CN102707365A - Positive and negative strip-shaped phase diaphragm, 4f phase-coherent nonlinear imaging system employing same and nonlinear refractivity measurement method - Google Patents

Abstract

The invention provides a positive and negative strip-shaped phase diaphragm, a 4f phase-coherent nonlinear imaging system employing the same and a nonlinear refractivity measurement method, relates to a 4f phase-coherent nonlinear imaging system of a phase diaphragm and a nonlinear refractivity measurement method, which belong to the optical field, and aims to solve the problems that a traditional phase diaphragm has difficult manufacture process and low measuring accuracy. The phase delays of a positive strip-shaped phase object and a negative strip-shaped phase object of the positive and negative strip-shaped phase diaphragm are respectively 2m Pi+Pi/2 and 2n Pi-Pi/2, wherein m and n are integral numbers. The 4f phase-coherent nonlinear imaging system employing the positive and the negative strip-shaped phase diaphragm comprises a measurement system and an energy reference system; and the method is used for measuring the nonlinear refractivity.

Description

Positive and negative bar shaped phase diaphragm and adopt this diaphragm the non-linear imaging system of 4f phase coherence and to the nonlinear refractive index measuring method

Technical field

The present invention relates to a kind of non-linear imaging system of 4f phase coherence of phase diaphragm and, belong to optical field the nonlinear refractive index measuring method.

Background technology

The Z scan method needs moving axially of nonlinear sample, and the requirement of light path adjustment is high.In addition, because the essence of its multiple-pulse irradiation causes it very high to the stability requirement of laser pulse on time, space, energy, otherwise will bring very big measuring error.Moreover, the Z scanning system has limited the scope of its application because near the lasting high illuminated the along can cause irreversible damage to nonlinear sample.Though the non-linear imaging technique of 4f phase coherence utilizes phase diaphragm to realize the synchro measure of nonlinear refractive index size and symbol; But make circular phase diaphragm will originally with regard to little circular iris center plating size littler and phase delay be the homogeneous transparent deielectric-coating of pi/2; The difficulty of manufacture craft is very big, causes the phase object out-of-shape, and the edge is jagged; Phase delay is inhomogeneous, has reduced measuring accuracy.Though and the bar shaped phase diaphragm has solved the difficult problem of manufacture craft, be that circular phase diaphragm or bar shaped phase diaphragm all can not make full use of positive negative phase-contrast principle, therefore measure sensitivity and be difficult to improve.

Summary of the invention

The difficulty that the objective of the invention is to conventional phase diaphragm manufacture craft is big, and the problem that measuring accuracy is low provides a kind of positive and negative bar shaped phase diaphragm and adopts the non-linear imaging system of 4f phase coherence of this diaphragm and to the nonlinear refractive index measuring method.

Positive and negative bar shaped phase diaphragm, the bit phase delay of the positive and negative bar shaped phase object at this phase diaphragm center is respectively 2m π+pi/2 and 2n π-pi/2, and wherein m, n are integer.

The non-linear imaging system of 4f phase coherence of positive and negative bar shaped phase diaphragm, it comprises measuring system and energy frame of reference;

Wherein measuring system is made up of 1/2nd wave plates, polarizing prism, beam expander, positive and negative bar shaped phase diaphragm, beam splitter, convex lens, No. two convex lens, neutral attenuator and imageing sensor;

/ 2nd wave plates, polarizing prism, beam expander, positive and negative bar shaped phase diaphragm, beam splitter, convex lens, nonlinear sample to be measured, No. two convex lens and a neutral attenuator are placed on the optical axis of incident light successively in proper order; On the photosurface of light beam after the neutral attenuator decay, form hot spot at imageing sensor; And the convex lens that convex lens and No. two convex lens are coaxial confocals, nonlinear sample to be measured is positioned on the focus of convex lens and No. two convex lens;

The energy frame of reference is made up of a catoptron, No. two neutral attenuators, No. three convex lens, No. two catoptrons and No. three catoptrons;

Catoptron, No. two neutral attenuators, No. three convex lens, No. two catoptrons and No. three catoptron order placements successively; Catoptron reflexes to neutral attenuator No. two with the beam splitter beam reflected; Light beam after No. two neutral attenuator decay is incident to convex lens No. three; Light beam through the projection of No. three convex lens is incident to catoptron No. two, on the photosurface that is incident to imageing sensor after No. three mirror reflects, forms hot spot through the light beam of No. two mirror reflects, and said hot spot and hot spot are not overlapping.

