The interplay between the polarization state of light and
transverse effects in lasers is analyzed through
an amplitude equation description of an atomic transition
between spin sublevels. Linearly polarized travelling
waves are found, whose stability is restricted by a phase
instability associated with the direction of polarization.
The instability persists for polarization stabilized lasers.
Novel states of laser light such as standing waves with a
spatially periodic linear polarization or coexisting
travelling waves with different wave numbers and
circular polarizations are also found.
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