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Lasers are one of the essential core attachments in modern laser processing systems. Although there are many kinds of lasers, they all generate laser light through excitation and stimulated radiation, so the basic attachment of the laser is fixed, that is, the working substance, the excitation source, and the optical resonator are composed of three parts. The pump source provides the light source for the laser, and the gain medium (also known as the working substance) absorbs the energy provided by the pump source and amplifies the light. The resonator is the loop between the pump light source and the gain medium. The resonator oscillates and selects the mode to output the laser.
Source of motivation. In order to make the particle number inversion appear in the working substance, a certain method must be used to excite the particle system to increase the number of particles at high energy levels. The gas discharge method can be used to excite the working substance by using electrons with kinetic energy, which is called electrical excitation; the pulsed light source can also be used to irradiate the working substance to generate excitation, which is called optical excitation; there are thermal excitation, chemical excitation, etc. Various incentives are figuratively called pumping or pumping. In order to continuously obtain the laser output, it is necessary to continuously "pump" to maintain the number of particles in the excited state. As an energy source, the pump source is used to generate photons to excite the gain medium. The photons emitted by the pump source pump the particles in the gain medium from the ground state to high energy levels to achieve population inversion. The excitation mechanism includes optical excitation (optical pumping), gas discharge excitation, chemical excitation, and nuclear energy excitation. At present, the pump source generally adopts high-power semiconductor laser (LD), and its main function is to complete the conversion of electrical energy into light energy.
Laser working substance. The generation of the laser must select a suitable working substance, which can be gas, liquid, solid or semiconductor. Population inversion can be achieved in this medium to create the necessary conditions for obtaining laser light. Obviously, the existence of metastable energy levels is very beneficial to the realization of population inversion. There are nearly 1,000 kinds of working materials, and the laser wavelengths that can be generated cover the vacuum ultraviolet band to the far infrared band, which is very wide. The gain medium is used to achieve population inversion and amplify the light, while determining the wavelength of the output laser. The gain medium can be liquid, gas and solid. Liquids such as organic solutions, gases such as carbon dioxide, and solids such as rubies. The basic requirement of the gain medium is to generate photons instead of photothermal conversion after being excited, and the particles in it need to be in a relatively isolated state before the transition between energy levels can occur.
Optical resonator. With a suitable working substance and excitation source, population inversion can be achieved, but the intensity of the stimulated emission produced in this way is too low to be applied. It was then thought that optical resonators could be used to "amplify" the stimulated radiation. The optical resonator is composed of two mirrors with certain geometric shape and optical reflection characteristics combined in a specific way. The resonator mainly plays the role of "storing" and "purifying" the laser. At the same time, the resonator can make the photons in the cavity have the same frequency/wavelength, phase and running direction, so that the laser has good directivity and coherence. Its main function is, firstly, to provide optical feedback capability, so that the stimulated radiation photons travel back and forth in the cavity multiple times to form a coherent continuous oscillation. Second, 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.
