R. Neubecker

Transverse pattern formation in liquid crystal light valve feedback system

Abstract Presented are the first experimental results on transverse pattern formation with a liquid crystal light valve under optical feedback. The setup is reduced to the essential elements, retaining maximum symmetry. Formation of regular and perturbed hexagonal structures and optical turbulence is found. Analogy to an existing model, predicting such pattern formation, can be shown.

Fourier space control in an LCLV feedback system

We show that a control technique, based on feedback filtered at a Fourier plane, can stabilize the spatio-temporally disordered output of a nonlinear optical system. We demonstrate this in an experiment with a LCLV feedback system and in a theoretical model. We stabilize the system and select square and roll patterns. The technique is non-invasive in that the control signal becomes small when control is achieved. A combination of real- and Fourier-space filtering can stabilize patterns in any chosen region of the transverse space.

Phase defects in a nonlinear Fabry-Pérot resonator

Abstract Optical phase singularities have been observed in a nonlinear Fabry-Perot, with a nematic liquid crystal film serving as dispersive-nonlinear medium. The singularities appear as a concomitant phenomenon of low-complexity amplitude pattern formation. While creation and motion of the singularities is strongly related to the amplitude pattern, the latter one seems not to be significantly influenced by the singularities.

Control and manipulation of solitary structures in a nonlinear optical single feedback experiment

We report on the addressing and control of the lateral positions of optical spatial solitary structures in a single feedback experiment with a saturable Kerr nonlinearity. Solitary structures allow a locally self-confined switching between a dark background and a bright state. This binary character can be of use to all-optically route information in optical networks. In general the lateral positions of solitary structures are strongly influenced by mutual interactions and system inhomogeneities. For potential photonic applications these interactions must be controllable. Therefore a noninvasive interferometric control method based on spatial Fourier filtering is studied in order to position solitary structures. Numerical and experimental results show that solitary structures can be aligned to periodic grids of different scale with hexagonal or square geometries.

Manipulation of Solitary Structures in a Nonlinear Optical Single Feedback Experiment

Interest in spatial solitary structures originates from their characteristics to enable a locally self-confined switching between a dark background state and a bright spot. We report on the control and addressing of solitary structures in a single feedback experiment with a liquid crystal based reflective optically addressable spatial light modulator (OaSLM) as nonlinearity. In general the lateral positions of solitary structures are strongly influenced by mutual spot interaction and system inhomogeneities. Pinning, drifting and spontaneous disappearance of solitary spots determines the system behavior. For photonic applications these effects must be minimized. A control method based on a Fourier filtering method was studied in order to position solitary spots. Numerical and experimental results show that solitary spots can be aligned to periodic grids of hexagonal or square geometries.

Noninvasive experimental control of beam profiles in nonlinear optics

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.