Enhanced spin orbit interaction of light in highly confining optical fibers for mode division multiplexing.

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.

On the scalability of ring fiber designs for OAM multiplexing.

The promise of the infinite-dimensionality of orbital angular momentum (OAM) and its application to free-space and fiber communications has attracted immense attention in recent years. In order to facilitate OAM-guidance, novel fibers have been proposed and developed, including a class of so-called ring-fibers. In these fibers, the wave-guiding region is a high-index annulus instead of a conventional circular core, which for reasons related to polarization-dependent differential phase shifts for light at waveguide boundaries, leads to enhanced stability for OAM modes.

Asymptotic theory of strong spin-orbit coupling in optical fiber.

The spin-orbit coupling of light propagating in optical fiber can be dramatically enhanced by the presence of a high-contrast interface in the refractive index profile, even for modes that are highly paraxial. The resulting modes have spatial and polarization structures that depart greatly from the weak coupling form, and, in particular, are neither orbital nor spin angular momentum eigenstates. We explain the physical origins of this strong-coupling regime with a vector geometric theory of diffraction expansion.

Complex mode amplitude measurement for a six-mode optical fiber.

We propose a measurement protocol and parameter estimation algorithm to recover the powers and relative phases of each of the vector modes present at the output of an optical fiber that supports the HE₁₁, TE₀₁, HE₂₁, and TM₀₁ modes. The measurements consist of polarization filtered near-field intensity images that are easily implemented with standard off-shelf components. We demonstrate the accuracy of the method on both simulated and measured data from a recently demonstrated fiber that supports stable orbital angular momentum states.

Control of orbital angular momentum of light with optical fibers.

We present a fiber-based method for generating vortex beams with a tunable value of orbital angular momentum from -1ℏ to +1ℏ per photon. We propose a new (to our knowledge) method to determine the modal content of the fiber and demonstrate high purity of the desired vortex state (97% after 20 m, even after bends and twists). This method has immediate utility for the multitude of applications in science and technology that exploit vortex light states.