Designing a Four Tube Hollow Core Fiber for Mid-IR Ultra-Low Loss Transmission

Abstract This paper introduces a novel hollow-core negative curvature fiber (HC-NCF) that can transmit broadband mid-infrared (MIR) light within the 3-12 µm range. Four nested silica tubes, separated by optimized air gaps, form the antiresonant layered cladding. A finite-difference time-domain (FDTD) approach is used to conduct numerical simulations. Aiming to reduce confinement and bending losses, the core and cladding dimensions are fine-tuned. By 3 µm, the confinement loss has reached 10⁻¹⁵ dB/km, but at bend radii up to 4 km, the bending loss stays below 10⁻⁴ dB/km. Strong air-core guidance is indicated by the fact that the effective index remains close to unity in all modes. We examine mode profiles, dispersion features, and stability of polarization. For MIR applications like gas sensing, spectroscopy, and laser delivery, the suggested design combines low loss, high modal purity, and structural simplicity, which is promising.