The Soliton Self Frequency Shift.

We describe experimental discovery of a continuos down shift in the optical frequency of a soliton pulse as it travels along the fiber. We show that 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. In a several hundred meter long fiber, the net frequency shifts of subpicosecond pulses can amount to several percent or more of the optical frequency.