超快激光成丝现象研究综述 
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1 引 言
高功率超快激光在透明介质(如空气、水、玻璃)中传输时,在各种光学效应的综合作用下,会发生自引导传输的物理现象。激光本身在传播过程中不会产生明显的发散,其传输距离可以远远超越衍射极限。同时,激光光强也不受传输距离的限制,保持在一个很高的水平,在传播过程中会电离介质,从而产生等离子通道[1-8]。这一等离子通道通常被称为“光丝”,这一过程相应被称为成丝现象。成丝现象的另一种显著特征是入射激光光谱会被极大展宽,展宽后的光谱可以覆盖从微波到紫外的超宽范围,被称为白色激光或者超连续谱[1-8],如
图 1. 实验室观测到的超快激光成丝现象。(a)空气中; (b)石英玻璃中
Fig. 1. Ultrafast laser filamentation formation observed in laboratory. (a) In air; (b) in quartz glass
激光成丝现象的研究最早可以追溯至1964年,当时研究者使用调Q脉冲激光器在玻璃中产生了一串直径为数微米的破坏点[9]。随后,人们提出利用基于衍射散焦效应和光克尔自聚焦效应之间动态平衡的自陷模型来解释这一实验现象[10]。该平衡点的激光功率后来被定义为自聚焦阈值功率Pcr,它和波长平方成正比,但反比于非线性折射率系数[11-12]。但根据这一连续波模型,激光成丝实验现象只能在一个自聚焦阈值功率下才能产生,这与实际观测结果并不相符。后来,随着研究工作的深入,研究人员指出,一旦入射功率超过自聚焦阈值功率,激光将会聚于自聚焦焦点处,焦点位置正比于衍射长度,并和功率平方根成反比关系[11-12]。1967年,Marburger等[13]进一步考虑到脉冲激光的入射条件,提出由于脉冲激光在时域中不同强度的切片会在空间中不同的位置自聚焦,因此会形成一系列纵向分布的自聚焦焦点,如
随后的30年,相关方面的研究一直未能取得突破性进展,主要是受限于激光技术的发展。一方面,激光峰值功率低,一般只在玻璃或液体中才能观察到成丝现象;另一方面,脉冲宽度长,电离一旦发生就会导致击穿,不仅掩盖了很多光学效应(特别是等离子体[18-19])的作用,而且大大限制了人们对于其相关应用潜力的认识。这一情况在超快激光啁啾脉冲放大技术被发明后才得到了根本性改变[20]。因为飞秒激光脉冲的峰值功率从此可以轻易地超过空气中的自聚焦阈值(~5 GW)[21],所以成丝现象在空气等气体介质中也能很容易地产生[22-24]。1995年,密歇根大学Mourou教授课题组[22]首次发现了空气中的成丝现象。而且由于脉冲很短(目前普遍<50 fs),传统的光学击穿(主要指介质被完全电离)能被避免,介质被破坏不再是成丝现象出现的主要后果,它的潜在价值也陆续被人们发现,引起了超快激光成丝现象的研究热潮,一直持续至今[1-8]。
概括来说,超快激光成丝现象的主要特点如下:1)光强高,比如空气中产生的光丝的光强一般为5×1013 W/cm2左右[25];2)具有显著导电特性,电离率一般为1‰~1%[26-28];3)光谱宽,可覆盖微波到紫外频谱范围[1];4)传输距离长,甚至从在轨卫星发射的激光有望在大气层中产生成丝现象[29-30]。基于以上优势,人们重点关注的超快激光成丝现象的应用包括:利用脉冲压缩技术获得少周期激光脉冲[31-32],基于四波混频(FWM)效应的紫外、可见、红外甚至THz波等不同频率范围的超快脉冲产生[33-34],基于超连续吸收光谱的大气遥感[1],基于光丝诱导荧光谱的大气污染探测[35-38],包括放电控制、人工降雨雪等在内的人工干预天气技术[27,39-40],以及三维波导激光加工[41]、大幅面或异型面飞秒激光超精细加工[42-43]等。
成丝现象既包含自聚焦、自相位调制、自陡峭、多光子电离和隧穿、高次谐波产生Cherenkov辐射等丰富的非线性光学效应,又处于超高激光强度、超快时间尺度、超远工作距离、多样化的大气条件等极端的微观与宏观环境条件中。相关研究的空间尺度从微米跨越到千米,时间尺度从飞秒跨越至毫秒,还融合了激光非线性传输、大气湍流、大气吸收散射、等离子及气体热力学弛豫等多种物理过程,其复杂性和全面性前所未有。为应对这一挑战,经过近30年的发展,科研人员发展了丰富的实验表征技术、理论模型和相干调控方法。由于其包含丰富的物理原理,具有广泛的应用前景,因此对于成丝现象的研究已经超出了非线性光学的一个简单分支的范畴,正在形成一个新颖的交叉了物理、化学、生物和电子等诸多科学的光学分支——成丝非线性光学,它是当前世界物理研究的最新前沿之一[4]。
本文主要是从作者在相关领域的研究工作经验和认识的角度出发,回顾超快激光成丝现象研究的主要进展,包括:基于荧光光谱法的光丝光强、等离子体密度、非线性折射率系数等重要物理参数的实验表征方法;光强钳制效应、背景能量池作用、空间模式自改善、非线性效应自稳定,及超连续谱产生等光学现象的物理机制或模型;基于时空聚焦、π相位板等激光相位整形的光丝光强分布和多丝相互作用的调控方法等。本文最后重点针对未来成丝现象研究在大气远程应用、空气激光、强THz波产生等领域的应用所面临的主要挑战和攻关方向进行了进一步的展望。
2 成丝动力学过程表征实验研究进展
