  We experimentally demonstrate that the spectral sensitivity of a Mach-Zehnder
(MZ) interferometer can be enhanced through structural slow light. We observe a
20 times enhancement by placing a dispersion-engineered-slow-light
photonic-crystal waveguide in one arm of a fibre-based MZ interferometer. The
spectral sensitivity of the interferometer increases roughly linearly with the
group index, and we have quantified the resolution in terms of the spectral
density of interference fringes. These results show promise for the use of
slow-light methods for developing novel tools for optical metrology and,
specifically, for compact high-resolution spectrometers.
