CN101718937A - Device and method for increasing filamentation length of femtosecond laser in air - Google Patents

Abstract

The invention provides a device and method for increasing filamentation length of femtosecond laser in the air. The device is characterized in that adjustable apertures, a phase plate and a focusing lens are sequentially arranged on a light path for outputting collimated femtosecond laser. The experiments show that after passing through the phase plate, the high-energy femtosecond laser pulse changes the focusing characteristic of the focused zone due to interference effect, thus greatly increasing the length of the focused zone. Compared with the common Gaussian beam focusing, the method of the invention ensures supercontinuum radiation to focus in the axial direction, thus ensuring more energy to be conveyed to farther places, which is of great significance in laser triggered lightning, remote atmospheric sounding and acquisition of superstrong and supershort few-cycle laser pulse.

Description

The prolongation femtosecond laser becomes the apparatus and method of filament length degree in air

Technical field

The present invention relates to the high field laser physics, particularly a kind ofly prolong femtosecond laser becomes the filament length degree in air apparatus and method, the long-range atmosphere detection of laser, laser induced lightning, acquisition ultrashort, super strong laser pulse are had major application.

Background technology

Along with the appearance of femtosecond laser, people's research has all obtained a lot of important achievements in research at each subjects such as chemistry, physics, biologies.Yet, once a very long time people thought that superpower ultrashort laser was not suitable for long-distance transmissions in air, for example owing to the influence of reasons such as diffraction, GVD (Group Velocity Dispersion), with a tight waist for 5mm, pulsewidth are the laser pulse of 30fs, intensity can decay about 5 * 10 after 1000 meters of the transmission in air ³But it is found that in actual experiment femtosecond laser transmits and can form weak plasma channel in air, and the length of plasma channel can very long (tens meters to several kms), passage is that the light intensity of chevilled silk the inside is also up to 10 ¹³-10 ¹⁴W/cm ²Magnitude.The one-tenth silk phenomenon of Here it is ultrashort ultra-intense laser, become the appearance of silk phenomenon to cause a lot of people's concern, because become silk to have very important application prospects (can consult document S.L.Chin, F.Th é berge for long-range atmosphere detection, laser induced lightning, the ultrashort superpower few cycle laser pulse of acquisition, and W.Liu, Appl.Phys.B.86,477,2007 and A.Couairon, A.Mysyrowicz, Phys.Rep.441,47,2007).So how a lot of people prolong plasma channel length and higher laser energy are sent to farther distance in concern all the time.

The method of the simplest and direct prolongation chevilled silk length is to utilize pine to focus on to prolong Rayleigh length, and then prolongs the length of silk.Yet under the situation that pine focuses on, the focus on the hot spot can be easy to into silk, the multifibres structure can occur like this, and particularly under the higher situation of incident light energy, this is restricted to the concrete application meeting that becomes silk.In addition, experiment showed, that in the anomalous dispersion medium length of silk also can prolong greatly, yet commonly used now and femtosecond laser wavelength comparative maturity is a normal dispersion all near 800nm in air.Recently, the length that people such as P.Polynkin utilize the Bessel light beam to prolong silk greatly in air, and by optimizing length that warbling of laser optimize silk (referring to document P.Polynkin, M.Kolesik, A.Roberts, D.Faccio, P.D.Trapani, and J.Moloney, Opt.Exp.16,15733,2008 and P.Polynkin, M.Kolesik, J.Moloney, Opt.Exp.17,575,2009).What obtain in their test is accurate Bessel light beam, the Gauss light beam is obtained by a conical mirror, thereby the length that has prolonged focal zone greatly the longlyest extends to 2.5 times.

Summary of the invention

The present invention aims to provide a kind of femtosecond laser becomes the filament length degree in air apparatus and method that prolong, have simple and more effective characteristics, and the present invention also provides a kind of and makes super continuous radiation concentrate in the axial direction method, and this has important effect to atmosphere remote probe, laser induced lightning and the ultrashort, super strong laser pulse that utilizes a compression method to obtain the cycle magnitude.

Core concept of the present invention is the length that prolongs silk by the wavefront position that changes incident light mutually, with transmission-type and reflective position mutually regulative mode all can, as long as can control the wavefront position phase of incident pulse according to designing requirement.

