The excitation mechanism is the source of energy that raises the atoms in the active medium into their excited state, thus creating population inversion.
In accordance to the law of conservation of energy, the electromagnetic radiation out of the laser is always less then the input energy by the excitation mechanism. There are lasers with efficiency of much less than 1% (!), while others with efficiencies approaching a 100%.
There are few types of excitation mechanisms:
a) Optical pumping - Excitation by photons:
The electromagnetic radiation source can be of different kinds: