Thermal grating effects in infrared degenerate four-wave mixing fro trace gas detection

Degenerate four-wave mixing (DFWM) in the infrared region using a single mode CO2 laser has been used for the detection of ethylene, sulfur hexafluoride and mixtures of these gases with nitrogen and argon in the pressure range up to one atmosphere. The investigations showed that for the low pressure regime (<6.5 × 103 Pa) the formation of the DFWM signal was dominated by population gratings whereas for high pressures (>25 × 103 Pa) mostly contributions from thermal gratings gave rise to the observed DFWM signal. This behaviour is reproduced by a simple model prediction for these relaxation processes.