实验表征技术研究的重点是测量成丝过程中的激光参数、介质物理特性和超快演化过程,从而为丰富实验认识、理解成丝机制及研究调控原理提供关键数据支持。实验表征的最大困难在于光丝中高强度的激光足以打坏任何探测器,无法利用各种常规仪器进行直接测量。为了解决这一困难,人们尝试了多种间接测量方法。
2.1 激光光强测量方法
为避免激光破坏问题,最常用的实验方法就是在光丝中掠入射式放置玻璃片,将第一表面反射的激光导入相机、功率计、光谱仪等实验仪器进行测量。经过一系列必要的反射角度几何校正、反射率标定等步骤,该方法可以简单有效地测量激光的能量、空间分布和光谱等特性。这一方法的缺点在于测量结果容易受到玻璃片前表面质量或者后表面反射光等因素影响。更显著的局限性在于:由于成丝过程中激光脉冲在空间和时间上都经历了复杂的演化,因此它无法对激光强度这一关键参数进行测量。为此,研究人员相继提出通过测量高次谐波截止频率[44]、玻璃板烧蚀深度[45]、热敏纸灰度[46]等多种由激光强度决定的物理参数来间接推断激光强度的技术途径。实验结果表明,利用中心波长在800 nm附近的飞秒激光在空气中产生的光丝的直径在100 μm 左右,光丝内激光强度约为5×1013 W/c
图 3. 通过不同实验方法记录的光丝内激光强度空间分布。(a)玻璃板烧蚀深度[44];(b)热敏纸灰度[46]
Fig. 3. Spatial distributions of laser intensity inside the filament recorded by different methods.(a) Ablation depth of glass plate[44]; (b) gray scale of thermal-sensitive paper[46]
最近,科研人员基于氮气荧光测量法实现了光丝内激光强度原位非侵入式测量。
该方法的优势在于可实现对激光光强的原位和非侵入式测量,甚至可应用于激光光强的远程探测。
图 4. 典型的光丝诱导氮气分子荧光谱[49]
Fig. 4. Typical fluorescence spectra of nitrogen molecules induced by filament[49]
上述基于光丝诱导多光子荧光光谱的方法不仅可用于激光强度的定量测量,还可用于成丝动力学过程的“可视化”研究[50-52]。相比于控制液面高度从而获得不同传输距离的激光参数的实验方法[53],这种“可视化”研究方法因装置简单,在液体成丝动力
学研究中尤其显著。在溶解有染料的甲醇溶液中产生光丝时,通过收集光丝激发的三光子荧光信号,观察到了多次自聚焦现象及多丝的动态竞争过程,分别如
图 5. 利用溶解有染料的甲醇溶液中的光丝激发的三光子荧光所观测到的多次自聚焦现象[50]
Fig. 5. Multiple self-focusing phenomena observed by three photon fluorescence excited by filament in the methanol solution with dissolved dye[50]
图 6. 在溶解有染料的甲醇溶液中,利用光丝激发的三光子荧光观测到的三种典型的多丝动态竞争情况[51]
Fig. 6. Three typical dynamic competition situations of multifilament observed by three photon fluorescence excited by filament in the methanol solution with dissolved dye[51]
基于光丝诱导荧光的“可视化”研究方法也是测量光丝长度和光丝直径的最常见技术途径之一[49,54]。光丝在空气中诱导的氮气荧光的寿命在纳秒量级,因此背向荧光谱也常常被应用于长距离光丝长度的测量,其基本原理是飞行时间法,将背向散射荧光信号到达探测器的不同时间延迟转换为空间距离[55]。另外,光丝产生的超声信号、微波信号都可以应用于光丝长度的表征[56],与荧光信号测量方法的结果有较好的一致性,如
图 7. 光丝长度测量结果。(a)背向氮气荧光的飞行时间测量法[55];(b)侧向超声和微波信号测量法[56]
Fig. 7. Filament length measurement results. (a) Time-of-flight measurment method of backward nitrogen fluorescence[55]; (b) measurement method of the lateral signals of ultrasound and microwave[56]
2.2 激光脉冲时域特征演变表征方法
脉冲时域特征是超快激光的另一个重要参数。但是由于成丝过程中激光脉冲时域变化过于剧烈,脉宽达到亚周期量级[57],频谱覆盖紫外到微波波段,脉冲时域测量问题一直以来都未能得到很好解决。一般会利用掠入式反射器件将光丝中的激光导入到自相关仪或者频率分辨光快门(FROG)等仪器中进行测量,从而得到粗略结果。近期,Odhner等[58]提出利用瞬态频率分辨光快门(TG-FROG)技术可以实现光丝内激光时域包络和相位的原位测量,如
