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 (Δneff) 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, HE1,1 and HE2,1 are used in the formation of SMs and SOAM modes. Simulations show that all the nine SOAM groups originating from HE1,1 mode and the first five SOAM groups stemming from normal coupling of HE2,1 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 (Aeff) value reaches up to 638.88µm2 at 1.55µm, which is nearly eight times larger compared to that of conventional ring-core MSFs.