CN102692382A - High-sensitivity frequency domain filtering baffle plate Z-scan method for measuring material nonlinearity - Google Patents

Abstract

The invention discloses a high-sensitivity frequency domain filtering baffle plate Z-scan method for measuring the material nonlinearity, and belongs to the fields of nonlinear photonic material and nonlinear optical information processing. The method of the invention comprises: the focus of a lens forms an image through another lens, and a filtering pinhole is arranged at the point of the image; and a circular diaphragm forms an image through a lens set formed by the above two lenses, and a coaxial circular baffle plate is arranged in the image plane. During the action of pulse laser, a sample moves from side to side near the focus of the lens. Through measuring the non-linear transmittance of an opening, the baffle plate, and the filtering pinhole, the non-linear absorption coefficient and the non-linear refraction coefficient of the material can be determined. The method of the invention is characterized by simple work measurement system optical path, obvious stray light influence filtering effect, high measuring sensitivity, and simple data processing, and can measure non-linear absorption and non-linear refraction at the same time.

Description

A kind of method of high sensitivity frequency domain filtering baffle plate Z scanning survey material nonlinearity

Technical field

Involved in the present invention is a kind of optical non-linear method of measuring material, belongs to non-linear photon and learns material and nonlinear optics field of information processing.

Background technology

Along with the develop rapidly of art such as optical communication and optical information processing, the research of nonlinear optical material becomes more and more important.The realization of functions such as optical logic, optics memory, optical transistor, photoswitch and phase place complex conjugate mainly depends on the progress of nonlinear optical material.Measuring material optical nonlinearity method has a lot, like non-linear interferometric method, and degeneration four-wave mixing, nearly degeneracy three wave mixings, elliptic polarization method, beam aberration method etc.Z scan method (Mansoor Sheik-Bahae, Ali A.Said, Tai-Hui Wei wherein; David J.Hagan, E.W.Van Stryland. " Sensitive measurement of optical nonlinearities using a single beam ", IEEE J.Quantum Elect; 26; 760-769 (1990)) light path is simple, highly sensitive, is present the most frequently used measurement material optical non-linear method, PO Z-scan technology (Junyi Yang and Yinglin Song; " Direct observation of the transient thermal-lensing effect using the phase-object Z-scan technique "; Optics Letters, 34:157-159 (2009)) is exactly on the basis of traditional Z-scan, adds a phase object in the position of the front focal plane of lens.Compare with traditional Z-scan, the result of measured nonlinear refraction coefficient of materials has become unimodal or single paddy characteristic curve by the peak valley characteristic curve of traditional Z-scan.Flat top beam Z scans (zhao w.Palffy-Muhoray P., " z-scan technique using top hat beams ", Appl Phys lett; 63:1613-1615 (1993)), the sensitivity of z scanning survey is improved 2.5 times, solar eclipse method Z scans (T.Xia; D.J.Hagan; M.sheik-Bahae, andE.W.Van Stryland, " eclipsing z-sdan measurement of λ/10 ^-4Wave-front distortion, Optics letters, 19:317-319 (1994)) utilize disk to replace the far field aperture of common Z scanister, and, can measure about λ/10 with the lens collection edge light beam entering detector that is placed on its back ^-4Wavefront distortion, but since the edge light intensity a little less than, edge noise has reduced the measurement signal to noise ratio (S/N ratio), further improves thereby limited sensitivity.

Summary of the invention

The method that the purpose of this invention is to provide a kind of high sensitivity frequency domain filtering baffle plate z scanning survey material nonlinearity through the frequency domain filtering technology, can reduce system noise greatly, measures two magnitudes of remolding sensitivity common z scanning raising thereby make.

