CN101187770B - Femtosecond pulse compression device - Google Patents

Abstract

The femtosecond pulse compression device comprises a first grating and a second grating which are arranged in parallel, wherein one surface of each of the two gratings with grating stripes is positioned on the inner side of a grating pair, the first grating and the second grating are both sub-wavelength deep etching transmission gratings with the same structure and the period of d, the second grating is positioned on the direction of the first grating transmission negative first-order diffraction order, and femtosecond laser pulses to be compressed are incident to the first grating at a Bragg angle theta and satisfy a grating equation: sin theta ═ lambda₀(ii)/2 d, wherein: theta is the incident angle, lambda₀The center wavelength of the femtosecond laser pulse. The femtosecond pulse compression device formed by utilizing the sub-wavelength deep etching transmission type grating has the characteristics of compact structure and high efficiency.

Description

Femto second compression device

Technical field

The present invention relates to femto-second laser pulse, particularly a kind of high-level efficiency sub-wavelength that utilizes loses the femto second compression device that transmission-type grating constitutes deeply.

Technical background

Femto-second laser pulse has advantages such as peak power height and duration weak point, therefore is being widely used aspect physics, biology, chemistry and little manufacturing and the little processing.

Generally, the laser pulse that produces in the femtosecond resonator cavity is because the chromatic dispersion of material such as interacvity gain medium, thereby just has and warble.For the influence of compensative material chromatic dispersion, reach the purpose of compression pulse, generally can insert the negative dispersion element in the inside and outside of cavity.Prism is to being the most widely used dispersive compensation element at present, it has the low advantage of energy loss, but because the angular dispersion of prism is smaller, therefore for material dispersion in the compensated cavity, prism between distance generally can be very big, must use a plurality of catoptrons just can make the light path compactness, light path is just complicated like this, the huge and difficult adjusting of space structure.

Technology 1[E.B.Treacy formerly, IEEE J.Quantum Electron.QE-5,454-458 (1969)] and technology 2[O.E.Martinez formerly, J.Opt.Soc.Am.B.3,929-934 (1986)] in the method for using the reflective holographic grating pair to carry out pulse compression has been proposed.Formerly technology 3[Zhou Changhe ices in vain, utilizes Darman raster to producing multipulse device, patent of invention, publication number: CN1786750] adopt the low-density grating pair to connect the structure of catoptron again, realize conllinear beam splitting and compression to femtosecond pulse.Technology 4[Zhou Changhe formerly, general Zheng, femto second compression device, patent of invention, publication number: CN200959058] propose to utilize the double density grating pair to carry out the femtosecond pulse compression, thus further simplified device.

What formerly technology 3,4 was considered is to utilize the low-density grating to realize the femtosecond pulse compression, and their shortcoming is exactly that capacity usage ratio is low, and apparatus structure is big.

High dencity grating, is widely used in chirped pulse and amplifies in (CPA) system therefore as stretcher and compressor reducer owing to can produce very big angular dispersion, and the grating that is adopted generally was the reflective holographic grating in the past.Because the material dispersion in the femtosecond laser resonator cavity is smaller, high dencity grating between distance can be very little, the clear aperture of light beam is restricted, this makes the reflective gratings structure have very large technical difficulty in actual applications.And use the transmission-type grating structure that problem is more easily solved.If but use the transmission-type holographic grating, and at first can not guarantee the high-level efficiency that-1 order diffraction level is inferior, secondly the second best in quality transmission hologram grating is difficult to make the problem to be solved in the present invention that Here it is.

Summary of the invention

Purpose of the present invention provides a kind of femto second compression device in order to overcome the difficulty of above-mentioned prior art, and this femto second compression device should have compact conformation, the characteristics that efficient is high.

Technical conceive of the present invention is: high-level efficiency and the big dispersive power of utilizing sub-wavelength to lose transmission-type grating deeply constitute a kind of new femto second compression device, reach compact conformation, the purpose that efficient is high.

Technical solution of the present invention is as follows:

A kind of femto second compression device, its formation comprises first grating and second grating of two parallel placements, the one side that these two blocks of gratings have grating fringe is positioned at the inboard of grating pair, it is that the sub-wavelength of d loses transmission-type grating deeply that described first grating and second grating are the identical cycle of structure, described second grating is positioned at the negative first-order diffraction level power of the first grating transmission upwards, and femto-second laser pulse to be compressed incides first grating and satisfies grating equation with Bragg angle θ:

$\sin \mathrm{\θ}=\frac{{\mathrm{\λ}}_0}{2d}$

Wherein: θ is an incident angle, λ ₀Centre wavelength for femto-second laser pulse.

The cycle d of described grating satisfies:

$\frac{1}{2}{\mathrm{\&lambda;}}_0<d<{\mathrm{\&lambda;}}_0$

Upwards and with this diffraction direction first catoptron is set vertically at the negative first-order diffraction level power of the described second grating transmission, this first catoptron can vertically rotate one less than 5 ° angle.

The light path of returning and exporting from described first grating through described first catoptron is provided with second catoptron.

Technique effect of the present invention:

Apparatus of the present invention are by selecting the sub-wave length grating of suitable line density, because it is binary optical elements that sub-wavelength loses grating deeply, make than being easier to, by the degree of depth, isoparametric optimization of cycle are obtained high-level efficiency, this is different with in the past holographic grating on diffraction mechanism.Since sub-wave length grating between distance very little, usually less than a millimeter, the right negative dispersion amount of prism in the past just can be provided, therefore this compression set structure is very small and exquisite, and pass through raster density, the optimization of etching depth and dutycycle can make sub-wavelength lose grating efficiency deeply and bring up to more than 95%.

Description of drawings

Fig. 1 is the structural representation of apparatus of the present invention embodiment 1.

Fig. 2 is the structure schematic top plan view of apparatus of the present invention embodiment 2.

Fig. 3 is the structure schematic side view of apparatus of the present invention embodiment 2.

Spectrogram when Fig. 4 is the experiment measuring of the embodiment of the invention.

Fig. 5 is the amplitude-time plot of the embodiment of the invention.

Embodiment

Below in conjunction with embodiment and accompanying drawing apparatus of the present invention are further described.

Consult Fig. 1 earlier, as seen from the figure, the present invention utilizes sub-wavelength to lose the device that transmission-type grating is realized the femtosecond pulse compression deeply, comprises first grating 1 and second grating 2.Described two gratings are that sub-wavelength loses transmission-type grating deeply, and two grid stroke density equate.Described two parallel placements of grating, and the one side with grating fringe is positioned at the inboard of grating pair.Move described second grating 2 to regulate the spacing of two gratings, can satisfy the compression of the femtosecond pulse of different dispersion measures.

Femtosecond pulse to be compressed incides first grating 1 with the Bragg angle, and satisfies grating equation:

$\sin \mathrm{\&theta;}=\frac{{\mathrm{\&lambda;}}_0}{2d}$

Wherein θ is an incident angle, λ ₀Centre wavelength for femtosecond pulse.Described second grating 2 is positioned at the negative first-order diffraction level power of first grating, 1 transmission upwards.If walk when less demanding for the spatial frequency spectrum of femtosecond pulse, femto-second laser pulse only just can obtain femtosecond pulse near transform limit through apparatus of the present invention.

Fig. 2 and Fig. 3 are the structural representations of apparatus of the present invention embodiment 2, present embodiment is by first grating 1, second grating 2, first catoptron 3 and second catoptron 4 constitute, first grating 1 and second grating, 2 sub-wavelengths lose transmission-type grating deeply, two grid stroke density equate, cycle is d, described two parallel placements of grating, and the one side with grating fringe is positioned at the inboard of grating pair, described second grating 2 is positioned at the negative first-order diffraction level power of first grating, 1 transmission upwards, upwards and with this diffraction direction first catoptron 3 is set vertically at the negative first-order diffraction level power of described second grating 2 transmissions, this first catoptron 3 can vertically rotate one less than 5 ° angle.The light path of returning and exporting from described first grating 1 through described first catoptron 3 is provided with second catoptron 4.

Femto-second laser pulse to be compressed incides first grating and satisfies grating equation with Bragg angle θ:

$\sin \mathrm{\&theta;}=\frac{{\mathrm{\&lambda;}}_0}{2d}$

Wherein: θ is an incident angle, λ ₀Centre wavelength for femto-second laser pulse.

The cycle d of described grating satisfies:

$\frac{1}{2}{\mathrm{\&lambda;}}_0<d<{\mathrm{\&lambda;}}_0$

Because the wide range characteristics of femtosecond pulse, incident pulse can produce the diffraction of different directions, i.e. angular dispersion after through first grating 1.By second grating 2, angular dispersion is compensated, and has introduced negative group velocity dispersion effect, but still has the influence of space chirp.In order to eliminate space chirp, outgoing pulse returns by former road after 3 reflections of first catoptron, thereby realizes pulse compression, has remedied the influence of space chirp and angular dispersion simultaneously.

Described light and the mechanism that separates of input light of will exporting is along on the incident ray direction, with first catoptron 3 vertically angle of fine rotation (less than 5 the degree), just can obtain exporting pulse from described second catoptron 4 so that emergent light separates with incident light.

