CN101419157B - Accurate measurement method for optical parameter of edible oil by terahertz time-domain spectrum - Google Patents

Abstract

The invention discloses a method for accurately determining optical parameters of edible oil by utilizing a terahertz time-domain spectroscopy. By utilizing a transmission terahertz time-domain spectroscopy (THz-TDS) device, a THz time-domain spectroscopy of a sample cell without a sample is measured as a reference signal, and then a THz time-domain spectroscopy of the sample cell with the sample is measured as a sample signal; and a measurement value of a transmission coefficient of the sample in a terahertz waveband can be obtained by evaluating the ratio of Fourier transforms of the sample signal to the reference signal, then a three-layer transmission function model containing a container is utilized, and a Nelder-Mead search method and a trust region smoothing method are used to fit the measurement value of the transmission coefficient of the sample in the terahertz waveband, thereby accurately determining the refractive index and the absorption coefficient of the edible oil in a corresponding terahertz waveband. The method fully considers multiple reflection effect during the propagation of THz wave in a plurality of layers of media, considers the influence of noise and measurement error, effectively improves the measurement accuracy of the optical parameters of the edible oil, and helps to apply a THz-TDS technology to quantitative detection occasions such as component analysis of the edible oil and so on.

Description

A kind of method of utilizing terahertz time-domain spectrum accurately to measure optical parameter of edible oil

Technical field

The invention belongs to the THz wave technical field, relate to a kind of method of utilizing terahertz time-domain spectrum accurately to measure optical parameter of edible oil.

Background technology

Terahertz (Terahertz or THz) ripple typically refers to the electromagnetic wave of frequency in 0.1～10THz interval, and the energy of its photon is about 1～10meV, just in time with molecular vibration and rotational energy level between the energy of transition roughly suitable.Radiation has strong absorption to THz for most of polar molecules such as hydrone, amino molecule etc., and the vibrational energy level of many organic macromolecules (DNA, protein etc.) and the transition between the rotational energy level are also just in time in the THz wavelength band.Therefore, the THz spectrum of material (comprising emission, reflection and transmitted spectrum) includes abundant physics matter and chemical information, it absorbs and dispersion characteristics can be used for doing the detection and the identification of chemistry such as explosive, medicine and biological sample, has important use value at aspects such as physics, chemistry, biomedicine, uranology, material science and environmental sciences.

The action mode that THz spectrum and imaging technique are unique to biomolecule, water, nonpolar object etc. can make this novel detection technique make a breakthrough at agricultural product and food quality detection range.From as seen-fluorescence spectrum, Fourier transform infrared spectroscopy (FTIR), ultra-violet absorption spectrum, to the x-ray imaging analysis etc., spectral analysis and imaging technique are indispensable important means in agricultural product and the food analysis research.With respect near infrared and middle-infrared band, the advantage of THz radiation is that its wavelength is longer, and the scattering of object is less, and the most of nonpolar objects of THz radiation energy transmission are transparent and have only few medium near infrared and intermediate infrared radiation.Utilize comprehensively optical parametric such as refractive index, absorption coefficient and specific inductive capacity etc. that THz time-domain spectroscopic technology (THz-TDS) can obtain sample simultaneously, for quantitatively and multianalysis more useful informations are provided.THz can obtain widespread use at agricultural product and food inspection field as a kind of novel quick, harmless, in the future cheap detection technique.

Vegetable oil is important agricultural product and raw-food material, can calculate very important optical parametrics such as absorption coefficient, refractive index, specific inductive capacity by measuring its THz time domain spectrum, is used for quality evaluation and analysis of components, has important practical value.Existing Terahertz spectrum computation model is a kind of simplification computation model, does not consider multipath effect, does not also consider the measuring error influence, is unfavorable for detection by quantitative occasions such as analysis of components.

Summary of the invention

The present invention seeks to overcome the deficiency of existing Terahertz spectrum simplified model, a kind of method of utilizing terahertz time-domain spectrum accurately to measure optical parameter of edible oil is provided.

