The chemical laser is a member of the family of Gas Dynamic Lasers:

Gas dynamic lasers are based on rapid expansion of hot, high pressure gas, through nozzles into a near vacuum. This rapid expansion reduce the gas temperature.

As a result, since the transfer of the molecules to the ground state takes more than the time of rapid expansion, we get at low temperature many molecules at excited levels. Thus, "population inversion".

The gas usualy flow through the nozzles in a transverse flow (perpendicular to the optical axis of the laser), so many nozzles can operate at the same time, yielding high power from the laser.

The lasing action of the chemical laser is usually based on vibrational transitions of diatomic molecule.