has two main uses: 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 remote lighting or projection, as well as focusing systems. Divergence angle of the a beam expanded varies inversely with the expansion ratio. Compared with the beam without beam expansion, the beam after beam expansion can be focused smaller. The magnification of the laser beam expanding collimator is the magnification of the beam diameter. The product of the spot size and divergence angle of the laser beam output from the laser is an optical invariant, which is approximately a certain value. When the waist radius is expanded by x times, the divergence angle is compressed to 1/X of the original, and the compressed divergence angle is actually the collimation of the laser. The beam quality is the product of waist radius and divergence angle; When the divergence angle is compressed, the spot size increases with the increase of waist radius. The laser marker is often used in combination with the flat field focusing lens.