Degenerate optical parametric oscillators can exhibit both uniformly
translating fronts and non-uniformly translating envelope fronts under the
walk-off effect. The nonlinear dynamics near threshold is shown to be
described by a real convective Swift-Hohenberg equation which provides the
main characteristics of the walk-off effect on pattern selection. The
predictions of the selected wave-vector and the absolute instability
threshold are in very good quantitative agreement with numerical solutions
found from the equations describing the optical parametric oscillator.
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