Einstein described the stimulated emission of atoms in the 1930 s, and people have been wondering whether this phenomenon can be used to strengthen the light field, because the premise is that the medium must have the state of population inversion (or population inversion). In a two-layer system, this is impossible. The first thing people think of is to use a three-stage system, and the calculation proves the stability of radiation. 1958, American scientists Sholo Schawlow and Townes discovered a magical phenomenon: when they shine the light from neon lamp on a rare earth crystal, the molecules of the crystal will emit bright light, and this light will always gather together. According to this phenomenon, they put forward the "laser principle", that is, when a substance is excited by the same energy as its molecular natural oscillation frequency, it will produce this indiscriminate strong light-laser. They published an important paper on this. (1964 nobel prize in physics) after the publication of the research results of Luo Xiao and downs, scientists from all over the world put forward various experimental schemes, but all failed. 1960 may 16, Mayman, a scientist at Hughes laboratory in California, announced that he had obtained a laser with a wavelength of 0.6943 micron, which was the first laser ever obtained by human beings, and therefore Mayman became the first scientist in the world to introduce lasers into practical fields. 1960 On July 7th, Mayman announced the birth of the world's first laser. Meman's plan is to stimulate Ruby with a high-intensity flash tube. Because ruby is only a kind of corundum doped with chromium atoms in physics, it will emit red light when stimulated. Drill a hole in the surface of the ruby, and install a reflector on the surface of the ruby to let the red light overflow from the hole, thus producing a fairly concentrated and slender red beam, which can reach a temperature higher than the surface of the sun when hitting a certain point. Nikolai Basov, a scientist from the former Soviet Union, invented the semiconductor laser in 1960. The structure of semiconductor laser is usually composed of P layer, N layer and active layer, forming a double heterojunction. It is characterized by small volume, high beam combining efficiency, fast response, wavelength and size suitable for fiber size, direct modulation and good coherence. In the late1980s, semiconductor technology made it possible to use more efficient and durable semiconductor laser diodes, which were used in low-power CD and DVD drives and optical fiber data lines. 1990s realized the principle of high-power laser excitation, such as sheet laser and fiber laser. Because of the new processing technology and high power of 20kW, the latter has been continuously applied in the field of material processing, thus partially replacing CO2 laser and Nd:YAG laser. In 2000, the nonlinearity of laser was used to generate X-ray pulses (to track the process inside atoms). On the other hand, blue light and ultraviolet laser diodes began to enter the market. In 2009, China developed a crystal named KBBF, which can be used to excite deep ultraviolet laser. Once successfully applied, the capacity of each CD can exceed 1tb, and the circuit density that can be stored on the semiconductor can be greatly improved [1]. Now, lasers have become important equipment in industry, communication, science and electronic entertainment.