Noninvasive experimental control of beam profiles in nonlinear optics

A self-regulating method for the control of spontaneous instabilities of th e transversal beam profile in nonlinear optical systems is experimentally r ealized. The control method is the high-dimensional analogue of classical n egative feedback regulators. An all-optical implementation with its capabil ities of parallel processing is essential for the experimental feasibility. Unstable system-inherent transversal patterns are stabilized, including al so the stationary homogeneous state. Even spatio-temporal disorder is remov ed in favour of the highly symmetric, stationary patterns and nearly withou t losses of output power. The manipulation of the output state is demonstra ted to be noninvasive. This allows investigations of the otherwise inaccess ible unstable patterns, for which an example is given.