CN2916581Y - Optical fiber Bragg grating sensing device for eliminating cross sensitivity - Google Patents

Abstract

A fiber Bragg grating sensing device that eliminates cross sensitivity is used to measure multiple physical quantities such as external strain, temperature and stress. It is mainly composed of a broadband light source, a sensing grating, a 2×2 coupler, three optical splitters, three photodetectors, two demodulation fiber Bragg gratings, a wavelength adjuster and a computer. The sensing system of the utility model has the characteristics of low cost, fast, simple and practical, can overcome some shortcomings of the existing demodulation system, and can solve the problem of cross sensitivity of fiber Bragg grating to temperature and strain.

Description

Eliminate the optical fiber Bragg grating sensing device of cross sensitivity

Technical field

The utility model belongs to optical fiber grating sensing, optical testing technology field, particularly a kind of optical fiber Bragg grating sensing device of eliminating cross sensitivity, be used to measure extraneous strain and temperature, measure with the utility model again after also the variation of other physical quantitys can being converted to the variation of temperature or strain.

Background technology

Fiber grating is to set up a kind of space refractive index cycle to distribute in optical fiber as a kind of novel optical device, and making therein, the propagation characteristic of light is changed.When wideband light source incides in the Fiber Bragg Grating FBG (FBG), its catoptrical central wavelength lambda _BProvide by Bragg equation:

λ _B＝2nΛ

Wherein, n is the effective refractive index of fibre core, and Λ is the grating cycle.

When physical quantitys such as the temperature of FBG environment of living in and strain change, can cause the variation in grating refractive index and cycle, thereby cause λ _BVariation, measure Δ λ _BPromptly can draw the situation of change of measured physical quantity, therefore, FBG is widely used in measuring physical quantitys such as temperature, strain, stress, pressure, pressure and displacement.Δ λ _BLinear with the variable quantity of measured physical quantity within the specific limits, when the variation of ambient temperature amount is Δ T, corresponding wavelength shift Δ λ _BTCan be expressed as:

Δλ _BT＝λ _B(α+ξ)ΔT

Wherein α and ξ are respectively the thermal expansivity and the thermo-optical coeffecient of optical fiber.For common silica fibre, when 1550nm, the temperature variant sensitivity coefficient of wavelength is about 13pm/ ℃.

When FBG is subjected to longitudinal strain to be Δ ε, corresponding wavelength shift Δ λ _BSCan be expressed as:

$\mathrm{\Δ}{\mathrm{\λ}}_{\mathrm{BS}}={\mathrm{\λ}}_B\{1-\frac{n^2}{2}[{\mathrm{\ρ}}_{12}-v({\mathrm{\ρ}}_{11}-{\mathrm{\ρ}}_{12})]\}\mathrm{\Δ\ϵ}$

Wherein, ρ ₁₁And ρ ₁₂Be the stress tensor unit of optical fiber, v is a Poisson ratio.For common silica fibre, when 1550 nm, wavelength is about 1.15pm/ μ ε with the sensitivity coefficient of strain variation.

In the FBG sensing technology,, how simply, fast, accurately the small accurate demodulation of wavelength amount of movement is come out, be vital problem in the FBG sensing system because measured signal is Wavelength-encoding.The method of the most direct measurement wavelength variable quantity is utilized spectrometer exactly, yet spectrometer price height, precision is low, volume is big, is not suitable for in-site measurement.For head it off, the researchist has developed some Wavelength demodulation technology in succession, mainly contains several:

(1) interfere people such as demodulation techniques Kersey to propose in 1992 to come the signal of demodulating fiber bragg grating sensing with the method for non-equilibrium Mach-Zehnder interference, the phase place of interferometer is directly proportional with the strain of grating or temperature, the phase change of stellar interferometer can be learnt the strain or the temperature of grating.

(2) the tunable optical source demodulation techniques utilize live width less than the continuously adjustable within the specific limits laser instrument of FBG bandwidth, output wavelength as light source, carry out spectral scan by the output wavelength of tuned laser.Obtain maximum output intensity when the output wavelength of laser instrument overlaps with the centre wavelength of FBG, at this moment, the centre wavelength of FBG can uniquely determine, by with the initial wavelength of FBG relatively can obtain Δ λ _B

(3) linear edge demodulation by filter technology adopts wideband light source as testing light source, the reflected signal of FBG is through the wave filter of a live width broad, transmitance linear change, the intensity that sees through signal changes with the variation of FBG centre wavelength, measure the intensity of this signal, just can obtain the centre wavelength of FBG.

