CN103364105A - Optical fiber refractive index and temperature sensor based on multiple-mode interference and measuring method thereof - Google Patents

Abstract

The invention discloses an optical fiber refractive index and temperature sensor based on multiple-mode interference and a measuring method of the optical fiber refractive index and temperature sensor. The optical fiber refractive index and temperature sensor comprises a broadband light source, an optical fiber circulator, a measurement sensing head and a spectrometer (4). The broadband light source is connected to an input port of the optical fiber circulator, a first output port of the optical fiber circulator is connected to the measurement sensing head through optical fibers, and a second output port of the optical fiber circulator is connected to the spectrometer (4) through optical fibers. When the optical fiber refractive index and temperature sensor carries out measurement, the multiple-mode interference is generated on light inside the measurement sensing head, the Fresnel reflection is caused on an interface of the measurement sensing head and matter to be measured, the light returns to the interior of the measurement sensing head to be continuously transmitted, the multiple-mode interference is generated on the light, finally the light is transmitted to the spectrometer (4), the loss peak power and the loss peak wave length of an interference fringe can be measured through the spectrometer (4), and then the refractive index and the temperature of the matter to be measured can be obtained through calculation. The optical fiber refractive index and temperature sensor can achieve high-precision and large-scale refractive index and temperature measurement and is simple in structure and convenient to operate.

Description

Optical fibre refractivity and temperature sensor and measuring method thereof based on multiple-mode interfence

Technical field

The present invention relates to a kind of refractive index and temperature sensor, relate in particular to a kind of optical fibre refractivity based on multiple-mode interfence and temperature sensor and measuring method thereof.

Background technology

Fibre Optical Sensor was studied widely in recent years, and they have many advantages, and for example size is little, highly sensitive, anti-electromagnetic interference (EMI) etc.They have attracted the great interest of people at remote measurement and process control field, can be used for measuring temperature, stress, refractive index, displacement and other physical quantitys.Recent years, the multiple-mode interfence phenomenon is widely used in sensor field, for example utilizes single mode-multi-mode-single mode (SMS) optical fiber structure, single mode-multi-mode-single mode optical fiber structure cascaded optical fiber Bragg grating, the multimode optical fiber Bragg grating of 3 ° of inclinations, multimode-centreless-multimode optical fiber structure.All above these methods all are based on the multiple-mode interfence phenomenon that occurs in the optical fiber, but, the method of these propositions is that one-parameter is measured and used single mode-multi-mode-single mode optical fiber structure mainly is transmission-type mostly, because the single-mode fiber after the welding and the fusion point of multimode optical fiber are at the lower easy fracture of the excessive situation of bending, so this transmission-type structure is not easy to operate.In addition, traditional multiple-mode interfence phenomenon of utilizing is measured the method for sensing of refractive index, generally to remove wholly or in part the covering of multimode optical fiber, even to erode the part fibre core, to allow the abundant contact measured material of multimode optical fiber fibre core, make test substance serve as the covering of multimode optical fiber fibre core, the movement that causes multiple-mode interfence resonance wavelength realizes the measurement of refractive index, the shortcoming of this method is because the covering of optical fiber is removed, the remitted its fury that can bear, stability reduces, and range of application is limited, make complexity, cost rises simultaneously.The method of traditional measurement temperature, the method of traditional measurement temperature generally will be used Fiber Bragg Grating FBG, perhaps at the terminal temperature variant material of certain refractive index meeting that covers of single-mode fiber tail optical fiber, to change the Fresnel reflection rate on single-mode fiber end and this kind material interface, movement by measuring bragg wavelength or the variation of Fresnel reflection rate realize the measurement of temperature, the shortcoming of these methods is that cost is high, makes complexity, is not easy to large-scale application.

Summary of the invention

The object of the invention is to overcome the prior art above shortcomings, optical fibre refractivity and temperature sensor and measuring method thereof based on multiple-mode interfence are provided, concrete technical scheme is as follows.