The method of utilizing terahertz time-domain spectrum accurately to measure optical parameter of edible oil comprises the steps:

1) on terahertz time-domain spectrum transmission measurement device, the terahertz time-domain spectrum of measuring sample cell when not placing sample is measured the terahertz time-domain spectrum of sample cell when placing sample as sample signal again as the reference signal;

2) utilize the ratio of following formula calculation sample signal and reference signal Fourier transform, obtain the measured value of sample at the terahertz wave band transmission coefficient:

$H_{\exp }\left(\mathrm{\ω}\right)=\frac{E_{\mathrm{Smaple}}^{\exp }\left(\mathrm{\ω}\right)}{E_{\mathrm{Ref}}^{\exp }\left(\mathrm{\ω}\right)}$

Wherein

Be sample signal Fourier transform and

It is the Fourier transform of reference signal;

3), utilize the optical parametric complex index of refraction of material according to electromagnetic theory $\tilde{n}=n-\mathrm{ik}$ (n is real refractive index, and k is an extinction coefficient), utilize to set up and to comprise container at three layers of interior transition function model:

$H_{\mathrm{Thoery}}\left(\mathrm{\ω}\right)=\frac{E_{\mathrm{Smaple}}^{\mathrm{th}}\left(\mathrm{\ω}\right)}{E_{\mathrm{Ref}}^{\mathrm{th}}\left(\mathrm{\ω}\right)},$ At this moment

$E_{\mathrm{ref}}\left(\mathrm{\ω}\right)={\mathrm{\η}}_0\left(\mathrm{\ω}\right)E_0\left(\mathrm{\ω}\right)T_{10}\left(\mathrm{\ω}\right)P_0\left(\mathrm{\ω}\right)F{P}_0^{13}\left(\mathrm{\ω}\right)T_{03}\left(\mathrm{\ω}\right),$

$E_{\mathrm{Sample}}\left(\mathrm{\ω}\right)={\mathrm{\η}}_2\left(\mathrm{\ω}\right)E_0\left(\mathrm{\ω}\right)T_{12}\left(\mathrm{\ω}\right)P_2\left(\mathrm{\ω}\right)F{P}_2^{13}\left(\mathrm{\ω}\right)T_{23}\left(\mathrm{\ω}\right),$

Wherein $R_{\mathrm{ab}}\left(\mathrm{\ω}\right)=\frac{{\tilde{n}}_a-{\tilde{n}}_b}{{\tilde{n}}_a+{\tilde{n}}_b}$ With $T_{\mathrm{ab}}\left(\mathrm{\ω}\right)=\frac{2{\tilde{n}}_a}{{\tilde{n}}_a+{\tilde{n}}_b}$ Be respectively transmission and the reflection coefficient of incident medium a and emergent medium b; Medium 0 is an air, and medium 1,3 is chamber wall (SiO ₂), medium 2 is edible oils, η _xBe that the THz ripple is at chamber wall (SiO (ω) ₂) transmission, propagation and the multipath reflection factor:

${\mathrm{\η}}_x\left(\mathrm{\ω}\right)=T_{01}\left(\mathrm{\ω}\right)P_1\left(\mathrm{\ω}\right)F{P}_1^{0x}\left(\mathrm{\ω}\right)P_3\left(\mathrm{\ω}\right)F{P}_3^{x0}\left(\mathrm{\ω}\right)T_{30}\left(\mathrm{\ω}\right);$

4) utilization contains container at three layers of interior transition function model, uses Nelder-Mead optimizing method match sample to determine optical parametric at the measured value of terahertz wave band transmission coefficient;

5), determine statistic bias that noise gate and measuring repeatability cause trusted zones, level and smooth optical parametric according to Error Theory then as smoothing algorithm by repeatedly measuring two above sample signals and reference signal.

Described utilization contains container at three layers of interior transition function model, uses Nelder-Mead optimizing method match sample to determine the optical parametric step at the measured value of terahertz wave band transmission coefficient:

1) utilize mistiming of the main peak of reference signal and sample signal to determine the initial value n of sample refractive index ₀, the initial value k that recently determines the sample extinctivity of the amplitude of the main peak of use reference signal and sample signal ₀

2) at 0～2n ₀, 0～2k ₀Adopt convex polygon search procedure search optical parametric n, k in the scope, make and try to achieve H _Thoery(ω) near H _Measure(ω).

