Abstract
Light carries both orbital angular momentum (OAM) and spin angular momentum (SAM), related to wavefront rotation and polarization, respectively. These are usually approximately independent quantities, but they become coupled by light’s spin-orbit interaction (SOI) in certain exotic geometries and at the nanoscale. Here we reveal a manifestation of strong SOI in fibers engineered at the micro-scale and supporting the only known example of propagating light modes with non-integer mean OAM. This enables propagation of a record number (24) of states in a single optical fiber with low cross-talk (purity > 93%), even as tens-of-meters long fibers are bent, twisted or otherwise handled, as fibers are practically deployed. In addition to enabling the investigation of novel SOI effects, these light states represent the first ensemble with which mode count can be potentially arbitrarily scaled to satisfy the exponentially growing demands of high-performance data centers and supercomputers, or telecommunications network nodes.
Highlights
Light carries both orbital angular momentum (OAM) and spin angular momentum (SAM), related to wavefront rotation and polarization, respectively
The eigenmodes of optical fibers, which share the cylindrical symmetry of free-space paraxial optical systems, are conventionally described in a similar fashion
Lightmatter interaction at dielectric interfaces leads to spin-orbit interactions (SOI)[5], one of the most well-known manifestations of which is a spin-dependent correction to the modal propagation constants through the photonic Spin Hall Effect[6,7,8]
Summary
Light carries both orbital angular momentum (OAM) and spin angular momentum (SAM), related to wavefront rotation and polarization, respectively. We reveal a manifestation of strong SOI in fibers engineered at the micro-scale and supporting the only known example of propagating light modes with non-integer mean OAM. The spin[1] (S) and orbital[2,3] (L) angular momenta of light can be treated as independent in paraxial optical systems typically encountered in the lab In such a basis, neglecting the Gouy phase, one can describe propagating optical fields as: E^sl;mðr; φ; zÞ 1⁄4 σ^seilφFl;mðr; zÞeiβz ð1Þ. (r, φ, z) are cylindrical coordinates, and Fl,m(r; z) is a function describing the mode’s electric field distribution and whose z dependence includes only beam scaling and phase curvature evolution due to diffraction In this representation, l and s are respectively identified as the photon OAM and SAM (in units of reduced Planck constant, ħ) along the propagation direction, z. Exact fiber modes can be readily calculated by a brute-force solution to Maxwell’s equations without special consideration of SOI effects, a physically intuitive means to describe this impact of SOI is by expressing the influence of the confinement on the modes’ angular momenta as a first-order correction of the unperturbed (i.e., paraxial free space) modes[9]
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