the laser tube (resonator) with an active laser medium (e.g. a crystal, a gas, a liquid or a semiconductor)
a mirror on both ends, one reflecting, the other partially reflecting (90 %)
a pump source, e.g. a flashlight, an electron beam, a diode or a pump laser
sometimes additional external optics to focus/defocus the laser beam or to create a parallel/collimated beam
To start the laser activity the atoms of the laser medium have to be stimulated by the pum-ping process. The electrons of the atoms are elevated to energetically higher levels. Falling back to a lower level they emit their energy in form of a photon which is the smallest energy quantum of electromagnetic radiation. These photons in turn stimulate other atoms. The amount of light is being continually increased in intensity as it oscillates back and forth be-tween the mirrors, creating an avalanche effect. A certain part of the photons pass through the partially reflecting/partially transmitting mirror and forms the laser beam.
