CN102279164A - Difference measurement method and device for low moisture content of fiber bragg grating with double wavelengths and double light paths - Google Patents

Abstract

The invention relates to a difference measurement method and device for the low moisture content of a fiber bragg grating with double wavelengths and double light paths and belongs to the fields of sensors and detection techniques. The device provided by the invention comprises a broadband light source 11, two fiber couplers 12 and 19, two self-focusing lenses 13 and 15, a sample absorption pool 14, a fiber circulator 16, a measuring fiber grating 17, a reference fiber grating 21, index-matching fluid 18 and 22 and a spectrum analyzer 20. The device provided by the invention is characterized in that the Bragg reflection wavelength of the measuring fiber grating 17 is within the near-infrared absorption wavestrip range of water; and the difference between the Bragg reflection wavelength of the reference fiber grating 21 and the Bragg reflection wavelength of the measuring fiber grating is at least larger than 1nm. Thus, the difference measurement method for the low moisture content of double wavelengths and double light paths is realized by utilizing a fiber chain structure which is skillfully designed, thereby solving the influence on measurement because of light source power fluctuation and the like.

Description

A kind of fiber grating low-water-content difference measurement method and apparatus of dual wavelength double light path

Technical field

The present invention relates to a kind of fiber grating low-water-content difference measurement method and apparatus of dual wavelength double light path, belong to the Sensors ﹠ Testing Technology field.

Background technology

Strength of fluid is one of key parameter that characterizes characteristics of liquids, and the measurement of strength of fluid is controlled, improved the quality of products etc. for current environmental monitoring, food security, the production automation and all has high realistic meaning.The measurement of water percentage also has important application background and researching value in a lot of fields.In recent years, high-precision low-water-content detection technique is subjected to people's extensive concern.For example, (Guang-Hui huang, Da-Jun Yan, Pei-Li Qiu, Application of crude oil water content analyzer, Oil and Gas Surface Engineering, Vol.19, No.6,2000) proposed a kind ofly based on microwave and ultrashort wave (USW) method low-water-content to be measured, measuring accuracy has reached 0.1%.(Xue-Yan Hao, Yan-Qiu Yao, Xue-Song Li, Research of low-water-content sensor for crude oil, Information Technology, No.7,2002) proposed a kind of coaxial cylindrical condenser type low-water-content sensor construction, realized in percent by volume 0～3% scope ± 0.05% measuring accuracy.More than the measuring accuracy of two kinds of methods do not reach desirable effect.(Yong Zhao, Jian Yang, Jin-Qi Wang, Fang-Xiao Gui, High-accuracy low-water-content measurement of crude oil based on near-infrared spectral absorption method, Optical Engineering, Vol.43, No.10,2004) proposed a kind of optical fiber sensing system, the measurement resolution of low-water-content has been reached 0.01% based on the near infrared absorption principle.But owing to needing high-precision optical filter and PbS detector to make system cost higher, and the light source that adopts is the bromine tungsten filament lamp light source, is difficult to realize higher and stable coupling efficiency with optical fiber in this system.

Summary of the invention

The objective of the invention is in order to overcome the weak point of prior art, realize a kind of simple in structure, cost is low, precision is higher practical low-water-content measuring method and device.

The present invention proposes a kind of fiber grating low-water-content difference measurement method and apparatus of dual wavelength double light path, comprises absorption of sample pool structure and spectroanalysis instrument.It is characterized in that, comprise following content:

