Lasers can be divided into 4 types according to the working substance. ①Solid-state laser: Its working substance is solid. Commonly used are ruby lasers, neodymium-doped yttrium aluminum garnet lasers (ie YAG lasers) and neodymium glass lasers. They have roughly the same structure, and are characterized by their small size, robustness, and high power. Neodymium glass lasers are the devices with the highest pulse output power, reaching tens of megawatts. ②Gas laser: its working substance is gas. Now there are various gas atom, ion, metal vapor, gas molecule lasers. Commonly used are carbon dioxide lasers, helium neon lasers and carbon monoxide lasers, which are shaped like ordinary discharge tubes, and are characterized by stable output, good monochromaticity, and long life, but with low power and low conversion efficiency. ③ Liquid laser: It can be divided into chelate laser, inorganic liquid laser and organic dye laser, the most important of which is organic dye laser, its biggest feature is that the wavelength is continuously adjustable. ④Semiconductor laser: It is a relatively young laser, and the more mature one is the GaAs laser. It is characterized by high efficiency, small size, light weight and simple structure, and is suitable for carrying on airplanes, warships, tanks and infantry. Can be made into rangefinders and sights. However, the output power is small, the directionality is poor, and it is greatly affected by the ambient temperature.