Bounds and optimisation of orbital angular momentum bandwidths within parametric down-conversion systems F.M. Miatto, D. Giovannini, J. Romero, S. Franke-Arnold, S.M. Barnett and M.J. Padgett Eur. Phys. J. D, 66 7 (2012) 178 Published online: 10 July 2012 DOI: 10.1140/epjd/e2012-20736-x