For achieving the above object, the technical scheme that the present invention adopts is:

A kind of method of high sensitivity frequency domain filtering baffle plate Z scanning survey material nonlinearity; After comprising the steps: beam of laser carried out beam-expanding collimation, be incident to a circular iris, the light beam through diaphragm is divided into two bundles by first semi-transparent semi-reflecting lens: wherein a branch ofly be monitoring light; Planoconvex lens is assembled the back by the first detector record; Another bundle to testing sample, makes testing sample produce optical nonlinearity through first convex lens focus for surveying light; Said testing sample can pass through near the displacement platform move left and right focal plane of first convex lens; Be divided into two bundles through said testing sample modulated laser pulse by second semi-transparent semi-reflecting lens: wherein a branch ofly assemble the back through second convex lens and received (perforate) by second detector, another bundle is assembled back entering the 3rd detector (closed pore) through the 3rd convex lens; What after the lens combination that said circular iris is formed via first convex lens and the 3rd convex lens, form images places a coaxial circular baffle plate as the planimetric position, places a coaxial filtering aperture at said first concave lens focus through the 3rd convex lens imaging image point position; Concrete measuring process is:

(1) placing testing sample away from the first concave lens focus position; In pulsed laser action simultaneously; Make testing sample by direction or by direction motion to the left to the right on the focus right side in the focus of first convex lens left side, in the record sample motion process diverse location by second detector, the 3rd energy that detector is surveyed respectively with first energy ratio that detector is surveyed;

(2) the ratio curve that obtains in the step (1) is handled, the optical nonlinearity that obtains required test material absorbs and the nonlinear refraction coefficient.

In the technique scheme; Correlative value curve in the said step (2) is handled and is comprised: initial value in the ratio ordered series of numbers that obtains in the step (1) and this ordered series of numbers is divided by; Obtain the normalized non-linear transmittance curve of sample, normalized non-linear transmittance curve is carried out theoretical fitting obtain non-linear absorption and nonlinear refraction coefficient.

In the technique scheme, the transmitance after said circular iris and the circular baffle plate combination is less than 0.1.

In the technical scheme of the present invention, after nonlinear sample received the effect of pulsed light, the absorption and the refractive properties of material changed, and produced optical nonlinearity.Under the approximate condition of thin sample; Energy is only relevant with non-linear absorption, and nonlinear refraction can be ignored to the influence of energy, because the perforate measurement is the variation of whole energy; Irrelevant with nonlinear refraction, so the transmitance of perforate is relevant with the non-linear absorption of material.On the other hand, the nonlinear phase shift of sample generation changes with the intensity variations of laser.Like this, near the nonlinear phase shift that sample is equivalent to add a variation to the light pulse modulation focal plane.Additional nonlinear phase shift will cause that the light distribution of far field construction hot spot changes, thereby will cause the variation of the transmitance of baffle plate.The variation of far field construction hot spot edge energy is desirable bigger than the variation that main spot sees through the aperture energy, therefore replaces aperture in the far field with baffle plate, and the sensitivity of gained normalized nonlinear transmitance is higher than seeing through aperture normalized nonlinear transmitance.Since edge diffraction light a little less than, receive ground unrest bigger in the experiment, the convergence of rays through the 3rd convex lens among the present invention is in the filtering aperture position; After energy probe places the filtering aperture, be that the light through baffle plate is for assembling light wave with solar eclipse method difference; Need not to re-use lens and collect hot spot, greatly reduce ground unrest, place sample near other first concave lens focus; Can't realize frequency domain filtering; And the filtering aperture is placed in frequency plane imaging back through the 3rd convex lens, and be equivalent to carry out LPF at frequency domain, can effectively eliminate ground unrest influences measurement result.

Produce nonlinear phase shift in the first convex lens focal plane position in the sample moving process and reach maximal value; Normalized non-linear transmitance is also maximum; So; If sample is placed on focal plane position, under a monopulse light action, also can measuring samples non-linear absorption coefficient and nonlinear refraction coefficient through measuring whole energy changing.

The inventive method has realized the measurement to optical nonlinearity with a kind of new way, compares with other nonlinear optics measuring techniques, has the following advantages:

1, the present invention is optimized traditional z scanning, and sample is belonged to the frequency plane imaging and the filtering aperture is set, and improves system's anti-noise ability greatly.

2, incident circular iris of the present invention and baffle combination reduce linear background greatly, will measure sensitivity and improve 2 magnitudes.

