  The capacity of optical communication channels can be increased by space
division multiplexing in structured optical fibers. Radial core optical fibers
allows for the propagation of twisted light--eigenmodes of orbital angular
momentum, which have attracted considerable attention for high-dimensional
quantum information. Here we study the generation of entangled photons that are
tailor-made for coupling into ring core optical fibers. We show that the
coupling of photon pairs produced by parametric down-conversion can be
increased by close to a factor of three by pumping the non-linear crystal with
a perfect vortex mode with orbital angular momentum $\ell$, rather than a
gaussian mode. Moreover, the two-photon orbital angular momentum spectrum has a
nearly constant shape. This provides an interesting scenario for quantum state
engineering, as pumping the crystal with a superposition of perfect vortex
modes can be used in conjunction with the mode filtering properties of the ring
core fiber to produce simple and interesting quantum states.
