The physical reason for the difference between the small-scale self-focusing of nanosecond and femtosecond pulses is that the typical intensity of the latter is three orders of magnitude higher, i.e., TW/cm2 versus GW/cm2. This causes a significant shift of the growth-rate maximum of the Bespalov–Talanov instability to the region of high spatial frequencies. During free propagation, a decrease in the spectral density of noise and the self-filtering of the beam lead to the noise-density decrease in the region of the maximum growth rate and, therefore, slowing of the self-focusing development. This is shown to shift the restriction on using the transmissive optical elements in the superpower lasers towards high powers.

中文翻译：

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超功率飞秒激光器小尺度自聚焦的发展特点

纳秒和飞秒脉冲的小尺度自聚焦差异的物理原因是后者的典型强度高三个数量级，即TW/cm2 与GW/cm2。这导致 Bespalov-Talanov 不稳定性的最大增长率显着转移到高空间频率区域。在自由传播过程中，噪声谱密度的降低和光束的自滤波导致最大增长率区域的噪声密度降低，从而减缓了自聚焦发展。这表明可以将超功率激光器中使用透射光学元件的限制转向高功率。