The nonlinear interaction of ultra-short pulse high brightness lasers with a uniform plasma is studied numerically in 1-D. Apart from the usual nonlinear steepening and periodic lengthening of the plasma wave, modification of the laser pulse also occurs if the pulse has a relatively long trailing edge compared to the leading edge. This induced modulation results in the formation of spikes on the laser pulse which are coincident with the Langmuir wave density maximum.