  We experimentally demonstrated that infrared light imprinted with orbital
angular momentum (OAM) was linearly converted into visible light using
four-wave mixing (FWM) via a ladder-type configuration in 85Rb atoms.
Simultaneously, we theoretically simulated this linear conversion process, and
the theoretical analysis was in reasonable agreement with the experimental
results. A large single-photon detuning process was used to reduce the
absorption of the atoms to the up-converted light and to avoid pattern
formation in the FWM process. The multi-mode image linear conversion process is
important for applications including image communications, astrophysics and
quantum information.
