Out-of-plane coupling at mode cutoff in tapered hollow waveguides with omnidirectional reflector claddings.

We describe the theoretical and experimental analysis of light propagation in tapered, air-core waveguides with omnidirectional reflector claddings. For light within the omnidirectional band, nearly vertical out-ofplane radiation at wavelength-dependent positions along the length of the taper was observed. The coupling positions correspond to the core sizes at which individual modes approach cutoff. The leaky nature and low scattering loss of the waveguides enabled the direct imaging of modal interference and standing waves. The out-coupling experiments were corroborated by in-coupling experiments and by a theoretical analysis. The mechanism described might find application to three-dimensional optical integration, on-chip spectroscopy, and wavelength division multiplexing.