CN1972042A - A high rate multi-stage pulse compression method based on nonlinear cascade process - Google Patents

Abstract

This invention relates to one multiple impulse compression method based on non-linear process in laser technique field, wherein, each degree is composed of one block of non-linear transistor and one pair of grating. This invention provides one simple method to improve compression times to get fess impulse from ps.

Description

A kind of high rate multi-stage pulse compression method based on nonlinear cascade process

Technical field

The invention belongs to laser technology field, be specifically related to a kind of passing through and improve, obtain the method for femto-second laser pulse easily high power psec light pulse compression multiple.

Background technology

Femtosecond laser is a kind of laser with the impulse form running, and the duration is short especially, and very high instantaneous power is arranged.Greatly promoted ultrafast optical physics, femtochemistry, the research of femtosecond biology and nonlinear optics etc.

Present High Power Femtosecond Laser device complex structure, price are comparatively expensive.Therefore, the light pulse compression becomes a kind of important technology that produces femtosecond pulse gradually.Present pulse compression technique mainly comprises: (1) monomode fiber compression (J D Kafka, B HKolner, T Baer, D M Bloom, Compression of pulses from a continuous-wave mode-locked Nd:YAGlaser.Opt Lett 1984,9:505-506).This technology mainly is subjected to the restriction of the damage threshold that high-order nonlinear and optical fiber can bear, and therefore can only reach the pulse energy of nJ.(2) utilization is filled with pulse compression technique (the M Nisoli of the capillary hollow of high pressure clean gas, S De Silvestri, O Svelto, Generation of high energy 10 fs pulses by a new pulsecompression technique.Appl Phys Lett 1996,68:2793-2795).Shortcoming is gas ionization can be produced serious self-focusing effect simultaneously when its pulse energy reaches 1mJ.(3) based on the impulse compression method of the third-order non-linear of large scale material (C Rolland, P B Corkum, Compression of high-power optical pulses.J Opt Soc Am B 1988,5:641-647).This method seriously is subjected to the restriction of self focusing, and the quality of deformation influence output pulse also can take place light beam.(4) frequency multiplication compression (Y J Wang, B Luther-Davies, Frequency-doubling pulse compressor forpicosecond high-power neodymium laser pulses.Opt Lett 1992,17:1459-1461).It is the compression to frequency doubled light, can't obtain the first-harmonic pulse laser of short pulse, and the group velocity of three ripples is required comparatively strictness, experimental implementation difficulty.

Summary of the invention

Exist problems such as complex structure, compression multiple are low at the technology of existing high power light pulses compression, the object of the present invention is to provide a kind of simple and easy to doly, can effectively improve the method for pulse compression multiple.

The method of the raising pulse compression multiple that the present invention proposes is to utilize a plurality of nonlinear cascade process.Specifically be a kind of second order pulse compression process, improve high power psec light pulse compression multiple, thereby obtain femto-second laser pulse easily by multistage nonlinear cascade.Wherein, each grade compression process realizes by the dispersor of a non-linear BiBo crystal and a pair of grating composition;

Make light pulse pass through the BiBo crystal, adjust the angle of the incident light and the optical axis of crystal, obtain the amount of mismatch of required position phase, sense the nonlinear phase shift of big value when light pulse is propagated owing to nonlinear cascade process in crystal;

The grating pair dispersor is made up of two blocks of gratings and two lens, presses the 4f system and arranges, by selecting the relative position of grating constant, the focal length of lens and each several part, obtain required dispersion measure, the frequency chirp of compensation negative value is realized the compression of paired pulses, to obtain bigger pulse compression multiple.

In the front and back stages compression process, also use a beam expander, this beam expander is made up of convex lens and concavees lens, select the focal length and the relative distance of two lens, the fundamental frequency photopeak value light intensity attenuation that makes previous stage compression output is to identical with the peak light intensity of initial light field, so that the enforcement of two stages of compression.