图 8. 瞬态频率分辨光快门实验[58]。(a)实验装置示意图;(b)光丝内激光时域包络、光谱和相位的测量结果
Fig. 8. TF-FROG experiment[58]. (a) Schematic of experimental setup; (b) measurement results of the time-domain envelope, spectra, and phase of the laser in filament
2.3 光丝自由电子密度测量方法
成丝现象实验研究的另一项重要内容是光学介质性质的表征,尤其是光丝中电离产生的自由电子的密度Ne的测量。自由电子的密度测量最早是通过测量光丝的导电率来实现的[27],后来研究人员陆续提出阴影图[60]、干涉图[61]、等离子空间衍射法[62]、超声波探测法[63]、原子荧光谱法[64]等一系列测量手段。阴影图、干涉图、等离子空间衍射等方法都属于超快泵浦-探测技术,利用一束低能量的探测光测量等离子体的复折射率。不同之处在于阴影图测量的是与复折射率虚数部分相对应的吸收系数,而干涉图、等离子空间衍射方法则都是测量的复折射率的实数部分。实验结果表明,阴影图和空间衍射法需要复杂的数据标定和处理过程,干涉图法灵敏度较低,电导率法的测量精度比较低,而超声波法则需要基于另一种独立技术的预校准。相比较而言,原子荧光谱法则在以上方面具有独特优势(
图 9. 原子荧光谱法测量自由电子密度实验[64]。(a)实验装置图;(b)光丝内激发的777 nm附近氧原子荧光谱及其Voigt线型拟合
Fig. 9. Experiment of free electron density measured by atomic fluorescence spectroscopy method[64]. (a) Experimental setup; (b) fluorescence spectrum of oxygen atom excited in filament at 777 nm and its Voigt line fitting
2.4 非线性折射率系数测量方法
另外,非线性折射率系数n2是决定成丝动力学过程的关键参数之一。目前,最简单有效的测量n2值的实验方法是焦点移动法[21]。当使用焦距为f的透镜聚焦超快激光时,它的自聚焦距离会发生改变,改变后透镜的焦距f'遵从透镜变换公式[54],即
式中:zf为介质中自聚焦焦点的位置。分析(2)式可知,当入射激光功率小于自聚焦阈值功率时,f'移动不明显,与f重合,而一旦入射激光功率超过自聚焦阈值功率,焦距f'将向入射方向移动,通过对激光在焦点处产生的等离子体成像即可测量出移动距离,发生明显位移的功率值即对应自聚焦阈值功率,从而可以进一步根据自聚焦阈值功率与n2的正比关系计算其具体数值。
研究结果还证实自聚焦阈值功率与激光脉宽有关[21]。这是因为空气的非线性折射率由两部分组成:光学克尔效应和拉曼效应各自导致的非线性折射率[66]。当入射脉冲宽度远大于拉曼弛豫时间时,其作用近似于连续光,对应的自聚焦阈值功率在5 GW左右;但当脉宽与弛豫时间相当甚至更短时,拉曼效应的作用明显减弱,自聚焦阈值功率则会相应地快速增加。
3 成丝动力学过程的基本物理机制
3.1 激光脉冲包络非线性传输方程
超快激光成丝动力学过程研究最常使用的理论模型是描述激光脉冲包络A时空演化的非线性传输方程,具体表达式为
式中:k0为光脉冲的波数,ω为光脉冲的中心频率,Δ⊥为xy平面的拉普拉斯算符,k2为群速度色散系数,n0为介质折射率,Δnnon为介质的非线性折射率,α为吸收系数。(3)式右侧分别代表时空耦合、衍射、群速度色散、自陡峭、非线性折射率和吸收等光学效应。非线性折射率项Δnnon包含光学克尔效应、拉曼效应、电离、四波混频等各种非线性光学过程。
值得强调的是,玻璃或水等凝聚态物质中的成丝动力学过程和空气等气体中的成丝动力学过程基本相同,主要差异是不同物质密度所带来的自聚焦阈值功率和电离机制不同[67]。因为非线性光学极化系数与物质密度成正比,所以自聚焦阈值功率大致与物质密度成反比关系。因空气中自由电子碰撞时间在皮秒量级,而目前常用的产生光丝的钛宝石飞秒激光器激光脉宽小于百飞秒,同时光丝内的激光光强所导致的电离机制以隧道电离为主[68],所以自由电子速率方程可以简化为一个等效的多光子电离方程[68]。但是对于玻璃或液体,自由电子碰撞时间只有几飞秒,光丝内的电离机制以多光子电离和碰撞电离为主。
需要指出的是,根据基于麦克斯韦方程组的完全电磁场数值模拟对比结果,可知非线性传输方程也适用于单周期激光脉冲的非线性传输动力学研究[69]。另一方面,由于非线性传输方程具有复杂性,一般没有解析解,因此主要依靠数值模拟方法对成丝动力学过程进行理论研究。
图 10. 空气成丝过程中激光脉冲时空变化的数值模拟结果[2]
Fig. 10. Numerical simulation results of the spatial and temporalvariation of laser pulse in the process of air filament[2]
图 11. 光丝内激光角谱分布数值模拟结果[70]
Fig. 11. Numerical simulation result of the laser angular spectrum distribution in filament[70]
