Reproduction of material from the journal in any form requires written permission of the publisher.
Teachers, students and researchers can individually use the materials on Physics-Uspekhi website for noncommercial use in a teaching or research activities.
Оригинал: Днепровский В С «Нелинейная фотоника» УФН 161 (6) 219–220 (1991); DOI: 10.3367/UFNr.0161.199106j.0219