The level and smooth optical parametric step of described use trusted zones smoothing method:

1) by repeatedly measuring two above sample signals and reference signal, determine the statistic bias that noise gate and measuring repeatability cause according to Error Theory then, and then the error range of transition function real part and imaginary part, as the trusted zones of further level and smooth optical parametric;

2) remove except first and the last point, do 2 running means, ask the H of each point then by refractive index and extinction coefficient that the Nelder-Mead match obtains _Thoery(ω) and H _MeasureDeviation (ω) if less than the trusted zones of this point, is then accepted the mean value of optical parametric, if greater than trusted zones then recover the original optical parameter value of this point before average;

3) repeating step 2) 5 to 10 times, obtain the level and smooth optical parametric of sample.

The present invention has taken into full account the multipath effect when the THz ripple is propagated in multilayered medium, and considered that noise and measuring error influence, set up the multilayered medium measurement model of edible oil THz spectrum, and use the Nelder-Mead optimizing method and based on the smoothing method of trusted zones, effectively improved the measuring accuracy of optical parameter of edible oil, can be used for the quantitative evaluation of its quality, help the THz-TDS technology is applied to detection by quantitative occasions such as edible oil analysis of components.

Description of drawings

Fig. 1 terahertz time-domain spectrum measurement mechanism figure;

Fig. 2 (a) reference signal measurement model synoptic diagram, among the figure: medium 0 is an air, medium 1, medium 3 are measuring vessel wall (quartz);

Fig. 2 (b) sample signal measurement model synoptic diagram, among the figure: medium 1, medium 3 are measuring vessel wall (quartz), medium 2 is sample (edible oil);

The terahertz time-domain spectrum oscillogram of Fig. 3 (a) corn oil;

The Fourier transform oscillogram of Fig. 3 (b) corn oil terahertz time-domain spectrum;

The refractive index curve of Fig. 3 (c) corn oil;

The absorbance curves of Fig. 3 (d) corn oil.

Embodiment

The present invention utilizes transmission-type terahertz time-domain spectrum (THz-TDS) device, and the THz time domain spectrum conduct of measuring sample cell when not placing sample is with reference to signal, and the THz time domain spectrum of measuring sample cell when placing sample again is as sample signal; Can obtain the measured value of sample by the ratio of asking sample signal and reference signal Fourier transform at the terahertz wave band transmission coefficient, utilize then and contain container at three layers of interior transition function model, use Nelder-Mead optimizing method and trusted zones smoothing method match sample measured value, thereby accurately determine the refractive index and the absorption coefficient of corresponding terahertz wave band edible oil at the terahertz wave band transmission coefficient.

As shown in Figure 1, it is that 800nm, repetition frequency are that 80MHz, pulse width are the LASER Light Source of 100fs that titanium sapphire femtosecond mode locking pulse laser instrument produces centre wavelength, output power 960mW.After entering the THz system, light beam is divided into stronger pump light and more weak detection light through beam splitter.Pump light is modulated by chopper, and directive photoconductive antenna gallium arsenide (GaAs) crystal excites the THz pulse after lens focus.The THz pulse is thrown face mirror collimation through two from the axle metal and is incided on the sample, throws the face mirrors through other two metals again and focuses on and arrive the thick zinc telluridse ZnTe crystal of 2mm, converges with detection light through lag line.At this moment the electric field of THz pulse of electromagnetic radiation is surveyed polarization state and is changed thereupon by the index ellipsoid of linear electro-optic effect modulation electric luminescent crystal ZnTe, is surveyed by the balance diode, and signal is sent into lock-in amplifier and amplified.And the method for passing through the change delay line length is surveyed the whole time domain waveform of THz signal.In order to prevent of the influence of water in air steam, be sealed in the casing that is filled with nitrogen to this section light path of crystal detection ZnTe from GaAs, the sample cell that produces the THz signal to the THz signal.Relative humidity in the case is less than 4%, and temperature is 294K.In the signal scanning process, the signal to noise ratio (S/N ratio) of experimental system is 1000, and spectral resolution is better than 40GHz.

