Helically twisted double-cladding-three-core photonic crystal fiber for generation of orbital angular momentum

Abstract

In view of the shortcomings of helically twisted single-cladding-few-core photonic crystal fibers in generating orbital angular momentum (OAM), the double-cladding and three-core structures with non-uniform inner and outer air holes were introduced into a photonic crystal fiber for the first time and realized the generation of high-order OAM modes through helical twisting. The fiber is expected to reduce the losses of the generated OAM modes by introducing a specially designed double-cladding structure, while the three cores distributed in a regular triangle around the center are expected to increase the number of generated OAM modes. On the basis of optical transformation theory, the optical fiber is systematically analyzed by the finite element method. It is found that when the twist rate <i>α</i>=7853.98 rad/m, the generated OAM modes included "OAM<sub>-4,1</sub>, OAM<sub>+9,1</sub>, OAM<sub>+10,1</sub>, OAM<sub>+11,1</sub>, OAM<sub>+13,1</sub>", where +13 is the highest order of the OAM modes currently generated by using helically twisted fibers. And the losses of OAM modes are all less than 1.64×10<sup>-3</sup> dB/m, all of which are at least two orders of magnitude lower than the lowest OAM mode loss in the existing references, and the OAM mode purity greater than 93%. Further studies show that the generation of orbital angular momentum depends on the resonant couplings between the core supermodes and the ring-core modes, and the parity of the orders of the generated OAM modes is related to the polarization direction of the core supermodes and the ring-core modes.