Conservation of orbital angular momentum in air core optical fibers

TL;DR

This paper demonstrates that orbital angular momentum (OAM) can be conserved in air-core optical fibers even under strong bend perturbations, enabling stable propagation of multiple high-order OAM modes for advanced communication applications.

Contribution

It introduces air-core fibers that preserve high-order OAM modes despite perturbations, showing increased mode stability with higher OAM magnitude and enabling long-distance multimode OAM transmission.

Findings

Conservation of OAM in air-core fibers under bend perturbations.

Successful propagation of 12 OAM modes over kilometers.

Over 98% purity and low loss for 8 high-order OAM modes.

Abstract

Light's orbital angular momentum (OAM) is a conserved quantity in cylindrically symmetric media; however, it is easily destroyed by free-space turbulence or fiber bends, because anisotropic perturbations impart angular momentum. We observe the conservation of OAM even in the presence of strong bend perturbations, with fibers featuring air cores that appropriately sculpt the modal density of states. In analogy to the classical reasoning for the enhanced stability of spinning tops with increasing angular velocity, these states' lifetimes with OAM magnitude. Consequently, contrary to convention wisdom that ground states of systems are the most stable, OAM longevity in air-core fiber increases with mode order. Aided by conservation of this fundamental quantity, we demonstrate fiber propagation of 12 distinct higher-order OAM modes, of which 8 remain low-loss and >98% pure from near-degenerate coupling after km-length propagation. The first realization of long-lived higher-order OAM states, thus far posited to exist primarily in vacuum, is a necessary condition for achieving the promise of higher-dimensional OAM-based classical and quantum communications over practical distances.