Principles and types of laser
What is laser?
LASER(Light Amplification by Stimulated Emission of Radiation) ; To get a better idea, take a look at the image below:

An atom at a higher energy level spontaneously transitions to a lower energy level and emits a photon, a process called spontaneous radiation.
Popular can be understood as: a ball on the ground is its most suitable position, when the ball is pushed into the air by external force (called pumping), the moment the external force disappears, the ball falls from a high altitude, and releases a certain amount of energy. If the ball is a specific atom, then that atom emits a photon of a specific wavelength during the transition.

Classification of lasers
People have mastered the principle of laser generation, began to develop different forms of laser, if according to the laser working material to classify, can be divided into gas laser, solid laser, semiconductor laser, etc..
1, gas laser classification: atom, molecule, ion;
The working substance of gas laser is gas or metal vapor, which is characterized by a wide wavelength range of laser output. The most common is a CO2 laser, in which CO2 is used as a working substance to generate an infrared laser of 10.6um by excitation of electrical discharge.
Because the working substance of the gas laser is gas, the overall structure of the laser is too large, and the output wavelength of the gas laser is too long, the material processing performance is not good. Therefore, gas lasers were soon eliminated from the market, and were only used in certain specific areas, such as laser marking of certain plastic parts.
2, solid laser classification: ruby, Nd:YAG, etc.;
The working material of the solid state laser is ruby, neodymium glass, Yttrium aluminum garnet (YAG), etc., which is a small amount of ions uniformly incorporated in the crystal or glass of the material as the matrix, called active ions.
The solid-state laser is composed of a working substance, a pumping system, a resonator and a cooling and filtering system.The black square in the middle of the image below is a laser crystal, which looks like a light-colored transparent glass and consists of a transparent crystal doped with rare earth metals. It is the special structure of the rare earth metal atom that forms a particle population inversion when illuminated by a light source (simply understand that many balls on the ground are pushed into the air), and then emits photons when the particles transition, and when the number of photons is enough, the formation of laser.In order to ensure that the emitted laser is output in one direction, there are full mirrors (the left lens) and semi-reflective output mirrors (the right lens). When the laser output and then through a certain optical design, the formation of laser energy.

3, semiconductor laser
When it comes to semiconductor lasers, it can be simply understood as a photodiode, there is a PN junction in the diode, and when a certain current is added, the electronic transition in the semiconductor is formed to release photons, resulting in laser. When the laser energy released by the semiconductor is small, the low-power semiconductor device can be used as the pump source (excitation source) of the fiber laser, so the fiber laser is formed. If the power of the semiconductor laser is further increased to the point that it can be directly output to process materials, it becomes a direct semiconductor laser. At present, direct semiconductor lasers on the market have reached the 10,000-watt level.

In addition to the above several lasers, people have also invented liquid lasers, also known as fuel lasers. Liquid lasers are more complex in volume and working substance than solids and are rarely used.