The non-linear imaging system of 4f phase coherence of positive and negative bar shaped phase diaphragm to the nonlinear refractive index measuring method, its performing step is following:

Step 1, under the situation of not placing nonlinear sample to be measured, laser pulse of light emitted is gathered a pulse diagram picture through imageing sensor then;

Step 2, nonlinear sample to be measured is placed between convex lens and No. two convex lens; And be positioned on the focus of No. two convex lens; Simultaneously neutral attenuator is placed between beam splitter and the convex lens, makes the light intensity that shines on the nonlinear sample be reduced to the range of linearity, with pulse diagram picture of CCD camera collection; This image is called linear image, demonstrates linear beam spot;

Step 3, neutral attenuator is moved on between No. two convex lens and the imageing sensor, gather a pulse diagram picture with the CCD camera, this image is called nonlinear images, demonstrates non-linear hot spot;

Step 4, nonlinear sample to be measured is taken away; With letting laser facula all get on the position on the energy meter probe between convex lens of energy meter placement and No. two convex lens; Laser pulse of light emitted is incident to the non-linear imaging system of 4f phase coherence; Measure this pulse with energy meter and get to the energy on the energy meter probe, gather the reference hot spot of reference path with imageing sensor 10 simultaneously;

Step 5, with linear beam spot as input, obtain the value of nonlinear refractive index through the non-linear hot spot of numerical fitting; Concrete grammar is following:

The monochromatic plane wave illumination of linear polarization is on positive and negative bar shaped phase diaphragm, and light field is E (t), the transmitance of bar shaped phase diaphragm be t (x, y),

Then the light field on surface is behind the bar shaped phase diaphragm: and O (x, y, t)=E (t) t (x, y) (1)

Nonlinear sample front surface light field is:

$S(u,v,t)=\frac{1}{{\mathrm{\&lambda;<figure-callout\; id="1"\; label="f"\; filenames="HDA00001804336800021.png,HDA00001804336800022.png"\; state="\{\{state\}\}">f</figure-callout>}}_1}\mathrm{FT}\left[O(x,y,t)\right]=\frac{1}{{\mathrm{\&lambda;<figure-callout\; id="1"\; label="f"\; filenames="HDA00001804336800021.png,HDA00001804336800022.png"\; state="\{\{state\}\}">f</figure-callout>}}_1}\&Integral;\&Integral;O(x,y,t)\exp [-2\mathrm{\&pi;i}(\mathrm{ux}+\mathrm{vy})]\mathrm{dxdy}---\left(2\right)$

FT represents Fourier transform in the following formula; U and v are respectively the spatial frequency of non-linear imaging system Fourier plane x of place of 4f phase coherence and y direction: u=x '/λ f ₁, v=y '/λ f ₁f ₁Focal length for incident lens in the 4f imaging system; λ is an optical maser wavelength, and i is an imaginary unit;

For three rank optical nonlinearities, approximate at thin sample with to become envelope slowly down approximate, the light intensity of pulse laser and phase change communication satisfaction in nonlinear medium:

$\frac{\mathrm{dI}}{{\mathrm{dz}}^{\&prime;}}=-(\mathrm{\&alpha;}+\mathrm{\&beta;I})I---\left(3\right)$

$\frac{\mathrm{d\&Delta;\&phi;}}{{\mathrm{dz}}^{\&prime;}}{=\mathrm{kn}}_{2}I---\left(4\right)$

Wherein, α and β are respectively linearity and non-linear absorption coefficient, and I is the light intensity in the sample, n ₂Be nonlinear refractive index, z' is the propagation distance of light beam in sample, and k=2 π/λ is the wave vector size, and Δ φ is a nonlinear phase shift, and the compound light field on surface is behind nonlinear medium:

$S_L(u,v,t)=S(u,v,t)e^{-\mathrm{\&alpha;L}/2}{[1+q(u,v,t)]}^{{\mathrm{<figure-callout\; id="2"\; label="ikn"\; filenames="HDA00001804336800013.png,HDA00001804336800021.png"\; state="\{\{state\}\}">ikn</figure-callout>}}_2/\mathrm{\&beta;}-1/2}---\left(5\right)$

Q in the formula (u, v, t)=β L _EffI (u, v, t), L _Eff=(1-e ^{-α L})/α is an effective length, and L is the thickness of nonlinear medium, and i is an imaginary unit; I (u, v are the intensity of light beam in sample t), are proportional to | S (u, v, t) | ², the complex amplitude response definition is:

Wherein is nonlinear phase shift, and its expression formula is:

At this moment, if α and β are 0, then equation (7) is reduced to

Formula (5) is reduced to

At the exit facet of the non-linear imaging system of 4f phase coherence, be as intensity:

I _im(x,y,t)=|U(x,y,t)| ²=|FT ^-1[S(u,v,t)T(u,v,t)H(u,v)]| ² (10)

In the formula, FT ^-1Be the inverse Fourier transform symbol, and H (u, v)=circ [(u ²+ v ²) ^1/2λ G/N _A] be the coherent optics transport function of aberrationless lens, circ (r) is a circular function, when r≤1, is 1, all the other situation are 0; N _ABe the numerical aperture of convex lens (6), G is the magnification of whole optical system,

Because the imageing sensor at place, picture plane only can distributions respond laser, obtains and can flow so need carry out time integral to light intensity:

$F(x,y)={\&Integral;}_{-\&infin;}^{+\&infin;}{I}_{\mathrm{im}}(x,y,t)\mathrm{dt}---\left(11\right)$

According to equation (1)-(11), can numerical simulation go out the ability distributions of the non-linear imaging system of 4f phase coherence, and then obtain the nonlinear refraction of nonlinear sample as the place, plane.

