Laser beam expander has two main purposes: one is to expand the diameter of the laser beam; The second is to reduce the divergence angle of the laser beam. Therefore, it is used for long-distance illumination or projection, as well as focusing systems. The divergence angle of an expanded beam varies inversely proportional to the expansion ratio. Compared to the unenhanced beam, the expanded beam can be focused smaller. The magnification of a laser beam collimating mirror is the magnification of the beam diameter. The product of the beam width of the laser beam output from the laser is approximately a certain value. When the waist radius is increased by x times, the divergence angle is compressed to 1/x of its original value. The compressed divergence angle is actually the collimation of the laser. The product of the spot size and the divergence angle is an optical invariant. Reducing the divergence angle does not improve the beam quality. The beam quality is the product of the waist radius and the divergence angle. While compressing the divergence angle, it will be accompanied by an increase in the waist radius, resulting in an increase in the spot size. It is commonly used in conjunction with a flat field focusing lens in laser marking machines.