Technical solution of the present invention is as follows:

A kind ofly prolong femtosecond laser becomes the filament length degree in air device, it is characterized in that the adjustable aperture, phase board and the condenser lens that set gradually constitute on the light path of collimation femtosecond laser output.

Described phase board is transmission-type phase board or reflective phase board.

A kind ofly prolong femtosecond laser becomes the filament length degree in air method, be characterised in that to comprise the following steps:

1. carry out linear analogue with the fresnel diffraction method and find the solution the inner circular shaped region radius r that obtains described phase board ₀, inner circular shaped region is with respect to the position phase transition amount of outer shroud

Utilize wet etching method or utilize ion beam etching or other lithographic methods etch a radius at the K9 glass surface and are r ₀Border circular areas, phase transition amount in the pairing position of etching depth is

2. on the light path of collimation femtosecond laser output, set gradually described adjustable aperture, phase board and condenser lens;

3. open the collimation femtosecond laser, adjust described adjustable aperture, make described femtosecond laser in air, become the filament length degree to reach the longest.

Experiment shows: apparatus of the present invention can prolong femtosecond laser becomes silk in air length greatly with method.Focusing is compared with common Gaussian beam, super continuous radiation is concentrated on axially, thereby more energy can be transported to farther place, and the long-range atmosphere detection of laser, laser induced lightning, acquisition ultrashort, super strong laser pulse are had major application.

Description of drawings

Fig. 1 is the sectional view and the vertical view of transmission-type phase board,

Among the figure: r ₀Be the inner circular shaped region radius,

Be the position phase transition amount of inner circular shaped region with respect to outer shroud;

Fig. 2 becomes the concrete index path of implementing of silk for prolonging laser pulse in air,

Among the figure: 1 is the superpower ultra-short pulse laser of incident, and 2 is the scalable aperture, and 3 is the transmission-type phase board, and 4 is condenser lens, and 5 is that ultrashort ultra-intense laser focuses on the chevilled silk (weak plasma channel) that the back forms in air;

Fig. 3 is incident field and the coordinate system synoptic diagram of waiting to ask light field.

Embodiment

The invention will be further described below in conjunction with embodiment and accompanying drawing, but should not limit protection scope of the present invention with this.

See also Fig. 1 earlier, Fig. 1 is the structural representation of transmission-type phase board, the left side be sectional view, the right be vertical view.The border circular areas that it is 5mm that the K9 glass surface that utilizes wet etching method (perhaps utilizing ion beam etching or other lithographic methods) to be about 1.8mm at thickness etches a diameter, etching depth is about 0.78 μ m.Incident light is by after this piece phase board, and the position of spot center zone and outer portion branch phase difference of pi is (interior ring position mutually π) in advance mutually.

Project organization that it should be noted that phase board is not unique, and the position that can be designed to any situation changes pattern mutually, decides according to own design result.In addition, phase board also is not limited to transmission-type, also can for example can make the various piece height difference of catoptron with reflective, makes the different light path of light experience of reflection, can realize the adjusting of a phase equally.If the time with the transmission-type phase board, pay special attention to strong femtosecond laser in transmission material self-focusing effect and destroy transmission material, so will pay special attention to the selection of material and the control of thickness.In the concrete test can measure spectrum after the phase board and pulsewidth whether change and have or not obvious white light judge material and thickness select whether suitable.Change if spectrum and pulsewidth have clearly, obvious white light is perhaps arranged, detail bit phase-plate material and thickness are selected improper, select the material of high destructive threshold value or reduce phase board thickness to weaken nonlinear effects such as self-focusing in phase board as far as possible.

Because laser instrument difference, spot size and facular model all have difference, so will optimize the length that focuses on back focal zone by continuous optimization to the position change pattern mutually of incident light with size according to the suitable phase board pattern of the laser parameter design of oneself, thereby realize prolonging the purpose of silk.Certainly, method should be come simulation electronic Density Distribution (A.Couairon, A.Mysyrowicz, Phys.Rep.441,47-189 (2007)) with the nonlinear transport equation accurately, thereby obtains more satisfactory result.Figure below is the process flow diagram of nonlinear computation machine simulation process.

Need carry out a large amount of calculating yet separate the nonlinear transport equation,, optimize this a large amount of time of process need cost with present PC computing power.