Advantage of the present invention is:

Numerical simulation result shows; Under the situation of identical incident intensity; In the nonlinear images of the positive and negative bar shaped phase diaphragm after the improvement, the mean intensity in phase object zone is almost equal in the nonlinear images of the mean intensity of the positive bar-shaped zone of phase delay pi/2 and bar shaped phase diaphragm.But in the nonlinear images of the phase diaphragm after improvement, the mean intensity of the negative bar-shaped zone of phase delay-pi/2 will weaken, so Δ T '>Δ T, the phase diaphragm after promptly improving makes the sensitivity of measuring system increase.

In the phase diaphragm after the improvement and do not require that the zone that phase delay is respectively pi/2 and-pi/2 must be strict symmetrical distribution.Numerical simulation proves that the variation of two-part relative size is very little to the sensitivity influence of system.Consider that the influence that the more little conversion diffraction of phase object brings is big more, it is proper choosing two parts phase object symmetry.

The raising of the sensitivity of the phase diaphragm after the improvement is different along with the variation of sample nonlinear phase shift.The variation of sensitivity is defined as Δ T '/Δ T; When the nonlinear phase shift that produces in the sample

, sensitivity can be improved.

Be negative situation for nonlinear phase shift, the increase of sensitivity is very obvious.

Description of drawings

Fig. 1 is a conventional phase diaphragm synoptic diagram, and phase object 16 is bar shaped among the figure, the light beam bit phase delay pi/2 of other part of optical beam ratio through phase object 16, and wherein the overall width of phase object is 2R _p, the diaphragm radius is R _a, the used major parameter of numerical simulation is the overall width of phase object and the ratio ρ=2R of diaphragm radius _p/ R _a

Fig. 2 is a phase diaphragm of the present invention; By negative bar shaped phase object 17 and half bar shaped phase object, the 18 common phase objects of forming a positive and negative bar shaped just; Wherein negative bar shaped phase object 17 produces phase delay-pi/2, and positive bar shaped phase object 18 produces the phase delay pi/2;

Fig. 3 is the structural representation of the non-linear imaging system of positive and negative bar shaped phase diaphragm 4f phase coherence of the present invention;

The nonlinear images of Fig. 4 for obtaining by the conventional strip phase diaphragm;

Fig. 5 is the sectional view of Fig. 4 along y=0;

The nonlinear images that Fig. 6 obtains for positive and negative bar shaped phase diaphragm;

Fig. 7 is the sectional view of Fig. 6 along y=0;

Fig. 8 is the Δ T of bar shaped phase diaphragm and the Δ T ' of positive and negative bar shaped phase diaphragm and the relation curve of nonlinear phase shift

, and the difference that the average energy in the nonlinear images that the bar shaped phase diaphragm produces outside the average energy of phase diaphragm position stream and the phase object flows is Δ T; And the nonlinear images that produces for positive and negative bar shaped phase diaphragm is that average energy stream and the bit phase delay of position of the positive bar shaped phase object of pi/2 is defined as Δ T ' for the difference of the mean intensity of the negative bar shaped phase object position of-pi/2 with bit phase delay; Therefrom find out in the scope of

; Δ T in positive nonlinear phase shift scope sensitivity greater than corresponding negative nonlinear phase shift; And Δ T ' is a centrosymmetric image about

; And the sensitivity of Δ T ' all is higher than Δ T in the scope of

, and is particularly evident for the increase of negative nonlinear phase shift sensitivity;

The hot spot two dimensional gray distribution design sketch of Fig. 9 when not having nonlinear sample in the carbon disulphide experiment;

Figure 10 is the linear distribution design sketch of hot spot in the carbon disulphide experiment;

Figure 11 is the nonlinear Distribution design sketch of hot spot in the carbon disulphide experiment.

Embodiment

Embodiment one: below in conjunction with Fig. 2 this embodiment is described, this embodiment is positive and negative bar shaped phase diaphragm, and the bit phase delay of the positive and negative bar shaped phase object at phase diaphragm center is respectively 2m π+pi/2 and 2n π-pi/2, and wherein m, n are integer.

