色散對雙晶交叉偏振濾波輸出特性的影響?

耿易星 李榮鳳 趙研英王大輝 盧海洋 顏學(xué)慶

(北京大學(xué)物理學(xué)院,核物理與核技術(shù)國家重點(diǎn)實(shí)驗(yàn)室,北京 100871)

色散對雙晶交叉偏振濾波輸出特性的影響?

耿易星 李榮鳳 趙研英?王大輝 盧海洋 顏學(xué)慶

(北京大學(xué)物理學(xué)院,核物理與核技術(shù)國家重點(diǎn)實(shí)驗(yàn)室,北京 100871)

(2016年9月13日收到;2016年10月7日收到修改稿)

在北京大學(xué)超小型激光等離子體加速器系統(tǒng)上,通過壓縮器對激光脈沖引入色散,研究了色散對雙晶交叉偏振濾波(XPW)輸出特性的影響.結(jié)果顯示,隨著正負(fù)色散的引入,XPW功率會減小、輸出光譜帶寬也會變窄、輸出光譜中心波長會發(fā)生藍(lán)移和紅移.與此同時,正負(fù)色散對于三者的影響具有不對稱性,負(fù)色散相對于正色散會更快減小XPW功率和輸出光譜帶寬.因此,對雙晶XPW輸出特性而言,正色散的影響要小于負(fù)色散.該結(jié)果對雙晶XPW技術(shù)在高對比度超強(qiáng)激光中的應(yīng)用提供了重要的實(shí)驗(yàn)數(shù)據(jù).

啁啾脈沖放大,交叉偏振濾波,光譜,色散

1 引 言

隨著超短超強(qiáng)激光的不斷發(fā)展,尤其是采用啁啾脈沖放大(chirp pulse amplification,CPA)技術(shù)后[1],飛秒激光聚焦的功率密度不斷提高,當(dāng)前可以達(dá)到的功率密度為1020W·cm-2,為研究強(qiáng)場物理提供新的技術(shù)手段[2-4],在如此高的功率密度下,激光的時域?qū)Ρ榷仍趶?qiáng)場物理實(shí)驗(yàn)中扮演著重要的角色.

但是,在高功率激光裝置中,由于選單元件消光比的限制以及放大過程中的自發(fā)放大輻射效應(yīng)等因素的影響,不可避免地會存在時域背景噪聲,從而導(dǎo)致激光脈沖時域?qū)Ρ榷认陆礫5].當(dāng)超強(qiáng)激光與固體靶相互作用時,如果飛秒激光的自發(fā)放大輻射或者預(yù)脈沖的功率密度超過1012W·cm-2,靶體會先被預(yù)脈沖離化形成預(yù)等離子體,進(jìn)而影響主脈沖與固體靶的相互作用以及最終的實(shí)驗(yàn)結(jié)果[6-8].為了得到高對比度的激光脈沖,目前發(fā)展了多種提高對比度的方法,如可飽和吸收體[9]、光學(xué)參量啁啾脈沖放大[10]、交叉偏振濾波(crosspolarized wave,XPW)[11,12]、等離子體鏡等[13,14].其中XPW技術(shù)結(jié)構(gòu)相對簡單,轉(zhuǎn)化效率高,提高對比度明顯,尤其是雙晶XPW技術(shù)的發(fā)展,可以有效地避免單晶帶來的飽和問題,獲得更高的轉(zhuǎn)化效率,更有益于該技術(shù)在超強(qiáng)激光中應(yīng)用[15].在雙啁啾脈沖放大激光系統(tǒng)中,雙晶XPW技術(shù)可有效地提高太瓦(TW)甚至拍瓦(PW)量級的激光對比度[16,17].

