In a nonlinear optical system with single-mirror feedback, several experiments displayed stationary hexagonal patterns. A linear stability analysis, however, predicts drifting patterns even for an arbitrarily small misalignment of the mirror. Studying the situation of small tilt angles we observe that the patterns remain stationary up to a critical value ␣ c of the angle (locking). Above this threshold they discontinuously start to drift with a velocity that depends linearly on the tilt angle. Both features are reproduced by numerical simulations with a Gaussian input beam. The existence of the locking region is traced back to the boundary conditions imposed by the spatially limited input beam. ͓S1050-2947͑97͒51312-6͔PACS number͑s͒: 42.65. Sf, 47.54.ϩr

An almost linearly polarized laser beam focused into a sodium vapor cell may split into different patterns ranging from two spots to a spot and a ring of opposite circular polarization. The patterns are controlled by static magnetic fields of the order of the earth s field. The behavior is explained, allowing for a longitudinal component of the dielectric polarization in the paraxial wave equation. Numerical results reveal the delicate balance between light shift, inhuence of the magnetic field, and the creation of ground-state orientation by the competing polarization components.PACS number(s): 42.50.Ne, 42.50. 6y, 42.65.An

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