  The light ray of a spatial soliton in an optical film whose refractive index
is smoothly modulated (wavelength much larger than the typical soliton width)
in both spatial directions is shown to possess chaotic regimes for which the
propagation is erratic. This is interpreted as a parametric driven pendulum,
obtained by a new perturbative approach of the Maxwell equation. These findings
are then demonstrated to compare well to the eikonal law of light ray
propagation (nonlinearity compensates diffraction).
