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Glossary of frequently used terms, acronyms and expressions in the field of lasers

Posted on05/08/2023

Read in this glossary the meaning of terms, acronyms, and expressions used to define qualities and characteristics of lasers.

Glossary of terms, acronyms and expressions used in the field of lasers

Absorption of radiation

Consists of the phenomenon whereby a material receives and absorbs electromagnetic radiation, this is transformed into a different form, usually heat. The manner in which the absorption process takes place depends on the wavelength of the electromagnetic radiation and the characteristics of the absorbing material.

Active medium

A material (set of atoms or molecules) that is suitable for stimulation to achieve the emission of electromagnetic radiation during a stimulated emission process.


Process in which by means of stimulation of the active medium contained in the optical cavity of the laser device additional electromagnetic radiation is emitted from it.


Measured from the equilibrium point, amplitude is the maximum value a wave reaches.


In a laser, an "aperture" is defined as a small hole through which electromagnetic radiation passes.


When a laser beam passes through an absorbing or scattering medium (material) the energy of the beam decreases; this decrease is called "attenuation."

Beam diameter

The diameter of a circular laser beam is measured at the point where the laser intensity is a fraction of its maximum intensity value. It is commonly defined as 1/e (0.368) and 1/e2 (0.135) of the maximum value.

Beam divergence

The laser beam is not perfectly cylindrical, the value of beam divergence indicates the angle of beam scattering measured in thousandths of radians. If the angle is small an approximate measurement can be represented using the ratio of the beam diameter to the distance to the laser aperture.

Black Body Radiation

Any object from its own surface can radiate heat to the ether and vice versa receive heat from the outside. If an object can receive and absorb all incident radiation that hits it regardless of its frequencies and directions, it is referred to as a "black body." For example, a hollow sphere equipped with a small hole is considered a black body, this is because any kind of radiation that penetrates it is ideally reflected only to its interior, thus it comes to be completely absorbed.

DOF (Depth of Focus).

For a laser the depth of focus is defined as the length of the segment of the beam starting before and ending after the ideal focus point whose intensity of the laser is approximately the same as measured at the focus point and within a maximum gap range of ±5%. For example, a laser focused at 1m at such a point has intensity X, depending on its characteristics the beam in the positive and negative directions from the focus point will have a segment within which the measured intensity is X±5% which will represent the depth of focus of the laser, e.g. a segment of N mm. long.

Electronic assembly or PCB

Simply it is an electronic board useful or necessary to control the operation of a laser. Usually in compact, low-power laser modules it is simple and composed of a number of electronic components (e.g., printed circuit board, chip ICs, resistors, capacitors, switches, diodes, cables, etc.). The PCBs can be integrated into the laser module or external and connected with cables of various types, if external they are easier to repair or replace but less practical, if integrated they are usually also miniaturized and very specific, much more practical but less manageable.

Evaporative laser cutting

By means of so-called evaporative laser cutting, a material is ablated by the direct vaporization method. Typically this method is used to process materials with low vaporization temperature and low thermal conduction.

Excimer or Exciplex lasers

Lasers are called excimer or exciplex when noble gas compounds are used as the active medium. They generate laser light in the ultraviolet and near-ultraviolet spectra from 0.193 to 0.351 microns. They are widely used for high-precision lithography for chip and electronic component manufacturing, micro-machining and laser eye surgery. Most excimer lasers use compounds of the noble gas Xenon, e.g., XeCl or other such compounds. Their very short wavelength allows to obtain a very small focus point and if useful a high energy density per area unit, thus it is possibile to use them for very high precision machining.

Gas laser

In gas lasers, the active medium is precisely a gas; the gas used can be a molecule, e.g. CO2, an atomic element, e.g. Helium, Neon, or an ion, e.g. Ar+ silver ions

Laser fusion cutting

Laser fusion cutting is carried out through fusion due to a powerful laser beam usually assisted by a gas jet that can perform various auxiliary functions, e.g., facilitate the flow of the fused material, prevent oxidation, protect the nozzle and optics used to focus the laser, cool the laser head, etc...

Ground state

The ground state is the lowest energy level of an atom or molecule.

Heat affected zone

The heat affected zone is the region of treated material adjacent to the point or area irradiated by the laser where there is a noticeable temperature change or a state of deformation due precisely to the temperature change.

Holography and Hologram

A hologram is an interference phenomenon captured on a photographic rigid plate or photographic film. By means of holography, holograms can be created that contain an enormous amount of information, e.g. by means of holographic techniques, information describing an object can be recorded in such a way that by observing the resulting hologram, an image of the object can be seen that appears three-dimensional and also shows the same object from different viewpoints. Holograms are usually created using lasers with certain characteristics.

Knudesen layer

The Knudesen layer is the thin region where a gas produced during a laser evaporation ablation process, near the laser phase interface, reaches translational equilibrium (when the laser insists on the material a strong evaporation is created, the gas thus created is not already in translational equilibrium, equilibrium is quickly reached after a few free mean paths of particles in a thin region, this region is called the Knudesen layer).


The word Laser is an acronym coined by Gordon Gould that represents the expression Light Amplification by Stimulated Emission of Radiation. A laser is a beam of electromagnetic waves that has special properties. Commonly known lasers emit light, which is an electromagnetic (EM) wave in the visible range.  Lasers are not necessarily visible; in fact, they can emit radiation at wavelengths ranging from infrared to ultraviolet to even dim X-rays. In general, the acronym laser is used to define devices that generate and amplify electromagnetic waves by emitting a highly coherent, monochromatic, directional beam.

