摘要
强飞秒激光在大气中的成丝过程伴随着自聚焦、群速度色散和等离子散焦等非线性光学现象,对于研究激光雷达、新型光源、人工降雨、大气污染物探测、激光远程探测和激光遥感等具有重要意义。飞秒激光在大气中传输时,通常会由于大气湍流导致的空气折射率扰动以及飞秒激光初始能量分布不均匀而产生随机多丝现象,从而影响了光丝的能量分布,缩短了光丝的传播距离并降低了光斑质量,限制了光丝的实际应用。本文介绍了近20年来国内外有关多丝调控研究的进展,分析了调节入射光束的椭圆率、改变激光场强梯度、调制激光相位、引入像散等多丝调控手段,旨在为研究飞秒激光多丝调控提供参考。这些调控手段都能在一定程度上消除多丝产生的随机性,但仍然存在多丝分布控制精度不高、激光能量损耗偏大、激光传输距离不够远等问题。因此,对多丝的调控还有待更加深入的研究。
Significance The filamentation process of ultra-intense femtosecond lasers in the atmosphere is accompanied by significant nonlinear optical effects such as self-focusing,self-steepening,and plasma defocusing.This is essential for studying lidar,new light sources,artificial rainfall,air pollution detection,and laser remote sensing.When the femtosecond laser pulse is propagated into the atmosphere,a random multifilament phenomenon occurs owing to air refractive index perturbation caused by atmospheric turbulence and the initial inhomogeneous energy distribution of the femtosecond laser.This affects the energy distribution of the filament,shortens the propagation distance of the filament,and reduces the spot quality of the beam,therefore limits the practical application of the filament.This review summarizes local and international research progress on multifilaments in the past two decades.A series of multifilament control methods are reviewed,including the introduction of the elliptical rate of the incident beam,variation of the laser field gradient,modulation of the laser phase,and introduction of image dispersion to establish a reference for the study of multifilament regulation in femtosecond lasers.Progress With continuous advancements in laser technology,the peak intensity of femtosecond laser pulse obtained in laboratory tests has far exceeded the relativistic threshold(1018 W/cm^(2))and even reaches 1023 W/cm^(2),which significantly reduces the difficulty of femtosecond laser atmospheric filamentation.This serves as a foundation for experimental research and the practical application of the filament.Researchers have found that the multifilament phenomenon is mainly caused by the perturbation of the refractive index of air and the initial uneven energy distribution of the femtosecond laser.Further studies have also shown that during the formation of femtosecond laser filaments,only a small portion of the laser energy is concentrated in the filament,and most of the laser energy is stored around the filaments as background energy,which is often called the background energy reservoir.In this regard,Mlejnek et al.proposed the theory of dynamic energy compensation for optical filament propagation.It is believed that an energy reservoir with a low light intensity concentrated around the optical filament provides energy for the propagation of the laser filament,and the interaction between background energy reservoirs can sustain the filament.This theory was experimentally confirmed in 2005.Liu et al.interrupted the transmission of background energy by shielding the filament s outer ring,immediately stopping the filament s propagation.In subsequent simulation studies,they found that the required background energy must be at least 50%higher than the total energy required to sustain the self-guided propagation of the filament.The main reasons for the multifilament phenomenon are atmospheric turbulence caused disturbance of air refractive index and the uneven distribution of the initial energy of the femtosecond laser.To effectively control the generation of a stable multifilament structure,researchers have proposed several methods;these methods include introducing ellipticity in the incident beam,changing the laser field intensity gradient(Fig.1,Fig.2),introducing astigmatism(Fig.3),modulating the wavefront phase(Fig.4),introducing axicon,introducing optical anisotropy of the introduced species(Fig.5),and using polarization axis symmetry breaking(Fig.6).These methods reduce and even eliminate the effect of random perturbations on femtosecond filament transmission by modulating the initial energy distribution of the femtosecond laser or the perturbation of the air refractive index cause by atmospheric turbulence,thereby achieving experimentally reproducible femtosecond laser transmission processes.In addition,by increasing the distance between the background energy reservoirs of the filaments and reducing the mutual interference between the energy pools,a multifilament structure with stable transmission can also be produced.Another method to control the orderly spatial distribution of femtosecond multifilaments involves inhibiting the generation of multifilaments,that is,turning the multifilaments into single filaments during laser transmission.Similar to regulating multifilaments,inhibiting multifilament production can also produce controllable filaments.One of the main ways of suppressing the generation of multifilaments is by making the initial light intensity distribution of the laser pulse as smooth as possible,thus reducing the influence of the initial uneven distribution of light intensity and preventing the generation of multiple hot spots.Another method is to reduce the distance between hot spots,causing the energy pools of each hot spot to overlap with each other so that the multifilaments are fused into a single filament.Based on these two techniques,researchers have proposed the use of telescopic systems for beam reduction(Fig.7),the introduction of astigmatism,the use of spatial light modulators or phase templates(Fig.8),and the use of iris diaphragms and axicons to control multifilaments.Conclusions and Prospects The formation process of femtosecond laser filaments is accompanied by rich optical effects such as fluorescence radiation,pulse self-compression,and supercontinuum generation.It has important application prospects in atmospheric pollution detection,new light sources,laser triggering,and terahertz radiation sources.Moreover,the study of femtosecond laser filamentation processes also benefits the development of the optics theory.Random multifilaments limit the practical applications of laser filamentation;hence,the significance of regulating multifilaments is to expand the practical applications of femtosecond lasers.Existing regulation methods for the multifilament phenomenon focus on generating controllable and stable structures and inhibiting multifilament production.Various research methods can be used to eliminate the randomness of multifilaments when the femtosecond laser is propagated to a certain extent in the atmosphere.However,there are still certain problems in multifilament control such as a low distribution control accuracy and shortened laser transmission distance due to laser energy loss.Therefore,the regulation of multifilaments needs to be studied further before it can be widely applied to various fields.
作者
程俊皓
胡理想
王铁军
罗剑
张芳沛
余同普
Cheng Junhao;Hu Lixiang;Wang Tiejun;Luo Jian;Zhang Fangpei;Yu Tongpu(College of Sciences,National University of Defense Technology,Changsha 410073,Hunan,China;State Key Laboratory of High Field Laser Physics,Shanghai Institute of Optics and Fine Mechanics and CAS Center for Excellence in UltraIntense Laser Science,Chinese Academy of Sciences,Shanghai 201800,China;Information Science Academy of China Electronics Technology Group Corporation,Beijing 100086,China)
出处
《中国激光》
EI
CAS
CSCD
北大核心
2023年第14期1-13,共13页
Chinese Journal of Lasers
基金
国家自然科学基金(11875319,U2130123,12105362)
湖南省科技创新计划(2020RC4020)
湖南省自科基金(2021JJ40653)
中国科学院国际合作重点项目(181231KYSB20200033)
上海市科技计划(21511105000)。
关键词
非线性光学
飞秒激光成丝
自聚焦
多丝操控
多丝抑制
nonlinear optics
femtosecond laser filamentation
self-focusing
multifilament manipulation
multifilament suppression
