CN103984114A - Small-size double-density grating pair femtosecond pulse compression device - Google Patents

Abstract

The invention discloses a small-size double-density grating pair femtosecond pulse compression device. The device is characterized in that a first grating and a second grating form a grating pair, the first grating and the second grating are parallel and opposite to each other, the first grating is a transmission type rectangular grating, the second grating is a reflection type rectangular grating, the period of the grating pair reaches the wavelength magnitude, the linear density of the second grating is twice that of the first grating, and the second grating is located on a light path of a negative level of diffraction light of the first grating. Femtosecond pulse light to be compressed reaches the back face of the first grating through vertical incidence, is reflected by the second grating and is returned along the original path. According to the small-size double-density grating pair femtosecond pulse compression device, the high-density gratings are adopted, and the large dispersion measure can be compensated for even though the distance between the two gratings is quite small. The device has the advantages that the structure is small and exquisite, the material dispersion is small, the weight is low, output pulses have no spectrum space walk-off or angular dispersion, and spectra are wide, and the device is used for femtosecond intracavity compression and enables the resonant cavity structure to be compact.

Description

Small-sized double density grating pair femto second compression device

Technical field

The present invention relates to Femtosecond Pulse Compression, particularly a kind of small-sized double density grating pair femto second compression device choosing.

Background technology

Femtosecond pulse peak power is high, the duration is short.Therefore aspect physics, chemistry, biology, micro-manufacture and micro-processing, be widely used.

The pulse producing in titanium jewel femto-second laser resonator cavity is because the dispersion of the materials such as interacvity gain medium is with just warbling.For compensative material dispersion carrys out compression pulse, generally to introduce negative dispersion element.Conventional negative dispersion element have chirped mirror, prism to and grating pair.Because chirped mirror is expensive, therefore generally all adopt prism to carrying out dispersion compensation.Prism is low to having advantages of energy loss, but the angular dispersion that prism causes also causes spectrum space walk-off effect, this makes to export a series of variations such as the distortion of pulse spot size, pulse droop, wavefront distortion, and this is unfavorable for the generation of pulse and control often.And because the angular dispersion amount of prism is smaller.For material dispersion in compensated cavity, prism between distance generally can be very large, just must use a plurality of catoptrons can make light path compact, make like this light path very complicated, bulky, take up room large and be difficult for regulating.

At technology 1[E.B.Treacy, IEEE.J.Quantum.Electron.QE-5,454-458 (1969)] and technology 2[O.E.Martinez, J.Opt.Soc.Am.B.3,929-934 (1986)] in the method for using reflective holographic grating pair to carry out pulse compression has been proposed.Formerly technology 3[week normal river. Bai Bing, utilize Darman raster to producing multipulse device.Patent of invention, publication number: CN1786750] adopt low-density grating pair to connect again the structure of catoptron, realize conllinear beam splitting and compression to femtosecond pulse.Technology 4[Zhou Changhe formerly, general's Zheng femto second compression device.Patent of invention publication number: CN200959058] propose to utilize double density grating pair to carry out Femtosecond Pulse Compression, thus further simplified device.Technology 5[Zhou Changhe formerly, Jia Wei femto second compression device, patent of invention, publication number: CN101187770] propose to utilize high dencity grating to connect mirror structure to realize the compression to femtosecond pulse.

What formerly technology 3 was considered is to utilize low-density grating to realize Femtosecond Pulse Compression, and their shortcoming is that capacity usage ratio is low, and dispersion measure is less, and apparatus structure is large.Formerly technology 5 utilize high density transmission-type grating to connect catoptron to carry out the femtosecond compression position that takes up room large, complex structure.

