  Any practical experiment utilising the innate D-dimensional entanglement of
the orbital angular momentum (OAM) state space of photons is subject to the
modal capacity of the detection system. We show that given such a constraint,
the number of measured, entangled OAM modes in photon pairs generated by
spontaneous parametric down-conversion (SPDC) can be maximised by tuning the
phase-matching conditions in the SPDC process. We demonstrate a factor of 2
increase on the half-width of the OAM-correlation spectrum, from 10 to 20, the
latter implying \approx 50 -dimensional two-photon OAM entanglement. Exploiting
correlations in the conjugate variable, angular position, we measure
concurrence values 0.96 and 0.90 for two phase-matching conditions, indicating
bipartite, D-dimensional entanglement where D is tuneable.