Embodiment two: this embodiment is described below in conjunction with Fig. 2; This embodiment is further specifying embodiment one described positive and negative bar shaped phase diaphragm; The described positive and negative bar shaped phase diaphragm of this embodiment is the bar shaped phase object that forms through the plating transparent dielectric film through in a circular iris, the light beam bit phase delay ± pi/2 of other part of optical beam ratio through this phase object.

Embodiment three: below in conjunction with Fig. 3 this embodiment is described, this embodiment is with embodiment one difference, the non-linear imaging system of 4f phase coherence of positive and negative bar shaped phase diaphragm, and it comprises measuring system and energy frame of reference;

Wherein measuring system is made up of 1/2nd wave plates 1, polarizing prism 2, beam expander 3, positive and negative bar shaped phase diaphragm 4, beam splitter 5, convex lens 6, No. two convex lens 8, neutral attenuator 9 and imageing sensor 10;

/ 2nd wave plates 1, polarizing prism 2, beam expander 3, positive and negative bar shaped phase diaphragm 4, beam splitter 5, convex lens 6, nonlinear sample to be measured 7, No. two convex lens 8 and neutral attenuator 9 order successively are placed on the optical axis of incident light; Light beam after neutral attenuator 9 decay forms hot spot 1 on the photosurface of imageing sensor 10; And the convex lens that convex lens 6 and No. two convex lens 8 are coaxial confocals, nonlinear sample 7 to be measured is positioned on the focus of convex lens 6 and No. two convex lens 8;

The energy frame of reference is made up of the neutral attenuator of catoptron 11, No. two 12, No. three convex lens 13, No. two catoptrons 14 and No. three catoptrons 15;

The neutral attenuator of catoptron 11, No. two 12, No. three convex lens 13, No. two catoptrons 14 and No. three catoptrons 15 order are successively placed; Catoptron 11 reflexes to neutral attenuator 12 No. two with beam splitter 5 beam reflected; Light beam after No. two neutral attenuator 12 decay is incident to convex lens 13 No. three; Light beam through No. three convex lens 13 projections is incident to catoptron 14 No. two; On the photosurface that is incident to imageing sensor 10 after No. three catoptron 15 reflections, form hot spot 2 through No. two catoptron 14 beam reflected, said hot spot 1 is not overlapping with hot spot 2.

Embodiment four: below in conjunction with Fig. 3 this embodiment is described, this embodiment is that the imageing sensor in the embodiment three 10 is further remarked additionally, and imageing sensor 10 is to adopt the CCD camera to realize.

Embodiment five: this embodiment is described below in conjunction with Fig. 3; This embodiment is further specifying position relation between the positive and negative bar shaped phase diaphragm in the embodiment three 4, convex lens 6 and the nonlinear sample to be measured 7; Distance between the described positive and negative bar shaped phase diaphragm 4 of this embodiment and the convex lens 6 equates with distance between convex lens 6 and the nonlinear sample to be measured 7, is f ₁, f ₁It is the focal length of convex lens 6.

Embodiment six: below in conjunction with Fig. 3 this embodiment is described, this embodiment is to embodiment five mid-focal length f ₁Further specify the described f of this embodiment ₁Representative value be 30cm.

Embodiment seven: this embodiment is described below in conjunction with Fig. 3; This embodiment is further specifying position relation between the nonlinear sample to be measured in the embodiment three 7, No. two convex lens 8 and the imageing sensor 10; Distance between the described nonlinear sample 7 to be measured of this embodiment and No. two convex lens 8 and No. two convex lens 8 equate with distance between the imageing sensor 10, are f ₂, f ₂It is the focal length of No. two convex lens 8.

Embodiment eight: below in conjunction with Fig. 3 this embodiment is described, this embodiment is to the focal distance f in the embodiment seven ₂Further specify the described f of this embodiment ₂Representative value be 20cm.

Embodiment nine: below in conjunction with Fig. 1 to Figure 11 this embodiment is described, this embodiment be positive and negative bar shaped phase diaphragm the non-linear imaging system of 4f phase coherence to the nonlinear refractive index measuring method, its performing step is following:

Step 1, under the situation of not placing nonlinear sample 7 to be measured, laser pulse of light emitted is gathered a pulse diagram picture through imageing sensor 10 then;

On step 2, the focus with nonlinear sample 7 convex lens 6 of placement to be measured and No. two convex lens 8; Simultaneously neutral attenuator is placed between beam splitter 5 and the convex lens 6; Make the light intensity that shines on the sample be reduced to the range of linearity; Gather a pulse diagram picture with the CCD camera, this image is called linear image, demonstrates linear beam spot;

Step 3, nonlinear sample to be measured 7 are placed on the focus of convex lens 6 and No. two convex lens 8; No. one in the step 2 neutral attenuator 9 is moved on between No. two convex lens 8 and the imageing sensor 10; Gather a pulse diagram picture with the CCD camera; This image is called nonlinear images, demonstrates non-linear hot spot;