A kind of optical fibre refractivity and temperature sensor based on multiple-mode interfence comprise wideband light source, optical fiber circulator, measurement sensing head and spectrometer; The input port of described optical fiber circulator is connected by optical fiber with wideband light source, and the first output port of optical fiber circulator is connected by optical fiber with the measurement sensing head, and the second output port is connected by optical fiber with the spectrometer input end; Light is being measured the inner multiple-mode interfence that occurs of sensing head, and Fresnel reflection occurs come back to and measure that sensing head is inner to be continued to propagate and the generation multiple-mode interfence measuring the interface of sensing head with test substance, finally be transferred to spectrometer, record loss peak power and the loss peak wavelength of interference fringe by spectrometer, calculate again refractive index and the temperature of test substance.

Above-mentioned optical fibre refractivity and temperature sensor based on multiple-mode interfence, measuring sensing head is the end face multimode stepped-index optical fiber of not removing covering vertical with shaft axis of optic fibre.Light enters multimode optical fiber through single-mode fiber, and the interface of and test substance terminal at multimode optical fiber generation Fresnel reflection comes back in the multimode optical fiber, finally be coupled into single-mode fiber, in this process, light inspires a plurality of eigenmodes at the end face of multimode optical fiber when entering multimode optical fiber from single-mode fiber, the light of these a plurality of patterns interferes when propagating in multimode optical fiber, finally be coupled in the single-mode fiber again, and be transferred to spectrometer.

In the above-mentioned optical fibre refractivity and temperature sensor based on multiple-mode interfence, described wideband light source is the fiber broadband light source of C-band (1520nm-1570nm), and the optical fiber that connects usefulness is general single mode fiber.

In the above-mentioned optical fibre refractivity and temperature sensor based on multiple-mode interfence, with the rule that the test substance variations in refractive index changes, calculate the refractive index of test substance according to the loss peak power of interference fringe; With the rule that the temperature variation of test substance changes, calculate the temperature of test substance according to the loss peak wavelength of interference fringe.

Utilize refractive index and the thermometry of above-mentioned optical fibre refractivity and temperature sensor, comprising: will measure sensing head and insert in the test substance; Light enters multimode optical fiber through single-mode fiber, and the interface of and test substance terminal at multimode optical fiber generation Fresnel reflection comes back to the multimode optical fiber kind, finally be coupled into single-mode fiber, in this process, light inspires a plurality of eigenmodes at the end face of multimode optical fiber when entering multimode optical fiber from single-mode fiber, the light of these a plurality of patterns interferes when propagating in multimode optical fiber, finally be coupled in the single-mode fiber again, and be transferred to spectrometer.The loss peak power of interference fringe changes with measuring the residing test substance variations in refractive index of sensing head, records the power of interference fringe loss peak by spectrometer, calculates the refractive index of test substance again; The loss peak wavelength of interference fringe changes with the temperature variation of measuring the residing test substance of sensing head, records the loss peak wavelength of interference fringe by spectrometer, calculates the temperature of test substance again.

In the above-mentioned measuring method, the loss peak power of described interference fringe is

$I=[{\mathrm{<figure-callout\; id="1"\; label="I"\; filenames="130712161613.png"\; state="\{\{state\}\}">I</figure-callout>}}_1+I_2+2\sqrt{I_1{I}_2}\cos \left(\frac{2\mathrm{\&pi;\&Delta;nL}}{\mathrm{\&lambda;}}\right)]{\left(\frac{n_{\mathrm{co}}-n_x}{n_{\mathrm{co}}+n_x}\right)}^2$

I wherein ₁, I ₂Be respectively the luminous power that eigenmodes 1 and 2 distributes, Δ n is the refringence between these two patterns, and L is the length of double multimode optical fiber, and λ is optical wavelength, n _CoThe refractive index of multimode optical fiber fibre core, n _xIt is the refractive index of test substance; The loss peak wavelength of described interference fringe is

${\mathrm{\&lambda;}}_{\min }=\frac{d^{2}m}{L}{n}_{\mathrm{co}}$

Wherein d is the core diameter of multimode optical fiber, and L is the length of double multimode optical fiber, n _CoBe the refractive index of multimode optical fiber fibre core, m is the exponent number of pattern.