Compare with technology 4 double density gratings formerly, the present invention proposes to utilize high dencity grating to form double density grating pair structure to carry out pulse compression, and wherein, the density of the second grating (2) is 2 times of first grating (1) density.First grating (1) is transmission-type rectangular raster, and second grating (2) is reflective rectangular raster.Grating pair spacing is very short just can produce very large dispersion measure, and the pulse Jiang Yanyuan road that enters the second grating (2) returns, and whole light channel structure is compactness simply.The high dencity grating diffraction efficiency that adopts is higher and have wide spectral characteristic, and the dispersion measure of introducing is large, and owing to adopting transmission+reflective gratings unobstructed to light beam to structure.This contrive equipment both can be used for compression in chamber, can be used for again chamber external compression.While compressing in chamber, can make laser cavity structure compact.

Summary of the invention

The present invention proposes a kind of miniaturization double density grating pair femto second compression device, that this femtosecond compression set has is simple and compact for structure, efficiency is high, wide spectrum, pulse without spectral space walk from etc. advantage.

The technology of the present invention solution is:

A kind of miniaturization double density grating pair femto second compression device, its feature is that this compression set is by two parallel, grating is staggered relatively the first grating and the second optical grating constitution grating pair, the first grating is transmission-type rectangular raster, the second grating is reflective rectangular raster, the cycle of grating pair is in wavelength magnitude, and the line density of the second grating is 2 times of line density of the first grating, femtosecond pulse to be compressed is from the back side vertical incidence of the first grating and meet grating equation:

$\sin {\mathrm{\θ}}_1=\frac{{\mathrm{\λ}}_0}{d_1}$

Negative first-order diffraction light after the first optical grating diffraction is incident to the second grating and meets grating equation:

$\sin {\mathrm{\θ}}_1+\sin {\mathrm{\θ}}_2=\frac{{\mathrm{\λ}}_0}{d_2}$

Wherein, θ ₁be the negative first-order diffraction angle of the first grating, θ ₂be the negative first-order diffraction angle of the second grating, d ₁be the cycle of the first grating, d ₂be the cycle of the second grating, and θ ₁=θ ₂.

Technique effect of the present invention is as follows:

Apparatus of the present invention are used double density deep etching grating, do not need to use catoptron, make compression set structure very small and exquisite, because deep etching grating is binary optical elements, comparison is easy.Low-density grating is in the past different, because the distance between grating pair is very little, conventionally in millimeter magnitude, just can provide the negative dispersion amount of prism to hundreds of millimeter in the past.The compression pulse producing is walked from without spectral space and is had wide advantages such as spectral characteristic.

Accompanying drawing explanation

Fig. 1 is the structural representation of femto second compression device of the present invention.

Under the positive chirped pulse incident of Fig. 2, output pulse width is with the variation of grating pair vertical interval.Incident pulse centre wavelength is 800nm, and pulsewidth is 89fs, and chirp value is 3.13 * 10 ^-4rad/fs ², the first grating 1 is transmission-type grating, and line density is 890 lines per millimeters, and the distance between grating pair is 0.3 millimeter can make compression pulse the shortest.

Fig. 3 is the time plot of pulse width before and after compression.

Embodiment

As shown in Figure 1, miniaturization double density grating pair femto second compression device of the present invention, by two the first grating 1 and the second gratings 2 parallel, that grating is staggered relatively, form grating pair, the first grating 1 is transmission-type rectangular raster, the second grating 2 is reflective rectangular raster, the cycle of grating pair is in wavelength magnitude, and the line density of the second grating 2 is 2 times of line density of the first grating 1, and femtosecond pulse to be compressed is from the back side vertical incidence of the first grating 1 and meet grating equation:

$\sin {\mathrm{\θ}}_1=\frac{{\mathrm{\λ}}_0}{d_1}$

Negative first-order diffraction light after the first grating 1 diffraction is incident to the second grating 2 and meets grating equation:

$\sin {\mathrm{\θ}}_1+\sin {\mathrm{\θ}}_2=\frac{{\mathrm{\λ}}_0}{d_2}$

Wherein, θ ₁be the negative first-order diffraction angle of the first grating 1, θ ₂be the negative first-order diffraction angle of the second grating 2, d ₁be the cycle of the first grating 1, d ₂be the cycle of the second grating 2, and θ ₁=θ ₂.

