This question is somewhat inaccurate. The structure of the laser head of a laser cutting machine is as follows:
From top to bottom, the lenses inside the laser head are, in order: collimating mirror, focusing mirror, protective mirror.
Collimating mirror: after focusing mirror after focusing spot area compared to the area from just out of the energy fiber from the point of light is twice as large, the energy density is the original 1/2.
Focusing mirror: the use of the laser beam of energy for cutting, the laser must be shot mountain of the original beam through the lens focusing, in order to form a high energy density of the spot. According to Gaussian optical theory, the highest power density at the focal point.
Protective mirror: when the laser cuts the material, the work surface will release a large amount of gas and spatter, these gases and spatter will cause damage to the laser cutting head in the focusing mirror, this time the need for protective mirror to protect the focusing mirror.
I hope my answer can help you~