Demonstration of spin and extrinsic orbital-angular-momentum interaction using a few-mode optical fiber

摘要

We demonstrate the spin and extrinsic orbital-angular-momentum interaction of light in a few-mode fiber. The interaction of spin and extrinsic orbital angular momentum splits the degeneracy of a degenerate subspace of the higher-order solution of Maxwell's equation and creates correction to the propagating constants of its degenerate eigenmodes that results in the formation of transmitting-length-dependent beat along the propagating trajectory. We examine the interaction of spin and extrinsic orbital angular momentum by the observation of the rotation of the polarization or intensity pattern of the propagating modes which are the combinations of these degenerate eigenmodes when altering the length of this transmitting-length-dependent beat. In experiment, the length of this transmitting-length-dependent beat is altered by the alteration of the pitch of a fiber helix which is made for the production of the extrinsic angular momentum and where the space inversion or mirror symmetry of the mode is broken and the interaction between the spin and extrinsic orbital angular momentum is realized. We demonstrate that the spin and extrinsic orbital-angular-momentum interaction induce the rotation of the polarization or intensity pattern of propagating modes in a way related with the order index of spin and orbital angular momentum carried by their constituent eigencomponents, which are circular vortex modes. This work may contribute to understanding the physical mechanism underlying the evolution of the modes in few-mode fiber or other complex systems for their various applications.