  We calculate the relative amplitude of orbital angular momentum (OAM)
entangled photon pairs from the spontaneous parametric down conversion. The
results show that the amplitude depends on both the two Laguerre indices l, p.
We also discuss the influences of the mostly used holograms and mono-mode
fibers for mode analyzation. We conclude that only a few dimensions can be
explored from the infinite OAM modes of the down-converted photon pairs.
