How lasers work

A laser is a device that can emit laser light. According to the working medium, lasers can be divided into four categories: gas lasers, solid-state lasers, semiconductor lasers and dye lasers. Recently, free electron lasers have also been developed. High-power lasers are usually pulsed. output.
 
With the exception of free electron lasers, the basic working principle of all lasers is the same. The essential conditions for laser generation are population inversion and gain greater than loss, so the essential components in the device are the excitation (or pump) source and the working medium with metastable energy levels. Excitation is the excitation of the working medium to an excited state after absorbing external energy, creating conditions for realizing and maintaining the population inversion. The excitation methods include optical excitation, electrical excitation, chemical excitation and nuclear energy excitation. The working medium has a metastable energy level so that the stimulated emission dominates, thereby realizing optical amplification. A common component in a laser is also a resonator, but the resonator (see Optical Resonator) is not an essential component. The resonator can make the photons in the cavity have a consistent frequency, phase and running direction, so that the laser has the same frequency, phase and direction. Good directionality and coherence. Moreover, it can well shorten the length of the working material, and can adjust the mode of the generated laser (ie, mode selection) by changing the length of the resonant cavity, so the general laser has a resonant cavity.

laser working substance

Refers to the material system used to achieve particle number inversion and generate stimulated radiation amplification of light, sometimes also called laser gain medium, which can be solid (crystal, glass), gas (atomic gas, ionic gas, molecular gas) ), semiconductors, and liquids. The main requirement for the laser working substance is to achieve a greater degree of population inversion between the specific energy levels of its working particles as much as possible, and to keep this inversion as effectively as possible during the entire laser emission process; To this end, the working substance is required to have suitable energy level structure and transition characteristics.

Incentive pumping system

It refers to a mechanism or device that provides an energy source for the realization and maintenance of the particle number inversion of the laser working material. Depending on the working material and the operating conditions of the laser, different excitation methods and excitation devices can be adopted, and the following four are common. ① Optical excitation (optical pump). The whole excitation device is usually composed of a gas discharge light source (such as xenon lamp, krypton lamp) and a concentrator. This excitation method is also called Lamp pump. ②Gas discharge excitation. The particle number inversion is realized by the gas discharge process that occurs in the gas working substance. The entire excitation device is usually composed of a discharge electrode and a discharge power source. ③ chemical incentives. Particle number inversion is achieved by using the chemical reaction process that occurs inside the working substance, and usually requires appropriate chemical reactants and corresponding initiation measures. ④ Nuclear energy incentives. It uses fission fragments, high-energy particles or radiation produced by small nuclear fission reactions to excite working substances and achieve population inversion.

Optical resonant cavity.

It is usually composed of two mirrors with a certain geometric shape and optical reflection characteristics combined in a specific way. The functions are: ① Provide optical feedback capability, so that stimulated radiation photons travel back and forth in the cavity for many times to form a coherent continuous oscillation. ② The direction and frequency of the reciprocating oscillating beam in the cavity are limited to ensure that the output laser has a certain directionality and monochromaticity. The effect of the resonant cavity depends on (1) the geometry (radius of curvature of the reflecting surface) and the relative combination of the two mirrors that make up the cavity; (2) a given resonant cavity type (which has different effects on light in different traveling directions and different frequencies in the cavity) selective loss characteristics).