Among the present invention, the progression of described multistage nonlinear cascade second order pulse compression process is 2 or 3.

Among the present invention, select frequency-doubling crystal length and grating pair, make nonlinear phase shift and the normal dispersion amount that provides reach best proportioning, thereby obtain optimum output light quality and pulse compression multiple.

Based on the second order pulse compression process of nonlinear cascade as shown in Figure 1, wherein each level all comprises a nonlinear cascade process and a dispersion compensation process (being realized by the grating pair dispersor).

According to the time bandwidth product of Fu Shi conversion limit decision, the spectrum of broad (Δ ν) can be supported shorter light pulse (τ).For in time domain picopulse being compressed to the femtosecond magnitude, key is how just can obtain wideer spectrum.Pulse can be sensed nonlinear phase shift (ΔΦ in cascade process ^Nl), cause frequency chirp, produce new frequency content, thus the spectrum of stretched pulse, the nonlinear phase shift value that this mode produces is big, and can avoid the influence of self focusing.In the compression process of each grade, the present invention all makes light pulse pass through a BiBO crystal (as C1, C2 among Fig. 2), adjust the angle of the incident light and the optical axis of crystal, obtain required position phase amount of mismatch, will sense the nonlinear phase shift of big value when light pulse is propagated owing to nonlinear cascade process in crystal, all represent this cascade process as 1 among Fig. 1 and 4.

During position phase mismatch, be equivalent to third-order non-linear χ in the first kind frequency multiplication production process ⁽³⁾Second order nonlinear cascade effect, in this cascade process, can make spectrum obtain abundant broadening.When considering the third-order non-linear effect, interactional coupledwave equation can be expressed as when fundamental frequency light and frequency doubled light were propagated in this process:

$(\frac{\&PartialD;}{\&PartialD;z}+\frac{{\mathrm{iL}}_{\mathrm{NL}}}{{2\mathrm{<figure-callout\; id="1"\; label="L\; D"\; filenames="A20061011810500081.png"\; state="\{\{state\}\}">L</figure-callout>}}_D}\frac{{\&PartialD;}^2}{{\&PartialD;t}^2}){\mathrm{<figure-callout\; id="1"\; label="E"\; filenames="A20061011810500081.png"\; state="\{\{state\}\}">E</figure-callout>}}_1={\mathrm{<figure-callout\; id="1"\; label="iE"\; filenames="A20061011810500081.png"\; state="\{\{state\}\}">iE</figure-callout>}}_1^{*}{\mathrm{<figure-callout\; id="2"\; label="E"\; filenames="A20061011810500081.png,A20061011810500091.png"\; state="\{\{state\}\}">E</figure-callout>}}_2\exp \left(\mathrm{i\&Delta;kz}\right)+i2\mathrm{\&pi;}\left(n_2{I}_0\right)\&times;\frac{L_{\mathrm{NL}}}{\mathrm{\&lambda;}}({\left|{\mathrm{<figure-callout\; id="1"\; label="E"\; filenames="A20061011810500081.png"\; state="\{\{state\}\}">E</figure-callout>}}_1\right|}^2+2{\left|{\mathrm{<figure-callout\; id="2"\; label="E"\; filenames="A20061011810500081.png,A20061011810500091.png"\; state="\{\{state\}\}">E</figure-callout>}}_2\right|}^2){\mathrm{<figure-callout\; id="1"\; label="E"\; filenames="A20061011810500081.png"\; state="\{\{state\}\}">E</figure-callout>}}_1$

