An air-core fiber imposed by torsion is investigated in this paper. We refer to this kind of fiber as twisted air-core fiber (TAF). It has been demonstrated that the eigenstates of the TAF consist of guided optical vortex waves with different propagation constants of a different effective index. With the increase of the twist rate, TAF could separate the OAM modes which are near degenerate or degenerate in the air-core fiber. The separation of OAM modes in TAF is conductive to ultralong distance propagation with low crosstalk. TAF could be considered as an ideal candidate fiber for OAM based optical communication. Moreover, we investigated the twisted air-core photonic crystal fiber (TAPCF) which can improve the relative energy distribution of the OAM modes. Compared with TAF, more energy is located in the ring shaped core, which is conductive to ultralong distance propagation. TAF and TAPCF are of potential interest for increasing channel capacity in optical telecommunications, and the result is also of interest to the photonic crystal community.

We proposed an approach for creating three-dimensional (3D) multifocal perfect vortices arrays by using a high numerical aperture objective. The position, orbital angular momentum states, number and diameter of the perfect vortices can be freely modulated by a special designed hybrid phase plate (HPP). HPP could be calculated by 3D phase shifting expression which is derived from Fourier transform theory of the Debye diffraction integral. Furthermore, we developed a novel pixel checkerboard method for adding phase information into the HPP. The segmentation of HPP is related to vortex quality and intensity uniformity. This method could fully use each pixel to modulate the light, since the spatial light modulator has to be used. Small size lattices could generate high quality and uniform intensity vortex arrays in tight focusing region, which may have potential applications in coupling, optical coding and decoding.

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We present a simple method of spatial-division multiplexing/demultiplexing with orbital angular momentum (OAM) based on a multi-ring optical fiber. Multi-ring fibers are ideal spatial division multiplexing carriers, providing more communication channels including orthogonal OAM multiplexing channels and spatial channels. We propose an OAM encoding scheme using multiplexing perfect vortex array (MPVA) to make full use of these channels and achieve high-capacity data communication links. For improving the coupling efficiency, perfect vortices whose radii independent of topological charge are employed to generate the intensity rings in each ring core region. Furthermore, a practical encoding scheme of multiplexing OAM is proposed to optimize the use of available OAM states. The encoded information in one core could reach L bits as there are L available OAM states in the fiber. To decode the multiplexing perfect vortex, an additional correction phase is employed to transform the perfect vortex into conventional optical vortex. By detecting the states of multiplexing OAM of each spot in this array with a perfect vortex detection phase plate, information encoded by perfect vortices is decoded. The efficient data encoding and decoding method with MPVA could be expanded to almost all kinds of multi-ring fibers to achieve high coupling efficiency and high-capacity data transmission.