A vectorial finite element method (VFEM) is adopted to investigate the novel single-polarization single-mode (SPSM) photonic crystal fiber (PCF) coupler which has asymmetric dual-core and two lines of enlarged air holes. It is demonstrated that the SPSM region of the designed fiber can be more than 250 nm wide with a set of optimized air-hole parameters and the width of the SPSM region could change slightly by fine adjustment of the inner large air holes. The coupling length between the odd and even x-polarization modes is also investigated through fine-tuning the large and small air-hole diameters.Photonic crystal fibers (PCFs) [1] are characterized by a set of microscopic air-holes in the transverse plane. Based on different air-hole diameters along the two orthogonal axes, PCFs with high modal birefringence can be easily achieved, because the index contrast is higher than that in conventional fibers. Recently, it has been shown that highly birefringent PCFs have the potential ability to realize wide-band singlepolarization single-mode (SPSM) fibers [2][3][4] . S-Q. Lou et al [2] have designed an ultra-broadband SPSM PCF with a wavelength range from 1.46 m to 2.04 m, nearly 580 nm in width. F. Zhang et al [3] have proposed a rectangular-lattice PCF which has a wide-band SPSM range and confinement loss less than 0.1 dB/km within the wavelength from 1.20 m to 1.66 m. The SPSM PCF coupler was first designed by Y. Yang et al [5] which had a SPSM coupling range more than 200 nm wide. Due to the inherent flexibility in designing the array of air holes in a PCF structure, the directional couplers based on triple-core index-guiding PCFs have been extensively investigated [6,7] . However, both the dual-core and triple-core designs are symmetrically arrayed.In this paper, a novel asymmetric dual-core PCF coupler with the SPSM functionality is proposed and investigated using a vector finite element method (VFEM) [8] with perfect match layer (PML) boundary conditions. It is demonstrated that the SPSM region of the designed fiber can be more than 250 nm wide. The coupling length between the odd and even x-polarization modes is also investigated through fine-tuning the large and small air-hole diameters.The cross section of the PCF is a triangular-lattice fiber structure as shown in Fig.1, consisting of two lines of four enlarged air holes (eight large air holes with diameter d') and two pure silica cores (one in the midpoint and the other in the third ring) which form the asymmetric dual-core. The asymmetric dual-core is designed for better splicing with a standard single mode fiber. The background material is regular silica with refractive index assumed to be 1.444. The hole pitch is =2.2 m (the hole-to-hole distance), and the small Fig.1 Transverse cross section of the designed PCF C