CN102735648A - Device and method for measuring third-order nonlinear index of refraction by using comparison method - Google Patents

Abstract

The invention discloses a device and a method for measuring a third-level nonlinear index of the refraction by using a comparison method. The device comprises a phase diaphragm, a first optical patch, and a CCD (charge coupled device) camera, wherein an optical 4f system is formed by the first optical patch. The device is characterized by being provided with a beam splitter and a second optical path, wherein the second optical patch forms an optical 4f system, the beam splitter is positioned behind the phase diaphragm and is used for splitting a beam into the first optical path and the second optical path, and the output light of the first optical path and the output light of the second optical path are respectively received by the CCD camera. By virtue of the comparison method, the third-order nonlinear index of the refraction of a sample to be tested can be measured. The method disclosed by the invention can be used for avoiding the complex fitting calculation of a single 4f system in the prior art, and has the advantages of convenience in use and accurate measurement results.

Description

A kind of apparatus and method of comparative method for measuring third-order non-linear refraction

Technical field

The present invention relates to a kind of optical non-linear method of measuring material, be specifically related to a kind of method that adopts three rank optical nonlinear refractions of comparative method for measuring material.

Background technology

Along with the develop rapidly of art such as optical communication and optical information processing, non-linear photon is learned investigation of materials and is become more and more important.The realization of functions such as light logic, optical storage, optical transistor, photoswitch mainly depends on the progress that non-linear photon is learned material.The measuring technique of Medium Optics nonlinear parameter is the gordian technique of research nonlinear optical material.4f phase coherence imaging system (G.Boudebs and S.Cherukulappurath; " Nonlinear optical measurements using a 4f coherent imaging system with phase object "; Phys.Rev.A; 69,053813 (1996)) be exactly a kind of new method of measuring nonlinear refraction coefficient of materials and absorption that proposes in recent years.

4f phase coherent imaging method is a kind of measuring method of beam aberration; This method is on 4f system object plane, to place a phase diaphragm; Non-linear object to be measured is placed on the Fourier plane, and on exit facet, receives the method for shoot laser pulse diagram picture with the CCD camera.This method can utilize monopulse to measure the size and the symbol of nonlinear refraction coefficient simultaneously.Phase diaphragm is to make the more phase object of small circular of an area at the center of a circular iris, and the phase delay of a pi/2 is arranged through other local light of light ratio of phase object.When the nonlinear refractive index of measured material when being positive, the nonlinear images that CCD receives since positive phase contrast strengthen around the strength ratio in the position of phase object.Opposite, when non-linear this refractive index of measured material for negative the time, a little less than wanting around the strength ratio of the position of the phase object of nonlinear images.

4f phase coherent imaging method utilizes phase diaphragm to realize the size of nonlinear refractive index and the measurement of symbol dexterously.But, in measuring process, need carry out complicated The Fitting Calculation.Therefore, be necessary the method for 4f phase coherent imaging method measurement optics nonlinear refractive index is improved.

Summary of the invention

The purpose of this invention is to provide does not a kind ofly need complicated loaded down with trivial details The Fitting Calculation and adopts the apparatus and method of the third-order non-linear refraction of comparative method for measuring material.

For reaching the foregoing invention purpose; The technical scheme that the present invention adopts is: a kind of device of comparative method for measuring third-order non-linear refraction; Comprise phase diaphragm, first light path, the CCD camera that mainly are made up of first convex lens, second convex lens and the first neutral attenuator, said first convex lens and second convex lens constitute optics 4f system; Be provided with beam splitter, the 3rd convex lens, the 4th convex lens, the second neutral attenuator and a plurality of catoptron; Said the 3rd convex lens, the 4th convex lens and the second neutral attenuator constitute second light path, and said the 3rd convex lens and the 4th convex lens constitute optics 4f system; Said beam splitter is positioned at said phase diaphragm rear, and with beam separation to said first light path and second light path, the output light of said first light path and second light path advances respectively by said CCD camera to receive.

In the technique scheme, being provided for of a plurality of catoptrons cooperates beam splitter that light beam is imported first light path and second light path, and the emergent light of first light path and second light path is imported the CCD camera.Catoptron the position is set and mode is the common practise of this area.

In the technique scheme, the focal length of the focal length of said first convex lens and the 3rd convex lens equates that the focal length of the focal length of said second convex lens and the 4th convex lens equates.

