LASER, which is an abbreviation consisting of the first letter of each word taken from the English Light Amplification by Stimulated Emission of Radiation. It means "light amplification by stimulated emission". The full English name of the laser has fully expressed the main process of manufacturing the laser. In 1964, according to the suggestion of the famous Chinese scientist Qian Xuesen, "optical stimulated emission" was renamed "laser".
Laser is another major invention of mankind since the 20th century, after atomic energy, computers, and semiconductors. It is called the "fastest knife", "the most accurate ruler", "the brightest light" and "singular laser". . Its brightness is about 10 billion times that of sunlight.
        The principle of  laser was discovered by the famous American physicist Einstein as early as 1917, but it was not until 1960 that the laser was successfully manufactured for the first time. Laser came into being under the background of the urgent need of theoretical preparation and production practice. As soon as it came out, it achieved extraordinary rapid development. The development of laser not only gave birth to the ancient optical science and optical technology, but also led to The emergence of a whole new industry. Laser enables people to effectively use unprecedented advanced methods and means to obtain unprecedented benefits and results, thereby promoting the development of productivity.
　 　Laser development history:
　　 In 1917, Einstein put forward a new set of technical theory ‘the interaction of light and matter’. This theory is that in the atoms that make up matter, there are different numbers of particles (electrons) distributed at different energy levels, and the particles at high energy levels are excited by a certain photon, and they will jump from the high energy level to (transition). At a low energy level, light of the same nature as the light that excites it will be radiated at this time, and in a certain state, there can be a phenomenon that a weak light excites a strong light. This is called "optical amplification by stimulated radiation", or laser for short.
　　 In 1953, American physicist Charles Townes used microwaves to realize the predecessor of lasers: microwave stimulated emission amplification (English acronym maser).
　　 In 1957, Gordon Gould coined the word "laser", which theoretically pointed out that light can be used to excite atoms.
In 1958, American scientists Schawlow and Townes discovered a miraculous phenomenon: when they put the light emitted by a neon light bulb on a rare earth crystal, the molecules of the crystal will emit bright, vivid colors. The bright light that always gathers together. Based on this phenomenon, they proposed the "laser principle", that is, when a substance is excited by the energy of the same natural oscillation frequency as its molecule, it will produce this kind of non-divergent strong light-laser. They published important papers for this and won the Nobel Prize in Physics in 1964.
        On May 15, 1960, scientist Mayman of the Hughes Laboratory in California, USA, announced that he had obtained a laser with a wavelength of 0.6943 microns. This was the first laser ever obtained by mankind. As a result, Mayman became the first in the world. Scientists who introduce lasers into practical fields.
　　 On July 7, 1960, Mayman announced the birth of the world's first laser. Mayman's plan was to use a high-strength flash tube to excite rubies. Since ruby is actually just a corundum doped with chromium atoms physically, when ruby is stimulated, it will emit a red light. A hole is drilled on the surface of a ruby with a mirror plated so that the red light can overflow from this hole to produce a fairly concentrated slender red beam. When it hits a certain point, it can reach more than the surface of the sun. High temperature.
At this time, it has been 43 years before Einstein proposed the laser theory to develop the first laser. The main reason is that the probability of stimulated radiation generated by particles in ordinary light sources is extremely small. The two processes of stimulated emission and stimulated absorption exist at the same time. The stimulated emission increases the number of photons, while the stimulated absorption reduces the number of photons. When the matter is in the thermal equilibrium state, the distribution of the particles at each energy level follows the uniform laser beam of the particles in the equilibrium state.
　　Calculate the distribution law. According to the law of statistical distribution, the number of particles at the lower energy level E1 must be greater than the number of particles at the higher energy level E2. In this way, when the light passes through the working substance, the energy of the light will only weaken but not strengthen. To make the stimulated radiation dominate, the number of particles at the high energy level E2 must be greater than the number of particles at the low energy level E1. This distribution is just the opposite of the particle distribution at equilibrium, and is called population inversion distribution, or population inversion for short. How to achieve population inversion technically is a necessary condition for laser generation.