The properties and uses of the laser

Light like no other

The whole mechanism of the laser — stimulated emission, inversion, cavity — produces light with exceptional properties that ordinary light (a light bulb, the Sun) does not possess. Three qualities set it apart.

1. Monochromatic (a single colour)

All the photons come from the same atomic transition, so they have exactly the same frequency. Laser light has an unrivalled purity of colour.

   lumiere blanche (ampoule) :  |###toutes les couleurs###|
   lumiere laser             :  |    |  <- une seule raie tres fine

2. Coherent (all the waves in phase)

Thanks to stimulated emission, which clones the phase, all the light waves oscillate together, in phase, like a single perfectly synchronized choir. This is coherence.

   lumiere incoherente :  \\/\\  /\\/  \\/\\   (ondes dephasees, en desordre)
   lumiere coherente    :  \\/\\/\\/\\/\\/\\/   (toutes en phase, alignees)

This coherence is what makes possible sharp interference patterns (holography) and ultra-precise distance measurements.

3. Directional (a fine pencil of light)

Thanks to the sorting by the cavity, all the photons travel parallel. The beam stays fine and barely divergent over very long distances — to the point that a laser pointed at the Moon forms only a spot a few kilometres across there, which made it possible to measure its distance to within a centimetre.

   lampe    :  rayonne dans TOUTES les directions  * * *
                                                   * (o) *
                                                    * * *
   laser    :  un pinceau concentre  ============>

A concentration of energy

These three properties together make it possible to concentrate an enormous amount of energy onto a tiny point, something no ordinary light can do (you cannot focus a light bulb onto such a fine point, because its light goes in all directions and mixes the colours).

Applications everywhere

   Domaine            Usage du laser
   -----------------  -------------------------------------------
   Quotidien          lecteurs CD/DVD/Blu-ray, code-barres, souris
   Telecommunications  fibre optique (internet a la vitesse lumiere)
   Medecine           chirurgie de l'oeil, bistouri laser, dermato
   Industrie          decoupe et soudure de metaux de precision
   Science            refroidissement d'atomes, mesures, fusion
   Metrologie         distances (Terre-Lune), interferometrie (LIGO)

The laser, born of a theoretical prediction by Einstein in 1917 and realized only in 1960, has become an invisible but ever-present technology. Every internet call over fibre, every barcode scan, relies on this fine mastery of stimulated emission.

In summary

Laser light is monochromatic (a single colour, a single transition), coherent (all the waves in phase, thanks to cloning by stimulated emission) and directional (a fine pencil of light, thanks to the sorting by the cavity). These three properties make it possible to concentrate energy in an unrivalled way, hence countless applications — from the DVD player to surgery, from optical fibre to the Earth-Moon measurement. A major and concrete application of quantum physics.