Far Infra-Red (FIR) lasers emit radiation in the Far-Infra-Red spectrum (wavelength range 12-1000 [mm].

The wavelength range greater than 100 [mm] is sometimes called sub-millimeter wave.

Far Infra-Red (FIR) lasers are gas lasers, and their lasing action occur between rotational levels of the gas molecules of the active medium. Usually these transitions are within the same vibrational level.

The active medium in FIR lasers is usually a gas of simple organic molecule such as:

Because of the very narrow width of each energy level of these materials, it is inefficient to optically pump them with ordinary light sources.
The best way to achieve population inversion in these lasers is to pump them with another laser at shorter wavelength. Usually CO₂ laser is used for pumping.