  We control the diffusion of light in a disordered photonic waveguide by
modulating the waveguide geometry. In a single waveguide of varying
cross-section, the diffusion coefficient changes spatially in two dimensions
due to localization effects. The intensity distribution inside the waveguide
agrees to the prediction of the self-consistent theory of localization. Our
work shows that wave diffusion can be efficiently manipulated without modifying
the structural disorder.
