  We measure localized and extended mode profiles at the band edge of
slow-light photonic-crystal waveguides using phase-sensitive near-field
microscopy. High-resolution band structures are obtained and interpreted,
allowing the retrieval of the optical density of states (DOS). This constitutes
a first observation of the DOS of a periodic system with weak disorder. The Van
Hove singularity in the DOS expected at the band edge of an ideal 1D periodic
structure is removed by the disorder. The Anderson-localized states form a
"tail" in the density of states, as predicted by Lifshitz for solid-state
systems.
