Rami Arieli: "The Laser Adventure" Chapter 3.2 page 1
3.2 The excitation mechanism

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:

In lasers with solid or liquid active medium, it is common to supply the excitation energy in a form of electromagnetic radiation (photons) which are absorbed in the active medium.

The electromagnetic radiation source can be of different kinds: