The three characteristics of laser are good monochromatic, good coherence and good orientation. In fact, the laser has four characteristics, but the last one is not so important. 1 Monochromaticity: the photons emitted by ordinary light sources are different in frequency, so they contain all kinds of colors. Laser is the best monochromatic light source because it emits the same photon frequency. Because the biological effect of light strongly depends on the wavelength of light, the monochromism of laser has been used in clinical selective therapy. In addition, the monochromatic characteristics of laser have been widely used in spectral technology and optical measurement, and have become an important means of basic medical research and clinical diagnosis. 2 good coherence: because the stimulated photons are consistent in phase, coupled with the mode selection of the resonator, there is a fixed phase relationship between the points on the cross section of the laser beam. So the spatial coherence of the laser is very good (the ordinary light generated by spontaneous emission is incoherent. The laser provides us with the best coherent light source. It is because of the advent of laser that the coherent technology is developed by leaps and bounds and holography is realized. (3) good directionality: the divergence angle of laser beam is very small, it is almost a parallel ray, the diameter of spot formed by laser beam on the moon is only about 1 km. In order to concentrate the ordinary light in a certain direction, a concentrator is often used, but even the best searchlights, such as projecting their light onto the moon, will expand to a diameter of more than 1,000 kilometers. The application of good directivity of laser beam in medicine is that laser energy can be highly concentrated in space, so laser beam can be made into laser scalpel. In addition, according to geometrical optics, the smaller the focal spot size of the beam with better parallelism and the better monochromatic laser, the less the speckle size is, and the smaller the focal spot size is, and the better the monochromatic laser is, and there is no dispersion aberration after focusing. It can even be used as a fine scalpel to cut cells or molecules. High brightness: the laser brightness is 1012-1019 times higher than the ordinary light source, is the brightest light source at present, the intense laser can even produce hundreds of millions of degrees of high temperature. High energy laser is one of the most valuable basic characteristics to ensure the effectiveness of laser therapy. The high energy of laser can also be used in laser processing industry and national defense, etc.