The Sliding-Frequency Guiding Filter: An Improved Form of Soliton Jitter Control

In ultralong distance soliton transmission, the limit to single- channel bit rate is set by jitter in pulse arrival times. The jitter can be reduced (and hence bit rate increased), through the use of narrow-band frequency-guiding filters, periodically distributed along the transmission line. Because extra gain must be employed to offset the loss the solitons experience from passage through the filters, however, amplifier spontaneous emission noise rises exponentially with distance. This tends to put a limit on the maximum usable filter strength, and hence on the practically attainable reduction in jitter. We show how that limitation can be substantially overcome, by the simple trick of gradually translating the peak frequency of the filters with distance: the silitons follow, while the noise is left behind. Thus, the exponential rise in noise is controlled, and it is possible to use much stronger filters.