The effects of laser fluence and foreign gas (N~2, CO~2, NO~2) pressure on the degenerate four-wave-mixing (DFWM) spectrum and signal intensity of I~2 have been studied. The dependence of DFWM saturation intensity on foreign gas pressure is measured and found to be I~s~a~t∝p^0^.^7. Results show that the foreign gas has little effect on the DFWM spectrum structure but has great effect on the signal intensity, which depends on the laser fluence. At high laser fluence, the addition of foreign gas can enhance the DFWM signal, and thermal grating contribution dominates. Using the relationship of I~s~a~t∝p^0^.^7 and the two-level model, the dependence of I~2 DFWM intensity on foreign gas pressure can be well fitted.

Key words: IODINE