XPW技術(shù)不但可以有效地提高飛秒激光時域?qū)Ρ榷?而且具有良好的光譜展寬和時域壓縮效應(yīng).在CPA系統(tǒng)中,由于增益窄化效應(yīng)會導(dǎo)致光譜帶寬越來越窄,可壓縮的脈沖寬度越來越寬,從而降低激光的功率密度.而XPW輸出光譜帶寬在特定條件下可以獲得倍的展寬,極大地?cái)U(kuò)展了CPA系統(tǒng)的光譜帶寬,使PW激光脈寬可壓縮到30 fs[18].目前已有色散對單晶XPW輸出特性的理論和實(shí)驗(yàn)研究[19-22],還沒有對雙晶XPW輸出特性的相關(guān)研究.因此我們在北京大學(xué)超小型激光等離子體加速器(compact laser plasma acceler-ator,CLAPA)系統(tǒng)上[23,24],通過改變壓縮光柵間距的方法為激光脈沖引入色散,研究了不同色散對雙晶XPW效率、輸出光譜帶寬以及輸出光譜中心波長的影響.研究表明,正負(fù)色散對雙晶XPW上述特征具有不對稱的影響,負(fù)色散更加快速地降低XPW效率和輸出光譜帶寬,并且對輸出光譜中心波長的影響也更加顯著.

2 XPW技術(shù)的原理

XPW過程是由晶體三階非線性張量χ3的實(shí)部各向異性所決定的三階非線性簡并四波混頻過程,輸出波的偏振方向垂直于入射波偏振方向.當(dāng)激光功率密度達(dá)到一定強(qiáng)度時,線偏振激光經(jīng)過非線性晶體后,其波矢會發(fā)生一定角度的旋轉(zhuǎn),產(chǎn)生與原來偏振方向垂直的交叉偏振波.由于對激光強(qiáng)度存在依賴效應(yīng),產(chǎn)生XPW所要求的功率密度一般要大于1012W·cm-2,這樣才能保證有較高的轉(zhuǎn)換效率.如果在光路中放置一對偏振正交的偏振片,主脈沖的功率密度較高,經(jīng)過非線性晶體后偏振方向發(fā)生旋轉(zhuǎn),其正交偏振分量會透過正交的偏振片,而脈沖中的預(yù)脈沖和自發(fā)放大輻射由于功率密度達(dá)不到產(chǎn)生XPW的閾值,不能發(fā)生三階非線性效應(yīng),偏振方向不發(fā)生偏轉(zhuǎn),不能透過正交偏振片,從而被濾掉,因此,XPW技術(shù)可以有效地提高超強(qiáng)激光脈沖的時域信噪比.

3 實(shí)驗(yàn)裝置

實(shí)驗(yàn)在北京大學(xué)CLAPA系統(tǒng)上進(jìn)行,該系統(tǒng)的前級為重復(fù)頻率1 kHz的再生鈦寶石放大器.輸入XPW的單脈沖能量為150μJ,脈沖寬度為40 fs,輸入光譜的中心波長λINI為796.5 nm,輸入光譜帶寬ΔλINI為35.5 nm.實(shí)驗(yàn)中通過改變壓縮光柵的間距引入色散,這里僅考慮光柵引入的二階色散,壓縮器引入色散φ2=±2000 fs2.圖1所示為雙晶XPW技術(shù)的實(shí)驗(yàn)設(shè)置圖,其中P1和P2是正交放置的格蘭棱鏡,消光比優(yōu)于10-6;F1和F2是聚焦系統(tǒng).壓縮后的飛秒激光脈沖以水平偏振經(jīng)過一塊格蘭棱鏡P1后進(jìn)入系統(tǒng),經(jīng)過F1聚焦系統(tǒng)聚焦后進(jìn)入兩塊BaF2晶體,產(chǎn)生的XPW脈沖由透鏡F2準(zhǔn)直后,通過第二塊偏振正交的格蘭棱鏡P2輸出.

圖1 (網(wǎng)刊彩色)雙晶XPW技術(shù)實(shí)驗(yàn)設(shè)置 其中P1和P2是一對正交放置的格蘭棱鏡,F1和F2是聚焦系統(tǒng)Fig.1.(color online)Experimental setup of XPW filter,P1,P2 are orthogonal positioned Glan prisms,F1 and F2 are focus system.

