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.

Except for free electron lasers, the basic working principles of all lasers are the same. The necessary conditions for generating lasers are particle number inversion and gain greater than loss, so the key 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 the excited state after absorbing external energy, creating conditions for achieving and maintaining particle number inversion. Excitation methods include optical excitation, electrical excitation, chemical excitation, and nuclear energy excitation. The working medium has a metastable energy level, which is mainly stimulated emission, thereby realizing light 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 consistent frequency, phase, and running direction, so that the laser has the same frequency, phase, and direction. Good directionality and coherence. And it can shorten the length of the working material very well, and the mode of laser generation can be adjusted by changing the length of the resonant cavity (i.e. mode selection), so general lasers have resonant cavities.

Laser working material

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

Excitation pumping system

Refers to the mechanism or device that provides an energy source for achieving and maintaining the particle number inversion of laser processing materials. According to the working material and the working conditions of the laser, different excitation methods and excitation devices can be used. The following four are common. ① Optical excitation (optical pump). The entire excitation device usually consists of a gas discharge light source (such as a xenon lamp, a krypton lamp) and a concentrator. This excitation method is also called lamp pump. ② Gas discharge excitation. Particle number inversion is achieved through the gas discharge process in the gas working material. The entire excitation device is usually composed of a discharge electrode and a discharge power supply. ③ Chemical inducement. Particle number inversion is achieved by utilizing the chemical reaction process inside the working material, which usually requires appropriate chemical reactants and corresponding initiation measures. ④ Nuclear energy excitation. It uses the radiation generated by fission fragments, high-energy particles or small nuclear fission reactions to excite the working material and achieve population inversion.

Optical resonant cavity.

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