区别于(3)式, Moloney课题组[71-72]提出描述激光脉冲电场而非包络的单向传输方程,并在此基础上发展出了相应的数值模拟方法。由于这种方法精确到激光脉冲电场复振幅的瞬时分布,计算量大大增加,其对数值计算平台性能的要求更高。
3.2 空间模式自改善机制
在实际研究工作中,根据研究所关注的主要物理过程,可以对(3)式进行不同程度的简化。比如,提出的自陷模型中只考虑了衍射与光克尔效应的相互作用[10]。虽然自陷模型对于成丝过程的理解具有局限性,但通过深入研究衍射与光克尔自聚焦的相互作用,可以深入认识成丝过程中空间模式的自改善现象,即非理想高斯型分布激光在经过成丝过程后,光丝的核心空间模式可能变为接近理想高斯分布(
图 12. 成丝过程中衍射与光克尔自聚焦相互作用的数值模拟结果[73](41 cm处的空间模式接近理想高斯型)
Fig. 12. Numerical simulation results of the interaction between diffraction and light Kerr self-focusing during the laser-filament process[73](laser mode at z=41 cm is near ideal Gaussian)
3.3 光强钳制效应产生机理
自引导模型只考虑了光克尔效应导致的自聚焦与等离子体引起的散焦(
图 13. 根据自引导模型模拟的空气中光丝内激光强度[22]
Fig. 13. Laser intensity inside air filament based on self-guiding model[22]
光强钳制效应产生的基本原理是光克尔非线性折射率和等离子体等效折射率之间达到平衡,即
式中:Δn为非线性折射率,n2为非线性折射率系数,I为脉冲强度,e为电子电荷,m为有效电离阶数,σ为有效电离横截面面积,ε0为真空中介电常数,me为电子质量,ω0为激光脉冲中心频率,N0为中性分子密度。因为(4)式中除光强外的其他参数均为常数,所以光丝内的激光强度一般为常数,与入射激光能量、传输距离等参数没有线性传输中的正比或平方反比关系,所以光丝内光强保持恒定的现象被称为光强钳制效应,它深刻地影响了成丝的动力学过程及应用所需调控方法的研究工作。(4)式还表明光丝内的钳制光强与泵浦激光的中心波长有关,研究结果表明波长越短,钳制光强越小[78]。根据理论计算结果,中心波长248 nm 泵浦激光产生的光丝内激光光强比中心波长800 nm的泵浦激光小一个数量级,这主要是因为紫外激光诱导分子电离的效率要远高于近红外激光。研究结果还证实了脉宽对于自由电子密度有影响,短脉宽激光对应的光丝中钳制光强更高[79]。
3.4 背景能量池作用
因为光强钳制效应的存在,激光能量不可能无限地聚焦于光丝内,实际能聚焦于光丝内的激光能量一般只有总能量的10%,大部分的能量围绕于光丝周围形成背景能量池[24]。文献[
48]在(3)式的基础上,主要考虑衍射、光克尔自聚焦和等离子体散焦作用,对成丝过程中激光能量的空间变化进行了数值模拟和分析,结果如
图 14. 在光丝中间插入不同直径光阑的数值模拟结果(当光阑直径大于2 mm时,其对于光丝产生过程的影响不明显,小于2 mm时则会不同程度地截断光丝)
Fig. 14. Numerical simulation results of different diameter apertures inserted in the middle of filament (when the diameter of aperture is larger than 2 mm, it hardly affects the generation of filament; when the diameter is smaller than 2 mm, the filament will be cut off)
图 15. 在光丝中间插入不同直径挡板时的数值模拟结果(光丝可以“穿过”亚毫米障碍物)
Fig. 15. Numerical simulation results of different diameter obstacles inserted in the middle of filament (the filament can pass through submillimeter obstacle)
3.5 超连续谱产生主要机制
代表光强钳制效应的(4)式也可以应用于对成丝现象中的超连续谱产生现象进行定性描述[50,87]。非线性传输过程中光谱的展宽Δω来源于自相位调制[88-89]。在激光脉冲的上升沿,自相位调制主要导致光谱的红移,而在脉冲下降沿,只要体现为光谱的蓝移。在激光脉冲上升沿,光克尔效应发挥主要作用,电离过程在逐渐积累自由电子,光谱的最大红移量与激光强度基本是正比关系;但在下降沿,等离子体在自相位调制中发挥重要作用,所导致的光谱最大蓝移量与激光强度成高阶非线性关系;两种效应导致超连续谱呈显著的不对称性,光谱的蓝移量明显高于红移量(
图 16. 在不同介质中,成丝过程所产生的超连续谱[90]
Fig. 16. Supercontinuum spectra induced by the laser filament in different media[90]