In the above-mentioned measuring method, when temperature changes Δ T, the corresponding variation will occur in the core diameter of multimode optical fiber, length, fiber core refractive index, cause the most at last the variation of interference fringe loss peak wavelength, be expressed as

${\mathrm{\&lambda;}}_{0\min }+\mathrm{\&Delta;}{\mathrm{\&lambda;}}_{\min }=\frac{{(d+\mathrm{\&Delta;d})}^{2}m}{(L+\mathrm{\&Delta;L})}(n_{\mathrm{co}}+\mathrm{\&Delta;}{n}_{\mathrm{co}})$

Δ d=k wherein ₁Δ T, Δ L=k ₁Δ T, Δ n _Co=k ₂Δ T, k ₁And k ₂Respectively thermal expansivity and the thermo-optical coeffecient of multimode optical fiber, λ _0minInitial loss peak wavelength, loss peak wavelength variations Δ λ _MinOnly relevant with temperature variation Δ T.

The present invention compared with prior art has following advantage and technique effect:

(1) sensor of the present invention can avoid measuring the caused cross-sensitivity of different physical effectively, has improved measurement accuracy.

(2) sensor construction of the present invention is simple, is easy to make, and cost is low, does not need optical fiber is done special processings such as removing covering, and is easy to operate.

(3) sensor of the present invention also can be used for remote measurement and Real Time Monitoring is carried out in industrial processes except being used for general liquid detecting.

This sensor can be realized high precision, on a large scale refractive index and temperature survey, and is simple in structure, easy to operate.

Description of drawings

Fig. 1 is based on optical fibre refractivity and the arrangement of temperature sensor schematic diagram of multiple-mode interfence.

Fig. 2 is for when sensing head is in the medium of different refractivity, the spectrum of the sensor-based system that records.

Fig. 3 is for when the refractive index of NaCl solution changes to 1.3534 from 1.3148, and interference fringe loss peak power is with the variation of solution refractive index.

Fig. 4 is in 5% the NaCl solution time for the concentration that is in different temperatures when sensing head, the spectrum of the sensor-based system that records.

When Fig. 5 changed to 95 ℃ for the temperature when 5% NaCl solution from 25 ℃, interference fringe loss peak wavelength was with the variation of solution temperature.

Embodiment

Below in conjunction with accompanying drawing implementation of the present invention is described in further detail, but enforcement of the present invention and protection domain are not limited to this, the present invention are done the identical replacement that is equal to of essence all belong to protection scope of the present invention.

Referring to Fig. 1, comprise wideband light source 1, optical fiber circulator 2, measure sensing head 3 and spectrometer 4 based on the optical fibre refractivity of multiple-mode interfence and temperature sensor.Wherein, wideband light source 1 is connected to the first input end mouth of optical fiber circulator 2, and the first output port of optical fiber circulator 2 is connected to the measurement sensing head, and the second output port is connected to spectrometer 4.Concrete measurement is the interference spectrum when going out to measure sensing head insertion measured matter by spectrometer measurement, obtains loss peak power and the loss peak wavelength of interference fringe, according to refractive index and the temperature of formula (1) and (2) acquisition detected solution.Measuring sensing head is comprised of the end face multimode stepped-index optical fiber vertical with shaft axis of optic fibre.

In invention, described wideband light source 1 is the C-band (wideband light source of 1520nm～1570nm).Transmission Fibers is single-mode fiber.

When measuring, measure sensing head and insert in the test substance (such as solution).Interference fringe loss peak power is as follows with the principle that the residing test substance variations in refractive index of measurement sensing head changes:

According to the Fresnel reflection law, the Fresnel reflection rate at the interface place of measuring sensing head end and test substance is:

$R_F={\left(\frac{n_{\mathrm{co}}-n_x}{n_{\mathrm{co}}+n_x}\right)}^2---\left(1\right)$

Wherein, n _CoThe refractive index of multimode optical fiber fibre core, n _xIt is the refractive index of test substance.

