Laser is the Acronyme English of Light Amplification by Stimulated Emission off Radiation (in French, amplification of the light by stimulated emission of rayonnement  ). The laser effect is a principle of coherent amplification of the Lumière by stimulated emission. The majority of the amplifying optics are based on the laser effect. A laser source is a temporally coherent source of light spatially and based on the laser effect. By extension, one calls laser a source of light based on the laser effect. Going down from the Maser, it was initially called optical maser.

A laser source associates an optical amplifier based on the laser effect with a optical Cavité, still called resonator, generally consisted of two Miroir S, of which at least one of both is partially reflective, i.e. part of the light leaves the cavity and the other part is reinjected towards the interior of the laser cavity. With certain long cavities, the laser light can be extremely directional. The geometrical characteristics of this unit impose that the emitted radiation is of a great spectral purity, i.e. temporally coherent. The spectrum of the radiation indeed contains a discrete whole of very fine lines, with wavelengths defined by the cavity and the amplifying medium. The smoothness of these lines is however limited by the stability of the cavity and the spontaneous emission within the amplifier (quantic background noise). Various techniques make it possible to obtain an emission around one only wavelength.

With the 21e century, the laser is more generally seen like a possible source for any electromagnetic radiation, to which belonged the visible Lumière. The wavelengths concerned were initially the Micro-onde S (Maser), then they extended to the fields from the Infrarouge, the visible , the Ultraviolet and even start to apply to the x-rays.

Phenomena concerned

To include/understand how a laser functions, it is necessary to introduce the concept of quantification of the matter : the electron S are distributed on discrete energy levels (“layers”). This assumption is fundamental and nonintuitive : if one considers the image according to which the electrons orbit around the core, in other words, they can be only on certain quite precise orbits.

In the continuation, one will consider an atom having only one electron, to simplify the discussion. This one is likely to be on several levels. The knowledge of the level on which this electron is defines the state of the atom . These states are numbered by order ascending of energy with an integer $n$, being able to take the values $1$, $2$,… the state $n\; =\; 1$ is thus the state of the lowest energy, corresponding to an electron on the “orbit” nearest to the core.

It is not essential to quantify the light (i.e. of speaking about Photon ) qualitatively to explain the operation of a laser, even if that proves to be essential to explain the concept of spontaneous emission. We will speak nevertheless about photons when that provides an image facilitating the comprehension of the evoked concepts.

Let us come to the principal processes of interaction between the light and the matter, namely the absorption , the stimulated emission and the spontaneous emission .

See also: Absorption (optical)

• the absorption . When it is lit by an electromagnetic radiation (the light), an atom can pass from a state $n$ to a state $n\text{'}\; >\; n$, by taking corresponding energy on the radiation. This process is resonant : the frequency of the radiation $\backslash \; omega$ must be close to a frequency of atomic Bohr so that it can occur. The atomic frequencies of Bohr are defined by $\backslash \; hbar\; \backslash \; omega\_\; \{\}\; =\; (E\_\; \{\}\; -\; E\_n)$, where $E\_\; \{\}\; >\; E\_n$ is energies of the states $n\text{'}$ and $n$. One can interpret this process like absorption of a photon of the radiation (of energy $\backslash \; hbar\; \backslash \; omega$) making pass the atom of the energy level $E\_n$ towards the energy level $E\_\; \{\}$. The condition of resonance corresponds then to the conservation of energy .

See also: stimulated Emission

• the stimulated emission . This process is symmetrical precedent: an atom in the state $n\text{'}$ can " désexciter" towards the level $n$ under the effect of an electromagnetic wave, which will be then amplified . As for absorption, this process is important only if the frequency of the radiation $\backslash \; omega$ is close to the frequency of Bohr $\backslash \; omega\_\; \{\}$. One can interpret it as the emission of a photon of energy $\backslash \; hbar\; \backslash \; omega$ which comes “to be added” to the radiation.

