CN101614654A - A kind of light hydrogen sensor based on light monopulse multi-reflection coherent - Google Patents

Abstract

A kind of light hydrogen sensor based on light monopulse multi-reflection coherent, this hydrogen sensor mainly make with light monopulse (L) and detect and encourage, by directional coupler (C ₁, C ₂) enter into signal arm (A ₁) and reference arm (A ₂), (e, effective length f) have increased catoptron (a respectively at the two ends of signal arm and reference arm in order to increase plating palladium fiber segment, b, c d), makes light pulse repeatedly to reflect in each arm, thereby under the condition of the physical length that does not increase plating palladium light section, realize the increase of equivalent action length, to carry out coherent interference from the light pulse of signal arm and reference arm output at last, thereby further improve the measuring accuracy of system, D ₁, D ₂Be photo-detector.

Description

A kind of light hydrogen sensor based on light monopulse multi-reflection coherent

Technical field

The present invention relates to the quick field of sensing technologies of optical fiber hydrogen, be specifically related to a kind ofly repeatedly reflect the amplification phase change, and realize detecting the optical fiber hydrogen sensor that small hydrogen concentration changes in conjunction with coherence detection based on the light monopulse

Background technology

The hydrogen composition is a kind of probe material important in the chemical-biological course of reaction.By detecting the concentration of hydrogen composition, can observe the state of this chemical-biological course of reaction, reach the effect of monitoring, but in course of reaction, the hydrogen composition of generation is very micro-, therefore need to use highly sensitive hydrogen detecting instrument in actual applications.Simultaneously, hydrogen as a kind of pollution-free energy source of using not to the utmost, is widely used again, but since hydrogen very easily explode, when the concentration of hydrogen in the air greater than 4% the time, meet naked light and will produce blast.Simultaneously, often electromagnetic interference (EMI) is very big need to carry out environment that hydrogen detects, and therefore studying a kind of safe and reliable, anti-interference, highly sensitive hydrogen concentration detecting sensor just has crucial meaning.

Because characteristics such as it is high temperature resistant, corrosion-resistant that optical fiber has, and long distance of signal transmission and anti-electromagnetic interference (EMI) are strong are highly suitable for single-point and the multi-point sensing of finishing hydrogen in the dangerous work environment and detect, so the optics hydrogen sensor are the developing direction of hydrogen sensing detection.Optical fiber also has been widely used in fields such as measurement gas density, especially is used for measuring the density of hydrogen, has also put forward various optical fiber hydrogen sensors, and these sensors all need to have characteristic such as can use in explosive environments.In order to measure the concentration of hydrogen in the mixed gas, most of hydrogen sensor has all utilized Metal Palladium (Pd) to absorb the characteristic that hydrogen causes the elongation effect in low dividing potential drop hydrogen environment, fiber lengths elongation and refractive index etc. are changed, thereby cause that the change of surveying light intensity and phase place etc. surveys the concentration of hydrogen.

Will produce stress when extending when Metal Palladium (Pd) absorption hydrogen, the stress δ that palladium (Pd) produces is δ=0.026x with the pass that absorbs hydrogen x; The local stress p that x and hydrogen produced satisfies the sievert rule in the α phase place, i.e. p ^1/2=Kx, wherein K is sievert (Sievert) coefficient, to sum up obtains the computing formula of the stress δ of palladium (Pd): $\mathrm{\δ}=0.026\frac{\sqrt{p}}{K}.$

Consider that normal force and torque influence are less, can ignore that this patent only calculates axial force.The suffered power of palladium (Pd) overlayer is δ Y _PdA _Pd, and the stress of fiber core is ε _F(Y _FA _F+ Y _PdA _Pd), Y wherein _PdAnd Y _FBe respectively the elastic modulus of palladium and silicon, A _FAnd A _PdIt is the area of optical fiber and palladium.Because fiber core and the tectal stress of palladium (Pd) are a pair of acting force reacting forces, therefore according to Newton first law, two stress equate, can obtain formula [1]:

$\mathrm{\ϵ}=\mathrm{\δ}\frac{(b^2-a^2)Y_{\mathrm{pd}}}{a^2{Y}_F+\left(b^2{-a}^2\right)Y_{\mathrm{pd}}}---\left(1\right)$

Wherein a and b are respectively the diameters of covering and fiber core.

Produce the elongation effect because Metal Palladium (Pd) absorbs hydrogen in low dividing potential drop hydrogen environment, make the length of optical fiber and refractive index that variation all take place.If the length of plating palladium optical fiber is l, refractive index is n, be Δ l because the effect of palladium (Pd) makes the length of optical fiber change, and change of refractive is Δ n.At first establish

Be dependent variable, so the pass of length l and dependent variable Δ l is: $\mathrm{\ϵ}=\frac{\mathrm{\Δl}}{l};$

According to elasto-optical effect, can derive obtains formula: $\frac{\mathrm{\Δn}}{n}=\left(\frac{n^2}{2}\right)[p_{12}-v(p_{11}+p_{12})]\left[2\right],$ Wherein ε represents stress; p ₁₁, p ₁₂It is the Pockel coefficient; V is the Poisson ratio; N is the refractive index of optical fiber.If p ₁₁=0.113; p ₁₂=0.252; N=1.482; V=0.16, the ε relation that therefore obtains refractive index n and dependent variable is: $\frac{\mathrm{\Δn}}{n}=-0.22\mathrm{\ϵ}.$

The optical fiber hydrogen sensor of existing report mainly comprises following several: interference-type optical fiber hydrogen sensor, fadout field pattern optical fiber hydrogen sensor, microlens type optical fiber hydrogen sensor and based on the optical fiber hydrogen sensor [3] of Fiber Bragg Grating type.No matter make any type, above hydrogen sensor all has a common ground--single effect, and promptly light is only by once plating palladium fiber segment, therefore in order to improve the sensitivity of hydrogen sensor, the length that just needs lengthening plating palladium fiber segment, but simple increase can cause other restricted problems.

In view of this shortcoming of the above hydrogen sensor of reporting, the present invention proposes a kind of novel smooth hydrogen sensor that repeatedly reflects the M-Z interferometer based on monopulse.In this hydrogen sensor, the palladium layer is plated on the bare fibre, increases by two mirror surfaces at the optical fiber two ends, make small light phase variable quantity amplify, be equivalent to the effective length that has improved the quick fiber segment of plating palladium hydrogen by repeatedly acting on.Carry out coherent interference from the two-beam of the reference arm output isolated with the signal arm of hydrogen effect with hydrogen, obtain because the variable quantity of the light phase that the effect of hydrogen palladium causes.Adopting highly sensitive coherent technique and pulse-echo repeatedly to act on the method that improves equivalent action length guarantees the hydrogen composition is carried out supersensitive detection.

[1]Boonsong?Sutapun，Massood?Tabib-Azar，Alex?Kazemi，Pd-coated?elastooptic?fiber?optic?Bragg?gratingsensors?for?multiplexed?hydrogen?sensing，Sensors?and?Actuators?B?60(1999)27-34

[2]A.D.Kersay，M.A.Davis，H.J.Patrick，M.L.LeBlanc，K.P.Koo，C.G.Aakins，M.A.Putanam，E.J.Friebele，Fibergraing?sensors，J.Lightwave?Technol.3(1997)1442-1462

[3] Zang Xinmei, Liu Jiansheng, Fan Huilong makes widely known. the progress of optic-fiber hydrogen sensor. optical communication technique .2005.04

Summary of the invention

Fig. 1 is the synoptic diagram of hydrogen sensor structure involved in the present invention, and this hydrogen sensor comprises light source (L), directional coupler (C ₁, C ₂), photo-detector (D ₁, D ₂), signal arm (A ₁), reference arm (A ₂) and differential amplifier (F).