The method of utilizing terahertz time-domain spectrum accurately to measure optical parameter of edible oil comprises the steps:

1) on terahertz time-domain spectrum transmission measurement device, the terahertz time-domain spectrum of measuring sample cell when not placing sample is measured the terahertz time-domain spectrum of sample cell when placing sample as sample signal again as the reference signal;

2) utilize the ratio of following formula calculation sample signal and reference signal Fourier transform, obtain the measured value of sample at the terahertz wave band transmission coefficient:

$H_{\exp }\left(\mathrm{\ω}\right)=\frac{E_{\mathrm{Smaple}}^{\exp }\left(\mathrm{\ω}\right)}{E_{\mathrm{Ref}}^{\exp }\left(\mathrm{\ω}\right)}$

Wherein

Be sample signal Fourier transform and

It is the Fourier transform of reference signal;

3) various optical parametrics can be summed up and asked complex index of refraction $\tilde{n}=n-\mathrm{ik},$ Wherein n is real refractive index, describes the dispersion situation of sample; K is an extinction coefficient, describes the absorption characteristic of sample.Reflection and refraction will take place in the THz pulse signal of supposing vertical incidence in the end of sample and container, its oscillation amplitude change is by reflection coefficient and transmission coefficient decision, and concrete numerical relation is provided by the Fresnel relation $R_{\mathrm{ab}}\left(\mathrm{\ω}\right)=\frac{{\tilde{n}}_a-{\tilde{n}}_b}{{\tilde{n}}_a+{\tilde{n}}_b}$ With $T_{\mathrm{ab}}\left(\mathrm{\ω}\right)=\frac{2{\tilde{n}}_a}{{\tilde{n}}_a+{\tilde{n}}_b}.$ Wherein,

Be respectively the complex index of refraction of incident medium and emergent medium.Suppose reflection and the projection of THz, consider the propagation factor of propagation distance L among the medium b through a, b, three layers of medium of c $P_b(\mathrm{\ω},L)=\exp (-i{\tilde{n}}_b\mathrm{\ωL}/c),$ And the FP effect that repeatedly reflects $F{P}_{b,\mathrm{\δ}}^{\mathrm{ac}}\left(\mathrm{\ω}\right)=\underset{k=0}{\overset{\mathrm{\δ}}{\mathrm{\Σ}}}{\left\{R_{\mathrm{bc}}\left(\mathrm{\ω}\right)P_b^2\left(\mathrm{\ω}\right)R_{\mathrm{ba}}\left(\mathrm{\ω}\right)\right\}}^k,$ This can set up in view of the above and comprise container at three layers of interior transition function model, and the THz impulse electric field strength expression formula that obtains reference signal and sample signal is $H_{\mathrm{Thoery}}\left(\mathrm{\ω}\right)=\frac{E_{\mathrm{Smaple}}^{\mathrm{th}}\left(\mathrm{\ω}\right)}{E_{\mathrm{Ref}}^{\mathrm{th}}\left(\mathrm{\ω}\right)},$ At this moment

$E_{\mathrm{ref}}\left(\mathrm{\ω}\right)={\mathrm{\η}}_0\left(\mathrm{\ω}\right)E_0\left(\mathrm{\ω}\right)T_{10}\left(\mathrm{\ω}\right)P_0\left(\mathrm{\ω}\right)F{P}_0^{13}\left(\mathrm{\ω}\right)T_{03}\left(\mathrm{\ω}\right)$

$E_{\mathrm{Sample}}\left(\mathrm{\ω}\right)={\mathrm{\η}}_2\left(\mathrm{\ω}\right)E_0\left(\mathrm{\ω}\right)T_{12}\left(\mathrm{\ω}\right)P_2\left(\mathrm{\ω}\right)F{P}_2^{13}\left(\mathrm{\ω}\right)T_{23}\left(\mathrm{\ω}\right)$

Its medium 0 is an air, and medium 1,3 is chamber wall (SiO ₂), medium 2 is edible oils, η _xBe that the THz ripple is at chamber wall (SiO (ω) ₂) transmission, propagation and the multipath reflection factor:

${\mathrm{\η}}_x\left(\mathrm{\ω}\right)=T_{01}\left(\mathrm{\ω}\right)P_1\left(\mathrm{\ω}\right)F{P}_1^{0x}\left(\mathrm{\ω}\right)P_3\left(\mathrm{\ω}\right)F{P}_3^{x0}\left(\mathrm{\ω}\right)T_{30}\left(\mathrm{\ω}\right);$

4) utilization contains container at three layers of interior transition function model, uses Nelder-Mead optimizing method match sample to determine optical parametric at the measured value of terahertz wave band transmission coefficient, and uses the level and smooth optical parametric of trusted zones smoothing method.

