CN103364105B - 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

Based on the optical fibre refractivity of multiple-mode interfence and temperature sensor and measuring method thereof

Technical field

The present invention relates to a kind of refractive index and temperature sensor, particularly relate 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 such as size is little, highly sensitive, electromagnetism interference etc.They have attracted the great interest of people at remote measurement and process control field, can be used for measuring tempeature, stress, refractive index, displacement and other physical quantitys.Recent years, multiple-mode interfence phenomenon is widely used in sensor field, such as, 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, 3 ° of multimode fibre Bragg grating, multimode-centreless-multimode fibre structures tilted.All these methods above are all based on the multiple-mode interfence phenomenon occurred in optical fiber, but, these methods proposed are that one-parameter is measured and used single mode-multi-mode-single mode optical fiber structure mainly transmission-type mostly, because the fusion point of the single-mode fiber after welding and multimode fibre ruptures bending excessive the lower easy of situation, therefore this transmissive type construction is not easy to operate.In addition, traditional multiple-mode interfence phenomenon that utilizes measures the method for sensing of refractive index, generally to remove the covering of multimode fibre wholly or in part, even want etch away parts fibre core, to allow the abundant contact measured material of multimode fibre fibre core, test substance is made to serve as the covering of multimode fibre fibre core, cause the movement of multiple-mode interfence resonance wavelength to realize 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 complicated, simultaneously cost increase.The method of traditional measuring tempeature, the method of traditional measuring tempeature generally will use Fiber Bragg Grating FBG, or covering certain refractive index at single-mode optical fiber pigtail end can temperature variant material, to change the Fresnel reflection rate on single-mode fiber end and this kind of material interface, the measurement of temperature is realized by the measurement movement of bragg wavelength or the change of Fresnel reflection rate, the shortcoming of these methods is that cost is high, makes complicated, is not easy to large-scale application.

Summary of the invention

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

Based on optical fibre refractivity and the temperature sensor of multiple-mode interfence, comprise wideband light source, optical fiber circulator, measurement sensing head and spectrometer; The input port of described optical fiber circulator and wideband light source are by Fiber connection, and the first output port of optical fiber circulator is with measurement sensing head by Fiber connection, and the second output port and spectrometer input pass through Fiber connection; Multiple-mode interfence is there is in light in measurement sensing head inside, and Fresnel reflection occurs on the interface measuring sensing head and test substance come back to and measure that sensing head is inner to be continued propagate and multiple-mode interfence occurs, finally be transferred to spectrometer, recorded loss peak power and the loss peak wavelength of interference fringe by spectrometer, 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 not removing covering that end face is vertical with shaft axis of optic fibre.Light enters multimode fibre through single-mode fiber, and on the interface of multimode fibre end and test substance, Fresnel reflection occurs and come back in multimode fibre, finally be coupled into single-mode fiber, in the process, light, when entering multimode fibre from single-mode fiber, the end face of multimode fibre inspires multiple eigen mode, interferes when the light of this multiple pattern is propagated in multimode fibre, finally be coupled in single-mode fiber again, and be transferred to spectrometer.

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

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

Utilize refractive index and the thermometry of above-mentioned optical fibre refractivity and temperature sensor, comprising: sensing head will be measured and insert in test substance; Light enters multimode fibre through single-mode fiber, and on the interface of multimode fibre end and test substance, Fresnel reflection occurs and come back to multimode fibre kind, finally be coupled into single-mode fiber, in the process, light, when entering multimode fibre from single-mode fiber, the end face of multimode fibre inspires multiple eigen mode, interferes when the light of this multiple pattern is propagated in multimode fibre, finally be coupled in single-mode fiber again, and be transferred to spectrometer.The loss peak power of interference fringe changes with the test substance variations in refractive index measured residing for sensing head, is recorded 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 variations in temperature of the test substance measured residing for sensing head, is recorded 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

$I=[I_1+I_2+2\sqrt{I_1{I}_2}\cos \left(\frac{2\mathrm{\π\ΔnL}}{\mathrm{\λ}}\right)]{\left(\frac{n_{\mathrm{co}}-n_x}{n_{\mathrm{co}}+n_x}\right)}^2$

Wherein I ₁, I ₂be respectively the luminous power that eigen mode 1 and 2 distributes, Δ n is the refringence between these two patterns, and L is the length of double multimode fibre, and λ is optical wavelength, n _cothe refractive index of multimode fibre fibre core, n _xit is the refractive index of test substance; The loss peak wavelength of described interference fringe is