$(\frac{\&PartialD;}{\&PartialD;z}+\frac{L_{\mathrm{NL}}}{L_{\mathrm{GVM}}}\frac{\&PartialD;}{\&PartialD;t}+\frac{{\mathrm{iL}}_{\mathrm{NL}}}{2{\mathrm{<figure-callout\; id="2"\; label="L\; D"\; filenames="A20061011810500081.png,A20061011810500091.png"\; state="\{\{state\}\}">L</figure-callout>}}_D}\frac{{\&PartialD;}^2}{{\&PartialD;t}^2}){\mathrm{<figure-callout\; id="2"\; label="E"\; filenames="A20061011810500081.png,A20061011810500091.png"\; state="\{\{state\}\}">E</figure-callout>}}_2={\mathrm{iE}}_1{\mathrm{<figure-callout\; id="1"\; label="E"\; filenames="A20061011810500081.png"\; state="\{\{state\}\}">E</figure-callout>}}_1\exp (-\mathrm{i\&Delta;kz})+i4\mathrm{\&pi;}\left(n_2{I}_0\right)\&times;\frac{L_{\mathrm{NL}}}{\mathrm{\&lambda;}}(2{\left|{\mathrm{<figure-callout\; id="1"\; label="E"\; filenames="A20061011810500081.png"\; state="\{\{state\}\}">E</figure-callout>}}_1\right|}^2+{\left|{\mathrm{<figure-callout\; id="2"\; label="E"\; filenames="A20061011810500081.png,A20061011810500091.png"\; state="\{\{state\}\}">E</figure-callout>}}_2\right|}^2){\mathrm{<figure-callout\; id="2"\; label="E"\; filenames="A20061011810500081.png,A20061011810500091.png"\; state="\{\{state\}\}">E</figure-callout>}}_2$

In the equation: E ₁, E ₂Represent the amplitude envelope of change slowly of fundamental frequency light and frequency doubled light respectively, E ₁ ^*Be E ₁Complex conjugate, and supposition fundamental frequency light is Gaussian pulse, I ₀Be the peak light intensity of initial fundamental frequency light, λ is the centre wavelength of fundamental frequency light, and initial frequency multiplication light field is made as E ₂=0, Δ k (ω ₀)=k _{2 ω}-2k _ωBe centre frequency ω ₀The ripple at place loses amount of mismatch, notes by abridging to be Δ k, k _{1 ω}, k _{2 ω}Represent the ripple of fundamental frequency light and frequency doubled light to lose L respectively _GVMBe group velocity mismatch (GVM) length between fundamental frequency light and the frequency doubled light, L _NLBe non-linear length, n ₂Be nonlinear refraction rate coefficient, L _D1With L _D2Represent the chromatic dispersion length when frequency is ω and 2 ω respectively, z is normalized propagation distance, and t is the normalized time.Because BiBO crystal second order nonlinear coefficient (d _Eff～3.7pm/V) big, the damage threshold light intensity is higher, the space deviation angle is less, therefore selects BiBO as frequency-doubling crystal.Simultaneously the effective nonlinear coefficient of BiBO crystal in the yz plane is relatively large, can obtain bigger nonlinear phase shift, then gets the yz plane and makes first kind frequency multiplication process based on e+e → o and realize spectrum widening to fundamental frequency light.

In near laser pulse peaks the main time range,, therefore can compensate the frequency chirp of negative value, thereby finally realize the compression of laser pulse by normal dispersion because the frequency chirp of second order cascade process induction is linear basically.The present invention selects grating to check colors to loose device that normal dispersion (as 2 among Fig. 1 and 5 two parts) is provided, the grating pair dispersor mainly by two blocks of gratings (as G1, the G2 of one-level compression section among Fig. 2, the G3 of secondary compression section, G4) and two lens (as L1, the L2 of one-level compression section among Fig. 2, the L3 of secondary compression section, L4) to form, they are pressed the 4f system and arrange.By selecting suitable grating constant, the relative position of the focal length of lens and each several part, obtain needed dispersion measure, the frequency chirp of compensation negative value, the compression that realizes paired pulses is to obtain bigger pulse compression multiple, wherein the effect of ridge mirror (RM1, RM2) is that incident and emergent ray are separated in the direction perpendicular to paper, and the light of dispersor outgoing can not blocked by completely reflecting mirror.

