A pulse of light arrived at the far end of an erbium fiber before it entered the near end.
In a Rochester University experiment, scientists sent a burst of laser light through an optical fiber that had been laced with the element erbium.
The peak of the pulse emerged from the other end of the fiber before the peak entered the front of the fiber, and well ahead of the peak of a reference pulse. This is possible because the leading edge of the pulse contained all of the information necessary to 'recreate' the pulse.
Boyd and his students had to cut back the fiber every few inches and re-measure the pulse peaks when they exited each pared-back section of the fiber. By arranging that data and playing it back in a time sequence, Boyd was able to depict that the pulse of light was moving backward within the fiber.
We sent a pulse through an optical fiber, and before its peak even entered the fiber, it was exiting the other end. Through experiments we were able to see that the pulse inside the fiber was actually moving backward, linking the input and output pulses. - Boyd
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