Abstract
For the traditional photonic crystal fibers with circular air holes, rectangular air holes are added to the fiber cladding. The periodic arrangement of the inner rectangular air holes allows the fiber structure to better match the annular mode field distribution of the vortex beam. The fiber structure was analyzed and calculated by COMSOL Multiphysics 5.4 finite element software, and the characteristics of fiber were analyzed, such as the dispersion, confinement loss, effective mode area and nonlinear coefficient. The results reveal that the photonic crystal fiber structure capable of carrying 50 orbital angular momentum (OAM) modes at the wavelength of 1.15 to 2.0 μm (850 nm). The effective refractive index difference Δneff between vector modes can reach 1 × 10-3, and larger difference can effectively separate the vector modes and improve the transmission performance of OAM modes. Moreover, the fiber has good performance, such as flat dispersion distribution of the low-order modes, low confinement loss below 10-9 dB·m-1, large effective mode field area and small nonlinear coefficient in the 850 nm wavelength range. Therefore, this fiber structure can be applied to the high-capacity communication system of fiber multiplexing OAM. In addition, the good characteristics of this fiber structure are of great significance for the transmission of vortex beam in fiber.
Highlights
The results reveal that the photonic crystal fiber structure capable of carrying 50 orbital angular momentum (OAM) modes at the wavelength of 1.15 to 2.0 μm (850 nm)
The vortex beam is a new type of beam with helical phase wavefront, and OAM modes with different topological charges are orthogonal in space, which can be used as independent channels
The photonic crystal fiber (PCF) structure designed in this study is calculated and analyzed by using the finite element software, the mode field distribution and the OAM modes supported in designed PCF were obtained
Summary
The effective refractive index difference between the vector modes is mostly on the order of 10−4, so that the number of OAM modes supported by the fiber is small. In 2016, Tian et al [12] designed a circular ring PCF structure that can support 26 OAM modes transmission in the core, and studied the characteristics of dispersion and confinement loss. In 2018, Nandam et al [13] proposed a spiral PCF with spirally arranged air holes that can support 14 OAM modes, and the effect of ellipticity on the index of fiber mode was discussed This fiber is relatively complex, which is not conducive to large-scale applications. The OAMl,m mode formed by the superposition of HEle+ve1n,m and HEol+d1d,m has the same circular polarization and field rotation direction, while the OAMl,m mode formed by the superposition of EH even l −1,m and odd l −1,m has the opposite behavior
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