4 實(shí)驗(yàn)結(jié)果與討論

4.1 XPW效率

我們首先測量了XPW效率η與色散φ2之間的關(guān)系,實(shí)驗(yàn)結(jié)果如圖2所示.當(dāng)φ2=0時,η為17%.隨著φ2不斷增大,η不斷減小,從初始的17%降至1%.實(shí)驗(yàn)中發(fā)現(xiàn),±φ2對雙晶XPW效率的影響呈現(xiàn)不對稱性.φ2＜0時,即引入的色散為負(fù),η隨著φ2的增加迅速下降,φ2=-2000 fs2,η=1%.而φ2＞0,即引入的色散為正,η隨著φ2的增加下降比較緩慢,φ2=2000 fs2,η=5%.當(dāng)|±φ2|=1120 fs2時,ηφ2=-1120=3%,ηφ2=1120=10%,ηφ2＜0明顯要小于ηφ2＞0.

圖2 XPW效率與色散φ2的關(guān)系Fig.2.Measured XPW efficiency as a function of quadratic spectral phase.

4.2 XPW輸出光譜帶寬

圖3 XPW輸出光譜帶寬ΔλXPW與色散φ2的關(guān)系 實(shí)線為測量的XPW輸出光譜帶寬,虛線為輸入光譜帶寬Fig.3.Measured spectral width(FWHM)of the XPW pulse as a function of quadratic spectral phase(full line).The dashed line is the spectral width of the initial pulse.

4.3 XPW輸出光譜中心波長

色散φ2不但對于XPW輸出光譜帶寬有影響,而且對于輸出光譜中心波長λXPW也有顯著的作用,如圖4所示,圖中實(shí)線是實(shí)驗(yàn)測量得到的輸出光譜中心波長λXPW,虛線是輸入光譜的中心波長λINI. 可以看出,φ2=0時,λXPW=775 nm＜λINI,存在藍(lán)移.引入色散φ2后,λXPW出現(xiàn)了明顯的移動,并且±φ2對λXPW的影響并不對稱.φ2＜0,λXPW隨著φ2的增加開始進(jìn)行紅移,φ2＜-350 fs2時,λXPW＞λINI,當(dāng)φ2＜-1000 fs2后,λXPW紅移的速率開始變緩,維持在806 nm附近.φ2＞0時,λXPW隨著φ2的增加開始緩慢藍(lán)移,但當(dāng)φ2＞1000 fs2時,λXPW維持在772 nm左右.引入不同色散測量得到的光譜如圖5所示.

圖4 XPW輸出光譜的中心波長λXPW與色散φ2的關(guān)系實(shí)線為測量的XPW 輸出光譜中心波長λXPW,虛線為輸入光譜中心波長λINIFig.4.Measured XPW pulse central-wavelength as a function of quadratic spectral phase(full line).The dashed line is the central-wavelength of the initial pulse.

圖5 不同色散情況下的XPW輸出光譜 (a)輸入光譜;(b)當(dāng)φ2=0 fs2時XPW輸出光譜;(c)當(dāng)φ2=-1960 fs2時XPW輸出光譜;(d)當(dāng)φ2=1960 fs2時XPW輸出光譜Fig.5.Experimental XPW pulse spectrum in different quadratic spectral phase：(a)Spectrum of the initial pulse;(b)XPW pulse spectrum measured whenφ2=0 fs2;(c)XPW pulse spectrum measured whenφ2=-1960 fs2;(d)XPW pulse spectrum measured whenφ2=1960 fs2.

5 結(jié) 論

在北京大學(xué)CLAPA系統(tǒng)上,通過改變壓縮光柵間距的方法為激光脈沖引入色散,研究了色散對雙晶XPW效率、輸出光譜帶寬以及輸出光譜中心波長的影響.結(jié)果顯示,正負(fù)色散的引入會導(dǎo)致XPW功率減小、輸出光譜帶寬變窄、輸出光譜中心波長出現(xiàn)藍(lán)移和紅移,除此之外,正負(fù)色散對于三者的影響具有不對稱性,負(fù)色散相對于正色散會更快減小XPW功率和輸出光譜帶寬,并且負(fù)色散會使輸出光譜中心波長快速紅移,而正色散對于輸出光譜中心波長的影響較小.對雙晶XPW輸出特性而言,正色散的影響要小于負(fù)色散.推測這種不對稱性源于雙晶XPW技術(shù)中的其他非線性效應(yīng)對XPW效率、輸出光譜帶寬和輸出光譜中心波長的影響.