Light enters multimode optical fiber through single-mode fiber, will inspire a plurality of eigenmodes in multimode optical fiber, and the light of these patterns will interfere in multimode optical fiber, and the luminous power that then finally is coupled into single-mode fiber is

$I=[I_1+I_2+2\sqrt{I_1{I}_2}\cos \left(\frac{2\mathrm{\&pi;\&Delta;nL}}{\mathrm{\&lambda;}}\right)]---\left(2\right)$

I wherein ₁And I ₂Be respectively the luminous power of eigenmodes 1 and 2, I is the luminous power that spectrometer records, and L is the length of double multimode optical fiber, and λ is optical wavelength, and Δ n is the refringence of these two patterns.

By formula (1) and (2), the loss peak power that can obtain interference fringe is

$I=[{\mathrm{<figure-callout\; id="1"\; label="I"\; filenames="130712161613.png"\; state="\{\{state\}\}">I</figure-callout>}}_1+I_2+2\sqrt{I_1{I}_2}\cos \left(\frac{2\mathrm{\&pi;\&Delta;nL}}{\mathrm{\&lambda;}}\right)]{\left(\frac{n_{\mathrm{co}}-n_x}{n_{\mathrm{co}}+n_x}\right)}^2---\left(3\right)$

Formula (3) shows, the loss peak power of interference fringe and optical mode refringence, multimode optical fiber length, optical wavelength, the multimode optical fiber fiber core refractive index, the test substance refractive index is relevant, and owing to optical mode refringence, multimode optical fiber length are easy to record, the multimode optical fiber fiber core refractive index can be consulted the Related product parameter and obtain, so by measuring the loss peak power of interference fringe, can obtain the refractive index of solution to be measured.

The loss peak wavelength of interference fringe is as follows with the principle that the temperature variation of measuring the residing test substance of sensing head changes:

According to circular symmetry and the desired collimation of input field, when input field enters multimode optical fiber, will only have LP _0mMould (L wherein and P _0mRefer to respectively? if the literary style that standard can be looked into can not change, and is no problem, the LP usefulness that connects together, in optical fiber, the pattern of transmitting is generally used LP ₀₁, LP ₀₂... representing, is a kind of general standard literary style) be excited, suppose LP _0mField distribution be F _m(r), then the field distribution on the multimode optical fiber end face is

$E(r,0)=\underset{m=1}{\overset{M}{\mathrm{\&Sigma;}}}{c}_m{F}_m\left(r\right)---\left(4\right)$

C wherein _mThe coefficient that excites for each pattern can be expressed as

$c_m=\frac{{\&Integral;}_0^{\&infin;}E(r,0)F_m\left(r\right)\mathrm{rdr}}{{\&Integral;}_0^{\&infin;}{F}_m\left(r\right)F_m\left(r\right)\mathrm{rdr}}---\left(5\right)$

When light was propagated in multimode optical fiber, the field distribution at the distance z place can be expressed as

$E(r,z)=\underset{m=1}{\overset{M}{\mathrm{\&Sigma;}}}{c}_m{F}_m\left(r\right)\exp \left(i{\mathrm{\&beta;}}_{m}z\right)---\left(6\right)$

β wherein _mBe the propagation constant of each eigenmode in the multimode optical fiber, multiple-mode interfence will occur when propagating in the light of these a plurality of patterns in multimode optical fiber, and at distance z=L _zThe place has with the identical field distribution of input field, and it is so-called from videoing phenomenon, L that Here it is _zCan be expressed as

$L_z=\frac{16n_{\mathrm{co}}{a}^2}{\mathrm{\&lambda;}}---\left(7\right)$

Wherein a is the fiber core radius of multimode optical fiber.

Like this when light when multimode optical fiber is coupled into the single-mode fiber again, the light of some wavelength is very strong, the light of some wavelength is but very weak even be zero, wherein interfering minimum is that the loss peak wavelength of interference fringe is

${\mathrm{\&lambda;}}_{\min }=\frac{d^{2}m}{L}{n}_{\mathrm{co}}---\left(8\right)$

Wherein d is the diameter of multimode optical fiber fibre core, and L is the Double Length of multimode optical fiber, n _CoIt is the refractive index of multimode optical fiber fibre core.