Femtosecond pulse to be compressed impinges perpendicularly on transmission-type the first grating 1, and its negative first-order diffraction light, through reflective the second grating 2 diffraction, returns finally by crossing the second former road of grating 1.Due to what select, be high dencity grating, therefore the distance between grating pair just can meet dispersion compensation in millimeter magnitude.

Grating pair vertical interval is D, for making outgoing and incident pulse separately, can make incident beam along low-angle of grating groove direction deflection, thereby makes the first grating 1 surface upper input hot spot and output facula separately.

According to grating equation, under vertical incidence situation, the diffraction angle i of diffraction light meets respectively:

sinθ ₁＝λ/d ₁，sinθ ₁+sinθ ₂＝λ/d ₂

According to raster density relation: 1/d ₂=2/d ₁obtain θ ₁=θ ₂, under vertical incidence situation, all diffraction lights are got under the situation of negative first-order diffraction, and light beam returns to strict former road.

Technology 2[O.E.Martinez formerly, J.Opt.Soc.Am.B.3,929-934 (1986)] and first technology 5[JangjunZheng, ChangheZhou, EnwenDai, J.Opt.Soc.Am.B.24,979-984 (2007)] compression of limited beam diameter ultrashort pulse light has been carried out to theoretical and experimental study.Consider that pulse to be compressed has positive linear chrip, and be Gaussian beam

$E_i(x,y,t)=\exp (-\frac{{2\ln 2t}^2}{{\mathrm{\τ}}^2})\exp \left({\mathrm{ibt}}^2\right)\exp (-i\frac{k(x^2+y^2)}{2q\left(z\right)})$

Wherein b is chirp factor, and for just.τ is the full width at half maximum (FWHM) of incident pulse, the complex parameter that q (z) is Gaussian beam: z is that light-beam position arrives distance with a tight waist, and σ is waist radius.The expression formula of the pulse frequency domain obtaining after this grating pair device when light beam is:

$\begin{array}{c}{E}_0(x,y,\mathrm{\ω})\∝E_i\left(\mathrm{\ω}\right)\exp \left(\mathrm{ik}{\mathrm{\β}}^2{\mathrm{\ω}}^{2}z\right)\\ \×\exp (-i\frac{k}{2}\frac{y^2}{q(d+2z)})\×\exp (-i\frac{k}{2}\frac{1}{q(d+2{\mathrm{\α}}^{2}z)}{x}^2)\end{array}$

Wherein, E _i(ω) be incident pulse frequency spectrum, what ω was frequency with pulse center frequency is poor, and β and ɑ all come from small angle approximation: Δ θ=α Δ γ+β ω

Wherein, $\mathrm{\α}=-\frac{\cos {\mathrm{\θ}}_1}{\cos {\mathrm{\θ}}_0},\mathrm{\β}=\frac{{-\mathrm{\λ}}^2}{2\mathrm{\π}\mathrm{cd}\cos \mathrm{\θ}}$ α under vertical incidence ²=1

$E_i\left(\mathrm{\ω}\right)=\exp (-\frac{\ln 2{\mathrm{\τ}}^2{\mathrm{\ω}}^2}{8{\left(\ln 2\right)}^2+{2b}^2{\mathrm{\τ}}^4})\exp (-i\frac{b{\mathrm{\τ}}^4{\mathrm{\ω}}^2}{{\left(4\ln 2\right)}^2+{\left(2b{\mathrm{\τ}}^2\right)}^2})$

The coupling with space item without frequency in above formula, this explanation output pulse is without space chirp and angular dispersion, and pulsewidth does not change with the distance of transmission, and the character of Gaussian beam is deferred in space transmission.According to Fourier transform, by obtaining the yet not coupling of life period item and space item of distribution of pulse time domain, there is not the distortion such as pulse droop, light beam superior in quality.

