A Mid-Infrared Ultrawide-Bandwidth Single-Polarization Single-Mode Fiber Based on a Twin-Tube Structure

A novel chalcogenide As
₂
Se
₃
fiber based on a twin-tube structure is proposed and numerically studied for mid-infrared (MIR) single-polarization single-mode (SPSM) operation. The proposed fiber consists of two identical thin-wall tubes, which are located symmetrically inside the cladding. The simulation results show that the y-polarization fundamental mode (YPFM) can be well confined between the two tube walls near the fiber center, while all of the unwanted modes, including the x-polarization fundamental mode (XPFM) and all of the higher order modes (HOMs), are effectively suppressed due to huge confinement loss (CL). The SPSM operation can be maintained in the wavelength range from 3 to 10 μm, covering almost the entire transparency region of an As
₂
Se
₃
fiber, with the maximum YPFM CL of 8.63 × 10
⁻³
dB/m at 10 μm. To the best of our knowledge, this is the first MIR fiber that can keep SPSM operation over such a wide wavelength region. Furthermore, the mode area of YPFM is quite small (e.g., 10.82 μm
²
at the wavelength of 6 μm), which is beneficial to realize high nonlinearity. Besides, the bending induced CL increment of YPFM at 10 μm is only 1 × 10
⁻⁵
dB/m even when the fiber bending radius is 1 cm. Owing to the simple structure and outstanding properties, the proposed fiber has practical applications in MIR photonics and nonlinear fiber optics.