Tunable Single-Polarization Single-Mode Negative-Curvature Fiber With an Asymmetrical Refractive Index Cladding for Mid-Infrared Region

We propose a hollow-core negative-curvature fiber implementing tunable single-polarization single-mode operation at mid-infrared region. This fiber owns a symmetrical geometry structure and asymmetrical refractive index cladding composed by two kinds of mid-infrared glass with different refractive indexes. Single-polarization single-mode can be achieved for any wavelength within the wavelength range from 2.0 to 4.0 μm by linear stretching fiber structure in the scale to wavelength, which means that this fiber structure possesses an excellent tunability. We mainly focus on analyzing the properties of the negative-curvature fibers for the wavelength of 2.0, 3.0, and 4.0 μm. Numerical results show that the main single-polarization single-mode regions for these three fibers are 2.020–2.033 μm, 2.998–3.016 μm and 3.998–4.018 μm, respectively. The widest bandwidth reaches 20 nm. The polarization extinction ratios at the wavelength of 2.0, 3.0, and 4.0 μm are 3180, 5085, and 2870, respectively. In addition, these fibers can maintain good single-polarization single-mode performance under bending state. The critical bend radius of the fiber for 3.0 μm is as low as 6 cm.