CN100354616C - Chirp pulse longitudinal diffraction interferometer - Google Patents

Abstract

The present invention relates to a longitudinal diffraction interferometer of a chirp pulse. The present invention is characterized in that the present invention comprises a grating pair; in the output light path of the grating pair, a half-wave plate, a beam-splitting plate, a polarizing filter, a diffraction lens and an area array CCD (charge-coupled device) spectrograph are sequentially arranged; a focusing lens is arranged at one side of the beam-splitting plate and is in parallel to the output light path of the grating pair so that drive beams which are perpendicular to the focusing lens can be collinear with detection light beams which transmit through the beam-splitting plate through plasma channels generated through the reflecting beams reflected by the beam-splitting plate. The signal intensity of the present invention directly depends on the magnitude of phase shift, and thus, the measuring sensitivity is very high, and the present invention can measure a minute change process of phase variation with time.

Description

The vertical diffraction interferometer of chirped pulse

Technical field

The present invention relates to the femtosecond time resolution measuring instrument.The vertical diffraction interferometer of particularly a kind of chirped pulse.

Background technology

Ultra-short pulse laser with the matter interaction process in can produce many (subpicosecond levels) fast physical process as: laser with the interaction process of gas, cluster and solid in, the evolution of the evolution of electron density, shock wave, laser cause that the variation of the structure of matter is as causing generation of THZ or the like from phase modulation (PM), Kerr effect and laser in the ionization process of atom, the plasma.To these fast physical process carry out the target that time-resolved diagnostic measures is very interested problem of people and pursuit always, and most important concerning research laser and matter interaction mechanism.Because the develop rapidly of ultrashort laser technology at present, the power of laser is more and more higher, pulsewidth is shorter and shorter, laser with the matter interaction process in the yardstick that develops in time of a lot of physical processes fast shorten to the hundreds of femtosecond from traditional quick process (picosecond magnitude), tens femtoseconds even arrived Ah's second-time, traditional measurement means such as streak camera, framing cameras etc. can only be in psec, (＞1ps) physical process is carried out diagnostic measures to changing at a slow speed on the time scale of nanosecond, and measuring system is very complicated, and price is very expensive.And for the diagnosis of the ultrafast dynamic process of subpicosecond magnitude, people are seeking and are developing the diagnostic techniques of multiple femtosecond time resolution, as frequency discrimination optical shutter (FROG:Frequency Resolved OpticalGating), multi-pulse interference frequency discrimination optical shutter (MI-FROG:MultipulseInterferometric FROG), the spectral phase interferometer (SPIDER:Spectral Phase Interferometry for Direct Electric fieldReconstruction) of direct light field reconstruct, the Pump-Probe technology and the frequency spectrum interferometer etc. of warbling, see [1] Craig W.Siders, George Rodriguez, Jennifer L.W.Siders, FiorenzoG.Omenetto, and Antoinette J.Taylor, " Measurement of UltrafastIonization Dynamics of Gases by Multipulse InterferometricFrequency-Resolved Optical Gating ", Phys.Rev.Lett.Vol.87,263002, (2001). and [2] K.Y.Kim, I.Alexeev, E.Parra, and H.M.Milchberg, " Time-Resolved Explosion of Intense-Laser-HeatedClusters ", Phys.Rev.Lett.Vol.90,023401, (2002).FROG, MI-FROG and SPIDER generally are used for the pulse waveform and the spectral phase of Laser Measurement; The Pump-Probe technology be owing to will adopt the method for sequence time-delay, thereby can not carry out real-time on-line measurement.Because ultra-short pulse laser itself has very wide frequency spectrum, different frequency contents can be modulated acquisition one good linear chrip pulse in time domain by chirp technology, make the time-domain information of laser to obtain embodying at frequency domain.Utilize ultrashort pulse to have these characteristics of wide spectrum, adopt frequency spectrum to interfere or the method for modulation, can ionization process, evolution of shock waves, the evolution of laser ablation material surface pattern and the THZ pulsewidth that laser produces or the like of physical process such as atom be carried out the real-time measurement of high time resolution fast to some.Because this method only needs single measurement, therefore can carry out on-line measurement in real time.For the method for interfering, owing to will observe the distortion of interference fringe, noticeable striped twist angle generally is greater than 1/8th fringe spacings, see Hui Yang, JieZhang, Yingjun Li, Jun Zhang, Yutong Li, Zhenglin Chen, Hao Teng, Zhiyi Wei, and Zhengming Sheng, " Characteristics of self-guidedlaser plasma channels generated by femtosecond laser pulses inair ", Phys.Rev.E Vol.66,016406, (2002). therefore the sensitivity of measuring is restricted.