1. the fiber grating low-water-content difference measurement method and apparatus of a dual wavelength double light path, mainly comprise wideband light source, 2 fiber couplers, 2 GRIN Lens, the absorption of sample pond, optical fiber circulator, the measuring optical fiber grating, the reference optical fiber grating, index-matching fluid, optical fiber link and spectroanalysis instrument, it is characterized in that: described optical fiber link is: the light that light source sends is divided into two-way after entering fiber coupler 1 through optical fiber, lead up to port 2 through the absorption of sample pond laggard port one of going into the optical fiber circulator, port 2 outgoing from optical fiber circulator arrive the measuring optical fiber grating then, the light that satisfies the specific wavelength of measuring optical fiber grating Bragg's condition of reflection will be reflected, remaining transmittance is refracted the absorption of rate matching fluid after crossing the measuring optical fiber grating, specific wavelength signal by the measuring optical fiber optical grating reflection is exported from the port 3 of optical fiber circulator, and the port one that enters fiber coupler 2 arrives port 3, and by the spectroanalysis instrument receiving record; Another road light by fiber coupler 1 outgoing passes through port 3 outgoing to the reference optical fiber grating, the light that satisfies the specific wavelength of reference optical fiber grating Bragg's condition of reflection will be reflected, remaining transmittance is refracted the absorption of rate matching fluid after crossing the reference optical fiber grating, specific wavelength signal by the reference optical fiber optical grating reflection is exported from the port 4 of fiber coupler 1, the port 4 that enters fiber coupler 2 again arrives port 3, and by the spectroanalysis instrument receiving record.

2. according to the fiber grating low-water-content difference measurement method and apparatus of the described a kind of dual wavelength double light path of claim 1, it is characterized in that: described measuring method is to utilize measuring optical fiber grating with different foveal reflex wavelength and reference optical fiber grating to know respectively to get to comprise measured information and do not comprise measured information, thus a kind of difference detecting method of realization; This method can effectively reduce the influence that light source power fluctuates and brings to measurement result.

3. according to the fiber grating low-water-content difference measurement method and apparatus of the described a kind of dual wavelength double light path of claim 1, it is characterized in that: the foveal reflex wavelength of described measuring optical fiber grating is 1550.009mn (being in the near infrared absorption band of water), and three dB bandwidth is 0.21nm; The foveal reflex wavelength of described reference optical fiber grating be 1534.085nm (with the difference of the foveal reflex wavelength of measuring optical fiber grating at least greater than 1nm), three dB bandwidth is 0.193nm.

4. according to the described a kind of method and apparatus that low-water-content is measured that is used for of claim 1, it is characterized in that: the output spectrum scope of described wideband light source is 1528.52～1557.32nm (must include the foveal reflex wavelength value of measuring optical fiber grating and reference optical fiber grating).

Description of drawings

Fig. 1 is the structure principle chart that the fiber grating low-water-content of a kind of dual wavelength double light path provided by the invention is measured.

Fig. 2 is the experimental system photo that utilizes the fiber grating low-water-content of a kind of dual wavelength double light path that the present invention realizes to measure.

Fig. 3 is the measured spectrogram that obtains of spectroanalysis instrument when utilizing low-water-content that the present invention realizes to measure.

When (a) being absolute ethyl alcohol in the sample cell, the spectrometer measurement spectrogram.

(b) in the sample cell, absolute ethyl alcohol is 350 μ L, when water is 5 μ L, and the spectrometer measurement spectrogram.

Fig. 4 utilizes the output signal that measuring method provided by the invention and device record and the relation curve of low-water-content.

Embodiment

The present invention proposes a kind of fiber grating low-water-content difference measurement method and apparatus of dual wavelength double light path, and accompanying drawings is as follows:

Because water is wideer in the absorption spectrum bandwidth of near-infrared band, therefore, the Differential Detection system of the dual wavelength double light path that the present invention proposes, the light of reference wavelength is without the sample cell that testing liquid is housed.