3, the present invention can realize single-pulse measurement, has solved testing sample easy damaged problem.

4, adopt method of the present invention, simultaneously the size of measuring samples non-linear absorption and nonlinear refraction.

5, measuring method of the present invention; Can be widely used in nonlinear optics measurement, research fields such as non-linear photon material, nonlinear optics information processing and photonics device; Especially key links such as the test of nonlinear optical functional material and modification are utilized the inventive method, can greatly reduce and measure cost (need not mobile platform and CCD); And can guarantee that test parameter is comprehensive, test result is accurate.

Description of drawings

Fig. 1 is the fundamental diagram of high sensitivity frequency domain filtering baffle plate z scanning survey material nonlinearity absorption of the present invention and nonlinear refraction coefficient method.

Fig. 2 is the device synoptic diagram of the inventive method.Wherein: 1, incoming laser beam; 2, expand the bundle convex lens; 3, collimation convex lens; 4, circular iris; 5, semi-transparent semi-reflecting beam splitter; 6, convex lens; 7, first detector; 8, convex lens; 9, testing sample; 10, one dimension translate stage; 11, convex lens; 12, second detector; 13, semi-transparent semi-reflecting beam splitter; 14, convex lens; 15, circular baffle plate; 16, filtering aperture; 17, the 3rd detector.

Fig. 3 is the circular iris synoptic diagram in the embodiment of the invention.

Fig. 4 is the circular baffle plate synoptic diagram in the embodiment of the invention.

Fig. 5 is the filtering aperture synoptic diagram in the embodiment of the invention.

Embodiment

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

Referring to shown in Figure 1, a kind of method of high sensitivity frequency domain filtering baffle plate Z scanning survey material nonlinearity, compositions such as optical routing beam splitter, convex lens, aperture, light tight baffle plate, energy-probe; Pulse laser focusing is on testing sample 9.

Accompanying drawing 2 is Experimental equipment of the method for a kind of high sensitivity frequency domain filtering of the present invention baffle plate Z scanning survey material nonlinearity.Experimental provision can be divided into beam-expanding system, measuring system and frame of reference three parts.Beam-expanding system is formed with collimation convex lens 3 by expanding bundle convex lens 2; Measuring system is made up of circular iris 4, convex lens 8, sample one dimension translate stage 10, semi-transparent semi-reflecting beam splitter 13, convex lens 14, circular baffle plate 15, filtering aperture 16, second detector 12, the 3rd detector 17 and convex lens 11.Wherein, Testing sample 9 is placed on the one dimension translate stage 10; At first expand bundle from the pulse laser of laser emitting through beam-expanding system; Laser behind the expansion bundle forms the flat-top light through circular iris 4 backs, and light beam planoconvex lens 8 converges to and is placed near the sample of its focus, changed by the light intensity and the phase place of testing sample 9 optical nonlinearities modulation afterpulse laser; Be divided into two bundles from the pulse laser of sample transmission through semi-transparent semi-reflecting beam splitter 13, a branch of planoconvex lens 11 is assembled the back and is received by second detector 12; Another Shu Jiguang planoconvex lens 14, circular baffle plate 15 and filtering aperture 16 backs are received by the 3rd detector 17.

Frame of reference is made up of semi-transparent semi-reflecting beam splitter 5, convex lens 6, first detector 7.The laser that comes out from circular iris 4 is divided into two bundles by beam splitter, and wherein a branch of planoconvex lens 6 is assembled the back and received by first detector 7, is reference light.

Fig. 2 is the synoptic diagram of circular iris 4.

Fig. 3 is the synoptic diagram of circular baffle plate 15, and baffle plate is light tight.

Fig. 4 is the synoptic diagram of circular filter aperture 16.

In the present embodiment, laser beam is Nd:YAG laser instrument (EKSPLA, PL2143B) the later 532nm laser of frequency multiplication, pulsewidth 21ps.Model for two detectors of (Rjp-765 energy probe) be connected energy meter (Rj-7620 ENERGY RATIOMETER, Laserprobe).Testing sample is carbon disulphide (CS ₂).