3.6 光丝内非线性作用自稳定机制
光强钳制效应[(4)式]的另一个重要影响是改变了光丝内的群速度色散关系,可以大大延长非线性作用的相干长度,从而显著提高高次谐波产生、四波混频等非线性光学效应的效率[33,75]。同样是因为光强钳制效应,在光丝内不仅这些非线性作用的效率有显著提高,而且其稳定性也大大增强[33,92]。
图 17. 光丝内四波混频过程中信号光能量抖动[33]。(a)泵浦钛宝石飞秒激光器输出能量抖动;(b)输入红外信号光能量抖动;(c)低功率下无光丝时空气中四波混频导致的可见光输出能量抖动;(d)高功率下有光丝时空气中四波混频导致的可见光输出能量抖动
Fig. 17. Energy fluctuation of signal light during the four-wave maxing process (FWM) in filament[33]. (a) Output energy fluctuation of the pumped Ti∶sapphire femtosecond lasers; (b) energy fluctuation of input infrared signal light; (c) output energy fluctuation of visible light generated by the FWM below the critical power for self-focusing in air; (d) output energy fluctuation of visible light generated by the FWM above the critical power for self-focu
3.7 高阶光克尔效应讨论
不久以前,欧洲研究人员曾提出利用四阶、六阶甚至十阶等高阶光克尔效应来补偿三阶光克尔效应,代替(4)式中的等离子体散焦作用[93],这一观点曾在相关研究领域引起广泛的关注和激烈的争论[94]。但是,无论是三阶非线性光学效应还是更高阶非线性光学效应都是基于波动方程的微扰理论分
析[88],其基本物理出发点是更高阶次非线性作用要远小于低阶次非线性作用,否则微扰分析法的结果将无法收敛,从而违背了基本物理原则。可以看出,人为引入高阶光克尔效应来补偿三阶光克尔效应事实上违背了上述基本原则,其合理性非常值得商榷[95]。仔细分析(4)式,它所阐述的主要是能量积聚过程和能量发散过程的平衡,除了等离子散焦,吸收、色散等能量发散过程也可以与光克尔自聚焦达到平衡[96]。但是与衍射和自聚焦之间到达平衡的情况类似,由于吸收或色散一般都与激光强度无关,它们与自聚焦达到平衡的条件非常苛刻,实际工作中并没有太多应用价值。
3.8 多丝相互作用物理机制
成丝现象的另一个重要特点是当激光峰值功率远高于自聚焦阈值功率时,通常会出现多丝现象[96]。多丝现象的出现一般是因为激光器输出光强的空间不均匀性或者气体中的湍流[97]或者介质中密度分布不均匀导致非线性折射率在空间上分布不均匀,影响了激光的波前相位[99],从而产生多个光强极大值(称之为“热点”)。以这些热点为中心,在光克尔效应的作用下,周围激光能量向热点积聚,如果功率超过了自聚焦阈值功率,就可能在热点位置产生光丝,多个热点就会相应产生多丝(
图 18. 普通数码相机拍摄的空气里光丝光斑分布[100]。(a)传输距离18 m处的光斑分布(多丝正在形成);(b)传输距离60 m处的光斑分布(多丝辐射的超连续谱发生干涉)
Fig. 18. Light spot distributions in the air captured by ordinary digital camera[100]. (a) Spot distribution at transmission distance of 18 m (multifilaments are forming); (b) spot distribution at transmission distance of 60 m (supercontinuum spectra of multifilament radiation interfere)
需要强调的是,多丝产生过程不是独立和静态的,而是取决于各个光丝之间的距离和功率,因此产生过程中会发生复杂的竞争、合并,甚至产生新光丝等行为[52,82,103-105]。由于这些行为表象的复杂性,多丝也曾被称为“光学湍流”[106]。决定多丝之间相互作用动力学过程的首要因素是距离。根据前面的介绍,由于光强钳制效应,光丝产生的过程本质上是光丝与背景能量池之间能量交换的过程。能量池的直径要远大于光丝自身直径,比如在空气中大于2 mm(
因此,如果光丝之间的距离足够大,特别是远大于背景能量池的距离时,它们之间不会发生显著的相互作用,其发展过程几乎是各自独立的。比如
图 19. 双光丝角辐射发生干涉产生新的“热点”[99]。(a)(b)实验结果;(c)(d)数值模拟结果
Fig. 19. New “hot spot” generated by interference of the angular radiation of two filaments[99]. (a)(b) Experimental results; (c)(d) numerical simulation results
