Nonlinear-optical device characteristics of gaseous saturable absorbers

Measurements have been made of the third-order nonlinear-optical susceptibilities of gaseous infrared saturable absorbers. These coefficients are relevant to mode locking of infrared lasers such as CO
2
. The measurements are in good agreement with a theory based on a four-level model of the absorber. Earlier mode-locking observations are interpreted in light of the new data. The value of the transition moment and some other parameters of SF
6
are deduced from the measurements. For SF
6
at
p = 1
torr, the third-order nonlinear coefficient is in the range
1-10 \times 10^{-23}
C.m/V
³
, when interacting waves differ in frequency by less than the molecular collision frequency. We find that the average dipole transition moment for SF
6
near the
P(16)
or
P(20)
lines of CO
2
is approximately 0.1-0.2 Debye. The nonlinear coefficient of CH
3
F is typically three orders of magnitude smaller than that of SF
6
.