Results of an experiment using an ultra-short laser pulse to create plasma waves through the forced laser wake-field process are described. The transmitted optical spectra are shown to exhibit both red- and blue-shifting, likely due to self-phase modulation through the interaction between the laser pulse and a large amplitude plasma wave. Spectral side-bands shifted by multiples of the plasma frequency associated with the forward Raman instability (FRS) are absent, indicating that the plasma waves which are observed to accelerate electrons are likely not generated through the FRS process. One- and two-dimensional particle-in-cell simulations using similar parameters as the experiment are discussed.