Nonlinear absorption of a transversely excited atmosphere CO_2 laser beam in CF_3Br molecules and measurement of the nonlinear refractive-index coefficient

Abstract

We investigate self-focusing and self-defocusing of a transversely excited atmosphere CO2 laser pulse (1.3 MW) in the medium of CF3Br molecules. Self-focusing of the 9R(30) line and self-defocusing of the 9R(26) and 9R(28) lines are observed. Nonlinear absorption cross sections (σ) are first measured for various fluences of the laser beam to permit us to obtain an accurate expression for the nonlinear absorption coefficient α. The beam-narrowing effect of the nonlinear absorption (α) modifies the far-field spatial profile of the beam. Finally, from the distorted profile we derive the nonlinear refractive-index coefficient n2 with an improved accuracy. The nonlinear refractive-index coefficients for the three lines are −7.9 ± 0.6 × 10−13 cm2/(W Torr) for 9R(26), −3.7 ± 0.5 × 10−13 cm2/(W Torr) for 9R(28), and 4.9 ± 0.4 × 10−12 cm2/(W Torr) for 9R(30).