飞秒laser脉冲在空气中从线性到非线性聚焦的能量极限
Energy limit for linear-to-nonlinear femtosecond laser pulse focusing in air
摘要：
紧聚焦的高功率超短激光脉冲在光学介质中的传播通常受到介质光学非线性的显著影响，介质光学非线性可以显著地影响非线性焦点周围的激光脉冲参数，并且导致焦腰的不可避免的并且通常不期望的空间失真。本文介绍了飞秒钛宝石激光脉冲在不同空间聚焦下在空气中传输的实验研究和数值模拟结果。我们集中研究了脉冲发生偏转时，从线性到非线性的不同聚焦状态下的光谱-角变换和空间变换。对于第一次，我们发现的激光脉冲数值孔径范围，即，从NA=0.002至0.005（激光脉冲能量为1 mJ），其中激光脉冲的频率-角谱和脉冲空间形状的失真是最小的。通过数值模拟，得到了在较宽的聚焦条件范围内，激光脉冲在空气中发生线性聚焦和强非线性聚焦的阈值能量和峰值功率。这种能量极限被证明是减少与脉冲数值孔径增强。我们的研究结果确定了激光脉冲的数值孔径和能量足以获得最大的激光强度与良好的光束质量周围的焦点适合于各种激光微图案化和微加工技术。
英文摘要：
Propagation of a tightly focused high-power ultrashort laser pulse in an optical medium is usually substantially influenced by the medium optical nonlinearity that can noticeably affect the laser pulse parameters around the nonlinear focus and lead to unavoidable and often undesirable spatial distortions of the focal waist. We present the results of our experimental study and numerical simulations on a femtosecond Ti-Sapphire laser pulse propagation in air under different spatial focusing. We concentrated our study on spectral-angular and spatial pulse transformations under different focusing regimes, from linear to nonlinear one, when pulse filamentation occurs. For the first time to the best of our knowledge, we found the laser pulse numerical apertures range,namely, from NA=0.002 to 0.005 (for laser pulse energy of 1 mJ), where the laser pulse distortions both in frequency-angular spectrum and pulse spatial shape are minimal. By means of the numerical simulations, we found the threshold pulse energy and peak power in a wide range of focusing conditions, within which a transition between the linear and strongly nonlinear laser pulse focusing in air takes place. This energy limit is shown to decrease with pulse numerical aperture enhancement. Our findings identify the laser pulse numerical apertures and energy adequate for getting a maximum laser intensity with a good beam quality around the focal point suitable for various laser micropatterning and micromachining technologies.
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