The structure principle chart that Fig. 1 measures for the fiber grating low-water-content of a kind of dual wavelength double light path of proposing in the literary composition.The measuring process of system is: the ASE light source sends the wide range light that comprises measuring optical fiber grating and reference optical fiber grating foveal reflex wavelength with certain bandwidth, through behind the input end of fiber coupler 1, be divided into two-way, exported by two output terminals of fiber coupler 1 respectively: the light of one of them output terminal output becomes directional light after collimating through GRIN Lens, enter the absorption of sample pond that testing liquid is housed, light in the absorption of sample pond with the liquid phase interaction, energy is absorbed a part, thereby light intensity reduces to some extent.The light that sees through the absorption of sample pond enters in the optical fiber by GRIN Lens, through optical fiber circulator (port one input, port 2 outputs), arrives the measuring optical fiber grating.Because fiber grating can reflect the light of specific wavelength, under the effect of measuring optical fiber grating, wavelength is λ ₁The light of (absorption peak of corresponding liquid) is reflected back by former road, whole permeametry fiber gratings of the light of its commplementary wave length and be refracted the rate matching fluid and absorb.The light that is reflected back toward enters optical fiber circulator by port 2, and by port 3 outputs, enters fiber coupler 2; Another road is without the absorption of sample pond, and as a reference, through the reference optical fiber grating, wavelength is λ ₂(with the foveal reflex wavelength X of measuring optical fiber grating ₁Difference at least greater than 1nm) light be reflected, the light transmission reference optical fiber grating of all the other wave bands and be refracted the rate matching fluid and absorb, the light that is reflected back toward enters fiber coupler 2 equally through behind the fiber coupler 1.Light (the λ of two wavelength ₁And λ ₂) in fiber coupler 2, be coupled into a branch of light by it, go forward side by side and analyze and reading into spectroanalysis instrument.Record the light intensity (luminous power) of the light of two wavelength respectively by spectroanalysis instrument, then the light intensity of the light of two wavelength is made ratio, obtain the relation of tested water percentage and output intensity ratio at last by the Differential Detection principle, calculate the size of tested water percentage.

According to Lambert-Beer's law, and consider the part interference factor:

I(λ ₁)＝I ₀(λ ₁)K(λ ₁)exp[-α(λ ₁)C7+β ₁l+γ ₁l+δ ₁](1)

I(λ ₂)＝I ₀(λ ₂)K(λ ₂)exp[-α(λ ₂)C7+β ₂l+γ ₂l+δ ₂](2)

I wherein ₀(λ) incide light intensity in the measured medium during for λ for wavelength, I (λ) is an output intensity, K (λ) is a scale-up factor, α (λ) is the unit concentration under the finite concentration, the absorption coefficient of unit length, C is a concentration, and l is the length of sample cell, and β is a rayleigh scattering coefficient, γ is a Mie scattering coefficient, and δ is the absorption coefficient that light source fluctuation causes.Making division by formula (1) and (2) can get:

$C=\frac{1}{[\mathrm{\α}\left({\mathrm{\λ}}_1\right)-\mathrm{\α}\left({\mathrm{\λ}}_2\right)]l}\{\ln \frac{I_0\left({\mathrm{\λ}}_1\right)K\left({\mathrm{\λ}}_1\right)}{I_0\left({\mathrm{\λ}}_2\right)K\left({\mathrm{\λ}}_2\right)}-\ln \frac{I\left({\mathrm{\λ}}_1\right)}{I\left({\mathrm{\λ}}_2\right)}+({\mathrm{\β}}_2-{\mathrm{\β}}_1)l+({\mathrm{\γ}}_1-{\mathrm{\γ}}_2)l+{\mathrm{\δ}}_2-{\mathrm{\δ}}_1\}---\left(3\right)$

Because λ ₁And λ ₂Be more or less the same, can think β ₁≈ β ₂, γ ₁≈ γ ₂, δ ₁≈ δ ₂Therefore

$C=\frac{1}{[\mathrm{\α}\left({\mathrm{\λ}}_1\right)-\mathrm{\α}\left({\mathrm{\λ}}_2\right)]l}\ln \frac{I_0\left({\mathrm{\λ}}_1\right)K\left({\mathrm{\λ}}_1\right)I\left({\mathrm{\λ}}_2\right)}{I_0\left({\mathrm{\λ}}_2\right)K\left({\mathrm{\λ}}_2\right)I\left({\mathrm{\λ}}_1\right)}---\left(4\right)$

Regulate optical system, make:

I ₀(λ ₁)K(λ ₁)＝I ₀(λ ₂)K(λ ₂)(5)

Then have

$C=\frac{1}{[\mathrm{\α}\left({\mathrm{\λ}}_1\right)-\mathrm{\α}\left({\mathrm{\λ}}_2\right)]l}\ln \frac{I\left({\mathrm{\λ}}_2\right)}{I\left({\mathrm{\λ}}_1\right)}---\left(6\right)$

