One of the advantages of laser cutting is the high energy density of the beam, generally 10W / cm2. Because the energy density is inversely proportional to the area, the focal spot diameter should be as small as possible to produce a narrow slit; at the same time, the focal spot diameter is also proportional to the focal depth of the lens. The smaller the focal depth is, the smaller the focal spot diameter is. However, there is spatter in cutting, and the lens is easy to be damaged when it is too close to the workpiece. Therefore, the focal length of 5 ″ ~ 7.5 ″ (127 ~ 190mm) is widely used in the industrial application of high-power CO2 laser cutting machine. The actual focal spot diameter is between 0.1 and 0.4mm. For high quality cutting, effective focal depth is also related to lens diameter and material to be cut. For example, a 5 "lens is used to cut carbon steel, and the focal depth is within + 2% of the focal length, i.e. about 5mm. Therefore, it is very important to control…