Femtosecond laser-induced optical and structural changes inside transparent materials are crucial to ultrafast laser micromachining. In this work, on-axis beam collapse has been predicted by theoretical simulations and further demonstrated by the experimental results where a femtosecond pulse was loosely focused into a fused silica with the energy far above the self-focusing threshold. A detailed discussion of the energy thresholds and the starting locations on the optical axis of beam collapse for different beam waist radiuses is given. In addition, the spatial distribution of the refractive index has been determined using a spectroscopic ellipsometer and a roughly uniform decrease of 0.1 has been observed in the laser-modified region. Further, the relationship between the spatial distribution of the refractive index and the pulse energy density has been discussed quantitatively. The observed modulation of the beam collapse and the refractive index distribution can be expected to be a useful tool for femtosecond laser micromachining.

飞秒激光诱导透明材料内部的光学和结构变化对于超快激光微加工至关重要。在这项工作中，轴上光束坍塌已通过理论模拟进行预测，并通过实验结果进一步证明，其中飞秒脉冲松散地聚焦到熔融石英中，其能量远高于自聚焦阈值。详细讨论了不同束腰半径的能量阈值和光束坍缩光轴上的起始位置。此外，已经使用光谱椭偏仪确定了折射率的空间分布，并且在激光修改区域中观察到大致均匀的 0.1 下降。更远，定量讨论了折射率的空间分布与脉冲能量密度之间的关系。观察到的光束坍缩和折射率分布的调制有望成为飞秒激光微加工的有用工具。