We report on interferometric measurements of nonlinear phase shifts of
the fundamental mode that are due to cascading in a second-harmonic-g
eneration experiment in lithium niobate channel waveguides. With tempe
rature tuning of the wave-vector mismatch the nonlinear phase shifts w
ere adjustable in sign and magnitude. Varying the wave-vector matching
condition along the waveguide leads to large phase shifts with low de
pletion and dispersion of the fundamental.