Epsilon-near-zero photonic crystal fibers for a large mode separation of orbital angular momentum modes

An orbital angular momentum (OAM) mode is a good candidate as an orthogonal basis mode set for mode division multiplexing (MDM) owing to its unique phase properties. Photonic crystal fibers support large numbers of OAM modes with notable performances. However, the effective index difference between high order OAM modes is not large enough. In this work, we propose two types of epsilon-near-zero (ENZ) materials-embedded circular photonic crystal fibers for high mode separation. Compared with previously developed photonic crystal fibers, the designed fibers exhibit a high refractive index contrast owing to the inserted ENZ materials. The designed fibers support up to 42 OAM modes. Most of these OAM modes provide a large effective index difference (Δneff > 10−3) while maintaining a radial sing mode condition over a wide wavelength range from 1.3 μm to 1.8 μm. The proposed fibers could potentially be exploited in MDM for stable transmission of OAM modes and other OAM-based applications.