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 D2 line. We report the origin of a circularly polarized radiation on V-type transitions of 87Rb F g = 2 → F e = 3 and 85Rb 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 85Rb F g = 2 → F e = 1 (Λ-type) radiation, as well as by different nature of intensity dependences of backward and fluorescence radiations.
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