Introduction - the discovery of the laser and its importance
In 2010, the lasercelebrated 50 years of its existence. Since then, the laser has penetrated into every possible sector of human activity. It finds its main use in industry, medicine, science and research, electronics, entertainment as well as in quite "ordinary" things like laser pointers, computer CD-ROMs, etc. In this series we would therefore like to gradually introduce the reader to the topic of lasers and their use with an emphasis on industrial laser applications .
Basic laser principle
The word LASERis an abbreviation for "Light Amplification by StimulatedEm ission of Radiation" from the English Light Amplification by Stimulated Emission of Radiation . Laser thus generally refers to an optical amplifier that generates electromagnetic radiation (light) by a process of stimulated photon emission based on the laws of quantum physics and thermodynamics.
A general schematic of a laser is shown in the figure below. The basis of a laser is an active medium that is excited in some way (optically, electrically, etc.). The excitation supplies energy to the laser, which is then emitted in the form of a laser beam through the process of stimulated emission. To do this, an optical resonator must be created, which is most often made up of reflective mirrors.
So how is the energy delivered to the resonator converted into a laser beam?
In general, the active medium always contains an elementthat can be in a ground state with lower energy or in an excited state with higher energy. This element is most often an atom, but this is not always the case (e.g. it can be the vibrational state of a molecule, a chemical bond, etc.). What is important for the moment is that during the transition from a higher to a lower energy state, this element emits a photon (a quantum of electromagnetic radiation). This radiant transition happens spontaneously by itself and the environment always tends to be in the lowest energy state - a state of thermodynamic equilibrium. It is through excitation that we break this state and bring the active environment into an excited state where most of our elements are in a higher energy state (this state is called population inversion).
It is only at this point that we can convert the energy supplied to the active environment into a laser beam (a stream of photons) through the process of stimulated emission, which is shown in the figure below. This is essentially an avalanche effect, where a photon incident on an excited atom causes (stimulates) it to move from an upper to a lower energy level, and in the process another photon is emitted.
As the photons travel through the resonator from one mirror to another their number increases rapidly and the avalanche effect occurs, releasing energy in the form of a photon stream (laser beam) - see figure below:
Unique properties of the laser beam
What makes the laser beam so unique ? Importantly, in the process of stimulated emission, the incident and emitted photon have the same energy (frequency), direction, polarization and phase. This gives rise to the three basic properties of a laser that distinguish it from other sources of radiation. A laser beam is:
- Collimated (i.e., it does not collide)
- Monochromatic ("monochromatic", i.e. the photons generated have the same frequency or wavelength)
- Coherent (the generated photons are so-called in phase both temporally and spatially)
The opposite is e.g. a classical light bulb, which generates radiation in a completely "chaotic" way and generates photons going in all directions, of different wavelengths and with random phase.
It is precisely these properties that have made the laser such a valuable tool in so many different applications. In industrial applications, it is particularly used to focus the laser beam to a small point and thus achieve the high areal energy density required to process the material (cutting, welding, marking, hardening, etc.). In other applications such as holography, coherence properties, etc. are mainly needed.
Basic division of lasers
The most common division of lasers you will encounter is according to the type of active medium:
Gas lasers
The active medium here is a gas that can be excited in various ways - electrically, by radio-frequency waves, optically, etc. A typical example is the HeHe (helium neon) and CO 2laser , which is well known in industry for cutting applications.
Solid-state lasers
The active medium is a solid, most often a single crystal. The excitation is most often optical, either by discharge tubes or laser diodes. A typical representative is Nd:YAG(the medium is a single crystal of yttrium aluminium garnet doped with neodymium atoms). Nd:YAG is mainly used in industry for laser cutting, marking and welding.
Fiber lasers
A special type of solid-state laser where the active medium is an optical fibre doped most commonly with erbium (Er) or ytterbium (Yr) atoms. Excitation is by laser diodes, whose radiation is fed back into the active fibre by the optical fibre. This is the so-called fibre-to-fibrearchitecture and the laser therefore does not contain any opto-mechanical elements such as mirrors etc. Power outputs today reach up to 40kW. Nowadays, it is the most advanced technology for industrial cutting, welding and marking and the market share of fiber lasers is constantly increasing.
Semiconductor (Diode) lasers
The active medium is an electrically pumped semiconductor diode. Diode lasers range in power from mW to kW. These lasers have high efficiency but suffer from low output beam quality. They can be miniature in size (low power) and are used in CD/DVD players, laser printers, etc. High power diode lasers are mainly used in industry for welding (metals and plastics) and hardening.
Chemical lasers
These lasers are excited by a chemical reaction and are capable of delivering huge amounts of energy in a short time. They are mainly of interest to the military for military purposes.
Excimer lasers
A special class of gas lasers excited by an electric discharge. The active element that generates the radiation is the so-called excimer (a special molecule where one of its components is in an excited state). These lasers operate in the ultraviolet (UV) region and their main use is in photolithography in the semiconductor industry and other applications where wavelengths from the UV region and high energies in the pulse are required.
Color lasers
An organic dye is used as the active medium. The advantage of these lasers is their tunability - they can emit at multiple wavelengths. Their use is mainly in science and research.
Other possible subdivisions of lasers
Another possible division of lasers is according to the type of output beam. Here the basic division is into continuous wave (CW)and pulsed lasers. CW laser generates continuous output power (mainly cutting, hardening,...), while pulsed laser generates laser pulses. Pulsed lasers are further subdivided according to the type of pulse generated. In industry, the most commonly used is the so-called Q-switching, where the laser generates pulses with a length of ns (especially for marking, engraving) or by pulse excitation (e.g. by discharge lamps) with pulses in the order of ms (for laser welding).
A special class is the so-called ultrafast lasers , which generate pulses in the ps and fs range (up to 10 -15s!), which are not yet widespread in industry due to their cost, but are nevertheless ideal for micro-machining applications in the future.
Conclusion
We hope that this information is sufficient for now to understand the basic principles of lasers and their main divisions, and in the next part of the series we will look in more detail at the main types of industrial lasers and their applications.