Frequencies of Optically Pumped Sub-Doppler Far-Infrared Laser Lines of Methanol

Sub-Doppler signals from the gain curves of four lines of a transversely pumped far-infrared methanol laser were used to stabilize the frequencies of the laser. The frequencies of four of the strongest FIR laser lines (70, 119, 123, and 570 μm) were measured. Lasers locked to these lines have a frequency reproducibility of a few parts in 108and an uncertainty of less than 30 kHz.