Concrete detection step is: (1) regulates that convex lens 2, convex lens 3, convex lens 8, convex lens 14, circular iris 4, circular baffle plate 15, filtering aperture 16 are coaxial, and sample is positioned on the one dimension translate stage 10, and the position is near convex lens 8 focuses.(2) sample in the pulsed light effect simultaneously; Focus opposite side to convex lens 8 moves; First detector 7, second detector 12, synchronous recordings are through beam splitter 5, beam splitter 13 and filtering aperture 16 energy sequence numerical value respectively for the 3rd detector 17, and its SF should be consistent with the pulsed laser pulse repetition rate.(3) second detector 12 is recorded energy sequence and record energy series, be the absorption of sample curve, carry out normalization with this ordered series of numbers numerical value divided by first element of this sequence again and handle, draw the sample non-linear absorption coefficient divided by first detector 7.(4) the 3rd detector 17 being recorded energy sequence and record energy series divided by first detector 7, is non-linear absorption, refraction curve after the normalization.(5) utilize the middle non-linear absorption in (3) gained non-linear absorption coefficient and (4), refraction curve calculation nonlinear refraction coefficient.

For CS ₂The experiment of nonlinear measurement and Theoretical Calculation detailed process are following:

Suppose that incident beam is the basic mode Gauss light, its field intensity expression formula is:

$E(r,t)=E_0\exp [-\frac{r^2}{{{\mathrm{\ω}}_e}^2}]\exp [-\frac{t^2}{2{\mathrm{\τ}}^2}]---\left(1\right)$

In the formula, E ₀Be the maximum field strength value of pulse laser, r is the radius of light beam, ω _eBe the waist radius of incident beam, τ is the time of pulsed light 1/e half-breadth.

The transmitance of circular iris 4 is:

T (r)=1 (r≤Ra) or t (r)=0 (r＞Ra) (2)

In the formula, R _aRadius for circular iris 4.

Field strength distribution behind the circular iris 4 is:

E ₀₁(r，t)＝E(r，t)t(r) (3)

The light field that propagates into sample surfaces can obtain through twice fresnel diffraction formula, is made as E ₀₂, in sample, consider to become slowly the situation that amplitude is approximate and thin sample is approximate, the amplitude of pulse laser and phase change communication satisfaction in sample

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

$\frac{\mathrm{d\Δ\φ}}{{\mathrm{dz}}^{\′}}=k{n}_{2}I---\left(4\right)$

In the formula, n ₂Be the nonlinear refraction coefficient of sample, α ₀Be the linear absorption rate of sample, β is the non-linear absorption coefficient of sample, I=|E ₀₂| ²(z '=0 place) is for acting on the light intensity on the sample.The light path that z ' laser is propagated in sample.

Then the light field on surface is behind the sample:

$E_{03}(r_1,t)=E_{02}(r_1,t)e^{-{\mathrm{\α}}_{0}L/2}{(1+q)}^{(\mathrm{ik}{n}_2/\mathrm{\β}-1/2)}---\left(5\right)$

When not considering that sample is non-linear, then the light field on surface is behind the sample:

E′ ₀₃＝E ₀₂ (6)

The light field that propagates into circular baffle plate and filtering aperture from the back surface of sample can obtain through the fresnel diffraction formula, is made as E ₀₄If do not consider that sample is non-linear, be linearity and see through, then be E ₀₄

The light intensity at baffle plate place is carried out the integration of room and time, can obtain energy through baffle plate.With this energy with do not considering that the energy that passes through baffle plate that obtains under the nonlinear situation of sample compares, just obtain normalized nonlinear transmitance through circular aperture and baffle combination:

$T=\frac{{\∫}_{-\∞}^{+\∞}{\∫}_{r_d}^{+\∞}2\mathrm{\π}{r}_2{\left|E_{04}\right|}^2{\mathrm{dr}}_2\mathrm{dt}}{{\∫}_{-\∞}^{+\∞}{\∫}_{r_d}^{+\∞}2\mathrm{\π}{r}_2{\left|{E^{\′}}_{04}\right|}^2{\mathrm{dr}}_2\mathrm{dt}}---\left(9\right)$

If the radius of baffle plate is got 0 (promptly not having baffle plate), then can obtain the transmitance situation of perforate.Normalized nonlinear transmitance to perforate and closed pore is carried out match, just can obtain the non-linear absorption and the nonlinear refraction coefficient of sample.

