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.
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