但是,如果光丝之间的背景能量池距离小,相互影响就取决于光丝之间的相位关系。相位关系可由相位板[99]或者初始“热点”的强度等实验参数确定[51]。当光丝之间同相位时,背景能量池之间是相长干涉,会产生新的热点,从而产生新的光丝,多丝的背景能量池发生合并,共同与这一“新生”光丝发生能量交换,表现为多丝互相吸引并融合的现象[103]。相反,如果光丝之间相位不同,尤其是相位差为π时,能量池之间是完全相消干涉,光丝之间会产生一个强度为零的“鸿沟”,阻断光丝之间的相互作用,从而光丝各自独立发展(
3.9 空气激光的发现
成丝过程中出现的另一个独特现象是“空气激光”,即在光丝内辐射的物质荧光谱有受激放大现象(
图 21. 荧光和信号激光在光丝中发生受激放大。(a)光丝中氮气分子的背向探测荧光信号受激放大[108];(b)不同波长信号光在光丝中被放大[111]
Fig. 21. Stimulated amplification of fluorescence and signal lasers in filaments. (a) Stimulated amplification of backscatteredfluorescence intensity of N2 in filament[108]; (b) amplification of signal light with different wavelengths in filament[111]
4 光丝相干调控方法研究进展
经过25年的发展,人们对于光丝中的基本物理机制及特点已经有了比较深入的了解,近期研究的重点是成丝现象在大气遥感、激光加工、太赫兹波产生等各个方面的应用基础研究。面向不同领域的应用,对于光丝相干调控的要求也不尽相同。总体来说,这些调控方法的具体目的有两类:一类是成丝过程中激光强度空间分布的调控,另一类是自由电子密度等介质物理参数的调控。激光强度分布调控方法又大致可以分为单丝品质(如光丝起始位置、长度、强度等参数)的调控,以及多丝有序分布的调控。
4.1 空间聚焦条件对光丝光强空间分布影响
光强分布调控方法的核心是激光脉冲时空相位整形。前面多次提到,成丝现象的最基本物理机制是自聚焦与散焦之间的动态平衡,其本质是激光能量聚集能力与发散能力之间的竞争。因此,几乎所有的时空相位整形调控方法都是围绕这一原则展开的。
在空间上,改变激光脉冲相位分布最简单的方法是使用不同焦距的透镜对激光脉冲进行预聚焦,它不仅可以用来调控光丝的空间位置[(2)式],还可以对光丝内激光光强进行一定的提升。
图 22. 利用光学望远系统调控光丝空间位置的实验装置(图中包括收集背向信号的激光雷达装置)[113]
Fig. 22. Experimental setup for adjusting the spatial position of filament by optical telescope system (the setup includes a lidar device that collects backward fluorescence signal)[113]
实验结果显示透镜焦距对于光丝内激光光强有一定调控作用[115],但这主要是在大数值孔径下才能实现[49,62]。根据大量实验经验,当数值孔径小于0.001时,聚焦透镜焦距对于激光光强不再有显著影响。聚焦数值孔径的另一个重要影响是改变光丝长度。小数值孔径聚焦条件下产生的光丝一般比大数值孔径条件下产生的光丝长。
需要注意的是,利用不同焦距透镜产生光丝时,入射数值孔径不能太大,否则会产生光学击穿效应,那样成丝过程就不发挥主要作用了[116-117]。光丝与击穿的竞争关系(
图 23. 光丝与击穿的竞争关系对于利用光丝在玻璃中直写波导的影响(只有区域4所对应的实验参数可以得到质量较高的波导)[118]
Fig. 23. Influence of the competition relationship between optical filament and breakdown on the direct writing waveguide in glass with filament(only the experimental parameters corresponding to region 4 can be used to obtain high quality waveguides)[118]
4.2 时域啁啾对光丝光强空间分布的影响
在时域,改变激光脉冲相位分布的最简单方法是引入啁啾[120-121],实验上最直接的方式是改变飞秒激光放大系统中压缩器内光栅对之间的距离。取决于初始啁啾与介质群速度色散的关系,啁啾对于光丝光强会有提升或减弱效果[120]。直观来看,啁啾的作用类似于透镜的聚焦作用。比如,在正色散介质中,如果激光脉冲具有初始负啁啾,在传输过程中脉冲首先会被压缩,从而峰值功率提高,等效于提高激光能量聚集能力,相当于使用了更大数值孔径的透镜聚焦,光丝内光强一般会有所提高,从而直接影响超连续谱产生、脉冲自压缩等非线性光学过程的效率[122-123]。需要说明的一点是,初始啁啾在光丝产生THz波过程中发挥着不同的作用[124-126]。光丝产生THz波的主要物理机制是激光瞬时振幅加速光丝中自由电子时所产生的光电流会辐射出低频电磁波[127-128]。因此THz波产生对于激光初始啁啾非常敏感。