Summary of the invention

The technical problem to be solved in the present invention is to overcome the restriction of above-mentioned prior art, provide a kind of chirped pulse vertical diffraction interferometer, the signal intensity of this instrument directly depends on the size of phase shifts, and the sensitivity of measurement is very high, can measure small phase place process over time.

Technical solution of the present invention is as follows:

The vertical diffraction interferometer of a kind of chirped pulse, it is characterized in that comprising a grating pair, on the output light path of this grating pair, be provided with half-wave plate, beam splitting chip, polaroid, diffraction lens and area array CCD spectrometer successively, one condenser lens is positioned at a side of described beam splitting chip and is parallel to the output light path of grating pair, makes perpendicular to the driving light beam of this condenser lens folded light beam and the detecting light beam conllinear that sees through beam splitting chip through the light splitting piece reflection.

One ultra-short pulse lasers (tens femtoseconds to the hundreds of femtosecond but spectrum width broad tens nanometers or wideer slightly) is through there being two bundle laser behind the beam splitting chip, wherein a branch of as pump beam, occupy most laser energy, scioptics focus on, be used for the generation of driving laser plasma or in material, cause the generation of nonlinear effect, cause change of refractive etc. thereby small variation takes place the structure of matter.

Here, we are example with the plasma channel, utilize Laser Driven to produce plasma channel in air.The laser of a branch of less energy is as detecting light beam in addition.In our design, detecting light beam is through grating pair broadening being aligned chirped pulse, and utilize the half-wave plate rotation to make detection polarisation of light direction vertical with the polarization direction of driving laser, see through the transmission of this beam splitting chip and pump beam conllinear then, pass plasma, because the refractive index of plasma is different with the refractive index of surrounding medium, therefore can cause the variation of phase shift to the regional area of exploring laser light wavefront.After this take out detecting light beam by polaroid, detecting light beam can produce diffraction fringe in the free space propagation and through a lens diffraction on the observation screen behind the segment distance.In the position of observation screen, we are with the change procedure of grating spectrometer record diffraction fringe on frequency spectrum, just can the reconstruct phase shift in time domain transient changing process in time, reach the purpose of femtosecond time resolution measurement.We have derived the analytic method of phase reconstruction, and this can be described below:

Be to survey the synoptic diagram that optical diffraction is propagated as shown in Figure 2.The exploring laser light waveform can think on time and space it all is Gaussian distribution, is a linear chrip pulse at time domain detecting light beam broadening.Can be written as at light field amplitude through exploring laser light behind the plasma slab:

U ₀(r，t)＝exp(-ar ²)×exp(-jφ(r，t))exp(-Tt ²+j(ω ₀t+bt ²)) (1)

2b is the chirp coefficient of exploring laser light, ω ₀It is the centre frequency of exploring laser light.A is relevant with the waist spot size and the pulsewidth of exploring laser light respectively with T.The phase shift that plasma slab causes can be expressed as over time:

$\mathrm{\φ}(r,t)=\left\{\begin{array}{cc}{\mathrm{\φ}}_0\left(t\right)& r\≤r_c\left(t\right)\\ 0& r>r_c\left(t\right)\end{array}\right.---\left(2\right)$

r _c(t) be the radius of plasma channel, φ ₀(t) be plasma slab to surveying the phase shift that causes before light wave process over time.According to the propagation diffraction theorem of light, survey light and after through lens, propagating a distance, d ₁The light field at place can be represented with fresnel integral:

U ₂(x，y，t)∝U(x，y，t)＝∫∫∫∫U ₀(ξ，η，t)exp(-jβ(ξ′ ²+η′ ²))exp(ja ₀[(ξ-ξ) ²+(η′-η) ²]×(3)

exp(ja ₁[(x-ξ′) ²+(y-η′) ²]dξdηdξ′dη′

A wherein ₀=π/λ d ₀,, a ₁=π/λ d ₁, β=π/λ f, d ₀Be the distance between plasma slab and the lens, show as Fig. 2.d ₁It is the distance between lens and the detector C CD.Coordinate (ξ, η), (ξ ', η '), (x, y) coordinate of difference representative plane, lens plane and detection plane.Can calculate in the intensity distributions of frequency domain diffraction fringe by the derivation of equation with spectrum:

