Far-Infrared (FIR) laser frequencies arise from rotational transitions in an excited vibrational state of a polar gas molecule at low pressure.

The optically pumped FIR laser relies on the selective absorption of tuneable infrared radiation to create a population inversion between rotational states and hence generation laser action. In practice, a few molecules are chosen to provide discrete frequencies across the entire FIR spectrum from 40 μm upwards. Tables of FIR lines are available on request.