In the present embodiment, projectile energy is 0.11 μ J, and circular iris 4 radiuses that produce flat-top light are 4.8mm, and its distance apart from convex lens 8 is 206mm; The focal length of convex lens 8 is 412mm; Convex lens 14 focal lengths are 305mm, and its length apart from convex lens 8 is 862mm; The radius R of circular baffle plate 15 _dBe 3.4mm, its length apart from convex lens 14 is 401mm; The diameter of filtering aperture 16 is 1mm, and its length apart from circular baffle plate 15 is 545mm.The normalized nonlinear transmitance that experiment records perforate approaches 1, the non-linear absorption that this sample is described very a little less than, can ignore.The normalized non-linear transmitance of baffle plate is 1200.Change sample nonlinear refraction coefficient n ₂, make the non-linear transmitance of baffle plate of Theoretical Calculation and matching that experiment records to get CS ₂Nonlinear refraction coefficient n ₂=1.9 * 10 ^-18m ²/ W and utilize the n of Z-scan gained ₂=3.2 * 10 ^-18m ²/ W is very approaching.

Claims (3)

1. the method for a high sensitivity frequency domain filtering baffle plate Z scanning survey material nonlinearity comprises the steps:

After beam of laser carried out beam-expanding collimation; Be incident to a circular iris (4); Light beam through diaphragm is divided into two bundles by first semi-transparent semi-reflecting lens (5): wherein a branch of planoconvex lens (6) is assembled the back by first detector (7) record in order to monitor light, and another bundle is detection light; Focus on the testing sample (9) through first convex lens (8), make testing sample (9) produce optical nonlinearity; Said testing sample (9) can pass through near the displacement platform move left and right focal plane of first convex lens (8); Be divided into two bundles through said testing sample (9) modulated laser pulse by second semi-transparent semi-reflecting lens (13): wherein a branch ofly assemble the back through second convex lens (11) and received by second detector (12), another bundle is assembled back entering the 3rd detector (17) through the 3rd convex lens (14); It is characterized in that: what the lens combination of forming via first convex lens (8) and the 3rd convex lens (14) at said circular iris (4) formed images places a coaxial circular baffle plate (15) as the planimetric position, places a coaxial filtering aperture (16) in said first convex lens (8) focus through the 3rd convex lens (14) imaging image point position; Concrete measuring process is:

(1) placing testing sample (9) away from first convex lens (8) focal position; In pulsed laser action simultaneously; Make testing sample (9) by direction or by direction motion to the left to the right on the focus right side in the focus of first convex lens (8) left side, in the record sample motion process diverse location by second detector (12), the 3rd detector (17) the energy of surveying respectively with first detector (7) energy ratio of surveying;

(2) the ratio curve that obtains in the step (1) is handled, the optical nonlinearity that obtains required test material absorbs and the nonlinear refraction coefficient.

2. the method for a kind of high sensitivity frequency domain filtering baffle plate Z scanning survey material nonlinearity according to claim 1; It is characterized in that: the correlative value curve in the said step (2) is handled and is comprised: initial value in the ratio ordered series of numbers that obtains in the step (1) and this ordered series of numbers is divided by; Obtain the normalized non-linear transmittance curve of sample, normalized non-linear transmittance curve is carried out theoretical fitting obtain non-linear absorption and nonlinear refraction coefficient.

3. the method for a kind of high sensitivity frequency domain filtering baffle plate Z scanning survey material nonlinearity according to claim 1 and 2 is characterized in that: the transmitance after said circular iris (4) and circular baffle plate (15) combination is less than 0.1.

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