  We report the first observation of coherent back-scattering (CBS) of light in
a transverse photonic disorder. The CBS peak is recorded in the far-field, at a
fixed propagation time set by our crystal length, and displays a contrast
approaching the ideal value of 1, which proves good coherence of transport in
our system. We study its dynamics for increasing disorder strength, and find a
non-monotonous evolution. For weak disorder, the CBS signal increases, and the
asymmetry of the momentum distribution becomes inverted compared to the initial
condition. For stronger disorder, we observe a resymmetrization of the momentum
distribution, confirmed by numerical simulations, and compatible with the onset
of strong (Anderson) localization.
