Polarization properties of fiber-based orbital angular momentum modes

Generally, orbital angular momentum (OAM) can be generated by combining two vector modes (VMs) or combining two linearly polarized modes (LPMs). The vortices generated by the first method are circularly polarized and carry spin angular momentum (SAM). While, the vortices generated by the second method are linearly polarized with no SAM. Although these two methods are closely related, they are essentially different. The difference and relation between the two methods are theoretically clarified in this paper. Two different kinds of OAM modes are experimentally demonstrated and validated in a two-mode fiber.