Manipulation and removal of defects in spontaneous optical patterns

Defects play an important role in a number of fields dealing with ordered structures. They are often described in terms of their topology, mutual interaction and their statistical characteristics. We demonstrate theoretically and experimentally the possibility of an active manipulation and removal of defects. We focus on the spontaneous formation of two-dimensional spatial structures in a nonlinear optical system, a liquid crystal light valve under single optical feedback. With increasing distance from threshold, the spontaneously formed hexagonal pattern becomes disordered and contains several defects. A scheme based on Fourier filtering allows us to remove defects and to restore spatial order. Starting without control, the controlled area is progressively expanded, such that defects are swept out of the active area.Comment: 4 pages, 4 figure

Spatial correlations in hexagons generated via a Kerr nonlinearity

We consider the hexagonal pattern forming in the cross-section of an optical beam produced by a Kerr cavity, and we study the quantum correlations characterizing this structure. By using arguments related to the symmetry broken by the pattern formation, we identify a complete scenario of six-mode entanglement. Five independent phase quadratures combinations, connecting the hexagonal modes, are shown to exhibit sub-shot-noise fluctuations. By means of a non-linear quantum calculation technique, quantum correlations among the mode photon numbers are demonstrated and calculated.Comment: ReVTeX file, 20 pages, 7 eps figure

Noninvasive experimental control of beam profiles in nonlinear optics

Abstract. A self-regulating method for the control of spontaneous instabilities of the transversal beam profile in nonlinear optical systems is experimentally realized. The control method is the high-dimensional analogue of classical negative feedback regulators. An all-optical implementation with its capabilities of parallel processing is essential for the experimental feasibility. Unstable system-inherent transversal patterns are stabilized, including also the stationary homogeneous state. Even spatio-temporal disorder is removed in favour of the highly symmetric, stationary patterns and nearly without losses of output power. The manipulation of the output state is demonstrated to be noninvasive. This allows investigations of the otherwise inaccessible unstable patterns, for which an example is given

Noninvasive experimental control of beam profiles in nonlinear optics

Abstract. A self-regulating method for the control of spontaneous instabilities of the transversal beam profile in nonlinear optical systems is experimentally realized. The control method is the high-dimensional analogue of classical negative feedback regulators. An all-optical implementation with its capabilities of parallel processing is essential for the experimental feasibility. Unstable system-inherent transversal patterns are stabilized, including also the stationary homogeneous state. Even spatio-temporal disorder is removed in favour of the highly symmetric, stationary patterns and nearly without losses of output power. The manipulation of the output state is demonstrated to be noninvasive. This allows investigations of the otherwise inaccessible unstable patterns, for which an example is given