Step 4, nonlinear sample 7 to be measured is taken away; With letting laser facula all get on the position on the energy meter probe between convex lens 6 of energy meter placement and No. two convex lens 8; Laser pulse of light emitted is incident to the non-linear imaging system of 4f phase coherence; Measure this pulse with energy meter and get to the energy on the energy meter probe, gather the reference hot spot of reference path with imageing sensor 10 simultaneously;

Step 5, with linear beam spot as input, obtain the value of nonlinear refractive index through the non-linear hot spot of numerical fitting; Concrete grammar is following:

The monochromatic plane wave illumination of linear polarization is on positive and negative bar shaped phase diaphragm, and light field is E (t), the transmitance of bar shaped phase diaphragm be t (x, y), then the light field on surface is behind the bar shaped phase diaphragm: O (x, y, t)=E (t) t (x, y) (1)

Nonlinear sample front surface light field is:

$S(u,v,t)=\frac{1}{{\mathrm{\&lambda;<figure-callout\; id="1"\; label="f"\; filenames="HDA00001804336800021.png,HDA00001804336800022.png"\; state="\{\{state\}\}">f</figure-callout>}}_1}\mathrm{FT}\left[O(x,y,t)\right]=\frac{1}{{\mathrm{\&lambda;<figure-callout\; id="1"\; label="f"\; filenames="HDA00001804336800021.png,HDA00001804336800022.png"\; state="\{\{state\}\}">f</figure-callout>}}_1}\&Integral;\&Integral;O(x,y,t)\exp [-2\mathrm{\&pi;i}(\mathrm{ux}+\mathrm{vy})]\mathrm{dxdy}---\left(2\right)$

FT represents Fourier transform in the following formula; U and v are respectively the spatial frequency of non-linear imaging system Fourier plane x of place of 4f phase coherence and y direction: u=x '/λ f ₁, v=y '/λ f ₁f ₁Focal length for incident lens in the 4f imaging system; λ is an optical maser wavelength, and i is an imaginary unit;

For three rank optical nonlinearities, approximate at thin sample with to become envelope slowly down approximate, the light intensity of pulse laser and phase change communication satisfaction in nonlinear medium:

$\frac{\mathrm{dI}}{{\mathrm{dz}}^{\&prime;}}=-(\mathrm{\&alpha;}+\mathrm{\&beta;I})I---\left(3\right)$

$\frac{\mathrm{d\&Delta;\&phi;}}{{\mathrm{dz}}^{\&prime;}}={\mathrm{kn}}_{2}I---\left(4\right)$

Wherein, α and β are respectively linearity and non-linear absorption coefficient, and I is the light intensity in the sample, n ₂Be nonlinear refractive index, z ' is the propagation distance of light beam in sample, and k=2 π/λ is the wave vector size, and Δ φ is a nonlinear phase shift, and the compound light field on surface is behind nonlinear medium:

$S_L(u,v,t)=S(u,v,t)e^{-\mathrm{\&alpha;L}/2}{[1+q(u,v,t)]}^{{\mathrm{<figure-callout\; id="2"\; label="ikn"\; filenames="HDA00001804336800013.png,HDA00001804336800021.png"\; state="\{\{state\}\}">ikn</figure-callout>}}_2/\mathrm{\&beta;}-1/2}---\left(5\right)$

Q in the formula (u, v, t)=β L _EffI (u, v, t), L _Eff=(1-e ^{-α L})/α is an effective length, and L is the thickness of nonlinear medium, and i is an imaginary unit; I (u, v are the intensity of light beam in sample t), are proportional to | S (u, v, t) | ², the complex amplitude response definition is:

Wherein

is nonlinear phase shift, and its expression formula is:

At this moment, if α and β are 0, then equation (7) is reduced to

Formula (5) is reduced to

At the exit facet of the non-linear imaging system of 4f phase coherence, be as intensity:

I _im(x,y,t)=|U(x,y,t)| ²=|FT ^-1[S(u,v,t)T(u,v,t)H(u,v)]| ² (10)

In the formula, FT ^-1Be the inverse Fourier transform symbol, and H (u, v)=circ [(u ²+ v ²) ^1/2λ G/N _A] be the coherent optics transport function of aberrationless lens, circ (r) is a circular function, when r≤1, is 1, all the other situation are 0; N _ABe the numerical aperture of convex lens (6), G is the magnification of whole optical system,

Because the imageing sensor at place, picture plane only can distributions respond laser, obtains and can flow so need carry out time integral to light intensity:

$F(x,y)={\&Integral;}_{-\&infin;}^{+\&infin;}{I}_{\mathrm{im}}(x,y,t)\mathrm{dt}---\left(11\right)$

According to equation (1)-(11), can numerical simulation go out the ability distributions of the non-linear imaging system of 4f phase coherence, and then obtain the nonlinear refraction of nonlinear sample 7 as the place, plane.