When temperature changes Δ T, the corresponding variation will occur in the core diameter of multimode optical fiber, length, fiber core refractive index, cause the most at last the variation of interference fringe loss peak wavelength, can be expressed as

${\mathrm{\&lambda;}}_{0\min }+\mathrm{\&Delta;}{\mathrm{\&lambda;}}_{\min }=\frac{{(d+\mathrm{\&Delta;d})}^{2}m}{(L+\mathrm{\&Delta;L})}(n_{\mathrm{co}}+\mathrm{\&Delta;}{n}_{\mathrm{co}})---\left(9\right)$

Δ d=k wherein ₁Δ T, Δ L=k ₁Δ T, Δ n _Co=k ₂Δ T, k ₁And k ₂Respectively thermal expansivity and the thermo-optical coeffecient of multimode optical fiber, λ _0minIt is initial loss peak wavelength.Can find out loss peak wavelength variations Δ λ from formula (9) _MinOnly relevant with temperature variation Δ T.

For further checking feasibility of the present invention, the spy carries out following experiment:

Experiment 1:

In experiment, use the spectrogram of fiber sensor measuring different refractivity material of the present invention, as shown in Figure 2, among the figure five curves respectively corresponding optical fiber sensor head be positioned over air, pure water, 2.5% concentration NaCl solution, 12.5% concentration NaCl solution, the spectrum in the 25% concentration NaCl solution.Wherein the core diameter of multimode optical fiber is 105 μ m, cladding diameter 125 μ m, length 60mm.As can be seen from Figure 2, the increase that interference fringe loss peak power is put the test substance refractive index with optical fiber sensor head reduces (for example, solution concentration is higher, and loss peak power is less).

Table 1 is the variation relation of same liquid (NaCl solution) refractive index of interference fringe loss peak power and variable concentrations.

Table 1

NaCl solution concentration (WT%)	Corresponding refractive index	Loss peak power/dBm
			2.5	1.3148	-66.93
5	1.3190	-67.47
			7.5	1.3234	-67.61
10	1.3277	-68.17
			12.5	1.3319	-68.59
15	1.3362	-68.93
			17.5	1.3405	-69.23
20	1.3448	-69.56
			22.5	1.3491	-70.19
25	1.3534	-70.58

Fig. 3 uses sensor of the present invention to data result and the linear fit result of the NaCl solution refractometry of variable concentrations.As can be seen from Figure 3, the measured data of experiment result presents good linear trends of change.

Experiment 2

In experiment, spectrogram when using fiber sensor measuring concentration of the present invention and be 5% NaCl solution different temperatures, as shown in Figure 4, among the figure three curves respectively corresponding optical fiber sensor head to be positioned over 25 ℃, 60 ℃, 95 ℃ concentration be spectrum in 5% the NaCl solution.As can be seen from Figure 4, interference fringe loss peak wavelength is put the test substance temperature with optical fiber sensor head and is raise and to become large.

Table 2 is the variation relation of same liquid (the 5% concentration NaCl solution) temperature of interference fringe loss peak wavelength and different temperatures.

Table 2

The NaCl solution temperature (℃)	Loss peak wavelength/nm
		25	1544.76
30	1544.80
		35	1544.84
40	1544.88
		45	1544.92
50	1544.96
		55	1545．00
60	1545.04
		65	1545.08
70	1545.12
		75	1545.16
80	1545.24
		85	1545.28
90	1545.32
		95	1545.36

Fig. 5 is that to use sensor of the present invention be data result and the linear fit result that 5% NaCl solution temperature is measured to the concentration of different temperatures.As can be seen from Figure 5, the measured data of experiment result presents good linear trends of change.

From top experiment as can be known, sensor of the present invention is feasible.

