Optical microsphere amplification system.

We demonstrated a compact optical amplification system using a fiber taper and rare-earth-doped microsphere. Both the signal and pump beams were injected into the microsphere via the fiber taper to increase the spatial matching between two whispering gallery modes. As the pump power was increased, the resonance dip in the transmission spectra due to intrinsic attenuation in the microsphere became filled and then transited to the resonance gain peak. An amplification factor of up to 2.5 was realized with a microsphere of 63 microm in diameter.

Slow light in coupled-resonator-induced transparency.

We performed optical pulse propagation experiments in a system in which two ultrahigh-Q silica microspheres of different diameters were coupled in tandem to a fiber taper to yield coupled-resonator-induced transparency. Nearly Gaussian-shaped optical pulses propagated with a large positive delay of 8.5 ns through a transparent frequency window, without significant attenuation, amplification, or pulse deformation, demonstrating classical analogy of the extremely slow light obtained with electromagnetically induced transparency.

Dynamics of fast and slow pulse propagation through a microsphere-optical-fiber system.

We discuss the mechanism of fast and slow light generation when optical pulses propagate through a fiber taper coupled to an ultrahigh-Q microsphere system in the time domain. Fast and slow light are explained as interference effects between ballistic light through the fiber and circulated light within the sphere. One of the striking effects predicted by this model is dynamic pulse splitting in the transmitted pulse at the critical coupling condition, where the coupling strength between the sphere and fiber equals the round-trip loss in the sphere. By realizing this critical coupling condition experimentally, we observed this dynamic pulse splitting effect.

Observation of fast light in Mie scattering processes.

We observed -4.9 nsec negative group delay in the optical wave-packet propagation through a fiber taper coupled with a single Mie sphere. The observed fast light under the Mie scattering process agrees excellently with the anomalous dispersion calculated on the basis of a directional coupling theory on the undercoupling condition, which also predicts a positive group delay on the overcoupling condition.