Broadband hollow-core single-polarization 3dB coupler based on a dual-hollow-core fiber embellished with semicircular silver nanowires by using localized surface plasmon resonance

A broadband hollow-core single-polarization 3dB coupler based on a dual-hollow-core anti-resonant fiber (DHC-ARF) embellished with semicircular silver nanowires is proposed. Semicircular silver nanowires are attached to the outer walls of two cladding tubes along the y-direction in the DHC-ARF to excite the localized surface plasmon resonance. By utilizing the mode coupling between y-polarized (y-pol) fundamental mode (FM) and surface plasmon polariton mode to increase the loss of y-pol FM, single-polarization operation can be implemented by filtering the y-pol FM and allowing only x-polarized (x-pol) FM to propagate with low loss between two cores. Finite element method combined with perfectly matched layer as a boundary condition is employed to optimize the properties of the DHC-ARF. Numerical results demonstrate that a hollow-core single-polarization 3dB coupler is constructed by only using a 1.82 cm long DHC-ARF within a wide bandwidth of 140 nm covering the wavelength range from 1480 nm to 1620 nm, in which the polarization extinction ratio (PER) is lower than − 20 dB and the coupling ratio is within 50 ± 2%. At the wavelength of 1550 nm, the PER reaches − 49.3 dB and the loss of the coupler are as low as 0.22 dB. To the best of our knowledge, this hollow-core single-polarization 3dB coupler exhibits the widest bandwidth so far, which is at least 12.7 times that of the current reports. This broadband hollow-core single-polarization 3dB coupler has great potential in polarization-sensitive fiber systems.