  We report on the formation and evolution of a dispersive shock wave in a
nonlinear optical medium. The experimental observations for the position and
intensity of the solitonic edge of the shock, as well as the location of the
nonlinear oscillations are well described by recent developments of Whitham
modulation theory. Our work constitutes a detailled and accurate bench-mark for
this approach. It opens exciting possibilities to engineer specific
configurations of optical shock wave for studying wave-mean flow interaction
and related phenomena such as analogue gravity.
