Asymmetric large-mode-area photonic crystal fiber structure with effective single-mode operation: design and analysis.

The asymmetrical structure of photonic crystal fiber has been reported for a large mode area with the single-mode operation. The design works on the principle of bend-induced mode filtering. The proposed structure can be designed (i) by introducing down-doped material rods in place of nine air holes of the inner ring near the core of the structure and (ii) by increasing the diameter of the rest of the three air holes of the same ring in the direction of bending. These three air holes together with nine down-doped material rods control the mode field inside the core region and hence the bending losses of the modes. The single-mode operation is ensured by introducing high bend loss for the first higher order mode and very low bend loss for the fundamental mode. The finite-element-method-based anisotropic perfectly matched layer boundary condition has been applied for accurate analysis of bend loss of the structure. Numerical results show that effective single-mode operation can be ensured with a mode area as large as 1530  μm at bend state with a bend radius of 30 cm. The proposed photonic crystal optical fiber with such a large mode area can have potential applications in compact high-power delivery devices such as high-power fiber lasers and amplifiers.