  We report an experimental observation of degenerate mirrorless lasing in
forward direction under excitation of a dilute atomic Rb vapor with a single
linearly polarized cw laser light resonant with cycling Fe > Fg atomic D2
transitions. Light polarized orthogonally to the laser light is generated for
the input light intensity exceeding a threshold value of about 3 mW/cm^2.
Application of a transverse magnetic field directed along the input light
polarization reveals a sharp about 20 mG wide magnetic resonance centered at B
= 0. Increasing the incident light intensity from 3 to 300 mW/cm^2, the
generated light undergoes rapid amplitude increase followed by a decline and
resonance broadening. Such nonlinear behavior of the observed magnetic
resonance is attributed to the population inversion on optical transitions
between magnetic sublevels established under linearly polarized excitation. We
present observations that indicate that a combination of nonlinear-optical
effects occurs in this system, including degenerate mirrorless lasing and
four-wave mixing.
