  We study the two-photon detection amplitude of the down-converted beams in
spontaneous parametric down-conversion when the physical variable of orbital
angular momentum is involved, taking into account both conservation and
non-conservation of angular momentum. Agreeing with experimental observations,
our theoretical calculation shows that spatial structure of the two-photon
detection amplitude of the down-converted beams carries important information
about conservation or non-conservation of orbital angular momentum in
spontaneous parametric down-conversion.
