Resonant cavity and laser beam intensity
Fundamental Physics of Solid-State Laser-Type Materials

As the length of the single-crystal rods cannot exceed several centimetres, the amplification provided by a single pass is too small to be noticeable. The beam amplified during a first pass must therefore be re-injected by means of mirrors forming an optical resonant cavity or resonator. After multiple passes, the intensity of the emerging beam increases significantly. This is why the Fabry-Pérot cavity is so useful for interferometric studies. There are, however, single-pass amplifying media needed to amplify once the emission of a pump laser (ruby, YAG) or the laser emission from the oscillating cavity of a dye laser. They are sometimes referred to as superradiant media.