Specific embodiment:

/ 2nd wave plates 1 are used in combination with polarizing prism 2 can be at the basic adjusted incident pulse energy that guarantees linear polarization state and polarization direction thereof.Still be Gaussian in time through beam expander 3 and the light that shines phase diaphragm 4, but spatially be top-hat light.Convex lens 6 and No. two convex lens 8 coaxial confocals constitute the 4f system.Positive and negative bar shaped phase diaphragm 4 is placed on the object plane of the non-linear imaging system of 4f phase coherence of positive and negative bar shaped phase diaphragm; Nonlinear sample 7 to be measured is on the Fourier plane, and CCD camera received pulse image on the picture plane of the non-linear imaging system of the 4f of positive and negative bar shaped phase diaphragm phase coherence.The Fourier transform of light beam planoconvex lens 6 converges on the testing sample 7 that is placed on the Fourier plane, because the nonlinear refraction character of testing sample makes the phase place of pulse of incident change.The pulse of surperficial outgoing is received by CCD camera 10 through the inverse Fourier transform of convex lens 8 behind the sample, is called main spot.

The energy frame of reference is made up of beam splitter 5, catoptron 11, neutral attenuator 12, convex lens 13, catoptron 14 and 15.Receive by CCD camera 10 at last, be called with reference to hot spot.

The measurement branch following steps of utilizing the non-linear imaging system of positive and negative bar shaped phase diaphragm 4f phase coherence to carry out nonlinear refractive index are carried out:

Step 1, take testing sample 7 away, gather a pulse diagram picture, be called image without image with the CCD camera.

Step 2, testing sample 7 is placed on the Fourier plane; Neutral attenuator 9 is placed between beam splitter 5 and the convex lens 6; Make the light intensity that shines on the testing sample 7 be reduced to the range of linearity, gather a pulse diagram picture, be called linear image with CCD camera 10.

Step 3, testing sample 7 is placed on the Fourier plane, the neutral attenuator 9 that uses when before gathering linear image moves on between No. two convex lens 8 and the imageing sensor 10, gathers a pulse diagram picture with CCD camera 10, is called nonlinear images.

Step 4, energy calibration are that nonlinear sample 7 is taken away, and a certain position that energy meter is placed between convex lens 6 and 8 makes laser facula can all converge on the light-sensitive surface of energy meter probe.Launch a laser pulse, measure the energy of pulse, gather the reference hot spot of reference path with CCD camera 10 simultaneously with energy meter.Because all devices all are linear units in the light path at this moment, so according to the size that just can know the incident pulse energy with reference to the power of hot spot.The energy that incides the pulse on the testing sample 7 in the nonlinear measurement process just can calculate through the reference hot spot that same laser pulse produces like this.

The used major parameter of numerical simulation is the overall width of phase object and the ρ=2R of diaphragm radius _p/ R _a=0.4mm/1.45mm=0.28, the testing sample nonlinear phase shift

The difference of the average energy stream of phase diaphragm position and the average energy stream outside the phase object is Δ T in the nonlinear images that definition bar shaped phase diaphragm produces.And the nonlinear images that produces for positive and negative bar shaped phase diaphragm is that average energy stream and the bit phase delay of position of the positive bar shaped phase object of pi/2 is defined as Δ T ' for the difference of the mean intensity of the negative bar shaped phase object position of-pi/2 with bit phase delay.Can see that from accompanying drawing 5 and accompanying drawing 7 bit phase delay in Δ T and the accompanying drawing 7 in the accompanying drawing 5 is that the average energy stream of position of the positive bar shaped phase object of pi/2 is more or less the same with the difference that average energy outside the phase object flows; Thereby Δ T '>Δ T, the phase diaphragm after promptly improving is improved the measurement sensitivity of system.

Fig. 8 is Δ T and the Δ T' of positive and negative bar shaped phase diaphragm and the relation curve of nonlinear phase shift

of bar shaped phase diaphragm.Therefrom find out in the scope of

, Δ T in positive nonlinear phase shift scope sensitivity greater than corresponding negative nonlinear phase shift.And Δ T' is a centrosymmetric image about

; And the sensitivity of Δ T ' all is higher than Δ T in the scope of

, and is particularly evident for the increase of negative nonlinear phase shift sensitivity.

