Parallel transport of fiber mode structure: orbit-orbit interaction

Abstract

That a paraxial light beam with spin angular momentum (SAM,σ ) propagating in a helical trajectory leads to the appearance of Rytov-Vladimirsky-Berry (RVB) phase has been a topic of extensive research for the past several decades. Recently, using geometrical optics approximation, it was shown that variations in the beam propagation direction leads to generic parallel transport law – a beam with intrinsic orbital angular momentum (IOAM, l ) behaves topologically similar to polarized beam containing only SAM but of magnitude proportional to the total angular momentum TAM = l ± σ . By considering the interaction of a beam with IOAM, propagating in a non-planar trajectory and hence with extrinsic orbital angular momentum (EOAM), in an inhomogeneous medium we study the parallel transport of fiber mode structure as a manifestation of orbit-orbit interaction. The resulting rotation of the transverse beam structure due to the parallel transport of the LP fiber mode propagating along non-planar ray direction is attributed to the ‘orbital’ Berry phase. The mode transformation is simulated based on the interference of the vector-vortex modes excited in the TMF. The LP mode rotation angle calculated as a function of the beam position at the fiber input is expected to show topological features that can be mapped onto orbital Poincare sphere.