Application of orbital angular momentum in optical measurement

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.