From amplifier to oscillator

Each amplifier can generate oscillations using a feedback system!

For a LASER this feedback is realized using a semitransparent mirror as a resonator.

Changing the arrangement of the mirrors one can vary the fraction of the amplified light.
To get a better ”quality” of the radiation one must increase the mirror losses.
The quality of the radiation is described by: the coherence (space and time), how parallel it is, plus time, ...