When the research was performed the author was with the Physical Institute, University of Erlangen-Nürnberg, Erlangen D-8520, Germany. He is now at the Laboratory for Digital Optics, Institute of Information Transmission Problems, Academy of Sciences of the USSR, Yermolovoj str. 19, Moscow 101447, Russia. Received 13 September 1991. 0003-6935/92/111677-03$05.00/0. © 1992 Optical Society of Amer...