In order to verify the validity of this system and method, we handle standard nonlinear kerr medium carbon disulphide.Fig. 9 is the hot spot two dimensional gray distribution plan when not having nonlinear sample in the carbon disulphide experiment; Figure 10 is the linear distribution figure of hot spot in the carbon disulphide experiment; Figure 11 is the nonlinear Distribution figure of hot spot in the carbon disulphide experiment.Corresponding experiment parameter is: projectile energy E _i=0.52 μ J, the phase shift of positive and negative bar shaped phase object

Thickness of sample L=2mm, convex lens 6 and 8 focal distance f ₁=30cm, f ₂=20cm, the refractive index that obtains at last according to fit procedure is n ₂=3.4 * 10 ^-18m ²/ W, the generally acknowledged value n under fitting result and the carbon disulphide picopulse 532nm excitation wavelength ₂=3.2 * 10 ^-18m ²/ W is very approaching.Here the main source of error is that linear beam spot and the not same laser pulse of non-linear hot spot excite; And the space distribution of different pulses and incomplete same (this error can be eliminated by the two 4f systems of parallel connection) have shown the feasibility and the reliability of the non-linear imaging system of 4f phase coherence of this positive and negative bar shaped phase diaphragm.

The present invention is not limited to above-mentioned embodiment, can also be the reasonable combination of technical characterictic described in above-mentioned each embodiment.

Claims (9)

1. positive and negative bar shaped phase diaphragm is characterized in that, the bit phase delay of the positive and negative bar shaped phase object at phase diaphragm center is respectively 2m π+pi/2 and 2n π-pi/2, and wherein m, n are integer.

2. positive and negative bar shaped phase diaphragm according to claim 1; It is characterized in that; It is the bar shaped phase object that forms through the plating transparent dielectric film through in a circular iris, the light beam bit phase delay ± pi/2 of other part of optical beam ratio through this phase object.

3. based on the non-linear imaging system of 4f phase coherence of the described positive and negative bar shaped phase diaphragm of claim 1, it is characterized in that it comprises measuring system and energy frame of reference;

Wherein measuring system is made up of 1/2nd wave plates (1), polarizing prism (2), beam expander (3), positive and negative bar shaped phase diaphragm (4), beam splitter (5), convex lens (6), No. two convex lens (8), a neutral attenuator (9) and imageing sensor (10);

/ 2nd wave plates (1), polarizing prism (2), beam expander (3), positive and negative bar shaped phase diaphragm (4), beam splitter (5), convex lens (6), nonlinear sample to be measured (7), No. two convex lens (8) and a neutral attenuator (9) order successively are placed on the optical axis of incident light; Light beam after a neutral attenuator (9) decay forms hot spot 1 on the photosurface of imageing sensor (10); And convex lens (6) and No. two convex lens (8) are the convex lens of coaxial confocal, and nonlinear sample to be measured (7) is positioned on the focus of convex lens (6) and No. two convex lens (8);

The energy frame of reference is made up of a catoptron (11), No. two neutral attenuators (12), No. three convex lens (13), No. two catoptrons (14) and No. three catoptrons (15);

Catoptron (11), No. two neutral attenuators (12), No. three convex lens (13), No. two catoptrons (14) and No. three catoptrons (15) order are successively placed; Catoptron (11) reflexes to No. two neutral attenuators (12) with beam splitter (5) beam reflected; Light beam after No. two neutral attenuators (12) decay is incident to No. three convex lens (13); Light beam through No. three convex lens (13) projection is incident to No. two catoptrons (14); On the photosurface that is incident to imageing sensor (10) after No. three catoptrons (15) reflection, form hot spot 2 through No. two catoptrons (14) beam reflected, said hot spot 1 is not overlapping with hot spot 2.

4. the non-linear imaging system of 4f phase coherence of the positive and negative bar shaped phase diaphragm of employing according to claim 3 is characterized in that, imageing sensor (10) is to adopt the CCD camera to realize.

5. the non-linear imaging system of 4f phase coherence according to claim 3; It is characterized in that; Distance between positive and negative bar shaped phase diaphragm (4) and the convex lens (6) equates with distance between convex lens (6) and the nonlinear sample to be measured (7), is f ₁, f ₁It is the focal length of convex lens (6).

6. the non-linear imaging system of 4f phase coherence according to claim 5 is characterized in that f ₁Representative value be 30cm.

7. the non-linear imaging system of 4f phase coherence according to claim 3 is characterized in that, the distance between nonlinear sample to be measured (7) and No. two convex lens (8) and No. two convex lens (8) equate with distance between the imageing sensor (10), are f ₂, f ₂It is the focal length of No. two convex lens (8).

8. the non-linear imaging system of 4f phase coherence according to claim 7 is characterized in that f ₂Representative value be 20cm.