A kind of method of comparative method for measuring third-order non-linear refraction, adopt said apparatus to realize that measuring process comprises:

(1) earlier two standard models is placed on the frequency plane place of first light path and second light path respectively, on the CCD camera, notes the nonlinear images of two standard models simultaneously, obtain the transmission change value Δ T of two standard models ₁With Δ T ₂Said transmission change value is in the nonlinear images, the average intensity that the phase object part average intensity of phase diaphragm and phase object are outer poor;

(2) take off the standard model at the frequency plane place of first light path, change testing sample, on the CCD camera, note the nonlinear images of testing sample and standard model simultaneously, obtain the transmission change value Δ T of testing sample _sTransmission change value Δ T with standard model ₂';

(3) the nonlinear refraction coefficient of testing sample does,

In the formula, n _sBe the nonlinear refraction coefficient of testing sample, n _rNonlinear refraction coefficient for standard model.

The present invention through with identical 4f reference path of original 4f main optical path parallel connection; In main optical path, place testing sample; Reference path is placed standard model; Utilize axle to go up nonlinear phase shift and the linear zone of transmission change, directly recently record the third-order non-linear refraction coefficient mutually with the non-linear transmission change of standard model, thereby avoid The Fitting Calculation through testing sample.

Its theoretical model analysis is following:

The normalized linear polarization monochrome plane wave that two-dimensional bodies are launched by pulse laser on the plane of incidence of 4f system (E (x, y, t)=E ₀(t) exp [j (ω t-kz)]+c.c.) irradiation.If the phase object transmitance be t (x, y), then behind phase object the surface be: O (x, y, t)=E (x, y, t) t (x, y), so sample front-surface field amplitude be O (x, y, spatial fourier transform t):

$S(u,v,t)=\frac{1}{\mathrm{\λf}}\mathrm{FT}\left[O(x,y,t)\right]$

$=\frac{1}{\mathrm{\λf}}\∫\∫O(x,y,t)\exp [-2\mathrm{\πj}(\mathrm{ux}+\mathrm{vy})]\mathrm{dxdy}$ (1)

The symbol of FT in the following formula---Fourier transform; U is confocal spatial frequency of locating the x direction of 4f system, u=x/ λ f; V is confocal spatial frequency of locating the y direction of 4f system, v=y/ λ f; F is lens L ₁With L ₂Focal length; λ is the excitation wavelength of incoming laser beam.

Only consider third-order non-linear, and sample thickness is far smaller than the diffraction length of light beam, then sample can be thought to approach, the amplitude of pulse laser and phase change communication satisfaction in sample:

$\frac{\∂I}{\∂z^{\′}}=-({\mathrm{\α}}_0+\mathrm{\βI})I$ (2)

$\frac{\mathrm{d\Δ\φ}}{{\mathrm{dz}}^{\′}}={\mathrm{kn}}_{2}I$

Q in the formula (u v) represents empty nonlinear phase shift, and q (u, v, t)=β L _EffI (u, v, t); L _EffBe effective length, L _Eff=[1-exp (α L)]/α; L is the thickness of medium; I (u, v, t) be the intensity of light beam in sample (be proportional to | S (u, v, t) | ²), α is linear absorption coefficient (m ^-1); β is two-photon absorption coefficient (m/W); n ₂Be nonlinear refractive index (m ²/ W).

So the multiple light field amplitude in surface can be written as behind nonlinear medium:

$S_L(u,v,t)=S(u,v,t)e^{-\mathrm{\&alpha;L}/2}{[1+q(u,v,t)]}^{(\mathrm{<figure-callout\; id="2"\; label="jk\; n"\; filenames="120716135107.png"\; state="\{\{state\}\}">jk</figure-callout>}{n}_{}{}_2/\mathrm{\&beta;}-1/2)}---\left(3\right)$

T (u, v t) by the complex amplitude response of non-linear generation, can be defined as:

Wherein is the nonlinear phase shift that medium causes, its expression formula is:

When medium is that α and β can ignore when can't harm kerr medium.Equation then; (5) can abbreviation be that

is similar; (3) be reduced to

Exit facet in the 4f system, can write as intensity becomes:

I _im(x,y,t)=|U(x,y,t)| ²=|λfFT ^-1[S _L(u,v,t)H(u,v)]| ² (6)

FT in the formula ^-1The expression inverse Fourier transform; H (u v) is not have to defocus or the coherent optics transport function of aberration lens group,

N _ABe lens L ₁Numerical aperture; G is the enlargement factor of total system.

