Feedback controlled periodic states for different kinds of photorefractive nonlinear response B. I. Sturman, A. S. Gorkunova and K. H. Ringhofer Eur. Phys. J. D, 23 2 (2003) 291-297 Published online: 04 March 2003 DOI: 10.1140/epjd/e2003-00057-1