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

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

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

${\mathrm{\λ}}_{0\min }+\mathrm{\Δ}{\mathrm{\λ}}_{\min }=\frac{{(d+\mathrm{\Δd})}^{2}m}{(L+\mathrm{\ΔL})}(n_{\mathrm{co}}+\mathrm{\Δ}{n}_{\mathrm{co}})$

Wherein Δ d=k ₁Δ T, Δ L=k ₁Δ T, Δ n _co=k ₂Δ T, k ₁and k ₂thermal coefficient of expansion and the thermo-optical coeffecient of multimode fibre respectively, λ _0mininitial loss peak wavelength, loss peak wavelength change Δ λ _minonly relevant with variations in temperature Δ T.

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

(1) sensor of the present invention can be avoided measuring the cross-sensitivity caused by different physical effectively, improves measurement accuracy.

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

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

This sensor can realize high accuracy, on a large scale refractive index and temperature survey, and structure is simple, easy to operate.

Accompanying drawing explanation

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

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

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

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

Fig. 5 is that interference fringe loss peak wavelength is with the change of solution temperature when the temperature of the NaCl solution of 5% changes to 95 DEG C from 25 DEG C.

Detailed description of the invention

Below in conjunction with accompanying drawing, specific embodiment of the invention is described in further detail, but enforcement of the present invention and protection domain are not limited thereto, all protection scope of the present invention is belonged to the equivalent replacement that the present invention makes essence identical.

See Fig. 1, comprise wideband light source 1, optical fiber circulator 2 based on the optical fibre refractivity of multiple-mode interfence and temperature sensor, 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 goes out to measure interference spectrum when sensing head inserts measured matter by spectrometer measurement, obtains the loss peak power of interference fringe and loss peak wavelength, according to refractive index and the temperature of formula (1) and (2) acquisition detected solution.Measure sensing head to be made up of the multimode stepped-index optical fiber that end face is vertical with shaft axis of optic fibre.

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

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

According to Fresnel reflection law, in the Fresnel reflection rate at the interface place measuring 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 fibre fibre core, n _xit is the refractive index of test substance.

Light enters multimode fibre through single-mode fiber, will inspire multiple eigen mode in multimode fibre, and the light of these patterns will interfere in multimode fibre, then the luminous power being finally coupled into single-mode fiber is

$I=[I_1+I_2+2\sqrt{I_1{I}_2}\cos \left(\frac{2\mathrm{\π\ΔnL}}{\mathrm{\λ}}\right)]---\left(2\right)$

Wherein I ₁and I ₂be the luminous power of eigen mode 1 and 2 respectively, I is the luminous power that spectrometer records, and L is the length of double multimode fibre, 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

$I=[I_1+I_2+2\sqrt{I_1{I}_2}\cos \left(\frac{2\mathrm{\π\ΔnL}}{\mathrm{\λ}}\right)]{\left(\frac{n_{\mathrm{co}}-n_x}{n_{\mathrm{co}}+n_x}\right)}^2---\left(3\right)$

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

The principle that the loss peak wavelength of interference fringe changes with the variations in temperature of the test substance measured residing for sensing head is as follows:

According to circular symmetry and the desired collimation of input field, when input field enters multimode fibre, LP will be only had _0mmould is excited, assuming that LP _0mfield distribution be F _mr (), then the field distribution on multimode fibre end face is

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

Wherein c _mexcite coefficient for each pattern, can be expressed as

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

When light is propagated in multimode fibre, the field distribution at distance z place can be expressed as

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

Wherein β _mbe the propagation constant of each eigen mode in multimode fibre, when the light of this multiple pattern is propagated in multimode fibre, 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

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

Wherein a is the fiber core radius of multimode fibre.

Like this when light is coupled into single-mode fiber again from multimode fibre, the light of some wavelength is very strong, and the light of some wavelength is but very weak is even zero, wherein interferes the loss peak wavelength of minimum i.e. interference fringe to be

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

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

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

${\mathrm{\λ}}_{0\min }+\mathrm{\Δ}{\mathrm{\λ}}_{\min }=\frac{{(d+\mathrm{\Δd})}^{2}m}{(L+\mathrm{\ΔL})}(n_{\mathrm{co}}+\mathrm{\Δ}{n}_{\mathrm{co}})---\left(9\right)$

Wherein Δ d=k ₁Δ T, Δ L=k ₁Δ T, Δ n _co=k ₂Δ T, k ₁and k ₂thermal coefficient of expansion and the thermo-optical coeffecient of multimode fibre respectively, λ _0minit is initial loss peak wavelength.As can be seen from formula (9), loss peak wavelength change Δ λ _minonly relevant with variations in temperature Δ T.