Since the group velocity mismatch between fundamental frequency light and the frequency doubled light can cause on the burst length walk from, cause nonlinear warbling, this non-linear chirp is difficult to compensate well with normal dispersion, pulse will produce little secondary lobe and pulse background.Mismatching of group velocity also can cause fundamental frequency light at the asymmetry of time domain and the broadening of spectrum simultaneously.For long frequency-doubling crystal, the influence of mismatching of group velocity can increase, though can obtain shorter pulse, loss also can increase, and the amount of distortion of pulse increases.

Because the influence of mismatching of group velocity, cascade process can produce non-linear frequency and warble, and influences effective compression of pulse.(because mismatching of group velocity) takes place before but when the phase misalignment dosage is enough big, the positive process of the nonlinear phase shift that produces negative value is separated in pulse with inverse process.The pulse compression multiple is proportional to the nonlinear phase shift that acts on the inceptive impulse in the ordinary course of things $\mathrm{\&Delta;}{\mathrm{\&phi;}}^{\mathrm{nl}}\&ap;-\frac{{\mathrm{\&Gamma;}}^2{\mathrm{<figure-callout\; id="2"\; label="L"\; filenames="A20061011810500081.png,A20061011810500091.png"\; state="\{\{state\}\}">L</figure-callout>}}^2}{\mathrm{\&Delta;kL}},$ Wherein $\mathrm{\&Gamma;}={\mathrm{\&omega;d}}_{\mathrm{eff}}\left|{\mathrm{<figure-callout\; id="0"\; label="E"\; filenames="A20061011810500091.png,A20061011810500101.png"\; state="\{\{state\}\}">E</figure-callout>}}_0\right|/\mathrm{<figure-callout\; id="2"\; label="c\; n"\; filenames="A20061011810500081.png,A20061011810500091.png"\; state="\{\{state\}\}">c</figure-callout>}\sqrt{n_{}}\sqrt{{}_{2\mathrm{\&omega;}}{n}_{\mathrm{\&omega;}}}$ (E ₀Be fundamental frequency light amplitude, n _ω, n _{2 ω}The refractive index in crystal for fundamental frequency light and frequency doubled light, C is the light velocity in the vacuum, L is a crystal length).Then when Δ kL more hour, nonlinear phase shift is just big more.And when Δ kL was big, though nonlinear phase shift is less relatively, the negative effect that the group velocity mismatch causes can be inhibited, and fundamental frequency light reduces to the conversion of frequency doubled light, thereby had reduced the insertion loss of entire device.In order to obtain higher pulse compression multiple and lower loss, need to select optimum Δ kL value to obtain best output pulse.The present invention is to draw among the conclusion figure by analog computation to the selection of Δ kL value, for example for the compression of the pulse of 30ps, from analog computation Fig. 4 and Fig. 5 as can be seen, in first order compression, should select Δ kL～40 π for obtaining optimum output result, (being generally Δ kL=40 π ± 5) obtains exporting pulse (Fig. 6 (a)) width and is～2920fs, peak light intensity is brought up to original～7.2 times.Select Δ kL～60 π in the compression of the second level, (being generally Δ kL=60 π ± 5), obtaining exporting pulse (Fig. 6 (b)) width is～235fs, peak light intensity improves～6.5 times, and as can be seen from the figure changing at Δ kL is not under the very big situation, its value changes the output influence and is little, can obtain more satisfactory impulse compressing result.

We find selecting under the identical situation of crystal total length in analog computation, and the comparable one-level process of secondary process obtains bigger nonlinear phase shift and wideer spectrum, and the pulse compression multiple just can be significantly improved, and the amount of distortion of pulse simultaneously also can reduce.So the present invention has adopted the cascade process of two-stage to carry out the compression of pulse, promptly under the constant situation of crystal total length, adopt the method for classification to obtain bigger pulse compression multiple, by process (6 and 7 represent the first order and partial compression process respectively) as shown in fig. 1, thereby can obtain high-power femtosecond pulse easily by cheap high energy picosecond pulse laser device.

