  We propose a quantitative model explaining the mechanism of light collimation
by leaky surface modes that propagate on a corrugated surface around the output
of a photonic crystal waveguide. The dispersion relation of these modes is
determined for a number of surface terminations. Analytical results obtained on
the basis of the model are compared to those of rigorous numerical simulations.
Maximum collimation is shown to occur at frequency values corresponding to
excitation of surface modes whose wave number retains a nonzero real part.
