A laser is made with a cavity containing solid or gaseous matter. This matter constitutes the active environment. It is doped with ions, in the solid case, or ionized in the gas case. These ions are the stimulated emission elements. Being excited, they amplify light passing through them.
The stimulated emission functioning is that fora photon exciting an ion, two photons, identical to the forst one will appear. This gives its coherence properties to the laser (starting with one photon, it will be amplified until having a coherent light beam).
The cavity is bounded by two parallel mirrors. One is generally totally reflecting, the other partially. The cavity is then excited by an electric current or another laser. Then, all the light naturally present inside the cavity (noise of the cavity) is amplified then reflected between the mirrors. At each travel, part of the light crosses the partially reflecting mirror.
As both mirrors are parallel, every light beam which is not in the laser axis will be expelled by successive rebounds. This is why a laser bean is so thin. In the fiber optic case, this effect will be given by the fiber characteristics.
Continuous and impulsionnal lasers