Claims (7)

1. based on optical fibre refractivity and the temperature sensor of multiple-mode interfence, it is characterized in that comprising wideband light source (1), optical fiber circulator (2), measure sensing head (3) and spectrometer (4); The input port of described optical fiber circulator (2) is connected by optical fiber with wideband light source (1), the first output port of optical fiber circulator (2) is connected by optical fiber with measurement sensing head (3), and the second output port is connected by optical fiber with spectrometer (4) input end; Light is being measured the inner multiple-mode interfence that occurs of sensing head, and Fresnel reflection occurs come back to and measure that sensing head is inner to be continued to propagate and the generation multiple-mode interfence measuring the interface of sensing head with test substance, finally be transferred to spectrometer (4), record loss peak power and the loss peak wavelength of interference fringe by spectrometer (4), calculate again refractive index and the temperature of test substance.

2. optical fibre refractivity and temperature sensor based on multiple-mode interfence according to claim 1 is characterized in that described measurement sensing head (3) is the perpendicular multimode stepped-index optical fiber tail optical fiber of not removing covering of end face and axis.

3. optical fibre refractivity and temperature sensor based on multiple-mode interfence as claimed in claim 1 is characterized in that described wideband light source is the wideband light source of C-band.

4. such as claim 1～3 each described optical fibre refractivity and temperature sensor based on multiple-mode interfence, it is characterized in that employed optical fiber is general single mode fiber except measuring sensing head (3).

5. utilize refractive index and the thermometry of the described optical fibre refractivity of claim 1 and temperature sensor, it is characterized in that: will measure sensing head and insert in the test substance; Light enters multimode optical fiber through single-mode fiber, and inspire a plurality of eigenmodes at the multimode optical fiber end face, the light of a plurality of eigenmodes interferes in multimode optical fiber, and at the other end of multimode optical fiber and the at the interface generation Fresnel reflection of test substance, and multiple-mode interfence occurs again, the loss peak power of interference fringe changes with the test substance variations in refractive index, the loss peak wavelength of interference fringe changes with the temperature variation of test substance, record loss peak power and the loss peak wavelength of interference fringe by spectrometer, calculate by analysis again refractive index and the temperature of test substance.

6. measuring method according to claim 5 is characterized in that the loss peak power of described interference fringe is

$I=[I_1+I_2+2\sqrt{I_1{I}_2}\cos \left(\frac{2\mathrm{\&pi;\&Delta;nL}}{\mathrm{\&lambda;}}\right)]{\left(\frac{n_{\mathrm{co}}-n_x}{n_{\mathrm{co}}+n_x}\right)}^2$

I wherein ₁, I ₂Be respectively the luminous power that eigenmodes 1 and 2 distributes, Δ n is the refringence between these two patterns, and L is the length of double multimode optical fiber, and λ is optical wavelength, n _CoThe refractive index of multimode optical fiber fibre core, n _xIt is the refractive index of test substance; The loss peak wavelength of described interference fringe is

${\mathrm{\&lambda;}}_{\min }=\frac{d^{2}m}{L}{n}_{\mathrm{co}}$

Wherein d is the core diameter of multimode optical fiber, and L is the length of double multimode optical fiber, n _CoBe the refractive index of multimode optical fiber fibre core, m is the exponent number of pattern.

7. measuring method according to claim 5 is characterized in that when temperature changes Δ T, and corresponding the variation will occur for the core diameter of multimode optical fiber, length, fiber core refractive index, cause the most at last the variation of interference fringe loss peak wavelength, be expressed as

${\mathrm{\&lambda;}}_{0\min }+\mathrm{\&Delta;}{\mathrm{\&lambda;}}_{\min }=\frac{{(d+\mathrm{\&Delta;d})}^{2}m}{(L+\mathrm{\&Delta;L})}(n_{\mathrm{co}}+\mathrm{\&Delta;}{n}_{\mathrm{co}})$

Δ d=k wherein ₁Δ T, Δ L=k ₁Δ T, Δ n _Co=k ₂Δ T, k ₁And k ₂Respectively thermal expansivity and the thermo-optical coeffecient of multimode optical fiber, λ _0minInitial loss peak wavelength, loss peak wavelength variations Δ λ _MinOnly relevant with temperature variation Δ T.

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