CN203465032U - Optical fiber refractive index and temperature sensor based on multiple-mode interference - Google Patents

Abstract

The utility model discloses an optical fiber refractive index and temperature sensor based on multiple-mode interference. The sensor comprises a broadband light source, an optical fiber circulator, a measurement sensing head and a spectrometer. 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, multiple-mode interference is generated on light inside the measurement sensing head, Fresnel reflection is caused on an interface of the measurement sensing head and a matter to be measured, the light returns to the inside of the measurement sensing head to be continuously transmitted and generate multiple-mode interference, 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 based on multiple-mode interfence and temperature sensor

Technical field

The utility model relates to a kind of refractive index and temperature sensor, relates in particular to a kind of optical fibre refractivity and temperature sensor based on multiple-mode interfence.

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, multiple-mode interfence phenomenon is widely used in sensor field, for example, utilize 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 are all based on the multiple-mode interfence phenomenon occurring in optical fiber, but, the method of these propositions is that one-parameter is measured and single mode-multi-mode-single mode optical fiber structure used is mainly transmission-type mostly, due to the fusion point of the single-mode fiber after welding and multimode optical fiber lower easy fracture the in 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 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, cause that the movement of multiple-mode interfence resonance wavelength realizes the measurement of refractive index, the shortcoming of this method is that the covering due to 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, or cover the temperature variant material of certain refractive index meeting at single-mode fiber tail optical fiber end, to change the Fresnel reflection rate on single-mode fiber end and this kind of material interface, by measuring the movement of 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.

Utility model content

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

Optical fibre refractivity based on multiple-mode interfence and a temperature sensor, 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 measurement sensing head, and the second output port is connected by optical fiber with spectrometer input end; Light is being measured the inner multiple-mode interfence that occurs of sensing head, and on the interface of sensing head and test substance, there is Fresnel reflection and come back to and measure the inner continuation of sensing head and propagate and multiple-mode interfence occurs measuring, finally be transferred to spectrometer, by spectrometer, record loss peak power and the loss peak wavelength of interference fringe, then calculate refractive index and the temperature of test substance.

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

In 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 use is general single mode fiber.

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

Refractive index and the thermometry of utilizing above-mentioned optical fibre refractivity and temperature sensor, comprising: will measure sensing head and insert in test substance; Light enters multimode optical fiber through single-mode fiber, and on the interface of multimode optical fiber end and test substance, there is Fresnel reflection and come back to multimode optical fiber kind, finally be coupled into single-mode fiber, in this process, light, when entering multimode optical fiber from single-mode fiber, inspires a plurality of eigenmodes on the end face of multimode optical fiber, and the light of these a plurality of patterns interferes while propagating in multimode optical fiber, finally be coupled in 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, then calculate the refractive index of test substance by spectrometer; 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, then calculate the temperature of test substance by spectrometer.

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

$I=[{\mathrm{<figure-callout\; id="1"\; label="I"\; filenames="130712162602.png"\; state="\{\{state\}\}">I</figure-callout>}}_1+I_2+2\sqrt{I_1{I}_2}\cos \left(\frac{2\mathrm{\&pi;\&Delta;nL}}{\mathrm{\&lambda;}}\right)]{\left(\frac{n_{\mathrm{co}}-n_x}{n_{\mathrm{co}}+n_x}\right)}^2$

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 above-mentioned measuring method, when temperature changes Δ T, will there is corresponding variation 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 utility model compared with prior art, has following advantage and technique effect:

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

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

(3) sensor of the present utility model, except for general liquid detecting, also can be used for remote measurement and industrial processes is monitored in real time.

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

Accompanying drawing explanation

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

Fig. 2 in the medium of sensing head in different refractivity time, the spectrum of the sensor-based system recording.

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 in the NaCl solution that the concentration of sensing head in different temperatures is 5% time, the spectrum of the sensor-based system recording.

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

Embodiment

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

Referring to Fig. 1, the optical fibre refractivity based on multiple-mode interfence and temperature sensor comprise wideband light source 1, optical fiber circulator 2, measure sensing head 3 and spectrometer 4.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 measurement sensing head, and the second output port is connected to spectrometer 4.Concrete measurement is the interference spectrum while 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 utility model, described wideband light source 1 is C-band (1520nm～1570nm) wideband light source.Transmission Fibers is single-mode fiber.

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

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

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

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 in multimode optical fiber, inspire a plurality of eigenmodes, and the light of these patterns will interfere in multimode optical fiber, and the luminous power that is finally 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

$I=[{\mathrm{<figure-callout\; id="1"\; label="I"\; filenames="130712162602.png"\; state="\{\{state\}\}">I</figure-callout>}}_1+I_2+2\sqrt{I_1{I}_2}\cos \left(\frac{2\mathrm{\&pi;\&Delta;nL}}{\mathrm{\&lambda;}}\right)]{\left(\frac{n_{\mathrm{co}}-n_x}{n_{\mathrm{co}}+n_x}\right)}^2---\left(3\right)$

Formula (3) shows, the loss peak power of interference fringe and optical mode refringence, multimode optical fiber length, optical wavelength, multimode optical fiber fiber core refractive index, test substance refractive index is relevant, and because optical mode refringence, multimode optical fiber length are easy to record, multimode optical fiber fiber core refractive index can be consulted 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 principle that the loss peak wavelength of interference fringe changes with the temperature variation of measuring the residing test substance of sensing head is as follows:

According to the circular symmetry of input field and desired collimation, when input field enters multimode optical fiber, will only have LP _0mmould is excited, and supposes LP _0mfield distribution be F _m(r), the field distribution on 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 is propagated in multimode optical fiber, the field distribution at 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 multimode optical fiber, when the light of these a plurality of patterns is propagated in multimode optical fiber, multiple-mode interfence will occur, and at distance z=L _zplace 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.

When light is when multimode optical fiber is coupled into single-mode fiber again, the light of some wavelength is very strong like this, and the light of some wavelength is but very weak is even zero, and 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, will there is corresponding variation 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.From formula (9), can find out loss peak wavelength variations Δ λ _minonly relevant with temperature variation Δ T.

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

Experiment 1:

In experiment, apply the spectrogram of fiber sensor measuring different refractivity material of the present utility model, as shown in Figure 2, in 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 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 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 is data result and the linear fit result of application sensor of the present utility model to the NaCl solution refractometry of variable concentrations.As can be seen from Figure 3, measured data of experiment result presents good linear trends of change.

Experiment 2

In experiment, spectrogram while applying the NaCl solution different temperatures that fiber sensor measuring concentration of the present utility model is 5%, as shown in Figure 4, in figure three curves respectively corresponding optical fiber sensor head be positioned over spectrum in the NaCl solution that the concentration of 25 ℃, 60 ℃, 95 ℃ is 5%.As can be seen from Figure 4, interference fringe loss peak wavelength is put 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

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

[0068]?

80	1545.24
		85	1545.28
90	1545.32
		95	1545.36

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

From experiment above, sensor of the present utility model is feasible.

Claims (4)

1. the optical fibre refractivity based on multiple-mode interfence and temperature sensor, 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 on the interface of sensing head and test substance comes back to and measure the inner continuation of sensing head and propagate and multiple-mode interfence occurs measuring, and is finally transferred to spectrometer (4).

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

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

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