Reference wavelength λ ₂Light not by Liquid Absorption, I (λ is arranged ₁)/I (λ ₂)＜1 is to ln (I (λ ₁)/I (λ ₂)) carry out Taylor expansion, omit higher order term:

$\ln \frac{I\left({\mathrm{\λ}}_2\right)}{I\left({\mathrm{\λ}}_1\right)}=-\ln [1+(\frac{I\left({\mathrm{\λ}}_1\right)}{I\left({\mathrm{\λ}}_2\right)}-1)]\≈\frac{I\left({\mathrm{\λ}}_2\right)-I\left({\mathrm{\λ}}_1\right)}{I\left({\mathrm{\λ}}_2\right)}---\left(7\right)$

Therefore tested water percentage can be expressed as:

$C=\frac{1}{[\mathrm{\α}\left({\mathrm{\λ}}_1\right)-\mathrm{\α}\left({\mathrm{\λ}}_2\right)]l}\frac{I\left({\mathrm{\λ}}_2\right)-I\left({\mathrm{\λ}}_1\right)}{I\left({\mathrm{\λ}}_2\right)}---\left(8\right)$

According to following formula as can be known, in wavelength X ₁And λ ₂Down, if absorption coefficient (λ ₁), α (λ ₂) determine, then by determining I (λ ₁) and I (λ ₂) value can determine the size of water percentage.Differential technique has been eliminated the disturbing factor of light path theoretically, has also eliminated the influence that the instability of source device output optical power is brought measurement result simultaneously.

The wavelength coverage of ASE light source of the present invention is 1528.52～1557.32nm, and Output optical power is 14.53dBm.The optical fiber circulator operation wavelength is 1550 ± 30nm, and fiber coupler is 2 * 2 fiber couplers of 3dB, and operation wavelength is 1550 ± 40nm.The foveal reflex wavelength of measuring optical fiber grating is 1550.009nm, and three dB bandwidth is 0.21nm, and reflectivity is greater than 90%, and side mode suppression ratio is greater than 15dB; The foveal reflex wavelength of reference optical fiber grating is 1534.085nm, and three dB bandwidth is 0.193nm, and reflectivity is 97.24%, and it is 20.44dB that rejection ratio is touched on the limit.It is the 2mm quartz colorimetric utensil that optical path length is adopted in the absorption of sample pond.GRIN Lens is that the GRIN Lens of P/4 is a pair of, and its operating distance is 10mm, and operation wavelength is 1550 ± 40nm, and the insertion loss is 0.16dBm.The wavelength resolution of spectroanalysis instrument is 0.02nm, and light intensity resolution is 0.01dBm.

According to structure principle chart shown in Figure 1 light path is connected, the experimental system of formation as shown in Figure 2.350 μ L absolute ethyl alcohols are injected in the cuvette with syringe, simultaneously with spectroanalysis instrument scanning, the light intensity of the light of the measurement wavelength (1550nm) of process GRIN Lens that measurement light source is sent and cuvette.

In cuvette, add water with microsyringe (max cap. is 5 μ L), whenever the water that adds 5 μ L is waited for a few minutes, treats that tested alcoholic solution mixes, be after output intensity is stablized, measure the light intensity and the record at 1550.009nm and 1534.085nm wavelength place by spectroanalysis instrument respectively.Until the water that adds is 50 μ L, just liquid water content be more than 10% till.The concentration that obtains based on differential principle and the relation (formula (8)) of light intensity:

Calculate the concentration of water in the tested alcohol.

Fig. 3 is in the experimentation, the output spectrum figure when absolute ethyl alcohol by spectroanalysis instrument record and liquid water content are 5 μ L.As seen from the figure, wavelength is the light of 1550.009nm, because the absorption of water, light intensity obviously diminishes after water is added absolute ethyl alcohol.And wavelength is the light of 1534.085nm, because without the absorption of sample pond, do not absorbed by aqueous solution, so light intensity almost remains unchanged.Constitute difference by this two paths of signals, the content of water in the ethanol is measured, shown in the table 1 is one group of experimental data of record in the experiment.By this table as can be seen, the light intensity of the 1550.009nm that spectroanalysis instrument records reduces with the increase of water cut in the ethanol, and the light intensity of 1534.085nm changes (variation is slightly thought what the fluctuation of light source caused) with the variation of liquid water content hardly, after water cut was increased to a certain degree, the beam intensity ratio of output had just remained unchanged substantially.

