NONLINEAR ABSORPTION OF A TRANSVERSELY EXCITED ATMOSPHERE CO2-LASER BEAM IN CF3 BR MOLECULES AND MEASUREMENT OF THE NONLINEAR REFRACTIVE-INDEX COEFFICIENT

We investigate self-focusing and self-defocusing of a transversely exc ited atmosphere CO2 laser pulse (1.3 MW) in the medium of CF3Br molecu les. Self-focusing of the 9R(30) line and self-defocusing of the 9R(26 ) and 9R(28) lines are observed. Nonlinear absorption cross sections ( sigma) are first measured for various fluences of the laser beam to pe rmit us to obtain an accurate expression for the nonlinear absorption coefficient alpha. The beam-narrowing effect of the nonlinear absorpti on (alpha) 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-ind ex coefficients for the three lines are -7.9 +/- 0.6 x 10(-13) cm2/(W Torr) for 9R(26), -3.7 +/- 0.5 X 10(-13) cm2/(W Torr) for 9R(28), and 4.9 +/- 0.4 X 10(-12) cm2/(W Torr) for 9R(30).