Large mode area microstructured fiber supporting 56 super-OAM modes

In this paper, a kind of super-mode orbital angular momentum microstructured fiber (SM-OAM-MSF) is proposed. By introducing 20 Ge-doped equiangular cylindrical inclusions in the ring-core region, mode coupling mechanism is employed in the formation of super-OAM (SOAM) modes. Specifically, the double degenerated out-of-phase SMs are first generated by the coupling of individual core mode, then the quadruple degenerated SOAM modes are formed by combining two components of the out-of-phase SMs with a phase difference of ±π/2. Theoretical analysis and numerical results reveal that the effective index difference (Δn_eff) between adjacent out-of-phase SM groups are strongly influenced by the parameters of the individual core except the ring-core's width. Therefore, large mode area and SOAM modes' index separation larger than 1.0×10^-4 can be achieved simultaneously in our proposed SM-OAM-MSF. Through careful fiber design, HE_1,1 and HE_2,1 are used in the formation of SMs and SOAM modes. Simulations show that all the nine SOAM groups originating from HE_1,1 mode and the first five SOAM groups stemming from normal coupling of HE_2,1 mode can be supported above 1.0µm, that are 56 SOAM modes in total. The highest purity is 99.86% for SOAM±2,1±,5 mode. And the maximum mode area (A_eff) value reaches up to 638.88µm² at 1.55µm, which is nearly eight times larger compared to that of conventional ring-core MSFs.