According to our design experiences, can carry out linear transfer optimization with the fresnel diffraction principle earlier, by continuous optimization the position change pattern mutually of incident light is optimized the length that focuses on the back focal zone, thereby realize prolonging the purpose of silk.Though though the structure of optimizing phase board with linear transfer is not very accurate, to a certain extent also with the experimental result coupling better.And (suppose that launching spot is distributed as Gaussian distribution) in our experience, when about 1/3rd and the interior position phase transition amount that accounts for whole light spot energy when the interior zone energy is π, just can obtain reasonable experimental result.

Be the concrete formula and the derivation of fresnel diffraction simulation below:

$U\left(p_0\right)=\frac{1}{i\·\mathrm{\λ}}\∫\∫U\left(p_1\right)\frac{e^{i\·k\·r_{01}}}{r_{01}}\cos \mathrm{\θ}\·\mathrm{ds}$

(1) be the fresnel diffraction formula, consult Fig. 3, wherein, U (p ₀) be the middle p of light field to be asked (scalar field) ₀(x, electric field complex amplitude y), U (p ₁) be p in the incident field ₁(λ is an optical maser wavelength for ξ, η) some electric field complex amplitude, $k=\frac{2\mathrm{\π}}{\mathrm{\λ}},$ r ₀₁Be p ₀, p ₁Distance between two points, θ are light p ₀p ₁Angle with the z axle has $\cos \left(\mathrm{\θ}\right)=\frac{z}{r_{01}},$ Z is the laser propagation distance.

Can obtain by (1)

$U(x,y)=\frac{z}{\mathrm{i\λ}}\∫\∫U(\mathrm{\ξ},\mathrm{\η})\·\frac{e^{{\mathrm{ikr}}_{01}}}{r_{01}^2}\·\mathrm{d\ξ}\·\mathrm{d\η}---\left(2\right)$

And $r_{01}=\sqrt{z^2+{(x-\mathrm{\ξ})}^2+{(y-\mathrm{\η})}^2}\≈z[1+\frac{1}{2}{\left(\frac{x-\mathrm{\ξ}}{z}\right)}^2+\frac{1}{2}{\left(\frac{y-\mathrm{\η}}{z}\right)}^2]$

So have

$U(x,y)=\frac{e^{\mathrm{ikz}}}{\mathrm{i\λz}}\∫\∫U(\mathrm{\ξ},\mathrm{\η})\·e^{\frac{\mathrm{ik}}{2z}[{(x-\mathrm{\ξ})}^2+{(y-\mathrm{\η})}^2]}\·\mathrm{d\ξ}\·\mathrm{d\η}$

$=\frac{e^{\mathrm{ikz}}}{\mathrm{i\&lambda;z}}\&CenterDot;e^{i\frac{k}{2z}({\mathrm{<figure-callout\; id="2"\; label="x"\; filenames="H200910198268XE00012.png"\; state="\{\{state\}\}">x</figure-callout>}}^2+y^2)}\&Integral;\&Integral;\{U(\mathrm{\&xi;},\mathrm{\&eta;})\&CenterDot;e^{i\frac{k}{2z}({\mathrm{\&xi;}}^2+{\mathrm{\&eta;}}^2)}\}\&CenterDot;e^{-\frac{i2\mathrm{\&pi;}}{\mathrm{\&lambda;}\&CenterDot;z}(\mathrm{x\&xi;}+\mathrm{y\&eta;})}\&CenterDot;\mathrm{d\&xi;}\&CenterDot;\mathrm{d\&eta;}---\left(3\right)$

For the incident and the diffractive light field that satisfy the circle symmetry, can establish:

x＝rcosα

y＝rsinα

ξ＝ρcosβ

η＝ρsinβ

Referring to Fig. 3, r is p ₀(x, y) to the distance of z axle, α is on the xy coordinate plane, p ₀(x, y) with the angle of x-y coordinate plane initial point line and x axle, ρ is p ₁(ξ, η) to the distance of z axle, β is on ξ-η coordinate plane, p ₁(ξ is η) with the angle of ξ-η coordinate plane initial point line and ξ axle.

