Bounds and optimisation of orbital angular momentum bandwidths within   parametric down-conversion systems
  The measurement of high-dimensional entangled states of orbital angular momentum prepared by spontaneous parametric down-conversion can be considered in two separate stages: a generation stage and a detection stage. Given a certain number of generated modes, the number of measured modes is determined by the measurement apparatus. We derive a simple relationship between the generation and detection parameters and the number of measured entangled modes. 
F. M. Miatto, D. Giovannini, J. Romero, S. Franke-Arnold, S. M.   Barnett, M. J. Padgett
quant-ph