The beating between two colinear Nd-YLF and Nd-YAG lasers in a homogen
eous plasma generates intense relativistic plasma waves associated wit
h a high longitudinal electric field of the order of 1 GV/m. It is sho
wn that these electron waves couple with ion waves in the regime of mo
dulational instability. Electric field amplitude and saturation time o
btained by Thomson scattering are in agreement with theoretical predic
tions taking this mechanism into account.