Discovery of the soliton self-frequency shift.

We describe the experimental discovery of a continuous shift in the optical frequency of a soliton pulse as it travels down the fiber. The effect is caused by a Raman self-pumping of the soliton, by which energy is transferred from the higher to the lower-frequency parts of its spectrum. For 120-fsec pulses, we have observed net frequency shifts as great as 10% of the optical frequency.