Why should you know the principle of lasers?
Knowing the differences between common semiconductor lasers, optical fibers, disks, and yags, you can also understand a little more when selecting models, and talk more at the same time.
The article mainly focuses on popular science: briefly introduces the principle of laser generation, the main structure of lasers, and several common lasers.
First of all: the principle of laser generation.
The English word "laser" is an abbreviation of "light amplification by stimulated emission of radiation", which means stimulated emission of light amplification. Originally, the Chinese translation of "laser" was directly transliterated as "laser" (and this name is still used in Hong Kong and Taiwan in China), and later under the advocacy of Qian Xuesen, a leader in the scientific and technological world, it was collectively referred to as "laser" in China
Lasers are produced by the interaction of light with matter, the so-called stimulated radiation light amplification; Understanding stimulated radiation light amplification requires an understanding of Einstein's concepts of spontaneous radiation, stimulated absorption, and stimulated radiation, as well as some necessary theoretical foundations.
Theoretical basis 1: Bohr model.
The Bohr model mainly provides the internal structure of the atom, which is convenient for understanding how laser light occurs. The atom is composed of the nucleus and the electrons outside the nucleus, the orbits of the electrons are not arbitrary, the electrons only have some definite orbitals, among which the innermost orbital is called the ground state; If the electron is in the ground state, then its energy is the lowest, if the electron jumps out of an orbit, it is called the first excited state, and the energy of the first excited state will be higher than the energy of the ground state; The outer orbit is called the second excited state;
Lasers can occur because electrons move in different orbits in this model, and if the electrons absorb energy, they can run from the ground state to the excited state; If an electron returns from the excited state to the ground state, it releases energy, which is often released in the form of a laser.
Theoretical basis 2: Einstein's stimulated radiation theory.
In 1917, Einstein proposed the theory of stimulated radiation, which is the theoretical basis for lasers and laser production: the absorption or luminescence of matter is essentially the result of the interaction between the radiation field and the particles that make up the matter, and its core essence is the transition between particles at different energy levels. There are three distinct processes in which light interacts with matter: spontaneous radiation, stimulated radiation, and stimulated absorption. For a system with a large number of particles, these three processes always exist simultaneously and are closely linked.
Spontaneous radiation:
As shown in the figure, an electron at the higher energy level E2 spontaneously transitions to the lower energy level E1 and emits a photon with energy HV, and HV=E2-E1; This spontaneous transition process that is not related to the outside world is called spontaneous transition, and the light waves emitted by the spontaneous transition are spontaneous radiation.
The characteristics of spontaneous radiation: each photon is independent, the direction and phase are different, and the occurrence time is also random and belongs to the chaotic incoherent light, which is not the light required by the laser, so the laser generation process should reduce this kind of stray light, which is also one of the reasons why the wavelength of the laser has stray light, the control is good, the proportion of spontaneous radiation of the laser is negligible, the purer the laser, for example, 1060, it is all 1060nm, so the laser will be more stable in terms of absorption rate and power.
Stimulated absorption:
Electrons at lower energy levels (lower orbits) move to higher energy levels (high orbits) after absorbing photons, a process called stimulated absorption. Stimulated absorption is very critical and is one of the key pumping processes, the pumping source of the laser is to provide photon energy to make the particles in the gain medium transition, and wait for stimulated radiation to a higher energy level to emit a laser.
Stimulated radiation:
When irradiated by the light of foreign energy (HV=E2-E1), the electrons at the high energy level are excited by the foreign photons and jump to the lower energy level (the high orbit runs to the low orbit), and at the same time, emit a photon that is exactly the same as the foreign photon, this process does not absorb the original excited light, so there will be two identical photons, which can be understood as the electron spits out the photon absorbed before, this luminescence process is called stimulated radiation, and stimulated radiation is the reverse process of stimulated absorption.
After the theory is clear, it is very simple to build a laser, as shown in the figure above: under normal conditions of material stability, most of the electrons are in the ground state, and the electrons in the ground state, and the laser is dependent on and stimulated radiation, so the structure of the laser is to let the stimulated absorption occur first, get the electrons to a high energy level, and then give an excitation, so that a large number of high-energy electrons are stimulated radiation, release photons, and then the laser can be generated, and the laser structure is introduced next.
