raster demodulating system that wavelength coverage is wide.

A kind of Bragg grating high-speed demodulating system based on cascade-connection long period fiber grating, it is characterized in that composed as follows: wideband light source links to each other with an output terminal of y-type optical fiber coupling mechanism by single-mode fiber, the input termination optical fiber Bragg raster of this y-type optical fiber coupling mechanism, this another output terminal of y-type optical fiber coupling mechanism connects cascade-connection long period grating, high-speed photodetector, modulate circuit, data collecting card, computing machine successively by another single-mode fiber; Above-mentioned cascade-connection long period grating, be in series by at least two long-period gratings, and each long-period gratings centre wavelength is identical, and the transmitted spectrum of the cascade-connection long period grating after the series connection and Bragg grating reflection spectrum intersect, and this intersection point is positioned at cascade-connection long period grating transmitted spectrum linearity range.

Because the cascade-connection long period grating is the transmission-type optical fibre device, the insertion loss is low, and system has utilized the transmitted light of cascade-connection long period, and optical fiber Bragg raster is the reflection type optical fiber device, the reflectivity height, and system has then utilized the reflected light of optical fiber Bragg raster.After the optical fiber Bragg raster center wavelength variation, respective change can take place by the filtered light intensity of cascade-connection long period grating in the optical fiber Bragg raster reflected light signal.Adopt high-speed photodetector that light signal (variable quantity of output intensity) is converted into electric signal, the light signal of light path part output a little less than, it is also fainter to obtain voltage signal after opto-electronic conversion, designed signal conditioning circuit to signal amplify, filtering etc.The counter variable quantity of releasing optical fiber Bragg raster centre wavelength of electric signal by modulate circuit output.

Described cascade-connection long period grating is in series by at least two long-period gratings, and each long-period gratings centre wavelength is identical, the transmitted spectrum of the cascade-connection long period grating after the series connection and Bragg grating reflection spectrum intersect, and this intersection point is positioned at cascade-connection long period grating transmitted spectrum linearity range.The transmitted spectrum of cascade-connection long period is different with the spectrum of common long-period gratings, because this system utilizes the linear filter characteristic of cascade-connection long period grating, so only the spectrum linearity range is analyzed.The slope of the spectrum linearity range of cascade-connection long period grating is much larger than single long-period gratings, and is therefore bigger through the filtered light intensity variable quantity of cascade-connection long period grating, makes things convenient for the photodetector monitoring, and a plurality of long period cascades can significantly improve the precision of demodulation.

The linearity range of the transmitted spectrum of the reflectance spectrum of described optical fiber Bragg raster and cascade-connection long period grating intersects, with the luminous power linear change after the assurance modulation.The wavelength coverage of cascade-connection long period spectrum linearity range adopts this scheme can increase the wavelength coverage of demodulation greater than single long-period gratings.

This utility model is an all optical fibre structure, and it is tuning to need not mechanical part, and demodulation speed depends in the bandwidth of photodetector and the processing speed of back-end circuit, and the bandwidth of high-speed photodetector is generally several GHz, therefore demodulation speed height of the present utility model.

Select the cascade-connection long period grating that matches according to the centre wavelength of optical fiber Bragg raster, the centre wavelength of optical fiber Bragg raster must be positioned at the left side linearity range or the right side linearity range of cascade-connection long period grating spectrum centre wavelength.When the centre wavelength of optical fiber Bragg raster during less than long-period gratings (being positioned at the left side linearity range), along with the increase of optical fiber Bragg raster centre wavelength, the luminous power of the resonance wavelength correspondence after the cascade-connection long period modulation reduces gradually, otherwise, the luminous power increase; The centre wavelength of optical fiber Bragg raster is during greater than long-period gratings, and along with the increase of optical fiber Bragg raster centre wavelength, the luminous power of resonance wavelength correspondence increases gradually, otherwise luminous power reduces.

The beneficial effects of the utility model are:

1, the demodulation scheme that the utility model proposes based on the cascade-connection long period grating, utilized the bandreject filtering characteristic of long-period gratings, the cascade-connection long period grating cycle is long, no back-reflection can not produce long-period gratings to light source and disturbs in light path, it is low to make simple cost, so the utility model is expected to replace existing expensive high speed fibre Bragg grating demodulation equipment.

2, the utility model adopts the light intensity demodulation method, is intensity variations with the change transitions of optical fiber Bragg raster centre wavelength, changes light signal into electric signal by photodetector, is convenient for measuring result's record, storage and control.Photodetector and modulate circuit are conveniently made module, change optical fiber Bragg raster and can adopt this modular circuit with the coupling long-period gratings, save cost.

