THERMAL GRATING EFFECTS IN INFRARED DEGENERATE 4-WAVE-MIXING FOR TRACE GAS-DETECTION

Degenerate four-wave mixing (DFWM) in the infrared region using a sing le mode CO2 laser has been used for the detection of ethylene, sulfur hexafluoride and mixtures of these gases with nitrogen and argon in th e pressure range up to one atmosphere. The investigations showed that for the low pressure regime (< 6.5 X 10(3) Pa) the formation of the DF WM signal was dominated by population gratings whereas for high pressu res (> 25 X 10(3) Pa) mostly contributions from thermal gratings gave rise to the observed DFWM signal. This behaviour is reproduced by a si mple model prediction for these relaxation processes.