Conservation of orbital angular momentum in air-core optical fibers

Abstract

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