In order for a laser to function properly, more atoms with stored energy must be present than atoms with free “storage space”, as more photons on average would otherwise be absorbed than the number of new ones being added. This state is known as inversion. In order to achieve this state, atoms are kept in an excited state by “pumping” the laser, and some photons are inserted. This causes some atoms to undergo stimulated emission, and the resulting photons cause other atoms to undergo stimulated emission, leading to a chain reaction.

 To make some of the photons pass through the laser medium more than once, a so-called resonator is used: Two mirrors are positioned to reflect and amplify the light between them before it leaves the laser as the output beam. One mirror is almost perfectly reflective and the second reflects only most of the incoming photons, allowing some of the photons to pass through it. This forms the laser beam.

more>>