Infrared–far‐infrared stabilized laser system with adjustable frequency offset for spectroscopic measurements

A double resonance infrared–far‐infrared spectroscopic system is described. The variation of the 118.8 μm wavelength gain line shape of the methanol far‐infrared laser on the diameter of the pump CO2 laser beam due to the induced nonlinear waveguiding effect has been measured. The high‐frequency stability and continuous tunability of the pump CO2 lasers necessary for such measurement were achieved by frequency offset locking them to a molecular line center stabilized reference laser. The long term frequency stability achieved for the offset locked lasers is 24 kHz.