Amplification of radiation in atomic vapor induced by a linearly polarized laser radiation

We present the results of spectroscopic and polarization studies of dilute rubidium vapor exposed to a single-frequency linearly polarized diode laser radiation in a spectral range of atomic D(2) line. We report the origin of a circularly polarized radiation on V-type transitions of (87)Rb F (g) = 2 -> F (e) = 3 and (85)Rb F (g) = 3 -> F (e) = 4, and amplification of this radiation in backward direction caused by a partial population inversion among magnetic sublevels of the ground and excited levels. This is confirmed experimentally by high directivity of backward radiation, absence in its spectrum of (85)Rb F (g) = 2 -> F (e) = 1 (I >-type) radiation, as well as by different nature of intensity dependences of backward and fluorescence radiations.