Negative curvature hollow-core fibers (NC-HCFs) are useful as gas sensors. We numerically analyze the single-mode performance of NC-HCFs. Both single-ring NC-HCFs and nested antiresonant fibers (NANFs) are investigated. When the size of the cladding tubes is properly designed, higher-order modes (HOMs) in the fiber core can be coupled with the cladding modes effectively and form high-loss supermodes. For the single-ring structure, we propose a novel NC-HCF with hybrid cladding tubes to enable suppression of the first two HOMs in the core simultaneously. For the nested structure, we find that cascaded coupling is necessary to maximize the loss of the HOMs in NANFs, and, as a result, NANFs with five nested tubes have an advantage in single-mode guidance performance. Moreover, a novel NANF with hybrid extended cladding tubes is proposed. In this kind of NANF, higher-order mode extinction ratios (HOMERs) of 105 and even 106 are obtained for the LP11 and LP21 modes, respectively, and a similar level of 105 for the LP02 modes. Good single-mode performance is maintained within a broad wavelength range. In addition, the loss of the LP01 modes in this kind of NANF is as low as 3.90 × 10−4 dB/m.
Broadband robustly single-mode hollow-core PCF by resonant filtering of higher-order modes