  We report spontaneous parametric downconversion having an unusually wide
spectral bandwidth. A collinear type-1 phase-matching configuration is employed
with degeneracy near the zero group velocity dispersion frequency. With a
spectral width of 1080 nm and degenerate wavelength 1885 nm, the source also
emits a high flux of photon pairs constrained to a cone of only 2 degree
half-angle. A rigorous theoretical approach is developed that confirms the
experimental observations. The source properties are consistent with an
ultra-short photon-pair correlation time and, for a narrowband pump, extremely
high-dimensional spectral entanglement.
