The geometry of photonic crystal waveguides with ring-shaped holes is optimized to minimize dispersion in the slow light regime. We found geometries with a nearly constant group index in excess of 20 over a wavelength range of 8 nm. The origin of the low dispersion is related to the widening of the propagating mode close to the lower band gap edge.
Säynätjoki, M., Mulot, M., Ahopelto, J., & Lipsanen, H. (2007). Dispersion engineering of photonic crystal waveguides with ring-shaped holes. Optics Express, 15(13), 8323-8328. https://doi.org/10.1364/OE.15.008323