The optical cavity

Amplifying is not enough: a cavity is needed

Population inversion makes it possible to amplify light on each pass through the medium. But a single pass is not enough to obtain an intense, pure beam. The laser's trick: make the light pass a very large number of times through the medium, using an optical cavity — two mirrors facing each other.

   Structure d'un laser :

   miroir 100%                          miroir partiel (~99%)
      |||                                     |||
      |||  [====== milieu amplificateur ======]  |||
      |||         ^^^^^^^^^^^^^^^^^                |||
      |||         pompage (energie)               |||
      |||                                          ||| ---> faisceau LASER
      totalement                              laisse sortir
      reflechissant                           ~1 % de la lumiere

The principle of the cavity

• One mirror is totally reflective (100%).
• The other is partially reflective (for example 99%): it sends almost all the light back into the cavity, but lets a small fraction leak out — it is this fraction that forms the useful laser beam.

The photons make round trips between the mirrors, passing each time through the gain medium, triggering new stimulated emissions on each pass. The intensity grows on each round trip.

   photon initial (emission spontanee)
        -> traverse le milieu : amplifie
        -> rebondit sur un miroir
        -> re-traverse : re-amplifie
        -> rebondit...
        -> avalanche de clones identiques
        -> ~1 % s'echappe a chaque passage : le FAISCEAU

The natural selection of photons

The cavity also performs a sorting. Only photons travelling exactly along the axis of the mirrors make many round trips; those that leave askew exit the cavity after one or two passes and are not amplified.

   photon sur l'axe    : rebondit des milliers de fois -> amplifie -> survit
   photon de travers   : sort tout de suite -> non amplifie -> elimine

The result: the final beam is extraordinarily directional — all the photons travel parallel to the axis. It is this geometric sorting, combined with cloning by stimulated emission, that gives the laser its very fine, barely divergent pencil of light.

The operating balance

The laser stabilizes when the gain (photons created by stimulated emission on each pass) exactly compensates for the losses (photons that escape through the partial mirror, plus various losses). The pumping must be strong enough to exceed a certain threshold; below it, the amplification does not compensate for the losses and the laser does not "start".

In summary

The optical cavity — two mirrors facing each other, one of them partially reflective — makes the light take multiple round trips through the gain medium, so that stimulated emission accumulates on each pass. It also selects the photons travelling along the axis, hence the great directionality of the beam. The laser works as soon as the gain exceeds the losses (above a pumping threshold), and a small fraction of the light escapes to form the beam.