Delays would not be limited by resonator ring-down times.
Suitably configured whispering-gallery-mode (WGM) optical resonators have been proposed as delay lines for trains of short light pulses. Until now, it has been common practice to implement an optical delay line as a coiled long optical fiber, which is bulky and tends to be noisy. An alternative has been to implement an optical delay line as a coupled-resonator optical waveguide (a chain of coupled optical resonators), which is compact but limits the width of the pulse spectrum to the width of an optical resonance and thereby places a lower limit on the duration of a pulse. In contrast, a delay line according to the proposal could be implemented as a single WGM resonator, and the pulses delayed by the resonator could be so short that their spectral widths could greatly exceed the spectral width of any single resonance.
Notably, it was found that in a microsphere optical resonator that supports a suitable combination of WGM modes, the group velocity for a train of light pulses could be positive, zero, or negative. A positive group velocity could be so small that the delay could be much longer than the ring-down time of the resonator; a delay of such great length is impossible for a single pulse interacting with either a linearly responding lossless resonator or a coupled-resonator optical waveguide.
The phenomenon of “stopped light,” which corresponds to a group velocity of zero (and, hence, to infinite delay), was demonstrated in experiments on a fused-silica microsphere of 300-μm diameter. Light from a diode laser at a nominal wavelength of 635 nm was swept through a 5-GHz frequency range at a repetition rate of 20 Hz and coupled into the resonator via an angle-cut optical fiber. The resonator was immersed in a solution containing a dye that fluoresces in response to evanescent-wave coupling of light from the WGM surface modes. When the tip of the angle-cut optical fiber was positioned and oriented to excite suitable combinations of WGM modes, stationary fluorescence patterns were observed (see figure).
This work was done by Andrey Matsko, Vladimir Iltchenko, Dmitry Strekalov, Anatoliy Savchenkov, and Lute Maleki of Caltech for NASA’s Jet Propulsion Laboratory.
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