We characterize coupling between two identical collinear hollow-core Bragg fibers, assuming a launching condition. Using a multipole method and a finite-element method, we have investigated the dependence of the beat length between supermodes of the coupled fibers and supermode radiation losses as a function of the interfiber separation, the fiber core radius, and the index of the cladding. We established that coupling is maximal when the fibers are touching each other and decreases dramatically during the first hundreds of nanometers of separation. However, residual coupling with the strength proportional to the fiber radiation loss decreased over a long range as an inverse square root of the interfiber separation and exhibited periodic variation with interfiber separation. Finally, we considered coupling between the modes with a view to designing a directional coupler. We found that for fibers with large enough core radii one can identify broad frequency ranges in which the intermodal coupling strength exceeds supermode radiation losses by 1 order of magnitude, thus opening the possibility of building a directional coupler. We attribute such unusually strong intermode coupling both to the resonant effects in the intermirror cavity and to proximity interaction between the leaky modes localized in the mirror.
© 2004 Optical Society of America