We present complete experimental characterisation of the temporal shape of an intense ultrashort 200-TW laser pulse driving a laser wakefield. The phase of the pulse was uniquely measured using (second order) frequency resolved optical gating (FROG). The pulses are asymmetrically compressed, and exhibit a positive chirp consistent with the expected asymmetric self-phase modulation due to photon acceleration/deceleration in a relativistic plasma wave. The measured pulse duration decreases linearly with increasing length and density of the plasma, in quantitative agreement with the intensity dependent group velocity variation in the plasma wave.