The sensitivity that the effective length of the quick optical fiber of growth plating palladium hydrogen can improve system.The related hydrogen sensor of this patent increases by two mirror surfaces respectively at the two ends of signal arm and reference arm, as shown in Figure 2, make light pulse in each arm, repeatedly to reflect, thereby under the condition of the physical length that does not increase plating palladium light section, realize the increase of equivalent action length, and then improve the degree of accuracy of measuring.Simultaneously, will carry out coherent interference from the light pulse of signal arm and reference arm output, thereby further improve the measuring accuracy of system.

As can be seen from Figure 2, the material of signal arm and reference arm and manufacture craft are identical, all are that the overlayer of optical fiber is removed, and plate one deck palladium on bare fibre, all embed catoptron at the two ends of signal arm and reference arm optical fiber then.Uniquely different be, reference arm is to be placed in the sealed space, and signal arm is to be placed on one not have in the sealed space, and hydrogen leads to into from the upper opening in signal arm (a) space and reacts with palladium (Pd), thereby the refractive index of optical fiber and length are changed.The consistance of signal arm and reference arm structure is can cancel out each other for the change that other external factor except that hydrogen cause on two arms, thus the stability of the system of assurance.Light for fear of the light of reflected back and input is overlapped in time, and the related hydrogen sensor of this patent adopts light pulse to detect, and in addition, the length of plating palladium fiber segment is less than the length between the catoptron.

Hydrogen sensor principle of work involved in the present invention is:

Produce the elongation effect because Metal Palladium (Pd) absorbs hydrogen in low dividing potential drop hydrogen environment, make the length of optical fiber and refractive index that variation all take place.If the length of plating palladium optical fiber is l, refractive index is n, be Δ l because the effect of palladium (Pd) makes the length of optical fiber change, and change of refractive is Δ n.At first establish

Be dependent variable, so the pass of length l and dependent variable is: $\mathrm{\ϵ}=\frac{\mathrm{\Δl}}{l};$ Know that by above-mentioned discussion formula (1) refractive index is that the pass of n and change of refractive amount Δ n is $\frac{\mathrm{\Δn}}{n}=-0.22\mathrm{\ϵ};$ And the pass of the local stress p that dependent variable ε and hydrogen are produced is $\mathrm{\ϵ}=0.026\frac{\sqrt{p}}{K}\frac{(b^2-a^2)Y_{\mathrm{pd}}}{a^2{Y}_F+(b^2-a^2)Y_{\mathrm{pd}}}$

Because the light phase that signal arm length and variations in refractive index cause transmitting in optical fiber changes, thereby draw the concentration of hydrogen for the change size that obtains phase place, set up a reference arm again, because reference arm does not feed hydrogen, so the light phase in the reference arm does not change.By the two-beam of exporting behind signal arm and the reference arm is interfered, thereby obtain the change of the signal intensity that signal arm output light phase difference causes.Because light phase is: $\mathrm{\φ}=\frac{2\mathrm{\π}}{\mathrm{\λ}}n\×l,$ So phase differential of two arm output light $s\left(t\right)={\mathrm{\φ}}_1-{\mathrm{\φ}}_2=\frac{2\mathrm{\π}}{\mathrm{\λ}}(n_1{l}_1-n_2{l}_2).$ Before bringing into use, at first calibrate two arms, make n ₁l ₁=n ₂l ₂, the phase differential when therefore beginning is zero.Hydrogen is fed in the signal arm, and the light phase difference is:

$s\left(t\right)=\frac{2\mathrm{\π}}{\mathrm{\λ}}\mathrm{nl}(\frac{\mathrm{\Δn}}{n}+\frac{\mathrm{\Δl}}{l})---\left(2\right)$

An advantage of hydrogen sensor involved in the present invention is: the two ends at signal arm and reference arm have all increased catoptron respectively, make light repeatedly to reflect in optical fiber, thereby increase the effective length of optical fiber.If light in the signal arm internal reflection m time, then optical path difference is (n+ Δ n) (2m-1) (l+ Δ l)-n (2m-1) l, so phase differential is:

$s\left(t\right)=\frac{2\mathrm{\π}}{\mathrm{\λ}}(2m-1)\mathrm{nl}(\frac{\mathrm{\Δn}}{n}+\frac{\mathrm{\Δl}}{l})---\left(3\right)$

Light pulse entering into three-dB coupler by signal arm and reference arm in the sensor through interfering, and the voltage that two arms of output of coupling mechanism are produced is input to a differential amplifier, so the input V of differential amplifier ₃Equal:

V ₃(t)＝2αV ₀cos[s(t)] (4)

Wherein, V ₀Be the voltage constant that is proportional to the relevant light intensity peak of input, α is the mixing effect coefficient relevant with the coupling mechanism splitting ratio with polarization state.

If two catoptrons have identical reflection coefficient and are ρ, the distance between two catoptrons is l ₁, the loss factor of optical fiber is β, therefore the voltage V of last output ₃(t) be

$V_3\left(t\right)=2\mathrm{\α}{V}_0{e}^{-(2m-1)\mathrm{\β}{l}_1}{\mathrm{\ρ}}^{(2m-1)}{(1-\mathrm{\ρ})}^2\cos [\frac{2\mathrm{\π}}{\mathrm{\λ}}(2m-1)\mathrm{nl}\×0.78\×0.026\frac{\sqrt{p}}{\mathrm{\κ}}\frac{(b^2-a^2)Y_{\mathrm{pd}}}{a^2{Y}_F+(b^2-a^2)Y_{\mathrm{pd}}}]$

Can it is evident that from above formula, when the concentration of extraneous hydrogen changes, will have influence on the dividing potential drop p that hydrogen produces, thereby cause voltage V ₃Change.By measuring V ₃Change, can access the value of hydrogen partial pressure, thereby obtain the concentration of hydrogen.

The course of work of hydrogen sensor involved in the present invention: at first signal arm and reference arm are placed in the identical environment.Then hydrogen to be measured is passed in the signal arm.Send a monochromatic light pulse from light source L, enter into two arms through directional coupler, the influence that is subjected to hydrogen owing to the palladium (Pd) of signal arm is extended, thereby cause the refractive index of optical fiber and length that variation has all taken place, make the light phase of signal arm output that change take place, interfere with the light of reference arm output then.Because the two ends of plating palladium optical fiber all are placed with light reflection mirror, light repeatedly reflects in optical fiber back and forth, and two light pulses that each transmission is gone out are interfered.Therefore along with the increase of order of reflection, the effective length of plating palladium optical fiber also increases, and the light phase difference constantly accumulates increase, thus the voltage V that makes sensor output ₃Change has taken place.By surveying the variation that voltage V3 is taken place, obtain palladium (Pd) the signal arm output change that light phase took place, obtain the variation of optical fibre refractivity and length and the elongation of palladium (Pd),, thereby can obtain hydrogen concentration because the elongation of palladium (Pd) and the concentration of hydrogen are proportional.

The present invention has the following advantages:

(1) hydrogen sensor involved in the present invention is owing to increased mirror surface respectively in plating palladium signal arm and reference arm, thereby increased the phase differential of output light, can detect the elongation of palladium (Pd), therefore the sensitivity that has improved hydrogen sensor greatly to very thin hydrogen.

(2), utilize highly sensitive coherent interference to detect the sensitivity that can further improve hydrogen sensor for the change of the light phase of side.

(3) the sensitive material palladium (Pd) of hydrogen sensor involved in the present invention is only effective to hydrogen, will survey light path simultaneously and seal, and therefore can reduce the influence of external environment condition to light path.

Description of drawings

Fig. 1: the hydrogen sensor structural representation, wherein L is a light source, C ₁, C ₂Be directional coupler; D ₁, D ₂Be photo-detector; A ₁, A ₂Be respectively signal arm and reference arm; F is a differential amplifier.

Fig. 2: the structural drawing of signal arm and reference arm; Wherein, a, b, c, d are light reflection mirror; E, f is plating palladium layer; C ₁, C ₂For coupling mechanism D is a photo-detector; L is light pulse; H is a hydrogen; A ₁, A ₂Be respectively signal arm and reference arm.