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

${\mathrm{\τ}}_0=\mathrm{\τ}{[{(1-\frac{2b{{\mathrm{\λ}}_0}^3{d}_1}{\mathrm{\π}{c}^2{({d_1}^2-{{\mathrm{\λ}}_0}^2)}^{\frac{3}{2}}}D)}^2+\frac{16{\left(\ln 2\right)}^2{{\mathrm{\λ}}_0}^6{d_1}^2{D}^2}{{\mathrm{\π}}^2{c}^4{({d_1}^2-{{\mathrm{\λ}}_0}^2)}^3{\mathrm{\τ}}^4}]}^{1/2}$

Wherein, ${\mathrm{\τ}}_{\min }=\mathrm{\τ}\sqrt{\frac{4{\left(\ln 2\right)}^2}{b^2{\mathrm{\τ}}^4+4{\left(\ln 2\right)}^2}}=2\ln 2\mathrm{\τ}\sqrt{\frac{1}{b^2{\mathrm{\τ}}^4+4{\left(\ln 2\right)}^2}}$

Condition that now must be satisfied is $D=\frac{\mathrm{\π}{c}^2{\mathrm{\τ}}^{4}b{({d_1}^2-{{\mathrm{\λ}}_0}^2)}^{3/2}}{2{{\mathrm{\λ}}_0}^3{d}_1[b^2{\mathrm{\τ}}^4+4{\left(\ln 2\right)}^2]},$ The compression effectiveness obtaining approaches the Fourier transform limit most.

High dencity grating is more responsive with grating change of distance on the ultrashort pulse impact of input.The dispersion measure that grating pair produces (B) depends on the transmission range z of grating and selected grid stroke density, B is proportional to quadratic sum transmission range long-pending of grid stroke density, change with raster density is faster, because grating is selected high dencity grating, it just can produce very high dispersion measure in the distance of 1mm, and the negative group velocity dispersion of introducing is just fast with change of distance, therefore high dencity grating can compensate the incident pulse with very high chirp value, compression effectiveness is more obvious.

According to theoretical modeling result, can find out, input pulsewidth 89fs, time domain is warbled as 3.13*10 ^-4rad/fs ²wavelength is 800nm, and the line density of the first grating 1 is 890 lines per millimeters, and the line density of the second grating 2 is 1780 lines per millimeters, through the shortest 44fs that is about of output pulse width after this grating pair compression set, is compressed into 50% of original pulse width.Now the vertical range between two grating pairs is 0.3mm, and compression effectiveness is very obvious.And two grating spaces are about 200mm decrement and reach minimum while formerly adopting low grating pair line density 50 lines per millimeter in technology 4.Thereby verified the superiority of this high density transmission-type+reflective rectangular raster to compression set.

Compression set compactness of the present invention is small and exquisite, and efficiency is high, and does not need catoptron, can produce high-quality without space chirp, without the compression femtosecond pulse of angular dispersion.Apparatus of the present invention can substitute prism pair, for chamber inner chamber external compression, thereby reduce the volume of resonator cavity, easily operation.There is good use value.

Claims (1)

1. a miniaturization double density grating pair femto second compression device, it is characterized in that this compression set forms grating pair by two parallel gratings the first grating (1) staggered relatively and the second grating (2), the first grating (1) is transmission-type rectangular raster, the second grating (2) is reflective rectangular raster, the cycle of grating pair is in wavelength magnitude, and the line density of the second grating (2) is 2 times of line density of the first grating (1), femtosecond pulse to be compressed is from the back side vertical incidence of the first grating (1) and meet grating equation:

$\sin {\mathrm{\θ}}_1=\frac{{\mathrm{\λ}}_0}{d_1}$

Negative first-order diffraction light after the first grating (1) diffraction is incident to the second grating (2) and meets grating equation:

$\sin {\mathrm{\θ}}_1+\sin {\mathrm{\θ}}_2=\frac{{\mathrm{\λ}}_0}{d_2}$

Wherein, θ ₁be the negative first-order diffraction angle of the first grating (1), θ ₂be the negative first-order diffraction angle of the second grating (2), d ₁be the cycle of the first grating (1), d ₂be the cycle of the second grating (2), and θ ₁=θ ₂.