根据以上研究思路,目前能够对光丝内光强进行调控的最有效方法是时空聚焦法[129]。如
图 24. 基于时空聚焦法的光丝光强调控方法[129]。(a)实验装置;(b)光谱数值模拟结果
Fig. 24. Filament intensity control method based on spatiotemporal focusing method[129].(a) Experimental setup; (b) spectral numerical simulation results
图 25. 基于空间色散和时间啁啾耦合的光丝时空相位调控方法示意图[131]
Fig. 25. Schematic of spatiotemporal phase control of filaments based on spatial dispersion and temporal chirp coupling[131]
4.3 多丝调控方法
抑制多丝的产生是光丝调控方法研究中另一项具有挑战性的工作,它常常可以同时获得光丝长度延长的效果。3.8节中介绍过多丝产生的重要条件之一是“热点”之间的距离。所以,抑制多丝的最简单方法是通过缩束将激光直径变小,从而使“热点”之间的距离足够小,背景能量池不会发生分裂,最终只会产生一个光丝[57,132]。另外一个思路是通过在空间上对激光相位进行分束来减小“热点”周围的能量,使得单个热点无法产生光丝。因为光丝结束的原因一般是光丝周围能量池已经不足以补偿主要由衍射和等离子散焦所带来的能量发散[133-134],所以这一分束方法不是简单的隔绝,而是在光丝周围形成一个独立的背景能量池,缓慢地将能量注入到中心产生光丝的背景能量池中,从而达到延长光丝的目的。常用的相位整形方法有π相位板[135]、空间光调制器产生涡旋光[136]、同心环光束(
图 26. 利用空间光调制器产生同心环光束来延长光丝长度的实验结果[137]。(a)实验装置图;(b)同心环光束的相位分布;(c)模拟得到的干涉图样;(d)实验得到的干涉图样;(e)~(h)利用同心环光束延长光丝的实验结果
Fig. 26. Experimental results of using spatial light modulator to generate phase-nested beam to extend the length of filaments[137]. (a) Experimental setup; (b) phase distribution of phase-nested beam; (c) simulation result of interference pattern; (d) experiment result of interference pattern; (e)-(h) experiment results of the filament extended by phase-nested beam
多丝调控的另一个重要任务是实现多丝空间分布的可控、有序控制,这对于改善基于光丝的超精细激光加工[141-142]、微波波导[143-144]、太赫兹产生[145]等应用具有重要价值。获得可控、有序分布的多丝的基本方法是采用周期分布特征的光学器件对入射飞秒激光进行分束,包括网筛、光阑、狭缝、π相位板、微透镜阵列、轴锥镜阵列、可变形镜;改变入射激光的椭偏率或者偏振态也可以产生有序排列的多丝[146]。
4.4 等离子体密度增强方法
除了光强空间分布调控,光丝调控的另一个重要内容是等离子体性质调控,这方面的主要技术途径是引入辅助脉冲。比如引入另一束纳秒激光,可以将等离子体寿命延长到微秒量级[147]。其主要物理机制是利用长脉冲激光对分子进行“加热”,提高了飞秒激光诱导电离的效率。这一方法还可被用来提高光丝辐射分子荧光谱的强度(
图 27. 纳秒激光辅助脉冲技术[148]。(a)实验装置图;(b)光丝辐射的荧光信号随纳秒激光能量的变化
Fig. 27. Nanosecond laser assisted pulse technology[148]. (a) Experimental setup; (b) curve of fluorescence signal of filament radiation with the energy of the nanosecond laser
图 28. 利用光丝中的四波混频产生深紫外超快激光[152]。(a)实验装置;(b)输出深紫外超快激光光谱
Fig. 28. Deep ultraviolet ultrafast laser produced by FWM in filaments[152]. (a) Experimental setup; (b) output deep ultraviolet ultrafast laser spectrum
5 结论及挑战
通过前面介绍,可以看出成丝过程蕴含着丰富的物理过程,成丝现象具有广泛的应用前景。至今成丝现象最成功的应用是利用光丝进行激光切割,它已经逐渐成为工业生产中玻璃切割的主流技术,被应用到显示屏、汽车、飞机等众多重要产业中。另外,基于光丝自压缩得到少数周期激光脉冲的技术也实现了商业产品化。