Record data in table 1 experiment

Fig. 4 has provided the curve of one group of measurement data, as seen from the figure, when the concentration of water less than 5% the time, I (λ ₁)/I (λ ₂) reduce with the increase of water concentration; And when concentration greater than 5% the time, I (λ ₁)/I (λ ₂) value almost do not change I (λ just ₁)/I (λ ₂) in small scope, fluctuate.That is to say that for the measurement of the content of water in the alcoholic solution, less than 5% o'clock, measuring system had higher sensitivity in concentration, so the method and apparatus that proposes of the present invention is fit to the measurement to low-water-content.

By water percentage is carried out error analysis less than 4.1% o'clock measurement data, obtain the result of table 2.

Table 2 experimental error

The error of absolute method of measurement of the low-water-content measuring method that proposes of the present invention and device is less than 0.03% as can be seen from Table 2, and water percentage is low more, and the absolute error of measurement is also more little.

Claims (4)

1. the fiber grating low-water-content difference measurement method and apparatus of a dual wavelength double light path, mainly comprise wideband light source, 2 fiber couplers, 2 GRIN Lens, the absorption of sample pond, optical fiber circulator, the measuring optical fiber grating, the reference optical fiber grating, index-matching fluid, optical fiber link and spectroanalysis instrument, it is characterized in that: described optical fiber link is: the light that light source sends is divided into two-way after entering fiber coupler 1 through optical fiber, lead up to port 2 through the absorption of sample pond laggard port one of going into the optical fiber circulator, port 2 outgoing from optical fiber circulator arrive the measuring optical fiber grating then, the light that satisfies the specific wavelength of measuring optical fiber grating Bragg's condition of reflection will be reflected, remaining transmittance is refracted the absorption of rate matching fluid after crossing the measuring optical fiber grating, specific wavelength signal by the measuring optical fiber optical grating reflection is exported from the port 3 of optical fiber circulator, and the port one that enters coupling mechanism 2 arrives port 3, and by the spectroanalysis instrument receiving record; Another road light by fiber coupler 1 outgoing passes through port 3 outgoing to the reference optical fiber grating, the light that satisfies the specific wavelength of reference optical fiber grating Bragg's condition of reflection will be reflected, remaining transmittance is refracted the absorption of rate matching fluid after crossing the reference optical fiber grating, specific wavelength signal by the reference optical fiber optical grating reflection is exported from the port 4 of fiber coupler 1, the port 4 that enters coupling mechanism 2 again arrives port 3, and by the spectroanalysis instrument receiving record.

2. according to the fiber grating low-water-content difference measurement method and apparatus of the described a kind of dual wavelength double light path of claim 1, it is characterized in that: described measuring method is to utilize measuring optical fiber grating with different foveal reflex wavelength and reference optical fiber to know respectively to get to comprise measured information and do not comprise measured information, thus a kind of difference detecting method of realization; This method can effectively reduce the influence that light source power fluctuates and brings to measurement result.

3. according to the fiber grating low-water-content difference measurement method and apparatus of the described a kind of dual wavelength double light path of claim 1, it is characterized in that: the foveal reflex wavelength of described measuring optical fiber grating is 1550.009mn (being in the near infrared absorption band of water), and three dB bandwidth is 0.21nm; The foveal reflex wavelength of described reference optical fiber grating be 1534.085nm (with the difference of the foveal reflex wavelength of measuring optical fiber grating at least greater than 1nm), three dB bandwidth is 0.193nm.

4. according to the described a kind of method and apparatus that low-water-content is measured that is used for of claim 1, it is characterized in that: the output spectrum scope of described wideband light source is 1528.52～1557.32nm (must include the foveal reflex wavelength value of measuring optical fiber grating and reference optical fiber grating).

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