Technology 2[O.E.Martinez formerly, J.Opt.Soc.Am.B.3,929-934 (1986)] and technology 5[Jiangiun Zheng formerly, Changhe Zhou, Enwen Dai, J.Opt.Soc.Am.B.24,979-984 (2007)] theoretical analysis and experimental study have been carried out in the compression of limited beam diameter ultrashort pulse light.Consider that pulse to be compressed has positive linear chrip, is expressed as:

$E_i(x,y,t)=\exp (-\frac{2\ln {2\mathrm{<figure-callout\; id="2"\; label="t"\; filenames="H2007100481853E00013.png,H2007100481853E00021.png"\; state="\{\{state\}\}">t</figure-callout>}}^2}{{\mathrm{\&tau;}}^2})\exp \left({\mathrm{ibt}}^2\right)\exp (-i\frac{k(x^2+y^2)}{2q\left(z\right)})$

B＞0 wherein, τ is the full width at half maximum (FWHM) of incident pulse, q (z) is the complex parameter of Gaussian beam:

$q\left(z\right)=z+i\frac{{\mathrm{\&pi;\&sigma;}}^2}{\mathrm{\&lambda;}}$

Z is that light-beam position arrives distance with a tight waist, and σ is a waist radius.After light beam was through this compression set (Fig. 1), the pulse frequency spectrum that obtains was expressed as:

$E_o(x,y,\mathrm{\&omega;})\&Proportional;E_i\left(\mathrm{\&omega;}\right)\exp \left({\mathrm{ik\&beta;}}^2{\mathrm{\&omega;}}^{2}z\right)$

$\&times;\exp \left\{\frac{-\mathrm{ik}}{2}\right[\frac{x^2}{q(d^{\&prime;}+{\mathrm{<figure-callout\; id="1"\; label="z"\; filenames="H2007100481853E00011.png,H2007100481853E00013.png"\; state="\{\{state\}\}">z</figure-callout>}}_1+{\mathrm{<figure-callout\; id="2"\; label="z"\; filenames="H2007100481853E00013.png,H2007100481853E00021.png"\; state="\{\{state\}\}">z</figure-callout>}}_2+2z)}+\frac{y^2}{q(d^{\&prime;}+{\mathrm{<figure-callout\; id="1"\; label="z"\; filenames="H2007100481853E00011.png,H2007100481853E00013.png"\; state="\{\{state\}\}">z</figure-callout>}}_1+{\mathrm{<figure-callout\; id="2"\; label="z"\; filenames="H2007100481853E00013.png,H2007100481853E00021.png"\; state="\{\{state\}\}">z</figure-callout>}}_2+2z)}\left]\right\}$

E wherein _o, E _iThe spectral domain that is outgoing pulse and incident pulse is respectively represented.Z is the distance that pulsed light is passed by between grating pair, and parameter beta is expressed as:

$\mathrm{\&beta;}=\frac{-{\mathrm{\&lambda;}}^2}{2\mathrm{\&pi;}\mathrm{cd}\cos \mathrm{\&theta;}}$

The full width at half maximum that can obtain compressing afterpulse by Fourier transform is:

${\mathrm{\&tau;}}_o=\mathrm{\&tau;}\sqrt{{(1-4\mathrm{bk}{\mathrm{\&beta;}}^{2}z)}^2+\frac{{(8\mathrm{<figure-callout\; id="2"\; label="ln"\; filenames="H2007100481853E00013.png,H2007100481853E00021.png"\; state="\{\{state\}\}">ln</figure-callout>}2\&times;{\mathrm{k\&beta;}}^{2}z)}^2}{{\mathrm{\&tau;}}^4}}$

Adjust the spacing of grating pair, can make to have the pulse compression of just warbling to Fourier transform limit, that is:

${\mathrm{\&tau;}}_{o\min }=\mathrm{\&tau;}\sqrt{1/[1+{({\mathrm{b\&tau;}}^2/2\ln 2)}^2]}$

In with embodiment shown in Figure 1, compare with embodiment 2, removed first catoptron 3 and second catoptron 4, therefore only unidirectional this compression set of process of pulse to be compressed, if the spatial spectrum separation requirement to femtosecond pulse is not high, still can realize tangible compression effectiveness.