In the two-stage pulse compression configuration of Fig. 1, also used beam expander, form (as Fig. 2) by convex lens (L5) and concavees lens (L6), select the suitable focal length of lens and the relative distance between two lens, the fundamental frequency photopeak value light intensity attenuation that just can make first order compression output is to identical with the peak light intensity of initial light field, so that the enforcement of two stages of compression and comparison.

Analog computation show pulse compression that the present invention passes through secondary in the ideal case to the psec light pulse can compress～130 times, can obtain～1000 times compression multiple by three grades processes.If therefore want to obtain shorter light pulse can reduce each piece crystal under the constant situation of total length of selecting crystal length, adopt the polylith crystal promptly to obtain more powerful pulse compression by the progression that increases pulse compression.But more number of compression stages also can cause the pulse distortion amount and strengthen, and makes experimental implementation become complicated, therefore, generally uses the cascaded compression method of secondary or three grades, and the pulse compression quality that obtains in this process still is more satisfactory (as Fig. 6).

Description of drawings

Fig. 1 is based on the secondary pulse compression schematic diagram of nonlinear cascade process.

Fig. 2 secondary pulse compression experiment index path.

The mark of distance diagram between each element of grating pair dispersor shown in Figure 3 and beam expander.

When Fig. 4 incident light pulse pulsewidth is 30ps, total position phase amount of mismatch Δ k ₁L is to the influence of first order compression.(a) curve A 1 representative compression back fundamental frequency light pulsewidth, curve B 1 have shown the ratio of squeezed light pulse and incident light pulse peak light intensity; (b) curve A 2 is represented the fundamental frequency optical loss, and curve B 2 demonstrates the normal dispersion amount that compensated linear is warbled required.Wherein peak light intensity is than being the ratio of emergent light peak value of pulse light intensity in the first order compression process with the incident light pulse peak light intensity.

The total position of Fig. 5 phase amount of mismatch Δ k ₂L is to the influence of second level compression, and (a) curve A 1 representative compression back fundamental frequency light pulsewidth, curve B 1 have shown the ratio of squeezed light pulse and incident light pulse peak light intensity; (b) curve A 2 is represented the fundamental frequency optical loss, and curve B 2 demonstrates the normal dispersion amount that compensated linear is warbled required.Wherein peak light intensity is than being the ratio of emergent light peak value of pulse light intensity in the compression process of the second level with the incident light pulse peak light intensity.

Fig. 6 (a) Δ kL=40 π, I ₀=30GW/cm ², the result of calculation of one-level pulse compression during L=3cm.(b) Δ k ₁L ₁=40 π, Δ k ₂L ₂=60 π, I ₀=30GW/cm ², L ₁=L ₂The result of calculation of secondary pulse compression during=3cm.Ordinate has shown emergent light peak value of pulse light intensity (I respectively among the figure ₁, I ₂) and initial light peak value of pulse light intensity I ₀The ratio.

Number in the figure: A is initial incident light pulse, and B is the emergent light pulse, and 1,4 all represent cascade process, and 3 is beam expander, and 2,5 is the grating pair dispersor, and 6,7 represent the first order and second level compression process respectively; A is initial incident light pulse, and B is the emergent light pulse, and C1, C2 are two BiBO crystal, and M1, M2 are completely reflecting mirror, and RM1, RM2 are the ridge mirror, and G1, G2, G3, G4 are grating, and L1, L2, L3, L4 and L5 are convex lens, and L6 is concavees lens.

Embodiment

Below in conjunction with accompanying drawing further with case description the present invention of analog computation.

