Working principle of laser pumping
Energy is absorbed in the medium, creating excited states in the atoms. Population inversion is achieved when the number of particles in an excited state exceeds the number of particles in the ground state or less excited states. In this case, a mechanism of stimulated emission can occur and the medium can be used as a laser or optical amplifier.
The pump power must be above the lasing threshold of the laser. Pump energy is usually provided in the form of light or electrical current, but more exotic sources such as chemical or nuclear reactions have been used.
Laser production conditions:
1. Gain medium: For laser generation, a suitable working substance must be selected, which can be gas, liquid, or solid. Population inversion can be achieved in this medium to create the necessary conditions for lasing.
Obviously, the existence of the metastable state energy level is very beneficial to realize the inversion of the number of particles. There are nearly a thousand kinds of working media, and the laser wavelengths that can be generated include a wide range from vacuum ultraviolet to far infrared. However, considering the laser performance of the laser output, there are certain requirements for the working substance used. The basic requirements are
(1) Uniform optical properties, good optical transparency, and stable performance;
(2) Energy levels with relatively long energy levels (called metastable energy levels);
(3) It has relatively high quantum efficiency.
2. Pumping source: In order to reverse the number of particles in the working medium, a certain method must be used to stimulate the atomic system to increase the number of particles in the upper energy level. Generally, gas discharge can be used to use electrons with kinetic energy to excite medium atoms, which is called electric excitation; pulse light sources can also be used to irradiate the working medium, which is called light excitation; there are also thermal excitation, chemical excitation, etc.
Various excitation methods are visually called pumping or pumping. In order to continuously obtain laser output, it must be continuously "pumped" to maintain more particles in the upper energy level than in the lower energy level.
3. Resonant cavity: With a suitable working substance and pump source, particle number inversion can be realized, but the intensity of stimulated radiation produced in this way is too weak to be practically applied. So people thought of using an optical resonant cavity to amplify.
The so-called optical resonant cavity is actually two mirrors with high reflectivity mounted on the two ends of the laser face to face. One is almost totally reflected, and one is mostly reflected and a small amount is transmitted, so that the laser can be emitted through this mirror.
The light reflected back to the working medium continues to induce new stimulated radiation, and the light is amplified. Therefore, the light oscillates back and forth in the resonant cavity, causing a chain reaction, which is amplified like an avalanche, and produces intense laser light, which is output from one end of the partially reflecting mirror.