Described utilization contains container at three layers of interior transition function model, uses Nelder-Mead optimizing method match sample to determine the optical parametric step at the measured value of terahertz wave band transmission coefficient:

1) utilize mistiming of the main peak of reference signal and sample signal to determine the initial value n of sample refractive index ₀, the initial value k that recently determines the sample extinctivity of the amplitude of the main peak of use reference signal and sample signal ₀

2) at 0～2n ₀, 0～2k ₀Adopt convex polygon search procedure search optical parametric n, k in the scope, make and try to achieve H _Thoery(ω) near H _Measure(ω).

The level and smooth optical parametric step of described use trusted zones smoothing method:

1) by repeatedly measuring two above sample signals and reference signal, determine the statistic bias that noise gate and measuring repeatability cause according to Error Theory then, and then the error range of transition function real part and imaginary part, as the trusted zones of further level and smooth optical parametric;

2) remove except first and the last point, do 2 running means, ask the H of each point then by refractive index and extinction coefficient that the Nelder-Mead match obtains _Thoery(ω) and H _MeasureDeviation (ω) if less than the trusted zones of this point, is then accepted the mean value of optical parametric, if greater than trusted zones then recover the original optical parameter value of this point before average;

3) repeating step 2) 5 to 10 times, obtain the level and smooth optical parametric of sample.

Embodiment

Measure the optical parametric of corn oil

The vegetable oil sample is a corn oil, and sampling receptacle adopts the quartz colorimetric utensil that the THz wave band is had very little absorptivity, light path 2mm, wall thickness 1.2mm.Measure the terahertz time-domain spectrum of vacant sampling receptacle earlier as the reference signal, measure the terahertz time-domain spectrum of the sampling receptacle that is placed with corn oil again as sample signal (shown in Fig. 3 a), carry out Fourier transform (shown in Fig. 3 b) respectively, use method of the present invention to calculate the refractive index and the absorption coefficient (shown in Fig. 3 c and Fig. 3 d) of corn oil then, wherein dotted line is the result of general approximation method, and solid line is a result calculated of the present invention.

Claims (3)

1. a method of utilizing terahertz time-domain spectrum accurately to measure optical parameter of edible oil is characterized in that comprising the steps:

1) on terahertz time-domain spectrum transmission measurement device, the terahertz time-domain spectrum of measuring sample cell when not placing sample is measured the terahertz time-domain spectrum of sample cell when placing sample as sample signal again as the reference signal;

2) utilize the ratio of following formula calculation sample signal and reference signal Fourier transform, obtain the measured value of sample at the terahertz wave band transmission coefficient:

$H_{\exp }\left(\mathrm{\ω}\right)=\frac{E_{\mathrm{Sample}}^{\exp }\left(\mathrm{\ω}\right)}{E_{\mathrm{Ref}}^{\exp }\left(\mathrm{\ω}\right)}$

Wherein

Be sample signal Fourier transform and

It is the Fourier transform of reference signal;

3), utilize the optical parametric complex index of refraction of material according to electromagnetic theory N is real refractive index, and k is an extinction coefficient, sets up to comprise container at three layers of interior transition function model:

$H_{\mathrm{Theory}}\left(\mathrm{\ω}\right)=\frac{E_{\mathrm{Sample}}^{\mathrm{th}}\left(\mathrm{\ω}\right)}{E_{\mathrm{Ref}}^{\mathrm{th}}\left(\mathrm{\ω}\right)}$ At this moment

$E_{\mathrm{Ref}}^{\mathrm{th}}\left(\mathrm{\ω}\right)={\mathrm{\η}}_0\left(\mathrm{\ω}\right)E_0\left(\mathrm{\ω}\right)T_{10}\left(\mathrm{\ω}\right)P_0(\mathrm{\ω},L_0)F{P}_{0,\mathrm{\δ}}^{13}(\mathrm{\ω},L_0)T_{03}\left(\mathrm{\ω}\right)$

