Slow and fast light in a microsphere-optical fiber system

The dispersion relation in a system of a fiber taper coupled with a microsphere is investigated. On the undercoupling condition, in which the coupling strength between the sphere and the fiber is weak compared with the round-trip loss in the sphere, the system shows anomalous dispersion. On the other hand, on the overcoupling condition, in which the coupling strength is strong compared with the loss, the system shows normal dispersion. We performed pulse propagation experiments and observed both the slow light and the fast light relevant to the normal and anomalous dispersions in the single microsphere-fiber taper system, controlling the coupling strength between the sphere and the fiber. The observed pulse propagation times show good agreement with theoretical calculations based on a directional coupling theory. (c) 2006 Optical Society of America.