  We experimentally control the spectral structure of photon pairs created via
spontaneous four-wave mixing in microstructured fibers. By fabricating fibers
with designed dispersion, one can manipulate the photons' wavelengths, joint
spectrum, and, thus, entanglement. As an example, we produce photon-pairs with
no spectral correlations, allowing direct heralding of single photons in
pure-state wave packets without filtering. We achieve an experimental purity of
$85.9\pm1.6%$, while theoretical analysis and preliminary tests suggest 94.5%
purity is possible with a much longer fiber.