See also: spontaneous Emission

• the spontaneous emission . An atom in a excited state $n\text{'}$ can be de-energized towards a state $n$, even in the absence of radiation. The radiation is emitted in a random direction, and its frequency is equal to the frequency of Bohr $\backslash \; omega\_\; \{\}$. One can interpret this process like the emission of a photon of energy $\backslash \; hbar\; \backslash \; omega\_\; \{\}$ in a random direction.

Operation

A laser is basically an amplifying of light (functioning thanks to the stimulated emission) whose exit is connected on the entry.

The amplifier is a whole of atoms which one “pumps” in an excited state $n\text{'}$, by means of an external energy source (for example with another laser…). These atoms can then be de-energized towards the state $n$, by emitting photons of frequency around $\backslash \; omega\_\; \{\}$. Thus a radiation of frequency $\backslash \; Omega\; \backslash \; simeq\; \backslash \; omega\_\; \{\}$ passing through this medium can be amplified by processes of stimulated emission. Let us note that it can also be absorbed: there will be amplification only if the atoms are more numerous with being in the state $n\text{'}$ (likely to emit) that in the state $n$ (suitable for absorb): it is necessary to have a “inversion of population”.

The outgoing radiation of this amplifier is rebouclé on its entry by means of mirrors, which constitute a “cavity” (where the light is trapped). Of course, a device (as a partially reflective mirror) makes it possible to extract from the light of this system, to obtain the laser radiation usable. Thus a radiation initially present in the system will be amplified first once, then rebouclé, then réamplifié, etc One can thus build an extremely important radiation, even starting from an extremely weak radiation (like only one photon emitted spontaneously in the cavity).

One can compare this process with the Effet Larsen, which occurs when an amplifier (the HiFi chain) has its exit (the loudspeaker) “connected” on the entry (the microphone). Then a very weak noise collected by the microphone is amplified, emitted by the loudspeaker, is collected by the microphone, is réamplifié, and so on… Of course the intensity of the sound does not grow indefinitely (just like intensity of the light in a laser): the amplifier has limits (there exists a maximum volume of the sound which can be produced). Let us note that the frequency of the sound emitted by this process is quite particular, and depends on the amplifier, the distance between the loudspeaker and the microphone: it is the same for a laser.

History

The principle of the stimulated emission (or induced emission) is described as of 1917 by Albert Einstein. In 1950, Alfred Kastler (Nobel Prize of Physics in 1966) proposes an optical process of Pompage, which is validated in experiments by Brossel, Kastler and Winter two years later. But it is only in 1953 that the first Maser (maser with the gas Ammoniac) is designed by J.P. Gordon, H.J. Zeiger and CH. H. Townes. During six years following, many scientists such NR. G. Bassov, A. Mr. Prokhorov, A.L. Schawlow and CH. H. Townes contribute to adapt these theories to the wavelengths of the visible one. Townes, Basov, and Prokhorov divide the Nobel Prize of Physics in 1964 for their fundamental work in the field of quantum electronics, which leads to the construction of oscillators and amplifiers based on the principle of the Maser - Laser. In 1960, the American physicist Theodore Maiman obtains for the first time a laser emission by means of a crystal of Rubis. One year later, Ali Javan develops a laser with the gas (Hélium and Néon) then in 1966, Peter Sorokin builds the first laser with liquid.

The lasers find very early outlets industrial. The first application is carried out in 1965 and consisted in machining a drilling 4,7 mm in diameter and 2 mm depth in diamond with a laser with Rubis. This operation was carried out in 15 min, whereas a traditional application took 24 hours.

In 1967, Peter Houlcroft cuts out 2,5 mm of stainless steel at a speed of 1m/min, under di-oxygen with a CO₂ laser of 300 W and designs the first head of cutting. During the same time in 1963 of the American researchers such as White and Anderholm show that it is possible to generate a wave shock inside a metal following an impulse laser irradiation. The exerted pressures are about 1 GPa.

