Hollow Bragg fiber bundles: when coupling helps and when it hurts.

Coupling between leaky modes of collinear hollow-core Bragg fibers is considered. It is found that coupling is unusually strong because of resonant effects in the interfiber cavity when the fibers are touching each other and decreases dramatically with the first tens of nanometers of fiber separation. However, residual coupling with a strength proportional to the fiber radiation loss is long range, decreasing as an inverse square root of the interfiber separation and exhibiting periodic variation. The possibility of building a directional coupler from touching Bragg fibers is discussed in view of the findings.