Focused propagation of a high-power ultrashort laser pulse is strongly influenced by optical medium nonlinearity that can significantly affect pulse parameters in the focusing region as compared to the linear focusing regime. We present the results of our experimental study and numerical simulations of frequency-angular patterns and laser plasma distribution of a gigawatt-power femtosecond Ti-sapphire laser pulse propagating in air under various focusing conditions. For the first time to the best of our knowledge, we found experimentally and confirm theoretically the range of pulse numerical aperture (NA), from approximately NA = 2 ⋅ 10⁻³ to 5 ⋅ 10⁻³, when both frequency and angular spectra broadening during supercritical laser pulse focusing in air is minimal. We attribute this NA-range to the transition region between quasilinear and nonlinear pulse focusing regimes. Additionally, the energy threshold for different pulse focusing regimes in the wide range of focusing conditions is obtained for the first time through the numerical simulations. This threshold is demonstrated to decrease with pulse NA increase.

高功率超短激光脉冲的聚焦传播受到光学介质非线性的强烈影响，与线性聚焦机制相比，光学介质非线性会显着影响聚焦区域的脉冲参数。我们展示了在各种聚焦条件下在空气中传播的千兆瓦功率飞秒钛蓝宝石激光脉冲的频率-角模式和激光等离子体分布的实验研究和数值模拟结果。据我们所知，我们第一次通过实验发现并从理论上确认了脉冲数值孔径 (NA) 的范围，从大约 NA = 2 ⋅ 10 ^-3到 5 ⋅ 10 ^-3，当在空气中超临界激光脉冲聚焦期间频率和角光谱加宽是最小的。我们将此 NA 范围归因于准线性和非线性脉冲聚焦机制之间的过渡区域。此外，通过数值模拟首次获得了在广泛聚焦条件下不同脉冲聚焦方式的能量阈值。该阈值被证明随着脉冲 NA 的增加而降低。