Following formula substitution (3) can be got

$U(r,\mathrm{\&alpha;})=U\left(r\right)=\frac{e^{\mathrm{ikz}}}{\mathrm{i\&lambda;z}}\&CenterDot;e^{i\frac{k}{2z}{\mathrm{<figure-callout\; id="2"\; label="r"\; filenames="H200910198268XE00012.png"\; state="\{\{state\}\}">r</figure-callout>}}^2}\&Integral;\&Integral;\{U(\mathrm{\&rho;},\mathrm{\&beta;})\&CenterDot;e^{i\frac{k}{2z}{\mathrm{\&rho;}}^2}\}\&CenterDot;e^{\frac{i2\mathrm{\&pi;}}{\mathrm{\&lambda;}\&CenterDot;z}\mathrm{r\&rho;}(\cos \mathrm{\&alpha;}\cos \mathrm{\&beta;}+\sin \mathrm{\&alpha;}\sin \mathrm{\&beta;})}\&CenterDot;\mathrm{\&rho;d\&rho;}\&CenterDot;\mathrm{d\&beta;}$

$=\frac{e^{\mathrm{ikz}}}{\mathrm{i\&lambda;z}}\&CenterDot;e^{i\frac{\mathrm{<figure-callout\; id="2"\; label="k\; r"\; filenames="H200910198268XE00012.png"\; state="\{\{state\}\}">k</figure-callout>}{r}^{}{}^2}{2z}}\underset{0}{\overset{\&infin;}{\&Integral;}}\underset{0}{\overset{2\mathrm{\&pi;}}{\&Integral;}}\{U(\mathrm{\&rho;},\mathrm{\&beta;})\&CenterDot;e^{i\frac{k}{2z}{\mathrm{\&rho;}}^2}\}\&CenterDot;e^{\frac{i2\mathrm{\&pi;}}{\mathrm{\&lambda;}\&CenterDot;z}\mathrm{r\&rho;}(\cos \mathrm{\&alpha;}\cos \mathrm{\&beta;}+\sin \mathrm{\&alpha;}\sin \mathrm{\&beta;})}\&CenterDot;\mathrm{\&rho;d\&rho;}\&CenterDot;\mathrm{d\&beta;}$

$=\frac{e^{\mathrm{ikz}}}{\mathrm{i\&lambda;z}}\&CenterDot;e^{i\frac{{\mathrm{<figure-callout\; id="2"\; label="kr"\; filenames="H200910198268XE00012.png"\; state="\{\{state\}\}">kr</figure-callout>}}^2}{2z}}\underset{0}{\overset{\&infin;}{\&Integral;}}\mathrm{\&rho;d\&rho;U}(\mathrm{\&rho;},\mathrm{\&beta;})\&CenterDot;e^{i\frac{k}{2z}{\mathrm{\&rho;}}^2}\underset{0}{\overset{2\mathrm{\&pi;}}{\&Integral;}}{e}^{\frac{i2\mathrm{\&pi;}}{\mathrm{\&lambda;}\&CenterDot;z}\mathrm{r\&rho;}\left(\cos (\mathrm{\&alpha;}-\mathrm{\&beta;})\right)}\mathrm{d\&beta;}---\left(4\right)$

$=\frac{2\mathrm{\&pi;}\&CenterDot;e^{\mathrm{ikz}}}{\mathrm{i\&lambda;z}}\&CenterDot;e^{i\frac{{\mathrm{<figure-callout\; id="2"\; label="kr"\; filenames="H200910198268XE00012.png"\; state="\{\{state\}\}">kr</figure-callout>}}^2}{2z}}\underset{0}{\overset{\&infin;}{\&Integral;}}U\left(\mathrm{\&rho;}\right)\&CenterDot;e^{i\frac{{\mathrm{k\&rho;}}^2}{2z}}\&CenterDot;J_0\left(\frac{2\mathrm{\&pi;r\&rho;}}{\mathrm{\&lambda;}\&CenterDot;z}\right)\mathrm{\&rho;d\&rho;}$

Notice that numerical simulation solution formula (3) needs 4 to recirculate, and the derivation of equation (4) only needs 2 to recirculate and get final product.

