Robust transmission of orbital angular momentum mode based on a dual-cladding photonic quasi-crystal fiber

A dual-cladding photonic quasi-crystal fiber is theoretically proposed to robustly transmit orbital angular momentum (OAM) modes. This fiber, based on conventional silica background, can lift the effective index separation of constituent vortex modes to the order of 10⁻² in the band of  >200 nm, surpassing two orders of magnitude than that of common silica-based fibers. Moreover, the effective index separations in first order group (TE₀₁, HE₂₁, and TM₀₁) present an opposite and near-linear variation with the increase of air-filling fraction of inner and outer cladding. The mode TE₀₁ can simultaneously realize an ultra-flattened dispersion with a maximum variation of 19.736 ps/nm/km and low confinement loss of order of 10⁻² dB m⁻¹ within a 400 nm bandwidth covering the communication bands of E, S, C, L and U. Detailed investigation on bend-induced influence shows that this fiber can remain well OAM propagation property with the bending radius being more than 30 μm. In addition, the robustness of OAM property is also stressed with assuming  ±3% deviation of diameter of all air holes due to imperfect fiber preparation. This design is expected for the application of OAM optical communication system and to provide guidance for engineering the large-index-separation fiber. (paper)