DOUBLE-RESONANCE SPECTROSCOPY BY A TRANSVERSELY NONUNIFORM PUMP BEAM IN A 3-LEVEL MOLECULAR-SYSTEM

We present a theoretical analysis of the influence of the collimation of a strong pump beam on a small-signal-gain line shape in a three-lev el LAMBDA-type molecular system in the regime of high pump intensity w hen saturation and Stark (Autler-Townes) splitting of the levels of th e pumping transition take place. The cases of homogeneous and Doppler- broadened pumping transitions have been considered. A significant offs et, an asymmetric deformation, and a narrowing of the gain line, i.e., a spectrum focusing, have been obtained and explained as a result of the nonlinear (dispersion) and the active (gain) wave guiding of the s mall signal in the field of the strong pump beam.