Gauss's light field process phase board that process circular aperture diaphragm blocks and the light field behind the thin lens are:

$U\left(\mathrm{\&rho;}\right)=A_0{e}^{-\frac{{\mathrm{\&rho;}}^2}{W_0^2}}\&CenterDot;\mathrm{Aperture}\left(\mathrm{\&rho;}\right)\&CenterDot;t_{\mathrm{lens}}\left(\mathrm{\&rho;}\right)\&CenterDot;t_{\mathrm{PhasePlate}}\left(\mathrm{\&rho;}\right)---\left(5\right)$

Wherein: Aperture (ρ) is the aperture function, and little pore radius is Rr,

$\mathrm{Aperture}\left(\mathrm{\&rho;}\right)=\begin{array}{c}1,\&CenterDot;\&CenterDot;\&CenterDot;\&CenterDot;\&CenterDot;\mathrm{\&rho;}<=\mathrm{Rr}\\ 0,\&CenterDot;\&CenterDot;\&CenterDot;\&CenterDot;\&CenterDot;\&CenterDot;\mathrm{\&rho;}>\mathrm{Rr}\end{array}---\left(6\right)$

t _Lens(ρ) be lens to the light field transforming function transformation function, f is the focal length of lens, then

$t_{\mathrm{lens}}\left(\mathrm{\&rho;}\right)=e^{-i\frac{{\mathrm{k\&rho;}}^2}{2f}}---\left(7\right)$

t _PhasePlate(ρ) be the phase board function, r ₀Be phase board interior zone radius,

Figure below is for carrying out the process flow diagram of linear analogue with fresnel diffraction method (formula (4)-formula (8)):

See also Fig. 2 again, Fig. 2 becomes the concrete index path of implementing of silk for prolonging laser pulse in air.

In Fig. 2, incident laser pulse 1 passes through an adjustable aperture 2 earlier (because in the concrete experimentation, the parameter of using when experiment condition and design has difference, can utilize aperture 2 to optimize spot size and come the optimization experiment result, to decide according to concrete experiment), change the wavefront of incident light pulse then through the phase board 3 that designs, focus on through lens 4 again and form chevilled silk 5 in the air.Because the wavefront of incident light changes, various piece no longer has been an equiphase in the launching spot, like this in through the focusing process after the lens, the light of various piece can interfere, thereby the intensity to focal zone produces modulation, and the result makes at the geometrical focus place and reduced the focusing light intensity, thereby has reduced the plasma density at geometrical focus place, the chevilled silk of formation can comparatively fast not ended owing to strong plasma blooming effect, thereby the length of silk is prolonged greatly.

We have designed concrete experiment and have come our idea of preliminary identification, are described in detail as follows:

We have used a lens focus that focal length is 50cm, and the incident light energy is the 6mJ/ pulse, the about 40fs of pulsewidth, the about 9.7mm (1/e of spot diameter ²).In order to regulate the size of launching spot, we have put an adjustable aperture.In experimenting, we find out that, do not add under the situation of phase board, and when incident light all injects, the length of the silk 60mm that only has an appointment.We have measured and have become the later far-field spot (after 3m) of silk simultaneously, and the spot center energy is very low, and most of energy all is present on the ring with the form of cone of radiation.But added after the phase board, under the same case (aperture is all opened), the length of silk increases to about 190mm, and length has prolonged three times more than.And the far-field spot central energy is very high, and super continuous radiation (white light) also all concentrates on the center, and this has very important meaning for the ultrashort pulse that utilizes a compression method to obtain high-energy cycle magnitude.

Claims (3)

1. one kind prolongs femtosecond laser becomes the filament length degree in air device, it is characterized in that the adjustable aperture (2), phase board (3) and the condenser lens (4) that set gradually on the light path of collimation femtosecond laser output constitute.

2. prolongation femtosecond laser according to claim 1 becomes the device of filament length degree in air, it is characterized in that described phase board (3) is transmission-type phase board or reflective phase board.

3. one kind prolongs femtosecond laser becomes the filament length degree in air method, is characterised in that to comprise the following steps:

1. carry out linear analogue with the fresnel diffraction method and find the solution the inner circular shaped region radius r that obtains described phase board (3) ₀, inner circular shaped region is with respect to the position phase transition amount of outer shroud

Utilize wet etching method or utilize ion beam etching or other lithographic methods etch a radius at the K9 glass surface and are r ₀Border circular areas, phase transition amount in the pairing position of etching depth is

2. described by claim 1, on the light path of collimation femtosecond laser output, set gradually described adjustable aperture (2), phase board (3) and condenser lens (4);

3. open the collimation femtosecond laser, adjust described adjustable aperture (2), make described femtosecond laser in air, become the filament length degree to reach the longest.

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