基于四瓣高斯飞秒激光的多丝阵列产生与调控

Multifilament Array Generation and Control with a Four-petalGaussian Femtosecond Laser Beam

基于亥姆霍兹波动方程和非线性传输波动方程,模拟了四瓣高斯飞秒激光在空气中线性传输和非线性传输的光强空间分布,以期获得规则的多丝阵列稳定传输。研究结果表明,当初始入射激光功率相对较强时,基于四瓣高斯飞秒激光光束可获得规则的多丝阵列产生。通过改变初始束腰半径和光束阶数,实现对光丝阵列间距的调控。光丝阵列间距大于背景能量池尺寸时,光丝阵列将稳定传输且间距保持不变;小于背景能量池尺寸时,多丝相互融合进而形成稳定的单丝。当初始入射激光功率相对较弱且大于自聚焦阈值功率时,将会出现多次自聚焦现象,最终形成稳定的单丝传输。该研究提供了一种产生二维规则飞秒激光光丝阵列的方法,将为基于飞秒激光多丝阵列的实际应用,如太赫兹波增强、空气激光增强、遥感探测、微波通道以及微粒捕获等,提供理论依据。

When femtosecond laser beam power is much larger than the critical power for self-focusing,the beam breaks up into multiple filaments,which have appeared as a promising medium for multichannel white-light radiation,enhanced terahertz generation,enhanced air lasing,and waveguiding of microwave radiation.These applications rely on the realization of a high reproducibility and regular localization of multifilament array pattern.A four-petal Gaussian beam can evolve into a number of mirror symmetric petals in the far field and the petals of higher order beams can be equally spaced.Moreover,the space among the petals is determined by the beam order.Therefore,the four-petal Gaussian beam are a promising beam type for the generation and control of a regular multifilament array.In this study,the linear propagation of a four-petal Gaussian laser beam has been simulated based on Helmholtz wave propagation equation.The result shows that the four-petal Gaussian laser beam evolve into more petals in the far field.A 3×3 laser beam array is generated with beam waist of 0.5 mm and beam order of 2.Further,the spatial distribution of the laser filaments from a four-petal Gaussian femtosecond laser beam has been investigated based on the nonlinear wave propagation equation.When the total laser power of the four-petal Gaussian femtosecond laser beam is 32Pcr,a 2×2 regular multifilament array is generated finally.Here,Pcr is a critical laser power for self-focusing,which is a critical value for the formation of a laser filament.The waist radius and beam order of the four-petal laser beam are set to be 0.5 mm and 2 respectively.The separation between to closed filaments in the laser array would remain at 0.146 cm.If the beam order is set to be 5,the separation between to closed filaments would increase and remain at 0.23 cm.When the total laser power of the four-petal Gaussian femtosecond laser beam is 28Pcr,the waist radius and beam order of the four-petal laser beam are 0.5 mm and 2,the laser beam would split and emerge together during the self-focusing process,leading to a single laser filament occurrence finally.When the waist radius of the four-petal Gaussian femtosecond laser beam is 0.2 mm,the total laser power and beam order are 28Pcr and 2,a regular multifilament array is generated.The separation between two closed laser filaments is around 0.05 cm at the initial stage.Then the separation decreases versus the propagation distance and the multifilament array emerge into a single laser filament finally.The above results indicate that when the input power of a four-petal Gaussian femtosecond laser beam is relative strong,a regular multifilament array will be generated.At the source plane of the four-petal Gaussian laser beam,the separation of the four petals is directly proportional to both the square root of the beam order and the waist radius.Thus,the multifilament array space can be tuned by the initial beam waist and beam order.Generally,the size of the filament background energy reservoir is about 1 mm,which can influence the interaction between two closed filaments in the multifilament array.When the separation is larger than the size of the background energy reservoir,the multifilament array propagates stably and the separation remains constant.When the separation is less than the size of the background energy reservoir,the separation decreases versus propagation distance and the multifilament array evolves into a single filament finally.When the input power of the four-petal Gaussian femtosecond laser beam is relatively low and still larger than the critical power for self-focusing,the multiple self-focusing phenomenon will occur.The laser beam evolves into a single filament finally.This study has provided a new method for the generation of a regular multifilament array and could pave the way to some potential applications relying on multifilament arrays,such as enhanced terahertz generation,air lasing,and waveguiding.