Abstract
We report on the measurement of nonlinear phase shift phiNL induced by a strong pump at wavelength lamda p on a weak signal at lamda s through a cascade of two second-order interactions in a periodically poled lithium-niobate waveguide. The experiment is performed by use of an interferometric scheme especially designed to be insensitive to slow mechanical and thermal drift effects. The observed behavior of phiNL as a function of pump power and of wavelength difference lamda p - lamda s, is in good agreement with the theoretical predictions.