Azimuthal confinement: the missing ingredient in understanding confinement loss in antiresonant, hollow-core fibers

Antiresonant, hollow-core optical fibers are currently challenging or even exceeding the loss performance of conventional solid-core fibers. Despite this progress, there are aspects of the guidance mechanism in these fibers that are still not understood. For example, a physical mechanism to explain why negative curvature of the core surround is correlated with low loss remains elusive. It is shown that the glass elements of the cladding structure with an approximately radial orientation play a crucial role in determining the confinement loss by strongly shaping the wave fields in the azimuthal coordinate. This shaping, described as azimuthal confinement, can result in an evanescent field in the radial direction through the cladding, and this leads to a confinement loss that is substantially lower than would be the case without azimuthal confinement. A comprehensive theory of azimuthal confinement is developed, yielding an expression for the confinement loss of any fiber structure with a single antiresonant glass layer between the core and the outer glass jacket. This is tested by comparison with large-scale numerical simulations on two types of cladding structure. It is shown that negative curvature of the core surround has little or no intrinsic role in reducing confinement loss in fibers with a nodeless cladding structure. The power of azimuthal confinement is demonstrated in model structures where the confinement loss drops by more than two orders of magnitude as the radial width of the cladding is increased. It is anticipated that the concept of azimuthal confinement will be valuable in interpreting confinement loss in a wide range of existing antiresonant, hollow-core fibers and in the design of novel, low loss cladding structures.