Les lasers sont des dispositifs qui produisent des faisceaux intenses de rayonnement monochromatique cohérent. Le terme original «laser» (datant d’environ 1960) était un acronyme de «l’amplification de la lumière par émission stimulée de rayonnement», bien que son sens original a généralement été éclipsé par l’utilisation régulière du terme minuscule. Cet acronyme original, cependant, fournit une excellente description d’un laser puissant.
Les lasers sont essentiellement des sources lumineuses spécialisées qui émettent un faisceau étroit de lumière cohérente. Il est important de noter que le terme lumière ne se réfère pas seulement à la lumière visible, mais est plus précisément utilisé pour représenter les rayonnements électromagnétiques de toute nature; Les lasers sont également capables d’émettre des rayonnements non visibles (rayons ultraviolets, infrarouges, etc.).
La cohérence représente un état idéal pour toute onde et existe sous deux formes: temporelle et spatiale. Bien que la cohérence soit un sujet complexe, il peut être trop simplifié en disant qu’une onde pleinement cohérente est entièrement en phase avec elle-même. Lorsqu’on parle de lasers, la cohérence représente la capacité d’une fonction d’onde – qui décrit l’action des particules de photon – d’interférer avec d’autres fonctions d’onde. (Dans ce cas, ce type d’interférence est souhaitable.)
Les atomes et les molécules qui composent toute la matière peuvent être excités d’un état énergétique faible à élevé (habituellement) par la chaleur et émettent de la lumière lorsqu’ils reviennent à leur état d’énergie faible. S’ils sont excités par une source de lumière ordinaire, les atomes rayonnent la lumière de nombreuses longueurs d’onde différentes et à des moments différents. Si, cependant, la lumière d’une longueur d’onde exacte passe à travers l’atome à l’instant exact où l’atome est excité, l’atome est stimulé pour émettre et amplifier ce nouveau rayonnement; Ce rayonnement résultant est cohérent temporellement et spatialement. Ce dernier concept décrit efficacement le mécanisme de fonctionnement d’un pointeur vert a vendre et est en fait représenté par les trois dernières lettres du sigle laser: «émission stimulée de rayonnement».
La production par laser de ce faisceau cohérent est importante pour un certain nombre de raisons liées à leurs nombreuses applications:
- Il permet de focaliser la lumière sur un endroit très serré.
- La cohérence spatiale maintient le faisceau collimaté (en ligne) sur de longues distances.
- La cohérence temporelle permet une focalisation sur une longueur d’onde très étroite et permet des pulsations très rapides.
Tous les lasers se composent de trois composants principaux: une source d’énergie (également appelée pompe ou source de pompage), un milieu de gain (ou milieu laser) et un résonateur optique formé de deux lentilles optiques ou plus. L’image d’un pointeur laser 200mw déconstruit ci-dessous illustre ces trois composantes; Vous trouverez plus d’informations sur chaque composant sous l’image.
Dans le cas de l’image:
- La diode laser fonctionne comme source d’énergie (pompe).
- Le milieu de gain est un cristal d’oxyde de néodyme / yttrium-vanadium (représenté par le marqueur Nd: YVO).
- Une lentille expansible, une lentille de collimation et un filtre IR fonctionnent collectivement en tant que résonateur optique.
La plupart des propriétés d’un laser bleu puissant 10000mw sont déterminées par son milieu de gain, et pour cette raison, le milieu de gain définit le type et l’application du laser. Des centaines de matériaux sont appropriés comme supports laser, y compris:
- Liquides (colorants) – La configuration chimique d’une solution liquide détermine la longueur d’onde du laser.
- Gaz – dioxyde de carbone, argon, krypton, hélium-néon, etc. Les gaz sont généralement pompés par décharge électrique.
- Solides – cristaux et verres comme le grenat d’yttrium-aluminium (YAG) et les saphirs. L’hôte cristal / verre est dopé avec une des diverses impuretés pour le rendre approprié comme milieu de gain.
- Semiconducteurs – le mouvement des électrons à travers les jonctions de diode provoque l’action du laser.
La source d’énergie d’un laser lui fournit de la lumière. Les sources courantes sont les décharges électriques, les lampes, un autre laser et les réactions chimiques. La source de la pompe d’un laser dépend de son milieu de gain; Par exemple, un laser Nd: YAG serait fourni par une diode laser ou une lampe flash, tandis qu’un dispositif HeNe est alimenté par une décharge électrique.
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