Based on the non-linear imaging system of 4f phase coherence of the described positive and negative bar shaped phase diaphragm of claim 3 to the nonlinear refractive index measuring method, it is characterized in that its performing step is following:

Step 1, under the situation of not placing nonlinear sample to be measured (7), laser pulse of light emitted is gathered a pulse diagram picture through imageing sensor (10) then;

Step 2, nonlinear sample to be measured (7) is placed between convex lens (6) and No. two convex lens (8); And be positioned on the focus of No. two convex lens (8); Simultaneously a neutral attenuator (9) is placed between beam splitter (5) and the convex lens (6), makes the light intensity that shines on the nonlinear sample (7) be reduced to the range of linearity, gather a pulse diagram picture with the CCD camera; This image is called linear image, demonstrates linear beam spot;

Step 3, a neutral attenuator (9) is moved on between No. two convex lens (8) and the imageing sensor (10), gather a pulse diagram picture with the CCD camera, this image is called nonlinear images, demonstrates non-linear hot spot;

Step 4, nonlinear sample to be measured (7) is taken away; With letting laser facula all get on the position on the energy meter probe between energy meter placement convex lens (6) and No. two convex lens (8); Laser pulse of light emitted is incident to the non-linear imaging system of 4f phase coherence; Measure this pulse with energy meter and get to the energy on the energy meter probe, use imageing sensor (10) to gather the reference hot spot of reference path simultaneously;

Step 5, with linear beam spot as input, obtain the value of nonlinear refractive index through the non-linear hot spot of numerical fitting; Concrete grammar is following:

The monochromatic plane wave illumination of linear polarization is on positive and negative bar shaped phase diaphragm, and light field is E (t), the transmitance of bar shaped phase diaphragm be t (x, y),

Then the light field on surface is behind the bar shaped phase diaphragm: and O (x, y, t)=E (t) t (x, y) (1)

Nonlinear sample front surface light field is:

$S(u,v,t)=\frac{1}{{\mathrm{\&lambda;f}}_1}\mathrm{FT}\left[O(x,y,t)\right]=\frac{1}{{\mathrm{\&lambda;f}}_1}\&Integral;\&Integral;O(x,y,t)\exp [-2\mathrm{\&pi;i}(\mathrm{ux}+\mathrm{vy})]\mathrm{dxdy}---\left(2\right)$

FT represents Fourier transform in the following formula; U and v are respectively the spatial frequency of non-linear imaging system Fourier plane x of place of 4f phase coherence and y direction: u=x '/λ f ₁, v=y '/λ f ₁f ₁Focal length for incident lens in the 4f imaging system; λ is an optical maser wavelength, and i is an imaginary unit;

For three rank optical nonlinearities, approximate at thin sample with to become envelope slowly down approximate, the light intensity of pulse laser and phase change communication satisfaction in nonlinear medium:

$\frac{\mathrm{dI}}{{\mathrm{dz}}^{\&prime;}}=-(\mathrm{\&alpha;}+\mathrm{\&beta;I})I---\left(3\right)$

$\frac{\mathrm{d\&Delta;\&phi;}}{{\mathrm{dz}}^{\&prime;}}{=\mathrm{kn}}_{2}I---\left(4\right)$

Wherein, α and β are respectively linearity and non-linear absorption coefficient, and I is the light intensity in the sample, n ₂Be nonlinear refractive index, z ' is the propagation distance of light beam in sample, and k=2 π/λ is the wave vector size, and Δ φ is a nonlinear phase shift, and the compound light field on surface is behind nonlinear medium:

$S_L(u,v,t)=S(u,v,t)e^{-\mathrm{\&alpha;L}/2}{[1+q(u,v,t)]}^{{\mathrm{ikn}}_2/\mathrm{\&beta;}-1/2}---\left(5\right)$

Q in the formula (u, v, t)=β L _EffI (u, v, t), L _Eff=(1-e ^{-α L})/α is an effective length, and L is the thickness of nonlinear medium, and i is an imaginary unit; I (u, v are the intensity of light beam in sample t), are proportional to | S (u, v, t) | ², the complex amplitude response definition is:

Wherein

is nonlinear phase shift, and its expression formula is:

At this moment, if α and β are 0, then formula (7) is reduced to

Formula (5) is reduced to

At the exit facet of the non-linear imaging system of 4f phase coherence, be as intensity:

I _im(x,y,t)=|U(x,y,t)| ²=|FT ^-1[S(u,v,t)T(u,v,t)H(u,v)]| ² (10)

In the formula, FT ^-1Be the inverse Fourier transform symbol, and H (u, v)=circ [(u ²+ v ²) ^1/2λ G/N _A] be the coherent optics transport function of aberrationless lens, circ (r) is a circular function, when r≤1, is 1, all the other situation are 0; N _ABe the numerical aperture of convex lens (6), G is the magnification of whole optical system;

Because the imageing sensor at place, picture plane only can distributions respond laser, obtains and can flow so need carry out time integral to light intensity:

$F(x,y)={\&Integral;}_{-\&infin;}^{+\&infin;}{I}_{\mathrm{im}}(x,y,t)\mathrm{dt}---\left(11\right)$

According to equation (1)-(11), can numerical simulation go out the ability distributions of the non-linear imaging system of 4f phase coherence, and then obtain the nonlinear refraction of nonlinear sample (7) as the place, plane.

Images