Fig. 3: simulation result figure; Wherein: on behalf of H=0.1%---, straight line represent H=0.01% ... represent H=0.001%

Fig. 4: interpretation of result figure; Wherein: on behalf of H=0.1%---, straight line represent H=0.01% ... represent H=0.001%

Embodiment

By above discussion, the relation that has obtained voltage and hydrogen partial pressure is as follows:

$V_3\left(t\right)=2\mathrm{\α}{V}_0{e}^{-(2m-1)\mathrm{\β}{l}_1}{\mathrm{\ρ}}^{(2m-1)}{(1-\mathrm{\ρ})}^2\cos [\frac{2\mathrm{\π}}{\mathrm{\λ}}(2m-1)\mathrm{nl}\×0.78\×0.026\frac{\sqrt{p}}{\mathrm{\κ}}\frac{(b^2-a^2)Y_{\mathrm{pd}}}{a^2{Y}_F+(b^2-a^2)Y_{\mathrm{pd}}}]$

For the better checking hydrogen sensor involved in the present invention and the relation of hydrogen partial pressure, obtain as shown in Figure 3 graph of a relation by emulation.After plating palladium optical fiber fabrication was finished, except that dividing potential drop and voltage, its residual value all was a fixed value in the relational expression.If α=0.5; V ₀=1; λ=829nm; N=1.48; L=1mm; K=350Torr ^1/2Y _Pd=1.21 * 1011; Y _F=7.3 * 1010; A=35um; B=36.2um; ρ=0.99; l ₁=1m; β=0.1.Through the Literature Consult checking, when the number percent of density of hydrogen is 0.10%, the local partial pressure p=0.5Torr that hydrogen produced.Expression order of reflection m and output voltage V among Fig. 3 ₃Relation.In order to express the effect of catoptron better, three curves among the figure have been represented the simulation result of three varying number level concentration hydrogen respectively.Can find, along with the minimizing of density of hydrogen, curve descend speed also reduce gradually, reach some specific voltage, the number of times of needed reflection just increases.

Because the ambiguity of light coherence function, the actual detected process is only used half of first cycle, and interpretation of result as shown in Figure 4.

As can be seen from Figure 4, when output voltage was a certain fixed value, along with the minimizing of density of hydrogen, needed order of reflection constantly increased.

In actual applications, number of times is not easy to measure, but because number of times was directly proportional with the time, therefore, fixedly output voltage arrives the time value of this fixed value by output voltage under the observation different hydro concentration, thereby can obtain the concentration of hydrogen.Therefore, hydrogen sensor involved in the present invention detects the time detecting that is converted to easy realization precision measurement to concentration, thereby realizes that high sensitivity concentration detects.

Claims (6)

1. hydrogen sensor that comprises signal arm and reference arm, the effect of signal arm are the different elongations of density of hydrogen to be converted palladium (Pd) to the light phase that transmits change in optical fiber.The effect of reference arm is the reference that changes as the signal arm light phase.

2. in the hydrogen sensor structure described in the claim 1,, the output of two arms is interfered for the change with the light phase in the reference signal arm detects and convert to the variation of intensity.

3. in the hydrogen sensor structure described in the claim 1, all embed catoptron, small variable quantity is amplified by repeatedly reflecting, thereby increase the change of exporting light phase, the sensitivity that has improved system at the two ends of signal arm and reference arm optical fiber.

4. in the hydrogen sensor structure described in the claim 1, adopt light pulse excitation and detection.

5. in the hydrogen sensor structure described in the claim 1, plating palladium signal arm is identical with the manufacturing process of reference arm, uses identical fiber optic materials to be made, and difference is that reference arm seals, and signal arm is packing less.

6. in the hydrogen sensor structure described in the claim 1, can be by other materials responsive of plating to other material (can be solid, liquid, gas), the length of optical fiber and refractive index are changed, reach the effect of surveying this material.

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