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