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

Experiment 1:

In an experiment, apply the spectrogram of fiber sensor measuring different refractivity material of the present invention, 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 fibre is 105 μm, cladding diameter 125 μm, length 60mm.As can be seen from Figure 2, interference fringe loss peak power with optical fiber sensor head put the increase of test substance refractive index and reduce (such as, 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 that application sensor of the present invention is to the data result of the NaCl solution refractometry of variable concentrations and linear fit result.As can be seen from Figure 3, measured data of experiment result presents good linear trends of change.

Experiment 2

In an experiment, apply spectrogram when fiber sensor measuring concentration of the present invention is the NaCl solution different temperatures of 5%, as shown in Figure 4, in figure, three curves concentration that corresponding optical fiber sensor head is positioned over 25 DEG C, 60 DEG C, 95 DEG C is respectively spectrum in the NaCl solution of 5%.As can be seen from Figure 4, interference fringe loss peak wavelength is put the rising of test substance temperature with optical fiber sensor head and becomes 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 (DEG C)	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 applies the thermometric data result of NaCl solution and the linear fit result that sensor of the present invention to the concentration of different temperatures is 5%.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 invention is feasible.

Claims (1)

1., based on the optical fibre refractivity of multiple-mode interfence and the refractive index of temperature sensor and thermometry, the described 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); Input port and the wideband light source (1) of described optical fiber circulator (2) pass through Fiber connection, first output port of optical fiber circulator (2) is with measurement sensing head (3) by Fiber connection, and the second output port and spectrometer (4) input pass through Fiber connection; Multiple-mode interfence is there is in light in measurement sensing head inside, and Fresnel reflection occurs on the interface measuring sensing head and test substance come back to and measure that sensing head is inner to be continued propagate and multiple-mode interfence occurs, finally be transferred to spectrometer (4), recorded loss peak power and the loss peak wavelength of interference fringe by spectrometer (4), then calculate refractive index and the temperature of test substance;

It is characterized in that described measuring method comprises: sensing head will be measured and insert in test substance, light enters multimode fibre through single-mode fiber, and multiple eigen mode is inspired on multimode fibre end face, the light of multiple eigen mode interferes in multimode fibre, and there is Fresnel reflection in the interface of the other end of multimode fibre and test substance, and again there is multiple-mode interfence, the loss peak power of interference fringe changes with test substance variations in refractive index, the loss peak wavelength of interference fringe changes with the variations in temperature of test substance, loss peak power and the loss peak wavelength of interference fringe is recorded by spectrometer, calculate refractive index and the temperature of test substance more by analysis, the loss peak power of described interference fringe is

$I=[I_1+I_2+2\sqrt{I_1{I}_2}\cos \left(\frac{2\mathrm{\π\ΔnL}}{\mathrm{\λ}}\right)]{\left(\frac{n_{\mathrm{co}}-n_x}{n_{\mathrm{co}}+n_x}\right)}^2$

Wherein I ₁, I ₂be respectively the luminous power that eigen mode 1 and 2 distributes, Δ n is the refringence between these two patterns, and L is the length of double multimode fibre, and λ is optical wavelength, n _cothe refractive index of multimode fibre fibre core, n _xit is the refractive index of test substance; The loss peak wavelength of described interference fringe is

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

Wherein d is the core diameter of multimode fibre, and L is the length of double multimode fibre, n _cobe the refractive index of multimode fibre fibre core, m is the exponent number of pattern;

When temperature changes Δ T, will there is corresponding change in the core diameter of multimode fibre, length, fiber core refractive index, cause the change of interference fringe loss peak wavelength the most at last, be expressed as

${\mathrm{\λ}}_{0\min }+{\mathrm{\Δ\λ}}_{\min }=\frac{{(d+\mathrm{\Δd})}^{2}m}{(L+\mathrm{\ΔL})}(n_{\mathrm{co}}+{\mathrm{\Δn}}_{\mathrm{co}}),$

Wherein Δ d=k ₁Δ T, Δ L=k ₁Δ T, Δ n _co=k ₂Δ T, k ₁and k ₂thermal coefficient of expansion and the thermo-optical coeffecient of multimode fibre respectively, λ _0mininitial loss peak wavelength, loss peak wavelength change Δ λ _minonly relevant with variations in temperature Δ T.