Usually, the amplitude distribution of the cross section of the fundamental mode radiation field emitted by the laser resonator obeys Gaussian function, so it is called Gaussian beam. Gaussian beam is symmetrical about the origin, so a circle is formed at 1/e, and the radius envelope everywhere is hyperboloid, and hyperboloid has asymptote. The propagation characteristic of Gaussian beam is that it propagates at a specific angle along the propagation direction in the distance. This angle is the far-field divergence angle of the beam, that is, the included angle of a pair of asymptotes, which is proportional to the wavelength and inversely proportional to its waist radius.