What is the theory behind the LASER operation?

laser is the optical source which used to convert the signal from electrical to optical form using the stimulated emission theory ( light amplification with stimulated emission of radiation )
the laser manufactured by semiconductor alloy ( gallium , arsenid , phosphide ) and when the elcton hit the atom it absorb the energy of electon and last a decay time ( microseconds ) then it will loose this energy in another way ( photons ) emitted . then we reflect this by mirriors