DRIFT INSTABILITY AND LOCKING BEHAVIOR OF OPTICAL-PATTERNS

In a nonlinear optical system with single-mirror feedback, several exp eriments displayed stationary hexagonal patterns. A linear stability a nalysis, however, predicts drifting patterns even for an arbitrarily s mall misalignment of the mirror. Studying the situation of small tilt angles we observe that the patterns remain stationary up to a critical value alpha(c) of the angle (locking). Above this threshold they disc ontinuously start to drift with a velocity that depends linearly on th e tilt angle. Both features are reproduced by numerical simulations wi th a Gaussian input beam. The existence of the locking region is trace d back to the boundary conditions imposed by the spatially limited inp ut beam. [S1050-2947(97)51312-6].