The laser is generally composed of three parts, and the solid-state laser is no exception. The working matter is the core of the laser. Only the matter that can realize the energy level transition can be used as the working material of the laser. At present, there are thousands of laser working materials, laser wavelength from X-ray to infrared light. For example, in He-Ne lasers, two levels of neon atoms are reversed by the assistance of helium atoms. The role of the excitation energy (optical pump) is to give energy to the working material, that is, the atom is excited from the low energy level to the external energy at the high energy level. The method of realizing population inversion by irradiating working materials by strong light is called optical pump method. The ruby laser, for example, uses a high power flash lamp to illuminate the ruby (working substance) and achieve population inversion, resulting in the condition of laser generation. Usually there can be light energy, thermal energy, electric energy, chemical energy and so on.