1, so that it spreads far enough to measure the change of its spot size. This method is the simplest, but it takes a long distance, the longer the better.

2. Focusing with a convex lens depends on the focal length. The smaller the focal length, the better the parallelism of light. This method requires that the optical quality of convex lens is good enough.

3. First expand the beam with the beam expanding system, and then check its parallelism with the above method. This method is the most accurate, but it is a bit troublesome.