超快激光成丝现象相关研究未来的重要挑战就在于面向更多不同应用领域的需求,开展基础机制、关键技术和示范应用等方面的系统深入研究。下面基于作者的认识,对其中几个广受关注的代表性研究方向进行简要介绍。
5.1 复杂大气条件下的远程传输机制研究
成丝现象的最独特之处是可以突破衍射限制远程产生超高光强,这是其他任何一种激光技术所不具备的优势。因此,成丝现象最大的潜在应用是在大气中的远程应用,包括大气遥感、人工干预天气、激光通信等。为了满足这些应用的现实需求,必须结合大气光学、流体力学等其他领域,对复杂大气条件下的光丝远程传输机制开展深入研究。
最近的研究工作已经在在轨发射激光(
图 29. 从高度400 km的轨道向地球表面发射TW飞秒激光的数值模拟结果[30]。(a')~(a'?)光束直径与高度的关系;(b)(c)最大光强和等离子体密度与高度的关系
Fig. 29. Theoretical numerical calculation results of the TW femtosecond laser propagation from the orbit at an altitude of 400 km toward earth's surface[30]. (a')-(a'?) Beam diameter as a function of altitude; (b)(c) maximum intensity and plasma density versus altitude
5.2 光丝内的物理化学机制研究
这一方面研究工作中最受关注的是空气激光增益的物理机制研究。目前的主要理论包括粒子数反转[156]、受激拉曼[157]、碰撞激发[158]、种子光强饱和[159]、Kramers-Henneberger 态[160]、多态耦合[161]等,科研人员最新还发现了分子轨道角动量对荧光辐射有影响[162]。由于光丝中的增益现象不仅在氮气中,而且在气溶胶[163]、多种碳氢化合物[164-165]等各种物质中都有发现,具有相当普遍性,因此基于光丝的大气污染探测技术具有重要意义(
图 30. 光丝诱导的大气污染源模拟样品的特征指纹谱。(a)三种氟利昂主要成分[35];(b)蛋白固体粉末[168];(c)金属样品[169](插图为光丝激发的贫铀与高浓缩铀590 nm附近荧光谱[170]);(d)盐水气溶胶[171]
Fig. 30. Characteristic fingerprints of simulation sample of air pollution sources induced by filament. (a) Three main components of Freon[35]; (b) solid protein powder[168]; (c) metal sample[169](the inset is the 590 nm fluorescence spectra of depleted and highly enriched uraniums induced by laser filament[170
图 31. 空气中光丝诱导的现象。(a)人工降雨[166];(b)降雪[39]
Fig. 31. Phenomena induced by filament in air. (a) Artificial rainfall[166] ; (b) snowfall[39]
5.3 光丝产生THz波效率优化方法研究
超快激光成丝产生的THz波脉宽窄、峰值强度高(电场强度已达8 MV/cm[172])、频谱范围覆盖宽(最高达到200 THz[173]),在大气遥感领域具有光明的前景和巨大潜力(
图 32. 光丝产生THz波在大气遥感中的应用[174]
Fig. 32. Application of THz wave generated by filaments in atmospheric remote sensing[174]
除以上几个研究方向,受到科研人员广泛关注的重要方向还有面向雷电控制的如何提高等离子体寿命和长度的调控方法,面向少数周期脉冲产生的如何提高压缩后脉冲能量等。
综上所述,超快激光成丝现象研究融合了物理、化学、电子、生物、力学和工程技术等多个交叉学科,不仅蕴含丰富的基础科学问题,而且拥有广阔和光明的应用前景,期待广大科研人员在这一领域深入研究,推广实现它在多个领域的重大应用[1-8,179]。
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Article Outline
刘伟伟, 薛嘉云, 苏强, 陈瑞良. 超快激光成丝现象研究综述[J]. 中国激光, 2020, 47(5): 0500003. Weiwei Liu, Jiayun Xue, Qiang Su, See Leang Chin. Research Progress on Ultrafast Laser Filamentation[J]. Chinese Journal of Lasers, 2020, 47(5): 0500003.
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