Although the processes are shown, it is necessary to await their associations with adapted machines so that they are established in industrial environment. These conditions are met at the end of the Seventies. And the first punts industrial forms are established in France as of the Eighties. Consequently the laser is essential like an industrial production equipment in micromachining. Its main advantages are a machining: at high speed of about 10 m/min, and without contact, wear of tool.

The laser becomes a means of reading in 1974, with the introduction of the readers of codes bars. In 1978, the Laserdisc S are introduced, but the optical disks become of everyday usage only in 1982 with the Compact disk. The laser then makes it possible to read great volumes of data.

Various types of laser

One classifies the lasers according to six families, according to the nature of the excited medium.

Crystalline lenses (with solid, or ionic)

These lasers use solid media, such as crystals or Verre S like medium of emission of the photons. The crystal or glass is only the matrix and must be doped by a Ion which is the laser medium. Oldest is the ruby laser from which the emission comes from the ion Cr ³⁺. Other ions are very much used (the majority of the Rare earths: Nd, Yb, Pr, er, Tm…, the Titanium and the Chromium, inter alia). The wavelength of emission of the laser depends primarily on the doping ion, but the matrix influences too. Thus, glass doped with neodymium does not emit with the same wavelength (1053 Nm) only YAG doped with neodymium (1064 Nm). They function uninterrupted or in an impulse way (impulses of a few microseconds to some femtosecondes --millionth of billionth of second). They are able to emit as well in the visible one, the close relation Infrarouge as the Ultraviolet.

The amplifying medium can be a bar in the case of a Laser Nd-YAG (thus doped with Nd and the matrix is YAG: a garnet of aluminum and Yttrium), but it can also appear as a fiber in the case of the lasers to fiber (thus doped with Yb and the matrix is out of silica). Today, the amplifying medium more used to generate impulses femtosecondes is sapphire doped titanium. It has two absorption bands centered to 488 and 560 Nm. It has a broad emission spectrum centered to 800 Nm.

Beyond of a crystal dimension of optical quality acceptable, these lasers make it possible to obtain powers about kw uninterrupted and GW in pulsated. They are used for applications as well scientific as industrial, in particular for welding, the marking and the cutting of materials.

With dyes (molecular)

In the lasers with liquid, the medium of emission is coloring an organics (rhodamine 6G for example) in liquid solution locked up in a flask of glass. The emitted radiation can as well be continuous as discontinuous according to the mode of pumping. The emitted frequencies can be regulated using a regulating prism, which returns this type of very precise apparatus. The choice of the dye determines primarily the color of the ray which it will emit.

With gas (atomic or molecular)

The generating medium of photons is here a Gaz contained in a tube out of glass or quartz. The emitted beam is particularly narrow and the frequency of emission is very pure. The most known examples are the lasers with helium-neon which are used in the systems of alignment (public works, laboratories), and the lasers for spectacles.

To note that the lasers with Carbon dioxide are able to produce very strong powers (operation in impulse) about 10⁶ W. It is Laser marking more used in the world. The laser CO₂ can for example be used for the engraving or the cutting of materials.

Examples: the laser CO₂ (infra-red, with 10,6 µm) and the laser He - (Red, to 632,8 Nm).

There exists also a subfamily of the gas lasers: the lasers excimer which emit in the ultra-violet. In the majority of the cases, they are composed of at least a halogenous gas and also sometimes of a rare gas. The " term; excimer" comes from the English word " dimer" who means that the molecules are made up of two identical atoms (ex: F₂). However the lasers excimer use molecules made up of two atoms different (rare gas and halogen ex: ArF) which remains whole only in an excited state, us should call them " exciplex". But by abuse language, the physicists lend to them the name of Laser excimer. The electric excitation of the mixture of halogen and rare gas produces these molecules excimers. After emission of the Photon the excimer disappears because its atoms separate, therefore the photon cannot be reabsorbed by the excimer what allows a good output the laser.

