Laser.The word “laser” is an acronym for “light amplification by stimulated emission of radiation” and is the name of any device with certain principles of operation and unique properties. Lasers emit coherent and monochromatic light. Coherence means that emission of light is spatially and temporally coordinated, that is, almost all emission is concentrated in a narrow beam and it occurs simultaneously, at the same time and the same phase of light waves. Monochromatic radiation means that all emission occurs with the same single wavelength. And light means electromagnetic radiation of a particular wavelength in the range from far infrared to X-rays. Each laser produces emission of only one particular wavelength.The fundamental design of lasers may look very simple. The core of a laser, which forms the laser beam, is a substance called the gain medium. The gain medium can be of any state of aggregation – solid, liquid, gas and plasma. Then there is a source of “pump energy”. It “pumps” energy into the gain medium. Then there is a control device, which controls the operation of the source of pump energy. Certain types of lasers also have emission stimulators (also controlled by the control devices), but most lasers do not need stimulators, they emit spontaneously after sufficient energy is pumped into their gain media.
The principle of operation of lasers (extremely simplified) is as follows. The source of pump energy supplies energy in the form of light or other electromagnetic waves, or in the form or electric field and electric current. This energy is directed to a gain medium.
Atoms of the gain medium have electrons on different orbits. The number of the orbits and the number of electrons in each orbit depends on the position of a particular atom in the Mendeleev Table. Each orbit has one or more energy levels, which electrons on it can take. Usually, electrons on each orbit are in the lowest energy level. However, when external energy is applied to them, they come to higher energy levels of the orbit, or to the highest level. An atom with electrons at a higher energy level of a particular orbit (usually this is the external orbit) is said to be excited, as well as each electron at the higher energy level. These energy levels are of quantum nature. Reaching each energy level requires a definite additional energy. When an excited electron skips from a higher level to a lower or the lowest level, it emits exactly the same amount of energy, which was necessary for getting it excited to that level. The emission occurs in the form of a quantum of electromagnetic energy, that is, a photon of a particular wavelength determined by the energy of this quantum.
An atom gets excited when it absorbs energy from outside. In the case of a laser this external energy is supplied by the source of pump energy. When the majority of atoms (or almost all atoms) in the gain medium get excited, the laser is ready to produce its coherent and monochromatic light. Then, with the help of a stimulator or spontaneously, one of the electrons of one of the atoms skips to a lower or the lowest energy level and emits a photon of a particular wavelength. It hits an electron in a nearby atom, and this electron also skips to a lower level and emits a photon of the same wavelength and phase. Now two photons travel inside the gain media and hit two other electrons, as a result, four photons are traveling. Now four photons hit four electrons, and there are now eight photons. And so on. It’s an avalanche-like process. As a result, all or almost all excited electrons release their gained energy in the medium, and the medium emits the light beam. The medium is shaped as a tube or a rod, the length of which is much bigger than the diameter. For this reason, the probability that the emitted photons will move along the axes of the rod or tube is much higher than the probability of their motion in other directions. This is why the laser beam is narrowly focused without optical focusing. One of the ends of the tube or rod of the gain medium is blocked with a mirror, for this reason, almost all radiation occurs through the other end, in one direction. There is also some collateral radiation, which is much less powerful than the main beam, and it is not concentrated.
Remember: this is a very simplified explanation. It gives only a vague idea of how lasers work.
There are pulsed lasers and continued-wave lasers. Pulsed lasers emit light in very short (nanoseconds to microseconds) pulses at desirable intervals, typically from milliseconds to several seconds. Or, sometimes, they emit a single pulse. Continued-wave lasers emit light continuously.
These two types of lasers have different power of beams. Consider a continued-wave laser and a pulsed laser (with a pulse lasting 1 microsecond emitted ten times a second) consuming the same amount of pump energy in a unit of time – one joule per second (one watt), then both emit the same amount of energy (100% efficiency). Then the power of the beam of the continued-wave laser is one watt (1 W), while the power of each pulse of the pulsed laser is 100 thousand watts (100 kW). In reality, of course, the efficiency is much less than 100% and depends on the type of laser and its gain medium. A part of the external energy is dissipated as heat, and another part is taken away by collateral radiation.