Nonlinear wave propagation was investigated experimentally in coupled waveg
uides by means of the cascaded nonlinearity in quasi-phase-matched second-h
armonic generation. with a specially designed wave-vector-mismatch distribu
tion along the propagation axis, cascading was optimized for low fundamenta
l depletion. High-contrast, ultrafast all-optical switching with switching
powers of tens of watts was observed. (C) 1999 Optical Society of America.