It is very simple: switch on the button and there is a light! In more complicated sometimes something must be tuned. It works in the same way like this “singing pipe”: charge it with energy and she alone create a proper sound (frequency).
Laser is taken from a simple,
as usually with Einstein’s ideas, observation that atoms cannot to infinity
absorb radiation because they will be “warmer” than photon bath in which they are placed.
There is need to forced emmision;
existence of another photons forces atom to emission his photon.
In practical realization, laser is pumped by external energy (external flash of light, electrical discharge, current on the semiconductor junction) and has a well determined resonance frequency.
Name of the device means light amplifier: it travels many times between the mirrors and with every passage it is amplified.
Last need are proper energetic levels in atom. One of them must be long-lived. Practically for many electrons atoms like Ar and He, such levels always exist.
Nobel prize for laser discovery was given in 1964 to C.H Townes, N.G. Basov and A.M. Prochorov , but in fact it was made by Meinen in USA, but every trace of him is lost [story told by prof. L. Pitaevski].
First CO2 laser (1968) [Science Museum, Londyn]