$E_{\mathrm{Sample}}^{\mathrm{th}}\left(\mathrm{\ω}\right)={\mathrm{\η}}_2\left(\mathrm{\ω}\right)E_0\left(\mathrm{\ω}\right)T_{12}\left(\mathrm{\ω}\right)P_2(\mathrm{\ω},L_2){\mathrm{FP}}_{2,\mathrm{\δ}}^{13}(\mathrm{\ω},L_2)T_{23}\left(\mathrm{\ω}\right)$

H wherein _Theory(ω) be the theoretical value of THz wave transmission coefficient,

With

Be respectively reflection and the transmission coefficient of incident medium A and emergent medium B;

For electromagnetic wave at medium A propagation distance L _APropagation factor, c is a vacuum light speed; ${\mathrm{FP}}_{B,\mathrm{\δ}}^{\mathrm{AC}}(\mathrm{\ω},L_B)=\underset{k=0}{\overset{\mathrm{\δ}}{\mathrm{\Σ}}}{\left\{R_{\mathrm{BC}}\left(\mathrm{\ω}\right){P_B}^2(\mathrm{\ω},L_B)R_{\mathrm{BA}}\left(\mathrm{\ω}\right)\right\}}^k$ Be the FP effect that repeatedly reflects, medium 0 is an air, and medium 1,3 is SiO ₂Chamber wall, medium 2 is edible oils, η _xBe that the THz ripple is at SiO (ω) ₂The transmission of chamber wall, propagation and the multipath reflection factor:

${\mathrm{\η}}_x\left(\mathrm{\ω}\right)=T_{01}\left(\mathrm{\ω}\right)P_1(\mathrm{\ω},L_1){\mathrm{FP}}_{1,\mathrm{\δ}}^{0x}(\mathrm{\ω},L_1)P_3(\mathrm{\ω},L_3){\mathrm{FP}}_{3,\mathrm{\δ}}^{x0}(\mathrm{\ω},L_3)T_{30}\left(\mathrm{\ω}\right);$

4) utilization contains container at three layers of interior transition function model, uses Nelder-Mead optimizing method match sample to determine optical parametric at the measured value of terahertz wave band transmission coefficient;

5), determine statistic bias that noise gate and measuring repeatability cause trusted zones, level and smooth optical parametric according to Error Theory then as smoothing algorithm by repeatedly measuring two above sample signals and reference signal.

2. a kind of method of utilizing terahertz time-domain spectrum accurately to measure optical parameter of edible oil according to claim 1, it is characterized in that described utilization contains container at three layers of interior transition function model, use Nelder-Mead optimizing method match sample to determine that at the measured value of terahertz wave band transmission coefficient the optical parametric step is:

1) utilize mistiming of the main peak of reference signal and sample signal to determine the initial value n of the real refractive index of sample ₀, the initial value k that recently determines the sample extinction coefficient of the amplitude of the main peak of use reference signal and sample signal ₀

2) at 0～2n ₀, 0～2k ₀Adopt convex polygon search procedure search optical parametric n, k in the scope, make and try to achieve H _Theory(ω) near H _Exp(ω).

3. a kind of method of utilizing terahertz time-domain spectrum accurately to measure optical parameter of edible oil according to claim 1 is characterized in that the level and smooth optical parametric step of described use trusted zones smoothing method is:

1) by repeatedly measuring two above sample signals and reference signal, determine the statistic bias that noise gate and measuring repeatability cause according to Error Theory then, and then obtain the error range of three layers of transition function real part and imaginary part, as the trusted zones of further level and smooth optical parametric;

2) except and the last point, do 2 running means at first, ask the H of each point then by real refractive index and extinction coefficient that the Nelder-Mead match obtains _Theory(ω) and H _ExpDeviation (ω) if less than the trusted zones of this point, is then accepted the mean value of optical parametric, if greater than trusted zones then recover the original optical parameter value of this point before average;

3) repeating step 2) 5 to 10 times, obtain the level and smooth optical parametric of sample.

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