Rectangle Lattice Large Mode Area Photonic Crystal Fiber for 2 $\mu$ m Compact High-power Fiber Lasers

A rectangle lattice silica-host photonic crystal fiber with large mode area (LMA) is successfully fabricated for the first time. The introduction of rectangle lattice contributes to distinct leakage of second order modes and two large air holes provide an effective solution to suppress bend distortion. Numerical results demonstrate that a large mode field area (MFA) of 2500 μm² is achieved at the wavelength of 2 μm when the fiber is straight. While the fiber is bent to a radius of 30 cm, it confirms to single-mode (SM) operation conditions and the decrements of MFA at x polarization and y polarization are just 4.3% and 5%. Within a bend orientation angle of ± 10° and a wide wavelength range from 1.8 μm to 2.4 μm, the fiber can operate under single-mode (SM) conditions with low bend loss. LMA, SM operation, and small bend distortion make this fiber of great potential in developing 2 μm compact high-power fiber lasers.