This article shows what are the sources of monochromatic radiation and what are the advantages of a solid-state laser over other types. It describes how coherent radiation is generated, why a pulsed device is more powerful, and why engraving is needed. Also, here are considered three essential elements of the laser and the principle of its operation.
Zone Theory
Before talking about how a laser works (solid state, for example), some physical models should be considered. From school lessons, everyone remembers that electrons are located around the atomic nucleus in specific orbits, or energy levels. If at our disposal not one atom, but many, that is, we consider any volumetric body, then one complication arises.
According to the Pauli principle, in a given body with the same energy there can be only one electron. Moreover, even the smallest grain of sand contains a huge number of atoms. In this case, nature found a very elegant way out - the energy of each electron differs from the energy of the neighboring one by a very small, almost indistinguishable amount. Moreover, all electrons of the same level are “compressed” into one energy zone. The zone in which the electrons farthest from the nucleus are called the valence. The zone following it has a higher energy. In it, the electrons move freely, and it is called the conduction band.
Emission and absorption
Any laser (solid state, gas, chemical) operates on the principles of electron transfer from one zone to another. If light falls on the body, then the photon gives the electron enough strength to turn it into a higher energy state. And vice versa: when an electron passes from the conduction band to the valence one, it emits one photon. If the substance is a semiconductor or dielectric, the valence and conduction bands are separated by an interval in which there is no level. Accordingly, electrons cannot be there. This interval is called the forbidden zone. If the photon has sufficient energy, then the electrons jump over this interval.
Generation
The principle of operation of a solid-state laser is based on the fact that a so-called inverse level is created in the band gap of a substance. The electron lifetime at this level is higher than the time it spent in the conduction band. Thus, in a certain period of time, electrons “accumulate” on it. This is called inverse population. When a photon of the desired wavelength passes past such a level dotted with electrons, it causes the simultaneous generation of a large number of light waves of the same length and phase. That is, the avalanche electrons all simultaneously pass into the ground state, generating a beam of monochromatic photons of sufficiently high power. It is worth noting that the main problem for the developers of the first laser was the search for such a combination of substances for which an inverse population of one of the levels would be possible. The first working substance was doped ruby.
Laser Composition
The solid-state laser in its main components does not differ from other types. The working fluid, in which the inverse population of one of the levels is carried out, is illuminated by some source of light. It is called pumping. Often this can be an ordinary incandescent lamp or a discharge tube. Two parallel ends of the working fluid (a solid-state laser means a crystal, a gas laser means a rarefied medium) form a system of mirrors, or an optical resonator. It collects in a beam only those photons that run parallel to the outlet. Solid-state lasers are usually pumped using flashlights.
Types of Solid State Lasers
Depending on the method of output of the laser beam, continuous and pulsed lasers are distinguished. Each of them finds application and has its own characteristics. The main difference is that pulsed solid-state lasers have a higher power. Since for each shot the photons “accumulate”, as it were, one pulse is capable of delivering more energy than continuous generation over a similar period of time. The shorter the pulse, the more powerful each shot. At the moment, it is technologically possible to build a femtosecond laser. One of its impulses lasts about 10 -15 seconds. This dependence is connected with the fact that the processes of reverse population described above last very, very little. The longer it takes to wait before the laser “shoots”, the more electrons will have time to leave the inverse level. Accordingly, the concentration of photons and the energy of the output beam are reduced.
Laser engraving
Patterns on the surface of metal and glass things adorn the everyday life of a person. They can be applied mechanically, chemically or using a laser. The latter method is the most modern. Its advantages over other methods are as follows. Since there is no direct effect on the surface being treated, it is almost impossible to damage a thing in the process of applying a pattern or inscription. The laser beam burns out very shallow grooves: a surface with such an engraving remains smooth, which means that the thing is not damaged and will last longer. In the case of metal, the laser beam changes the very structure of the substance, and the inscription will not be erased for many years. If you use the thing carefully, do not immerse it in acid and do not deform, then for several generations the pattern on it will definitely remain. It is best to choose a solid-state pulsed laser for engraving for two reasons: processes in a solid are easier to control, and it is optimal in terms of power and price.
Installation
There are special settings for engraving. In addition to the laser itself, they consist of mechanical guides along which the laser moves, and monitoring equipment (computer). The laser machine is used in many fields of human activity. Above we talked about decorating household items. Personalized cutlery, lighters, glasses, watches will remain in the family for a long time and will remind you of happy moments.
However, not only household, but also industrial products need laser engraving. Large plants, such as automobiles, produce parts in huge numbers: hundreds of thousands or millions. Each such element should be marked - when and who created it. You can’t find a better way than laser engraving: numbers, production time, and service life will remain for a long time even on moving parts for which there is an increased risk of abrasion. The laser machine in this case should be distinguished by increased power, as well as safety. After all, if engraving even by a fraction of a percent changes the property of a metal part, it can react differently to external influences. For example, break down at the place of application of the inscription. However, for domestic use, a simpler and cheaper installation is suitable.