Ultrafast lasers have peak power densities and obtain micron-scale beams by focusing the head. When a light beam acts on a glass material, the light intensity at the center of the beam is lower than that at the edge, so that the refractive index change at the center of the material is greater than that at the edge, the propagation speed of the center of the beam is slower than that at the edge, and the nonlinear optical Kerr effect of the beam seems to produce self-focusing, continue Improve power density.