If consider that incident light is flat-top light (top-hat), place the transmitance of the circular iris of the 4f system plane of incidence to be defined as:

$t_a(x,y)=\mathrm{circ}[\sqrt{x^2+y^2}/R_a]---\left(7\right)$

The prerequisite of this hypothesis is that the radius of circular aperture diaphragm is R _aComparing with the spatial spread (waist radius of Gauss light) of incident light and to want much little, is to pass through diaphragm for the monochromatic plane wave energy of incident like this.Add that at the diaphragm center radius is L _P(L _P<r _a) square phase object, so have φ at the center of flat-top light _LPhase delay, the transmitance of circular phase diaphragm is written as:

$t(x,y)=t_a(x,y)\exp \left\{j{\mathrm{\&phi;}}_L\mathrm{circ}\right[\sqrt{x^2+y^2}/L_p\left]\right\}---\left(8\right)$

We can simulate the picture intensity that CCD detects on the 4f system exit facet by formula (1)-(8).

The nonlinear phase shift that produces for sample is provided by (5) formula.For harmless kerr medium, its axle is gone up nonlinear phase shift

and is done

I ₀Be that axle is gone up light intensity.Can see and remove n ₂In addition, when other conditions are constant,

With n ₂Be directly proportional.Interval at 0-0.65 π by accompanying drawing can be seen

, Δ T and

are linear.Testing sample is identical with standard model thickness.Therefore, can obtain

Promptly

$n_{2S}=\frac{{\mathrm{\&Delta;T}}_S}{{\mathrm{\&Delta;T}}_R}{n}_{2R}---\left(11\right)$

Thus, can simply record the third-order non-linear refraction coefficient of testing sample.

Because the technique scheme utilization, the present invention compared with prior art has advantage:

1. on the basis of the device of the present invention through measuring third-order non-linear in existing 4f light path, increase a 4f light path, light path can realize the size of third-order non-linear refractive index and the measurement and the judgement of symbol well as a reference.

2. adopt method of the present invention, can directly obtain the third-order non-linear refraction coefficient, do not need complicated The Fitting Calculation with relative method.

Description of drawings

Accompanying drawing 1 is the two 4f phase imaging systematic schematic diagrams in the embodiment of the invention one;

Accompanying drawing 2 is with the phase diaphragm synoptic diagram of circular phase object in the embodiment of the invention one;

Accompanying drawing 3 in the embodiment of the invention one with the nonlinear images of the phase diaphragm numerical simulation of circular phase object;

Accompanying drawing 4 is Fig. 3 sectional views along y=0;

Accompanying drawing 5 is Δ T relation curves with in the embodiment of the invention one;

Accompanying drawing 6 is nonlinear images sectional views of testing sample among the embodiment;

Accompanying drawing 7 is nonlinear images sectional views of standard model among the embodiment.

Embodiment

Below in conjunction with accompanying drawing and embodiment the present invention is further described:

Embodiment one: accompanying drawing 1 is the Experimental equipment of two 4f phase imaging systems.Experimental provision can be divided into main optical path and reference path two parts.Main optical path is by phase diaphragm A, beam splitter BS, mirror M ₁, convex lens L ₁, testing sample S, convex lens L ₂, neutral colour filter tf ₁, mirror M ₂, M ₄Form with the CCD camera.Convex lens L wherein ₁With convex lens L ₂Constitute main optical path 4f system, phase diaphragm A is placed on the object plane of 4f system, and testing sample S is placed on the Fourier plane, and CCD camera received pulse image on the picture plane of 4f system.Reference path is by phase diaphragm A, beam splitter BS, mirror M ₃And M ₅, convex lens L ₃, standard model R, convex lens L ₄, neutral colour filter tf ₂, mirror M ₆, M ₇Form with the CCD camera.Standard model R is placed on the Fourier plane, wherein convex lens L ₃With convex lens L ₄Constitute reference path 4f system.At first restraint (this part has omitted) through expanding among Fig. 1 from the laser that laser instrument sends, the laser pulse that expands after restrainting forms nearly top-hat light through phase diaphragm, and light beam is through the beam splitter beam splitting, by convex lens L ₁, L ₃Fourier transform converge to respectively on the testing sample and standard model that is placed on the Fourier plane because the nonlinear refraction character of testing sample and standard model makes the phase place of pulse of incident change.The pulse of surperficial outgoing is through convex lens L behind the sample ₂, L ₄Inverse Fourier transform receive by the CCD camera, obtain the nonlinear images of testing sample and standard model.