Wherein ${\mathrm{<figure-callout\; id="0"\; label="A"\; filenames="C20041001786800101.png,C20041001786800111.png"\; state="\{\{state\}\}">A</figure-callout>}}_0=\frac{\mathrm{\&pi;}}{\mathrm{\&lambda;}{d}_1}(1-\frac{1}{1+{\mathrm{<figure-callout\; id="1"\; label="d"\; filenames="C20041001786800101.png,C20041001786800111.png"\; state="\{\{state\}\}">d</figure-callout>}}_1/d_0-{\mathrm{<figure-callout\; id="1"\; label="d"\; filenames="C20041001786800101.png,C20041001786800111.png"\; state="\{\{state\}\}">d</figure-callout>}}_1/f}),$ ${\mathrm{<figure-callout\; id="1"\; label="A"\; filenames="C20041001786800101.png,C20041001786800111.png"\; state="\{\{state\}\}">A</figure-callout>}}_1=\frac{\mathrm{\&pi;}}{\mathrm{\&lambda;}{d}_0}(1-\frac{1}{1+{\mathrm{<figure-callout\; id="0"\; label="d"\; filenames="C20041001786800101.png,C20041001786800111.png"\; state="\{\{state\}\}">d</figure-callout>}}_0/d_1-{\mathrm{<figure-callout\; id="0"\; label="d"\; filenames="C20041001786800101.png,C20041001786800111.png"\; state="\{\{state\}\}">d</figure-callout>}}_0/f}),$ $A_2=\frac{2\mathrm{\&pi;}}{\mathrm{\&lambda;}}({\mathrm{<figure-callout\; id="1"\; label="d"\; filenames="C20041001786800101.png,C20041001786800111.png"\; state="\{\{state\}\}">d</figure-callout>}}_1+d_0-d_0{\mathrm{<figure-callout\; id="1"\; label="d"\; filenames="C20041001786800101.png,C20041001786800111.png"\; state="\{\{state\}\}">d</figure-callout>}}_1/f),$ $b_0=A_2^{2}a/4(a^2+A_1^2),$ $b_1=A_2^2{A}_1/4(a^2+A_1^2).$ Subscript yes and no represent plasma channel respectively and do not have the optical field distribution of detection plane under two kinds of conditions of plasma channel.

Can calculate the phase shift distribution  of frequency domain by the diffraction fringe of following formula match record ₀(ω).Shown in Figure 4 as Fig. 3, we carry out the result of match to diffraction fringe.Can unique definite phase shift size and plasma channel diameter by match.By linear corresponding relation φ ₀(t (ω))= ₀(A (ω-ω ₀)) can calculate detecting light beam at phase shift evolutionary process φ in time through producing in the process of plasma slab ₀(t).Therefore, adopt this method, can obtain phase shift process over time by one-shot measurement, thus the production process of electronics in the research plasma channel.In addition, by measuring spectrum intensity and the phase shift distribution  that surveys light ₀(ω), adopt Fourier transform also can calculate phase shift evolutionary process φ in time ₀(t).The temporal resolution of this measuring method is mainly by the spectrum width decision V ω that surveys light, temporal resolution Vt ≈ 2 π/V ω.Therefore, the frequency spectrum of surveying light is wide more, and the temporal resolution of detection is high more.For example, if survey the gloss titanium precious stone laser, centre wavelength 800nm, if spectrum width (halfwidth) is 45nm, the temporal resolution of Ce Lianging can reach 20fs so.

Advantage of the present invention is:

1, adopts the method for vertical diffraction can increase the length (higher two more than the magnitude) of detecting area greatly,, so can improve the sensitivity of measurement greatly because phase shift is directly proportional with surveying length than lateral length.

2, adopt the method for vertical diffraction, the intensity distributions of the diffraction fringe of measurement is only relevant with phase shift, and does not rely on the degree of crook of striped, and therefore the sensitivity of measuring is very high.

3, adopt the linear chrip pulse laser can only need one-shot measurement just can access phase shift process over time as probe, the resolution of measurement be very high, can reach the 10fs magnitude.

4, in design invention process, derived the analytical expression of diffraction fringe, the size and the channel diameter of the phase shift of reconstruct easily with phase shift.

Description of drawings

Fig. 1 is the vertical diffraction interferometer structural representation of chirped pulse of the present invention.

Fig. 2 is that the present invention surveys the diffraction propagation synoptic diagram of light field after passing through a thin lens.

Fig. 3 is the result who adopts analytic method (formula 4) diffraction fringe to be carried out match to different phase shifts.

Fig. 4 is the result who adopts analytic method (formula 4) diffraction fringe to be carried out match to different plasma channel diameters.

Fig. 5 represents the phase shift φ of linear change in time ₀(t).

Fig. 6 is illustrated in the diffraction pattern with space distribution that detection plane is directly used the CCD cameras record.

Fig. 7 represents the intensity distributions of diffraction pattern in frequency domain and space.

Fig. 8 is that the diffracted intensity that reads along the center in the diffraction pattern shown in Figure 7 is with the wavelength change curve.

Embodiment

The present invention is used for high time resolution and measures small phase shift (low electron density＜10 ¹⁶Cm ^-3) in time transient changing process.See also Fig. 1 earlier, Fig. 1 is the synoptic diagram of one of the vertical diffraction interferometer arrangement works of chirped pulse of the present invention embodiment, as seen from the figure, the vertical diffraction interferometer of chirped pulse of the present invention, comprise a grating pair 2, on the output light path of this grating pair 2, be provided with half-wave plate 3 successively, beam splitting chip 4, polaroid 8, diffraction lens 9 and area array CCD spectrometer 10, one condenser lens 6 is positioned at these beam splitting chip 4 one sides and is parallel to the output light path of grating pair 2, makes the plasma channel 7 and detecting light beam conllinear that produce through the reflected light of beam splitting chip 4 perpendicular to the driving light beam of this condenser lens 6.