3, the utility model combines cascade-connection long period grating characteristic effectively in fiber-optic grating sensor centre wavelength monitoring technology, can be used for impact, the vibration monitoring of aerospacecraft, bridge, dam, submerged structure etc.

Description of drawings

Fig. 1 is based on the Bragg grating high-speed demodulating system of cascade-connection long period grating.

Fig. 2 is demodulation principle figure.

Fig. 3 is common long-period gratings and cascade-connection long period grating spectrum comparison diagram.

Fig. 4 is the spectrogram after the cascade-connection long period Grating Modulation.

Fig. 5 is a modulate circuit.

Label title among the figure: 1. wideband light source, 2. single-mode fiber, 3.Y type fiber coupler, 4. optical fiber Bragg raster, 5 single-mode fibers, 6. cascade-connection long period grating, 7. high-speed photodetector, 8. modulate circuit 9. data collecting cards, 10. computing machine, 11. zeroing circuit, 12. ac amplifier circuits, 13. filtering circuits.

Embodiment

As shown in Figure 1, the concrete composition of a kind of Bragg grating high-speed demodulating system based on cascade-connection long period fiber grating of the present utility model is, the light that wideband light source 1 (the wavelength available scope is the wideband light source of 1520nm-1570nm) sends incides on the optical fiber Bragg grating sensor 4 via Y type coupling mechanism 3 by single-mode fiber 2, and reflected light is coupled into cascade-connection long period grating 6 by single-mode fiber 5 through Y type coupling mechanism 3 and enters into high-speed photodetector 7.So far, light signal changes voltage signal into, by modulate circuit 8 signal is amplified, transfers to computing machine 10 by data collecting card 9 after the filtering and show.

The cascade-connection long period grating is in series by at least two long-period gratings, and each long-period gratings centre wavelength is identical, the transmitted spectrum of the cascade-connection long period grating after the series connection and Bragg grating reflection spectrum intersect, and this intersection point is positioned at cascade-connection long period grating transmitted spectrum linearity range.

Method of work based on the optical fiber Bragg raster high-speed demodulating system of cascade-connection long period fiber grating comprises the steps:

The first step, the light of wideband light source is by exciting light fiber Bragg grating sensor behind the coupling mechanism;

In second step, the light more than 90% reflects through optical fiber Bragg raster, modulates by being incident to the cascade-connection long period grating behind the coupling mechanism;

The 3rd step, after the center wavelength variation of optical fiber Bragg raster, the optical fiber Bragg raster output intensity generation linear change after the long-period gratings modulation;

In the 4th step, photodetector is converted into the variation of voltage signal with optical power change, handles through exporting computing machine to by data collecting card behind the signal condition.

The 5th step is by the anti-variation that pushes away the centre wavelength of optical fiber Bragg raster of the variable quantity of voltage;

The 6th step, the human-computer interaction interface that design is friendly;

In the 7th step, demodulating system is demarcated.

Fig. 2 is the demodulation principle synoptic diagram.If the centre wavelength of optical fiber Bragg raster is λ ₀, after the optical fiber Bragg raster center wavelength variation, establish variation delta I and the wavelength shift λ-λ of optical fiber Bragg raster reflected light signal by output intensity after the filtering of cascade-connection long period grating ₀Funtcional relationship be F (λ), in optical source wavelength was approximately linear scope, this filter function can be expressed as:

F(λ)＝A(λ-λ ₀) (1)

Suppose that Bragg optical grating reflection light is that spectrum width is the Gaussian distribution of Δ λ, reflective light intensity is

$I_s=\frac{1}{\sqrt{2}}{I}_0\mathrm{RA}---\left(2\right)$

I wherein ₀Be signal light intensity, R is the reflectivity of fiber grating, and Δ λ is the half-peak width of reflection peak, and A is the scale-up factor of linear filter, and luminous power becomes after the optical fiber Bragg raster reflectance spectrum sees through long-period gratings:

$I_R=\frac{1}{\sqrt{2}}{I}_0\mathrm{RA}({\mathrm{\λ}}_B-{\mathrm{\λ}}_0+\frac{\mathrm{\Δ\λ}}{\sqrt{\mathrm{\π}}})---\left(3\right)$

By (2), (3) Shi Kede:

$\frac{I_S}{I_R}=A({\mathrm{\λ}}_B-{\mathrm{\λ}}_0+\frac{\mathrm{\Δ\λ}}{\sqrt{\mathrm{\π}}})---\left(4\right)$

By measuring

Can obtain dynamic λ _BValue, this method is based on intensity monitor.

