Fig. 2: Finite element modelling of polarization properties of three antiresonant fibres (ARFs).

a Scanning electron micrograph cross-sections of the four-tube truncated double nested antiresonant nodeless fibre (tDNANF) and the five-tube nested antiresonant nodeless fibre (NANF) fabricated in-house, alongside the six-tube NANF reported in ref. ³⁸ (referred to as NANF-SI therein). The core diameters of the three ARFs are 28.2 μm, 27.9 μm, and 33.4 μm, respectively. The outer tube thicknesses are 1.08–1.16 μm, 0.41–0.51 μm, and 1.22 μm, respectively. The gap sizes are measured to be 5.4–6.2 μm, 5.3–7.1 μm, and 1.8–3.1 μm, respectively. b Simulated phase birefringence (B_p) and principal axis offsets (Δθ_A, with respect to that at 1545 nm) with no bend and in the wavelength range of 1510–1580 nm. The structural asymmetry residual in realistic ARFs—e.g., the variation of the membrane thickness of the outer tubes, as outlined in panel a—gives rise to wavelength-dependent B_p and Δθ_A. For the six-tube NANF, the experimental results of θ_A under a bend radius (R_b) of 8 cm are acquired from Fig. S4 in ref. ³⁸, and the simulation of B_p utilizes the structural parameters listed in Table S1 therein together with an invariant membrane thickness of 1.22 μm. For all the three ARFs, the simulated mode-field profiles of the two polarizations at 1545 nm are plotted below with the arrows indicating the vector direction of the transverse electric field. c Simulated B_p and Δθ_A of the four-tube tDNANF and the five-tube NANF as a function of wavelength (λ) and fibre bend direction (β) under R_b = 6 cm. Δθ_A are defined as the angle offsets from the principal axis directions at 1545 nm. The red arrow in panel a depicts the bend direction from the fibre toward the centre of curvature.