Shown in the accompanying drawing 2 is exactly the common form of phase diaphragm, and phase object is circular (dash area), the light beam bit phase delay pi/2 of other part of optical beam ratio through phase object.

The measurement that utilizes two 4f phase imaging systems to carry out nonlinear refractive index divides two parts to carry out, i.e. energy calibration and nonlinear measurement.The concrete steps of nonlinear measurement are:

The first step: all place standard model R on main optical path and the reference path Fourier plane, utilize energy meter can record energy ratio main, the ginseng light path.On the CCD camera, will write down their nonlinear images, and utilize nonlinear images can obtain beam intensity ratio main, the ginseng light path.

Second step: take off the standard model in the main optical path, testing sample is placed on the Fourier plane of main optical path.Reference path is still placed standard model.The CCD camera will write down the nonlinear images of testing sample and standard model.

Accompanying drawing 3 is the nonlinear images that obtained by the phase diaphragm with circular phase object, and accompanying drawing 4 is accompanying drawing 3 sectional views along y=0.The used major parameter of numerical simulation is the phase object radius and the ratio ρ=L of diaphragm radius _p/ R _a'=0.5mm/1.5mm ≈ 0.33, φ _L=pi/2, the testing sample nonlinear phase shift

Δ T is the poor of inner average intensity of phase object and outside average intensity, i.e. transmission change.

Accompanying drawing 5 are Δ T with

Relation curve, φ _L=pi/2,

Interval at 0-0.65 π, T is linear with Δ.

Accompanying drawing 6 and accompanying drawing 7 are sectional views of the nonlinear images of testing sample and standard model in the experiment.Testing sample is a toluene, referring to Fig. 6; Standard model is CS ₂, referring to Fig. 7.Experiment condition: laser wavelength lambda=532nm, φ _L=pi/2, the diaphragm radius R _a=1.5mm, phase object radius L _p=0.5mm.CS wherein ₂Third-order non-linear refraction coefficient n ₂=3.2 * 10 ^-18m ²/ W,

Can draw toluene third-order non-linear refraction coefficient is n ₂=1.28 * 10 ^-18m ²/ W.In error range with document in n ₂=1.1 * 10 ^-18m ²/ W coincide.

Claims (3)

1. the device of comparative method for measuring third-order non-linear refraction; Comprise phase diaphragm, first light path, the CCD camera that mainly are made up of first convex lens, second convex lens and the first neutral attenuator, said first convex lens and second convex lens constitute optics 4f system; It is characterized in that: be provided with beam splitter, the 3rd convex lens, the 4th convex lens, the second neutral attenuator and a plurality of catoptron; Said the 3rd convex lens, the 4th convex lens and the second neutral attenuator constitute second light path, and said the 3rd convex lens and the 4th convex lens constitute optics 4f system; Said beam splitter is positioned at said phase diaphragm rear, and with beam separation to said first light path and second light path, the output light of said first light path and second light path advances respectively by said CCD camera to receive.

2. the device of comparative method for measuring third-order non-linear refraction according to claim 1 is characterized in that: the focal length of the focal length of said first convex lens and the 3rd convex lens equates that the focal length of the focal length of said second convex lens and the 4th convex lens equates.

3. the method for a comparative method for measuring third-order non-linear refraction adopts the said device realization of claim 1, and measuring process comprises:

(1) elder generation is placed on two standard models respectively the frequency plane place of first light path and second light path; On the CCD camera, note the nonlinear images of two standard models simultaneously, obtain transmission change value

and

of two standard models; Said transmission change value is in the nonlinear images, the average intensity that the phase object part average intensity of phase diaphragm and phase object are outer poor;

(2) take off the standard model at the frequency plane place of first light path; Change testing sample; On the CCD camera, note the nonlinear images of testing sample and standard model simultaneously, obtain the transmission change value

of testing sample and the transmission change value

of standard model;

(3) the nonlinear refraction coefficient of testing sample does,

, in the formula, n _sBe the nonlinear refraction coefficient of testing sample, n _rNonlinear refraction coefficient for standard model.

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