Fig. 3 is the spectrogram of cascade-connection long period grating, common long-period gratings and optical fiber Bragg raster, and the long-period gratings of establishing centre wavelength 1530.360nm is LPFG1, and centre wavelength is that the long-period gratings of 1530.040nm is LPFG2.The cascade-connection long period grating is to be made of the LPFG that centre wavelength is respectively 1530.360nm and 1530.040nm, because the limitation of LPFG manufacture craft, can't obtain two on all four LPFG of centre wavelength, make cascade LPFG near the 1530nm wavelength, have spectral noise, so only consider the spectrum of wavelength in 1525nm-1529nm, 1531nm-1535nm scope.By the LPFG curve of spectrum as can be seen in 1526nm-1528nm, 1531nm-1533nm wavelength coverage luminous power and wavelength can be similar to and think linear, and the cascade LPFG curve of spectrum luminous power and wavelength in 1526nm-1529nm, 1531nm-1534nm wavelength coverage can be similar to and think linear, wherein LPFG1 transmission spectrum slope minimum in this section wavelength coverage; The spectrum slope maximum of cascade-connection long period.The centre wavelength of optical fiber Bragg raster reflectance spectrum is respectively 1527.050nm, 1527.843nm, and corresponding luminous power is respectively-15.321dB ,-15.923dB.The transmission spectrum curve intersection of optical fiber Bragg raster reflectance spectrum and LPFG2 and cascade LPFG as seen from Figure 3, promptly LPFG2 and cascade LPFG can be used as boundary filter optical fiber Bragg raster are modulated, and LPFG1 can not modulate optical fiber Bragg raster.Fig. 3 only is the spectrogram of two long-period gratings of cascade, and when a plurality of long-period gratings of cascade, slope variation (increase) can be more obvious.

Fig. 4 is the spectrogram after the cascade-connection long period Grating Modulation.Centre wavelength be 1527.050nm FBG wavelength variations 200nm simultaneously centre wavelength be under the situation of FBG wavelength shift 500nm of 1527.843nm, see shown in figure (a), the figure (b) through the outgoing spectrum situation of change of the FBG of LPFG2 and cascade LPFG modulation.Along with the increase of FBG centre wavelength, the luminous power of resonance wavelength correspondence reduces gradually, can get FBG by figure (a) under identical central wave length shift situation with the contrast of scheming (b) measurement result, and the light intensity variable quantity after cascade LPFG modulation is bigger.

Fig. 5 is the modulate circuit synoptic diagram, comprises three parts, is respectively zeroing circuit, ac amplifier circuit and filtering circuit.In electronic circuit, when feeble signal need be amplified, generally to use amplifying circuit, accurate in order to be operational amplifier operate as normal and amplification output voltage signal, need to adopt zeroing circuit.In the Dynamic Signal testing process, because photoelectric conversion module has bigger output voltage base value, when the relative voltage variable quantity hour, if directly the electric signal of photoelectric conversion module output is amplified, can cause that amplifier is saturated, corrupt data capture card etc., be unfavorable for data acquisition and analysis.The design zeroing circuit deducts the voltage base value, and under the constant situation of optical fiber Bragg raster centre wavelength, it is zero that the rheostat in the regulating circuit makes circuit output voltage.How to be interfered by the signal after opto-electronic conversion and the amplification, signal amplitude or phase place distort, and noise occurs, therefore need carry out filtering to signal.

Claims (1)

1. Bragg grating high-speed demodulating system based on cascade-connection long period fiber grating is characterized in that composed as follows:

Wideband light source (1) links to each other with an output terminal of y-type optical fiber coupling mechanism (3) by single-mode fiber (2), the input termination optical fiber Bragg raster (4) of this y-type optical fiber coupling mechanism (3), this another output terminal of y-type optical fiber coupling mechanism (3) connects cascade-connection long period grating (6), high-speed photodetector (7), modulate circuit (8), data collecting card (9), computing machine (10) successively by another single-mode fiber (5);

Above-mentioned cascade-connection long period grating (6), be in series by at least two long-period gratings, and each long-period gratings centre wavelength is identical, the reflectance spectrum of the transmitted spectrum of the cascade-connection long period grating after the series connection and Bragg grating (4) intersects, and this intersection point is positioned at cascade-connection long period grating transmitted spectrum linearity range.

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