Reference is technology 2[O.E.Martinez formerly, J.Opt.Soc.Am.B.3,929-934 (1986)] obtain, the distance of passing by between grating pair when femtosecond pulse satisfies:

$z=\frac{{\mathrm{b\&tau;}}^2{\mathrm{\&tau;}}_{o\mathrm{<figure-callout\; id="2"\; label="min"\; filenames="H2007100481853E00013.png,H2007100481853E00021.png"\; state="\{\{state\}\}">min</figure-callout>}}^2}{8{\mathrm{<figure-callout\; id="2"\; label="ln"\; filenames="H2007100481853E00013.png,H2007100481853E00021.png"\; state="\{\{state\}\}">ln</figure-callout>}}^{2}2k{\mathrm{\&beta;}}^2}$

The compression effectiveness that obtains is near Fourier transform limit, and the compression pulse width of this moment is:

${\mathrm{\&tau;}}_o^{\&prime;}={\mathrm{\&tau;}}_{o\min }{(1+\frac{8\mathrm{<figure-callout\; id="2"\; label="ln"\; filenames="H2007100481853E00013.png,H2007100481853E00021.png"\; state="\{\{state\}\}">ln</figure-callout>}2{\mathrm{\&beta;}}^2{z}^2}{{{\mathrm{\&tau;}}^2}_{o\min }{\mathrm{\&sigma;}}^2})}^{1/2}$

Generally speaking, in the formula β compare with limit pulse width very little, so spectral space walk from influence very little, can ignore.

In an embodiment, the optimal value of cycle for obtaining of our used grating through the rigorous coupled wave Theoretical Calculation, according to the rigorous coupled wave theory, the grating cycle be lower than incident wavelength very on a large scale in, by to the isoparametric optimization of etching depth, can reach high-level efficiency in incident pulse centre wavelength is the utmost point wide spectral range of 800nm, this is low-density grating and the incomparable advantage of holographic grating in the past.

Manufacturing process is the quartzy chromium sheet of holographic exposure, and it is certain to obtain the degree of depth with the method for inductively coupled plasma etching then, and the sub-wavelength that surface topography is good loses transmission-type grating deeply.By control etching speed and etching time, can obtain efficient up to the grating 95% or more (reference is technology 6[Shunquan Wang formerly, Changhe Zhou, Yanyan Zhang, Huayi Ru.App.Opt., 45,2567～2571 (2006)]).

In the experiment with the femto-second laser pulse of mixing the outgoing of titanium sapphire (Ti:Sapphire) laserresonator of coherent company as pulse to be compressed, after structure as shown in Figure 1, obtain compression pulse.Pulse after inceptive impulse and the compression is measured (with reference to technology 7[RickTrebino formerly by the FROG device of standard, " Frequency-Resolved Optical Gating:The Measurement of UltrashortLaser Pulsed; " Kluwer Academic Publishers, (2002)]), the results are shown in Fig. 4, Fig. 4 (1) and Fig. 4 (2) have provided the time spectrogram before and after the pulse compression respectively, and what Fig. 5 provided is the time curve of pulse width before and after the compression.

Obtain by the FROG measurement device, input pulse width (FWHM) is 73.9fs (femtosecond), the about 21.8nm of spectrum width, regulate the spacing of second grating 2 and first grating 1, the group velocity dispersion of input pulse is compensated substantially, the measurement output pulse width is 43.2fs, and is very approaching with the 42.6fs of Theoretical Calculation.The about 0.8mm of distance between this moment second grating 2 and first grating 1.

The present invention proposes to use sub-wavelength to lose the compression that transmission-type grating is realized femtosecond pulse deeply first, make compression set extremely small and exquisite, efficient is very high, deep simultaneously erosion sub-wave length grating has higher efficiency in wide spectral range, this point be in the past the High-Density Holographic grating do not have, it is right that apparatus of the present invention can substitute prism, is used for chamber inner chamber external compression, thereby dwindle the volume of laser instrument greatly, therefore have wide commercialization prospect.

Claims (4)

1. femto second compression device, be characterised in that its formation comprises first grating (1) and second grating (2) of two parallel placements, the one side that these two blocks of gratings have grating fringe is positioned at the inboard of grating pair, described second grating (2) is positioned at the negative first-order diffraction level power of first grating (1) transmission upwards, it is that the sub-wavelength of d loses transmission-type grating deeply that described first grating (1) and second grating (2) are the identical cycle of structure, and femto-second laser pulse to be compressed incides first grating (1) and satisfies grating equation with Bragg angle θ:

$\sin \mathrm{\&theta;}=\frac{{\mathrm{\&lambda;}}_0}{2d}$

Wherein: θ is an incident angle, λ ₀Centre wavelength for femto-second laser pulse.

2. femto second compression device according to claim 1 is characterized in that the cycle d of described grating satisfies:

$\frac{1}{2}{\mathrm{\&lambda;}}_0<d<{\mathrm{\&lambda;}}_0.$

3. femto second compression device according to claim 1, it is characterized in that upwards and with this diffraction direction first catoptron (3) being set vertically at the negative first-order diffraction level power of described second grating (2) transmission, this first catoptron (3) can vertically rotate one less than 5 ° angle.

4. femto second compression device according to claim 3 is characterized in that being provided with second catoptron (4) through the light path that described first catoptron (3) returns and exports from described first grating (1).

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