Design of Weakly Coupled Two-Mode Hollow-Core Antiresonant Fiber with Low Loss

We first design and optimize a hollow-core conjoined-tube antiresonant fiber (CT-ARF) for weakly coupled \mathrm{LP-{01}} and \mathrm{LP-{11}} modes transmission. The critical core radii for the weak-coupling condition in the CT-ARFs with 4 to 8 tubes are obtained. Using numerical simulation, we demonstrate that by tuning the glass sheets positions in cladding tubes the confinement losses (CLs) of higher-order modes (HOMs) \mathrm{LP-{21}}, \mathrm{LP-{02}}, and \mathrm{LP-{31}} can be maximized through core-cladding mode coupling. Among the CT-ARFs with 4 to 8 tubes, the 6-tube one is proved to have the lowest CLs and bending losses of \mathrm{LP}-{01} and \mathrm{LP}-{11} modes and the highest CL ratios between the undesired HOMs and the two guided modes. In the designed 6-tube CT-ARF, \mathrm{LP-{01}} and \mathrm{LP-{11}} modes have low CLs in the transmission band of 1.31-1.65 \mu \text{m}. The minimum CLs of \mathrm{LP-{01}} and \mathrm{LP-{11}} modes are \text{1.7}\times \text{10}^{-4} \text{ dB/m} and \text{3.1} \times \text{10}^{-3} \text{ dB/m} in the C-band (1.53-1.565 \mu \text{m}) respectively. A 10 cm bend has a weak influence on the two guided modes. The CL ratio between the lowest loss HOM and \mathrm{LP}-{11} mode keeps >\!\!150 in the wavelength range from 1.25-1.65 \mu \text{m}, which ensures the high purity of \mathrm{LP-{01}} and \mathrm{LP}-{11} modes. The fabrication tolerances of the fiber are discussed.