Fourfold truncated double-nested antiresonant hollow-core fiber with ultralow loss and ultrahigh mode purity

Hollow-core fibers (HCFs) are inherently multimode, making it crucial to filter out higher-order modes (HOMs) within the shortest possible fiber length for applications such as high-speed coherent communications and fiber-optic gyroscopes. However, current HCF designs face the challenges of simultaneously achieving ultralow fundamental mode (FM) loss and ultrahigh HOM suppression. In this study, we present a fourfold truncated double-nested antiresonant nodeless hollow-core fiber (4T-DNANF) structure that addresses this challenge. Our 4T-DNANF enables greater control over phase matching between core modes and air modes in the cladding, allowing for minimized FM loss and substantially increased HOM loss. Experimentally, we fabricated several HCFs: one with an FM loss of 0.1 dB/km and an HOM loss of 430 dB/km, and another achieving an FM loss of 0.13 dB/km with a HOM loss of 6500 dB/km, yielding a higher-order mode extinction ratio of 5×10 4 —the highest reported to date.