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PACS:06.60.Jn,42.65.—k,42.65.Re DOI:10.7498/aps.66.040601

Influences of quadratic spectral phase on characteristics of two crystal cross-polarized generation with femtosecond pulses?

Geng Yi-XingLi Rong-FengZhao Yan-Ying?Wang Da-HuiLu Hai-YangYan Xue-Qing

(State Key Laboratory of Nuclear Physics and Technology,Peking University,Beijing 100871,China)

13 September 2016;revised manuscript

7 October 2016)

The rapid developments of ultra-intense and ultra-short laser offer the possibility to study laser driven ion acceleration with using solid density target.However,the prepulse and amplified spontaneous emission generated in the amplification can create preplasma at the target front by heating,melting and evaporating a portion of a solid density.The main pulse then interacts with the preplasma,which would be harmful to laser ion acceleration.Therefore,many methods have been developed to enhance the temporal contrast of high power laser system,such as saturable absorber,cross polarized wave generation(XPW)and plasma mirror.With many advantages,such as high conversion efficiency,introducing neither spatial nor spectral distortions,and easy setup compared with other mechanisms,XPW has be√en used to clean the femtosecond laser system.Besides that,the spectrum of the XPW pulse could be broadened by3 times under the best condition compared with the initial spectrum.It can solve the spectrum narrowing problem during the laser amplification to obtain ultra-short femtosecond laser pulse.Here,we experimentally investigate the output power,spectrum bandwidth and center wavelength shift of the generated cross-polarized wave according to the input pulse quadratic spectral phase.

The femtosecond laser pulse in compact laser plasma accelerator system at Peking University is used to investigate the role of quadratic spectral phase in characterizing the two crystal cross-polarized generation.The Ti:Sapphire-based laser system has a central wavelength of 798 nm and bandwidth of 35.5 nm which allows the pulse to be compressed down to 40 fs duration(FWHM).Typical the input pulse energy of XPW is 150μJ and the laser system operates well at 1 kHz repetition rate.The quadratic spectral phase can be increased by changing the position of compressor grating.

The conversion efficiency,spectrum bandwidth and the central wavelength shift by changing the quadratic spectral phase are measured.The conversion efficiency is 17%when quadratic spectral phaseφ2=0,and decreases as quadratic spectral phase increases.The rapid decrease is caused by negative quadratic spectral phase.The spectrum bandwidth is 62 nm under the optimum condition,and the broadening effect exists when quadratic spectral phase is in a range of-280 fs2＜φ2＜1400 fs2.It is slowly blue-shifted whenφ2＞0 and stays at 772 nm whenφ2＞1000 fs2.It starts to be red-shifted whenφ2＜0 and stays at 806 nm finally.

In conclusion,with the increase of quadratic spectral phase,we observe the effects of conversion efficiency and spectrum bandwidth and the shift of central wavelength.Moreover,the influences of positive and negative quadratic spectral phase on characteristics of XPW are different.Our result shows that the negative quadratic spectral phaseis more effective at reducing the conversion efficiency and spectrum bandwidth than the positive one.

chirp pulse amplification,cross-polarized generation,spectrum,quadratic spectral phase

:06.60.Jn,42.65.—k,42.65.Re

10.7498/aps.66.040601

?國家自然科學(xué)基金(批準(zhǔn)號：11504009)和國家重大科學(xué)儀器設(shè)備開發(fā)專項(xiàng)(專項(xiàng)號：2012YQ030142)資助的課題.

?通信作者.E-mail：zhaoyanying@pku.edu.cn

*Project supported by the National Natural Science Foundation of China(Grant No.11504009)and the National Grand Instrument Project,China(Grant No.2012YQ030142).

?Corresponding author.E-mail：zhaoyanying@pku.edu.cn