Conservation of orbital angular momentum in air-core optical fibers

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. Analogous to the enhanced stability of spinning tops with increasing angular velocity, these states' lifetimes increase with OAM magnitude. Consequently, contrary to conventional 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 eight remain low loss and > 98% pure from near-degenerate coupling after kilometer-length propagation. The first realization of long-lived higher order OAM states, thus far posited to exist only in vacuum, is a necessary condition for achieving the promise of higher dimensional classical and quantum communications over practical distances. (C) 2015 Optical Society of America