Laser processing

Laser processing is a manufacturing process by which materials are processed, the operations that can be carried out with lasers are among the most diverse, drilling, cutting, welding, ablation of surface layers, engraving, 3D modeling, casting marking, holography and so on...

Standing wave

Standing wave is an oscillating wave that is stationary in space, that is, it does not propagate along a certain direction in space. It consists of the combination of two waves of equal amplitude and wavelength moving in opposite directions in the same space and axis, depending on how they interfere with each other the energy of the two can be summed or canceled.

Laser mode

The so-called laser mode indicates the possible standing electromagnetic waves in the laser resonator.

Longitudinal Axial Modes (Longitudinal Axial Modes).

Stationary axial electromagnetic waves present in the laser resonator.

Laser resonator or laser cavity

The laser resonator consists of optical mirrors, the pumping system (the energy source, such as a lamp) and the active medium (the material used to amplify the light). The resonator is also called a laser cavity. Resonators, or cavities, can be of a type called stable type when the oscillating beam is contained in the cavity, that is, it moves between the mirrors of the cavity without dispersion; it is called an unstable type when a portion of the beam is scattered in the cavity, that is, when a certain amount of the photons of the beam is blown out of the cavity (e.g., in some power lasers, this method is preferred to avoid overloads on the hard-to-manage optics).


The linewidth of the laser corresponds to the amplitude of the frequency of the electromagnetic wave of the laser beam, which is much narrower than that of ordinary light.

Liquid laser

The active medium of a laser can be a liquid loaded with large molecules of an organic dye. Lasers that use a liquid active medium precisely are called liquid lasers.

M2 of the beam

M2 of the beam is a quality indicator; it expresses the measure of the difference that is needed between a given real laser beam and the expected ideal Gaussian beam.

Mode locking (Mode locking)

So-called mode locking is a method of generating very short laser pulses. The phase difference of many modes (frequencies) in the laser resonator is made fixed or locked, very short pulses are thus created.


Called the "quantum" or "quantum" of electromagnetic radiation, it is the lowest known unit of discrete light energy. Also called particle of light. It behaves as both a wave and a particle, has no mass, no h to electric charge, in a vacuum it travels at the speed of light, believed to be the maximum possible to date.

Polarized light.

So-called polarization describes the direction of the planes, always perpendicular to each other, over which the electric and magnetic fields of an electromagnetic wave oscillate. When the wave's electric field oscillates only in one direction, such light is said to be polarized. Natural light for example has the vector E (of the electric field) oscillating in different random directions, so it is not polarized light. A laser beam consists of photons characterized by having the electric field oscillating in the same direction, so it is polarized light. The polarization of light can be created and adjusted using filters called polarizers. In nature, for example, clouds have the power to polarize light passing through them.

Population inversion

Under normal conditions in a material the amount of atoms that are in a high energy state is less than the number of atoms that are in a low energy state. When the number of atoms that are in the high energy state is greater than the number of atoms that are in the low energy state, population inversion is said to have occurred. To obtain a laser, it is necessary to stimulate the active medium for population inversion to occur; it is a necessary prerequisite for creating the laser phenomenon.


In lasers, pumping is the process of bringing the atoms of a material from a low to a high energy state, that is, causing the excitation of the electrons in thegi themselves and thus causing population inversion.


Q-Switching is a method to create high-energy laser pulses. By modulating the Q (quality) of the laser cavity in the first instance, population inversion is caused, after which the accumulated energy is allowed to be suddenly released, thereby generating high-energy pulses and resulting in even very powerful laser pulses.

Recombination radiation

When electrons in a semiconductor recombine with a hole, photons are emitted; this phenomenon is called recombination radiation or radiative recombination.


Resolution corresponds to the minimum possible increment in the scale of a measuring device; in digital instruments it is the least significant bit.

Solid-state lasers

When the active medium used is solid the laser is precisely called a solid-state laser, an example is the ruby bar laser (semiconductor lasers are usually not included in this category, this even though semiconductors are solid, some diode lasers are still said to be solid-state)

Semiconductor lasers

When the active medium used is a semiconductor, the laser is precisely called a semiconductor laser. Mostly combinations of elements from the third group of the Periodic Table, e.g., Al, Ga, In and the fifth group, N, P, As, Sb, are used; these are usually III-V compounds.

Spontaneous emission

According to quantum mechanics in atoms the electrons usually are in the lowest energy state, however they can assume different higher energy states, for example they go from the E1 state to E2, E3 and so on... Since electrons prefer the most stable energy states and these are the lowest the electrons always tend to decay to the lower energy state, in doing so the electrons give up a photon of energy equal to the energy difference between the higher and lower energy states to which they decay. This process is called spontaneous radiation.

Stable cavities and unstable cavities

The cavity, or resonator, can be called stable or unstable; if the mirrors in the cavity cause the oscillating beam to converge inside the cavity without scattering, the cavity is called stable; if, on the other hand, the mirrors are arranged in such a way that the beam scatters outside the cavity the cavity is called unstable.

Stimulated absorption

When an electron in an atom is at the fundamental energy level E1 is invested by a photon having a frequency corresponding to the higher energy level E2 the electron jumps to the higher energy level E2, i.e., it goes to the excited state, this phenomenon is called stimulated absorption.

Stimulated emission

When atoms that are in an excited energy state are hit by an incident electromagnetic field with a corresponding frequency, there is a certain probability that they can jump to corresponding lower energy levels, emitting electromagnetic waves or photons with the same frequency, direction and phase as the incident waves. This process is called stimulated emission and is the basic phenomenon by which laser beams can be created

Ultra-short pulse lasers

When the pulse duration is very short, less than 1 ns, the laser is precisely called ultra-short pulse laser

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