As shown in Figure 2, from the pulse duration of typical Nd:YAG mode-locked laser output is that 30ps, centre wavelength are that the Gaussian pulse of 1064nm incides on the first block length BiBO crystal through completely reflecting mirror M1, we carry out spectrum widening and pulse compression on the yz plane of BiBO crystal, it is 10.69 ° that the adjustment crystal makes incident light and y axle clamp angle, the phase that must put in place amount of mismatch Δ k ₁L～40 π.Grating G1, G2 (G3, G4), convex lens L1, L2 (L3, L4) and ridge mirror RM1 (RM2) constitute grating pair chromatic dispersion device, and normal dispersion is provided, to compressing from the pulse of crystal output.It is 72 ° that adjustment makes by light behind the crystal and grating G1 surface normal angle.Grating G1, G2 are respectively 200cm, 172cm to convex lens L1, L2 surface distance d1, d3, and between convex lens L1, the L2 is 400cm apart from d2, the dispersion measure that provides～2.5 * 10 ⁷Fs ²L1, L2, L3 and L4 are that focal length is the convex lens of 200cm.First BiBO crystal and first group of dispersor have constituted the compressibility of the first order, and second BiBO crystal and second group of dispersor have constituted partial compressibility.Use a telescopic system to expand bundle between one-level and secondary, the distance between convex lens L5 and the concavees lens L6 is 168cm, makes light intensity reduce to original 1/7.2.When inciding second BiBO crystal, adjusting and making incident light and y axle clamp angle is 10.45 °, makes Δ k equally ₂L is～60 π.It is 50 ° that adjustment makes by light behind the crystal and grating G3 surface normal angle.Grating G3, G4 are respectively 200cm, 196cm to convex lens L3, L4 surface distance d4, d6, and between convex lens L1, the L2 is 400cm apart from d5, and the dispersion measure that provides is～1.5 * 10 ⁵Fs ²The analog computation result shows that through behind second block of grating output pulse pulsewidth is～230fs.As can be seen from Figure 6 the quality of the pulse compression output pulse of process two-stage still is reasonable, and has obtained more satisfactory pulse compression multiple.

The parameter of each device that we select among Fig. 2 is: A is that pulsewidth is the initial incident light pulse of 30ps, and B is the emergent light pulse.C1, C2 are the BiBO crystal of two block length 3cm.M1, M2 are completely reflecting mirror, and RM1, RM2 are the ridge mirror, and G1, G2 are the grating of 1000 lines/mm, G1, G2 are the grating of 800 lines/mm, L1, L2, L3 and L4 are that focal length is the convex lens of 200cm, and L5 is that focal length is the convex lens of 268cm, and L6 is that focal length is-concavees lens of 100cm.

Claims (3)

1, a kind of high magnification multi-stage pulse compression method based on the non-line process of cascade is characterized in that improving high power picosecond laser pulse compression multiple by multistage nonlinear cascade second order pulse compression process, thereby obtains femtosecond laser; Wherein, each grade compression process realizes by the dispersor of a non-linear BiBo crystal and a pair of grating composition;

Make light pulse pass through the BiBo crystal, adjust the angle of the incident light and the optical axis of crystal, obtain the amount of mismatch of required position phase, sense the nonlinear phase shift of big value when light pulse is propagated owing to nonlinear cascade process in crystal;

The grating pair dispersor is made up of two blocks of gratings and two lens, presses the 4f system and arranges, by selecting the relative position of grating constant, the focal length of lens and each several part, obtain required dispersion measure, the frequency chirp of compensation negative value is realized the compression of paired pulses, to obtain bigger pulse compression multiple.

2, the high magnification multi-stage pulse compression method based on the non-line process of cascade according to claim 1 is characterized in that the progression of described multistage nonlinear cascade second order pulse compression process is 2 or 3.

3, the high magnification multi-stage pulse compression method based on the non-line process of cascade according to claim 1, it is characterized in that selecting frequency-doubling crystal length and grating pair, make nonlinear phase shift and the normal dispersion amount that provides reach best proportioning, thereby obtain optimum output light quality and pulse compression multiple.

Images