With semiconductors (ionic) - lasers diode - VCSEL

These lasers are mainly made up of a diode with semiconductor in order to produce a beam of light. Pumping is done using an electric current which enriches the generating medium in holes on a side and electrons of the other. The light is produced on the level of the junction by the recombination of the holes and the electrons. Often, this type of laser does not present mirrors of cavity: the simple fact of cleaving the semiconductor, of strong optical index, makes it possible to obtain a sufficient coefficient of reflection to start the laser effect.

It is this type of laser which represents the vast majority (of number and in sales turnover) of the lasers used in industry. Indeed, its advantages are numerous: first of all, it allows a direct coupling between electrical energy and the light, from where applications in Télécommunication S (with the entry of the networks of fiberoptics). Moreover, this energy transformation is done with a good output (about 30 to 40%). These lasers are inexpensive, very compact (the active zone is micrometric, even less, and the whole of the device has a size about the millimetre). One can now manufacture such lasers to obtain light on almost all the visible field, but the lasers delivering of the red or the infra-red close relation remain the most used and the least expensive. Their scopes of application are innumerable: optical readers (CD), telecommunications, printers, devices of “  pompage  ” for larger lasers (of lasers type to solid), pointers, etc Noter that regulations in force in France prohibited to manufacture some lighting beyond 1  000 meters.

Some flat all the same, the emitted light is in general less directional and less “  pure  ” spectralement that of other types of lasers (with gas in particular). What is not a problem in the majority of the applications.

A very close device in its operation, but which is not a laser, is LED: the device of pumping is the same one, but the production of light is not not stimulated , it is produced by spontaneous de-energizing, so that the produced light does not present the properties of coherence characteristic of the laser.

With free electrons (LEL)

This type of laser is very particular, because its principle is completely different from that higher exposed. The light is not produced there by beforehand excited atoms, but by a Synchrotron radiation produces by accelerated electrons. An electron beam, coming from an accelerator with electrons, is sent in a inverter creating a periodic magnetic field (thanks to an assembly of permanent magnets). This inverter is placed between two mirrors, as in the diagram of a conventional laser: the synchrotron radiation is amplified and becomes coherent , i.e. it acquires the characteristics of the light produced in the lasers.

It is enough to regulate the speed of the electrons to provide a light of frequency adjusted very finely on a very broad range, energy of the infra-red to x-rays, and the laser power can be also adjusted by the flow of electrons up to elevated levels. One can also have impulses laser of short and precise interval. All that returns this type of very general-purpose, and very useful laser in the applications of research. It is however more expensive to produce.

With fiber

The laser with fiber is the last born of the laser technology. Its design is enough revolutionist, because the active medium is a fiberoptic doped with a rare earth ion which is mainly the Ytterbium. This laser appreciably has the same wavelengths as the laser YAG. However, it is more compact, more stable and does not need mode of cooling. It also has a better quality of beam, the diameter of this last being weaker, it thus has a better resolution for applications of marking.

Safety

According to the power and the Wavelength of emission of the laser, this one can represent a real danger to the sight and cause irrevocable burns of the Rétine.

; Classify I: Lasers which are not dangerous uninterrupted for a vision or are manufactured to avoid a human vision. That typically relates to lasers of low power or lasers in cases (examples: Printing S, readers of CD-ROM and readers of DVD). ; Classify II: Lasers emitting a light visible causing a sufficient embarrassment with the eye, do not represent a danger to short periods. Those can be comparable with an intense source of light. ; Classify IIa: Lasers emitting a visible light not being made to be seen and not having to cause damage in the event of direct sight during less than 1000 seconds (for example, of the readers of S Code-bars). ; Classify IIIa: Lasers which should not normally be dangerous if seen temporarily, but could present a danger if seen through focusing optical apparatuses (examples: Magnifying glass S and Telescope S). ; Classify IIIb: Lasers which present a danger if directly seen, and can cause burns, as well directly as by reflection, but not by Diffraction other than at short distance. ; Classify IV: Lasers which represent a danger as well by direct sight as by reflection and diffraction. Can also cause Incendie S.

Applications

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