POPULATION INVERSION MECHANISMS IN GAS LASERS

In contrast the light emitted by a laser is generated by stimulated emission in an optical resonator. As a result the radiation has high spatial and temporal coherence which is not found in light emitted by any other source. In addition, the energy densities of radiation in laser beams 

In order that there should be a net amplification on a transition connecting non-degenerate states, it is necessary that the number density of atoms in the upper level should exceed that in the lower laser level. This fundamental requirement is known as a population inversion. 

The main purpose of this chapter is to examine the mechanisms by which population inversions are created in the most important of currently available gas laser systems. First, however, we derive detailed expressions for the gain coefficient in gas discharges and consider the conditions that must be satisfied by the radiative lifetimes and collision rates involved to ensure that gain should exist in steady state systems. 

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