(4) the tunable filtering demodulation techniques adopt wideband light source as testing light source, with a tunable optic filter (reflection or transmission) as restitution element, the light that light source sends directly enters tunable optic filter after the FBG reflection, the centre wavelength of regulating wave filter makes the light intensity of reflection (or transmission) reach maximum, this moment, the centre wavelength of tunable optic filter was the centre wavelength of FBG, by with the initial wavelength of FBG relatively can obtain Δ λ _B

In above-mentioned demodulation method, interfere the demodulation techniques measuring accuracy very high, but its measurement is relative value, and measurement range is subject to the Free Spectral Range of interferometer; Tunable optical source demodulation techniques input is convenient, the resolution height, but light source is difficult to make, and cost is higher; Linear edge demodulation by filter technical testing scope and resolution are inversely proportional to, and require the linearity of wave filter very good, and slope is very big, be difficult to make: tunable filtering demodulation techniques demodulation scope is big, but the scan period is long, and test speed is slow, and poor repeatability.Because the existence of above-mentioned shortcoming makes the Wavelength demodulation technology become one of major obstacle of FBG sensing technology industrialization.The FBG (FBG) demodulator that existing market is sold mainly is based on scanning F-P technology and interferential scanning technology, and it makes complexity, and prices are rather stiff.Therefore in addition, fiber grating is all very sensitive to strain and temperature, causes that by stress said method does not all solve the cross sensitivity problem of FBG to temperature and strain by temperature also the time when needing to distinguish the wave length shift of fiber grating.At present, the differentiation technology of multiple stress and temperature has been arranged, has mainly contained following several:

(1) with reference to the FBG method when measuring the strain of certain structure with a grating sensor S, can go to measure the temperature of S simultaneously with another sensor T (managing to keep it not to be subjected to external stress), Δ λ _S-Δ λ _TBe the wavelength shift that strain is induced, Δ λ _SWith Δ λ _TRefer to wavelength change on S and the T sensor respectively.

(2) dual wavelength stack FBG method is inscribed two gratings that resonant wavelength is different in the same position of optical fiber, if think that resonant wavelength drift and temperature and stress are linear respectively, then can obtain a linear equation in two unknowns group about strain and temperature, by the measurement of counter stress and temperature respectively, can try to achieve four coefficients in the equation, and then definite temperature and strain.

(3) the long period fiber grating method is utilized temperature or the strain that FBG and long period fiber grating (or swing wave filter) can be measured same point simultaneously to the different responses of temperature or strain.

(4) twin-core footpath FBG method is for identical temperature and strain, and the drift of the resonant wavelength of two FBG that core diameter is different is different.The FBG continued accesses that two core diameters is different together, the resonant wavelength of two FBG can be different, such as differing several nm, available WDM technology is separated them, can obtain like this can separation temperature and a system of equations of stress.

(5) method this structure in FBG F-P chamber has two reflection peaks that intensity is different in its principal reflection band, and the peak wavelength position of principal reflection spectrum can change with stress or temperature linearity well.By measuring the variation of catoptrical peak wavelength drift and intensity, measure when can realize temperature and strain.

Summary of the invention

The purpose of this utility model is to provide a kind of optical fiber Bragg grating sensing device of eliminating cross sensitivity, this device should have cost low, quick, simple and direct, practical characteristics, can overcome some shortcomings of existing demodulating system, and can solve the cross sensitivity problem of Fiber Bragg Grating FBG (FBG), promote the industrialization of FBG sensing technology temperature and strain.

A kind of optical fiber Bragg grating sensing device of eliminating cross sensitivity, this device comprises a wideband light source, the light that is sent by this wideband light source transfers to sensing grating through one fen terminal of first beam splitter and reflects, the light that this sensing grating reflects through another minute of first beam splitter terminal enter second beam splitter, after be divided into two bundles, link to each other with an input end of coupling mechanism and the input end of the 3rd beam splitter respectively, one output terminal of described coupling mechanism links to each other with first detector, another output terminal links to each other with the first demodulation grating, and the light that this first demodulation optical grating reflection is returned is received by second detector through described coupling mechanism; The terminal of closing of the 3rd beam splitter links to each other with the second demodulation grating, the light that the second demodulation optical grating reflection is returned is received by the 3rd detector after another minute of the 3rd beam splitter terminal, the described first demodulation grating is placed in the environment identical with sensing grating, experience the variation of environment temperature simultaneously with sensing grating, but the not strained centre wavelength that influences the second demodulation grating is by the control of wavelength regulation device, and described first detector, second detector, the 3rd detector and wavelength regulation device all link to each other with computing machine.

The described first demodulation grating adopts identical encapsulating material and packaged type with sensing grating, places same temperature environment during use.

Described wavelength regulation device is a temperature controller, or the strain tuner, or reflectance spectrum or transmission spectrum are similar to the adjustable light wave-filter of Gaussian distribution.

Described first beam splitter, second beam splitter and the 3rd beam splitter bundle device are circulator, or 2 * 2 coupling mechanism.

Ultimate principle of the present utility model is, adopt a sensing grating, two demodulation gratings, four optical branching devices, three photodetectors and a wavelength regulation device to constitute sensor-based system, the light that is sent by wideband light source enters sensing grating through first optical branching device, its reflected light is divided into two bundles, a branch of through entering first detector behind the first demodulation grating, another bundle is through entering second detector after second demodulation, the output valve of two detectors enters computing machine and handles.The first demodulation grating is placed in the environment identical with sensing grating, experience the variation of environment temperature simultaneously with sensing grating, but not strained influence makes the first demodulation grating play the effect that is similar to the temperature compensation grating simultaneously.Like this, the drift value Δ λ of the output valve of first detector and sensing grating centre wavelength is relevant with the drift value Δ λ 1 of the first demodulation grating centre wavelength, and the output valve of second detector is only relevant with the drift value Δ λ of sensing grating centre wavelength, just can try to achieve Δ λ and Δ λ according to the output valve of two detectors ₁Δ λ and Δ λ ₁Can be expressed as respectively:

Δλ＝0.78λ _Sε+λ _S(α+ξ)ΔT (1)

Δλ ₁＝λ _B1(α+ξ)ΔT (2)

λ and λ in the formula ₁Be respectively sensing grating and the centre wavelength of the first demodulation grating under zero strain, normal temperature, because the sensing grating and the first demodulation grating adopt same packaged type, can think sensing grating thermal expansivity and thermo-optical coeffecient all with and the first demodulation grating identical, be respectively α and ξ.Can obtain extraneous strain stress and temperature variation Δ T by last two formulas, solve the cross sensitivity problem.Simultaneously, the centre wavelength of the second demodulation grating can be regulated by the wavelength regulation device, is not acted upon by temperature changes with the demodulation scope that guarantees system.The output valve of first detector is used to eliminate the influence of light source shake to system's demodulation accuracy.

Advantage of the present utility model is:

(1) the utility model sensing device does not need special wavelength measurement equipment, only need just can calculate the foveal reflex wavelength of sensing grating according to the output valve of detector accurately, compare with initial foveal reflex wavelength again, just can draw the drift value of foveal reflex wavelength.

(2) the utility model sensing device, because a demodulation grating is used as the temperature compensation grating simultaneously, solved the cross sensitivity problem, have advantage simultaneously with reference to FBG method and matched filtering demodulation technology, compare with the demodulation of monochromatic light grid and to have enlarged dynamic range, and solved the diadic question in the monochromatic light grid demodulating system.

(3) the utility model sensing device, the reflected light that adopts the part sensing grating have been eliminated because the influence that light source fluctuation causes Wavelength demodulation as reference light.

(4) the utility model sensing device, use a computer and carry out data processing, can use software replace hardware, the error that hardware itself is introduced when having eliminated by the hardware handles data, and, use a computer and can realize Based Intelligent Control, convenient other parameter with testee is carried out comprehensively, thereby testee is comprehensively analyzed.

(5) the utility model sensing device except that the wavelength regulation device, all is made of optical fiber and fiber optic passive device, uses reliable and stable.The technological level of all components and parts is all very ripe, easy to make feasible, is convenient to promote the industrialization process of FBG sensing technology, can be widely used in various fields.

Description of drawings

Fig. 1 is the utility model fiber bragg grating sensing device synoptic diagram

Fig. 2 is the utility model fiber bragg grating sensing device three grating reflection spectrum synoptic diagram under various conditions

Wherein, (a) be room temperature, (b) be variation of ambient temperature after, (c) for after regulating by the wavelength regulation device; Dotted line is the reflectance spectrum of sensing grating FBG, and curve 1 ', 2 ' is respectively the reflectance spectrum of FBG4 and FBG10.

Embodiment

As shown in Figure 1, the utility model is eliminated the optical fiber Bragg grating sensing device of cross sensitivity, mainly is made of the coupling mechanism 6 of wideband light source 1,2 * 2, three beam splitters (first beam splitter 2, second beam splitter 5 and the 3rd beam splitter 9), three photodetectors (first photodetector 7, second photodetector 8 and the 3rd photodetector 11), two demodulating fiber bragg gratings (first demodulating fiber bragg grating 4 and second demodulating fiber bragg grating 10), wavelength regulation device 12, computing machine 13 and sensing gratings 3.The light that is sent by wideband light source 1 transfers to sensing grating 3 and reflects after a branch terminal of first beam splitter 2, the light that reflects is divided into two bundles after another minute terminal of first beam splitter 2 enters second beam splitter 5, a branch terminal with one of the input end of coupling mechanism 6 and the 3rd beam splitter 9 links to each other respectively.One output terminal of coupling mechanism 6 directly links to each other with first detector 7, and another output terminal links to each other with the first demodulation grating 4, and the light that the first demodulation grating 4 reflects is received by second detector 8 through coupling mechanism 6 backs; The terminal of closing of the 3rd beam splitter 9 links to each other with the second demodulation grating 10, and the light that the second demodulation grating 10 reflects is received by the 3rd detector 11 after another minute of beam splitter 9 terminal.The first demodulation grating 4 is placed in the environment identical with sensing grating 3, experiences the variation of environment temperature simultaneously with sensing grating 3, but not strained influence, and the centre wavelength of the second demodulation grating 10 is by 12 controls of wavelength regulation device.Described first detector 7, second detector 8 all link to each other with computing machine 13 with the 3rd detector 11 and wavelength regulation device 12, and the output valve of first detector 7 is used to eliminate the influence of light source shake to system's demodulation accuracy.About each grating centre wavelength relative position, as shown in Figure 2.During at room temperature with zero strain, should guarantee that second detector 8 and the 3rd detector 11 have output valve simultaneously, sensing grating 3 and two demodulation grating reflection spectrum all have light intensity that lap and they reflect in the detector sensitivity scope, shown in Fig. 2 (a).Suppose that environment temperature reduces Δ T with respect to room temperature, the whole blue shift of the first demodulation grating 4 and sensing grating 3 centre wavelengths, and the centre wavelength of the second demodulation grating 10 does not change, the relative position of their centre wavelength becomes Fig. 2 (b) by Fig. 2 (a), if Δ T is bigger, will have a strong impact on the demodulation scope of total system.In order to eliminate the influence of temperature to system's demodulation scope, with thermal tuning is example, can be by reducing the temperature of temperature controller 12, make the centre wavelength of the second demodulation grating 10 that the drift identical with the first demodulation grating 4 take place, realize that the first demodulation grating 4 and the second demodulation grating 10 mate again, see shown in Fig. 2 (c).Like this, even Δ T is very big, always can guarantee that the demodulation scope of system is unaffected so that the first demodulation grating 4 and the second demodulation grating 10 mate again.

The result that described first detector 7, second detector 8 and the 3rd detector 11 are measured carries out data processing via computing machine 13 according to formula 1 and 2, can obtain not have the temperature or the strain of cross sensitivity.

The utility model shows that through on probation this device has low, quick, simple and direct, the practical characteristics of cost, Some shortcomings of existing demodulating system can be overcome, and Fiber Bragg Grating FBG can be solved to temperature and should The cross sensitivity problem that becomes.

Claims (4)

1. A fiber Bragg grating sensor device for eliminating cross sensitivity, characterized in that the device includes a broadband light source (1), and the light emitted by the broadband light source (1) passes through a branch of the first beam splitter (2). The road end is transmitted to the sensor grating (3) for reflection, and the light reflected by the sensor grating (3) enters the second beam splitter (5) through the other branch end of the first beam splitter (2), and then Divided into two beams, respectively connected to an input end of the coupler (6) and an input end of the third beam splitter (9), and an output end of the coupler (6) is connected to the first detector (7) , the other output end is connected with the first demodulation grating (4), and the light reflected back by the first demodulation grating (4) is received by the second detector (8) through the coupler (6); the third The combiner end of the beam splitter (9) is connected to the second demodulation grating (10), and the light reflected back by the second demodulation grating (10) is passed through another split end of the third beam splitter (9). The third detector (11) receives, the first demodulation grating (4) is placed in the same environment as the sensing grating (3), and senses the change of ambient temperature simultaneously with the sensing grating (3), but Not affected by the strain, the center wavelength of the second demodulation grating (10) is controlled by the wavelength adjuster (12), the first detector (7), the second detector (8), the third detector ( 11) and wavelength regulator (12) are all connected with computer (13). 2. The sensing device according to claim 1, characterized in that the first demodulation grating (4) and the sensing grating (3) use the same packaging material and packaging method, and are placed in the same temperature environment middle. 3. The sensing device according to claim 1, characterized in that the wavelength adjuster (12) is a temperature controller, or a strain tuning device, or a tunable optical filter whose reflection spectrum or transmission spectrum is close to a Gaussian distribution device. 4. The sensing device according to claim 1, characterized in that the first beam splitter (2), the second beam splitter (5) and the third beam splitter (9) are circulators, or 2×2 couplers.