Drifting and locking behavior of optical patterns - an investigation usingFourier filtering

Stable hexagonal patterns have been observed by many groups in a nonlinear optical system with single-mirror feedback. In contrast, a linear stability analysis predicts drifting patterns if there is an arbitrarily small misal ignment of the feedback mirror. Studying the situation of small tilt angles we observe that the patterns remain stationary up to a critical value cc, of the angle (locking) and start to drift abruptly as a whole if a, is exce eded. Above this threshold the drift velocity depends linearly on the tilt angle. The locking is reproduced by numerical simulations with a Gaussian i nput beam and traced back to the boundary conditions imposed by the spatial ly limited input beam. Filtering of the feedback beam in Fourier space is u sed to generate stripe patterns which are normally unstable in the system. The investigation of the stripe patterns allows the verification of a dispe rsion relation predicted before and the confirmation that the locking behav ior is not restricted to hexagons. (C) 1999 Elsevier Science Ltd. All right s reserved.