  Photon pair entangled in high dimensional orbital angular momentum (OAM)
degree of freedom (DOF) has been widely regarded as a possible source in
improving the capacity of quantum information processing. The need for the
generation of a high dimensional maximally entangled state in the OAM DOF is
therefore much desired. In this work, we demonstrate a simple method to
generate a broader and flatter OAM spectrum, i.e. a larger spiral bandwidth
(SB), of entangled photon pairs generated through spontaneous parametric
down-conversion by modifying the pump beam profile. By investigating both
experimentally and theoretically, we have found that an exponential pump
profile that is roughly the inverse of the mode profiles of the single-mode
fibers used for OAM detection will provide a much larger SB when compared to a
Gaussian shaped pump.
