Physicists trace the mechanism of laser generation back to Einstein's hypothesis when he explained the law of blackbody radiation in 19 17, that is, the absorption and emission of light can go through three basic processes: stimulated absorption, stimulated radiation and spontaneous radiation. As we all know, the luminescence of any light source is related to the motion state of particles in its substance. When particles (atoms, molecules or ions) at a low energy level absorb external energy (light) at an appropriate frequency and are excited to jump to a corresponding high energy level (stimulated absorption), they always try to jump to a lower energy level and release excess energy in the form of photons. Without the action of foreign photons, light is spontaneously released (spontaneous radiation), and the light released at this time is ordinary light (such as electric lights and neon lights). ), which is characterized by inconsistent frequency, direction and rhythm of light. However, if the extra energy is released in the form of photons (stimulated radiation) when the foreign photons directly jump from high energy level to low energy level, the released photons are completely consistent with the foreign incident photons in frequency, phase and propagation direction, which means that the foreign light is strengthened, which is called optical amplification. Obviously, if the number of particles in the high-energy level is more than that in the low-energy level through stimulated absorption (the number of particles is reversed), this light amplification phenomenon will be more obvious, and it is possible to form a laser at this time.

It was not until 1958 that two American scientists in the microwave field, C.H. Towns and A. I. Schawlaw, broke the 40-year silence and published a famous paper "Infrared and Optical Laser", pointing out that the possibility and necessary condition for stimulated radiation to dominate luminescence is to realize "population inversion". Their papers immediately excited scientists working in the field of optics and put forward various experimental schemes to realize the inversion of particle numbers, thus opening up a brand-new field of laser research. In the same year, Soviet scientists Basov and prokhorov published a paper "Suggestions on Realizing Three-level Particle Number Inversion and Semiconductor Laser".

As early as 1976, Kononov et al. have found that the population inversion between the high-energy level and the low-energy level of xi ions only exists in a certain distance from the target surface. In 1968, Baco Br et al. used the method of simultaneous solution of nonlinear relaxation equation and one-dimensional adiabatic airflow equation to calculate the inversion of particle number in Laval nozzle. 1992, a research group observed the optical amplification signal without population inversion in sodium vapor for the first time, which took another important step for further study on optical amplification without population inversion.