The concrete course of work of the vertical diffraction interferometer of chirped pulse of the present invention is as follows:, one ultra-short pulse lasers 1 (tens femtoseconds to the hundreds of femtosecond but spectrum width broad tens nanometers or wideer slightly) sees through beam splitting chip 4 backs and makes to survey light, another Shu Zuowei pump light source 5, occupy most laser energy, scioptics 6 focus on after the space that is reflected in of beam splitting chip 4 produces plasma channel 7, or in material, cause the generation of nonlinear effect, cause change of refractive etc. thereby small variation takes place the structure of matter.

Here, we are example with the plasma channel, utilize Laser Driven to produce plasma channel 7 in air.The laser of a branch of less energy is as probe source 1 in addition.In our design, survey light 1 through grating pair 2 broadening being aligned chirped pulses, and utilize half-wave plate 3 rotations to make the polarization direction of surveying light 1 vertical mutually with the polarization direction of driving laser 5, then by the reflected light conllinear transmission of a beam splitting chip 4 with pump light 5, pass plasma channel 7, because the refractive index of plasma is different with the refractive index of surrounding medium, therefore can cause the variation of phase shift to the regional area of exploring laser light wavefront.After this take out by polaroid 8 and survey light 1, survey light 1 and on the observation screen behind the segment distance, can produce diffraction fringe in the free space propagation and through lens 9 diffraction.In the position of observation screen, we are with the change procedure of a grating spectrometer 10 record diffraction fringes on frequency spectrum, just can the reconstruct phase shift in time domain transient changing process in time, reach the purpose of femtosecond time resolution measurement.

In the process of implementing, there are two kinds of methods that the pump light and the detection light of conllinear transmission are made a distinction.A kind of method is to adopt the method for frequency multiplication, with pump light or detection optical sccond-harmonic generation; Another kind method is to adopt the method for polarization, makes it vertical with the polarization direction of pump light by changing detection polarisation of light direction, adopts polaroid 8 will survey light then and separates.

For example, for the phase shift φ of a linear change in time as shown in Figure 5 ₀(t)

${\mathrm{\&phi;}}_0\left(t\right)=\left\{\begin{array}{cc}\frac{(t+\mathrm{\&tau;})\mathrm{\&pi;}}{\mathrm{\&tau;}}& -\mathrm{\&tau;}\&le;t\&le;\mathrm{\&tau;}\\ 0& \mathrm{else}\end{array}\right.---\left(10\right)$

τ=700fs here, we can adopt the negative chirped pulse laser of a linearity to carry out time discrimination measurement as probe light.The spectrum width of exploring laser light is elected 45nm as, does not have under the condition of warbling, and laser pulse width is 20fs, can obtain the negative chirped pulse of a linearity behind process grating pair 2 broadenings, and pulse width can be elected 1.0ps as.Adopt experimental program as shown in Figure 1, if directly survey the intensity distributions of diffraction of light pattern with the space with the CCD cameras record in detection plane, can obtain diffraction fringe as shown in Figure 6 so, this diffraction fringe spatially is centrosymmetric.But, if we radially choose diffraction fringe in detection plane with a slit, taking records with grating spectrometer 10 then, we just can obtain as shown in Figure 7 diffraction pattern in the intensity distributions in frequency domain and space so.If read the space distribution intensity curve of diffraction fringe at different spectrum positions, the analytical analysis method above adopting is carried out the phase shift  that match can be calculated frequency domain ₀(ω) with the plasma channel diameter.Obtain phase shift conversion process φ in time by conversion then ₀(t).Fig. 8 is that the diffracted intensity that reads along the center in diffraction pattern shown in Figure 7 is with the wavelength change curve, for the phase shift φ of linear change in time ₀(t), its change curve is equivalent to a sinusoidal curve.

Claims (1)

1, the vertical diffraction interferometer of a kind of chirped pulse, it is characterized in that comprising a grating pair (2), on the output light path of this grating pair (2), be provided with half-wave plate (3), beam splitting chip (4), polaroid (8), diffraction lens (9) and area array CCD spectrometer (10) successively, one condenser lens (6) is positioned at these beam splitting chip (4) one sides and is parallel to the output light path of grating pair (2), makes perpendicular to the driving light beam (5) of this condenser lens (6) folded light beam and detecting light beam (1) conllinear that sees through beam splitting chip (4) through beam splitting chip (4) reflection.

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