The orbital angular momentum (OAM) of a vortex beam depends on the selection of its rotation axis. Therefore, the
associated OAM spectrum is subject to the alignment of beam axis as well. It has been demonstrated that a slight
misalignment can cause a large change in OAM spectrum. By investigating the relationship between misalignment and
the resultant OAM spectrum change, we propose a method of measuring the titling angle and displacement to the
reference axis. This approach can be used in correcting the alignment of free-space optical communication system.
In the optical wireless communication systems proposed by Gibson, et al, the information is encoded as states of orbital
angular momentum (OAM) of light and the transmitter unit can produce laser beam with single OAM-state in a time-slot.
Recently we have proved that it is possible to generate multiple OAM-states simultaneously by single spatial light
modulator. This method is adopted in our free-space optical wireless communication system and these OAM-states can
be detected in the receiving unit by a computer-generated hologram. Hence, the transmission capacity is enhanced
significantly without increasing the complexity of system.