In the cutting process of the laser cutting machine, the beam is focused into a small focal point through the lens of the cutting head, so that the focal point reaches a high power density, and the cutting head is fixed on the Z-axis. At this time, the heat input by the light beam far exceeds the part of the heat reflected, transmitted or diffused by the material, and the material is quickly heated to the melting and vaporization temperature. At the same time, a high-speed air stream blows out the melted and vaporized material from the coaxial or non-coaxial side, forming the holes cut in the material. With the relative movement of the focus and the material, continuous slit with narrow width is formed in the hole to complete the cutting of the material [1].
At present, the outer optical path of laser cutting machine mainly uses the flight optical path system. The beam of light emitted from the laser generator passes through the reflector 1, 2 and 3 to the focusing lens on the cutting head. After focusing, light spots are formed on the surface of the material to be processed. The reflector lens 1 is fixed on the body and does not move; Mirror 2 on the beam moves in the x direction with the movement of the beam; The mirror 3 on the z axis moves in the y direction as the z axis moves. It is not difficult to see from the figure that in the cutting process, the length of the optical path changes all the time as the beam moves in the x-direction and the Z-axis part moves in the Y-direction [1].
At present, due to the manufacturing cost and other reasons, the laser beam issued by the civil laser generator has a certain divergence Angle, a “cone”. When the height of the “cone” changes (equivalent to the length of the laser cutting machine light path), the cross-sectional area of the beam on the focusing lens surface also changes. In addition, light also has the property of a wave. Therefore, diffraction phenomenon will inevitably occur, which will cause the transverse expansion of the beam in the process of propagation. This phenomenon exists in all optical systems and can determine the theoretical limits of the performance of these systems. Due to the “conical” shape of Gaussian beam and the diffraction effect of light wave, when the length of optical path changes, the diameter of the beam acting on the lens surface changes all the time, which will cause changes in the size and depth of the focus, but has little effect on the location of the focus. If the focus size and focus depth change in continuous processing, will inevitably have a great impact on the processing, for example, will cause the width of the cutting seam is inconsistent, in the same cutting power will cut through or ablative plate