Laser structure:
The laser structure matches the laser generation conditions above
Occurrence conditions and corresponding structures:
1. There is a gain medium that provides amplification as the laser working medium, and its active particles have an energy level structure suitable for generating stimulated radiation (mainly to pump electrons to high-level orbits, and can exist for a certain period of time, and then release photons in one breath through stimulated radiation);
2. There is an external excitation source (pump source), which can pump the electrons of the lower energy level to the upper energy level, so that the number of particles between the upper and lower energy levels of the laser is reversed (that is, there are more particles at the higher energy level than at the lower energy level), such as the xenon lamp of the YAG laser;
3. There is a resonator to realize laser oscillation, which can increase the working length of the laser working substance, screen the light wave mode, control the propagation direction of the beam, and selectively amplify the stimulated radiation frequency to improve monochromaticity (to ensure that the laser is output again to a certain energy).
The corresponding structure is shown in the figure above, which is a simple structure of a YAG laser, and the others will be complicated, but the core is this, and the laser generation process is shown in the figure
Laser classification: generally according to the gain medium or according to the laser energy form.
Gain Media Classification:
Carbon dioxide laser: The carbon dioxide laser gain medium is helium and CO2, and the laser wavelength that occurs is 106um, belongs to the earliest laser products on the market, the early laser welding is mainly based on carbon dioxide lasers, currently mainly used in non-metallic materials (cloth, plastic, wood, etc.) welding and cutting, and is also used on lithography machines, carbon dioxide lasers can not be transmitted through optical fibers, take the space optical path, the earliest Trumpf is better, and a lot is used on cutting equipment;
YAG (yttrium aluminum garnet) laser: A YAG crystal doped with neodymium (ND) or yttrium (Yb) metal ions is used as the laser gain medium, and the emission wavelength is 106um, yag laser can output higher pulses, but the average power is low, the peak power can be up to 15 times the average power, mainly pulse lasers, can not achieve continuous light; However, it can be transmitted through optical fiber, and at the same time, the absorption rate of metal materials increases, and it begins to be applied to high-reflective materials, which is first applied to the 3C field;
Fiber laser: the mainstream of the current market, with ytterbium-doped fiber as the gain medium, the wavelength is 1060nm, and it is divided into fiber and disk lasers according to the shape of the medium; Fiber is represented by IPG, and disc is represented by TRUMPF.
Semiconductor laser: the gain medium is semiconductor PN junction, the wavelength of semiconductor laser is mainly at 976nm, the current semiconductor near-infrared laser is mainly used in cladding, the spot is above 600um, laserline is a representative enterprise of semiconductor laser.
According to the form of energy action, it is divided into: pulsed lasers (pulse), quasi-CW lasers (QCW), and CW lasers (CW).
Pulsed lasers: nanoseconds, picoseconds, femtoseconds, this kind of high-frequency pulsed lasers (ns that is, pulse width) can often achieve high peak energy, high frequency (MHz) processing, used for thin plate copper and aluminum dissimilar material processing, as well as cleaning, the use of high peak energy, can quickly melt the base metal, low action time, small heat affected zone, in the processing of ultra-thin materials have advantages (05mm or less);
Quasi-CW Lasers (QCW): Due to the high repetition rate and low duty cycle (less than 50%), the pulse width of quasi-CW lasers reaches 50 us-50 ms, and the QC fiber lasers make up for the gap between kilowatt-class CW fiber lasers and Q-switched pulse lasers. The peak power of quasi-CW fiber lasers can be up to 10 times the average power in CW mode operation. QCW lasers generally have two modes, one is continuous welding at low power, and the other is high peak power pulse laser welding with 10 times the average power, which can achieve thicker materials and more heat welding, and at the same time control the heat in a small range;
CW laser (CW): This is the most used, most of the CW lasers seen on the market are continuous lasers, which continuously output laser for welding processing, and fiber lasers are divided into single-mode and multi-mode lasers according to different core diameters and different beam qualities, which can be adapted for different application scenarios.
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2. With the development of lithium battery, photovoltaic and laser, more and more engineers have begun to contact lasers, considering that the laser process is a technology that is extremely dependent on practical operation, and the current market lacks relevant Xi information, this paper aims to disseminate the relevant basic knowledge of the application of the relevant laser process and promote the development of the industry.
3. This company has a corresponding laser process, welcomes process peers, communicates, learns from each other, and learns from each other, Xi WeChat, djfwill.