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Symmetric Dispersion-Managed Fiber

Dispersion-managed fiber (DMF) spans consisting of  positive-dispersion fiber (+D), followed by inverse-dispersion fiber (–D), have been originally developed for high-capacity, high-bit-rate submarine transmission, where the combination of high local dispersion of +D fiber (typically +17-20 ps/nm/km in C-band) and excellent dispersion and dispersion-slope compensation by –D fiber very efficiently suppresses nonlinearities without paying any dispersion penalties at high  bit rates. In order for this technology to be useful in next-generation terrestrial systems, where typical amplifier spacing is several times larger, the DMF spans must be compatible with Raman amplification (at least, in the backward-pumped configuration). While the +D fiber usually has effective area larger than that of standard single-mode fiber, typically near 110 µm2, the effective area of the –D fiber is much smaller than that of standard single-mode fiber, typically 20-30 µm2. In a backward-pumped classic +D/-D DMF, most of the Raman gain is generated in the small-effective-area -D fiber at the exit end of the span. At high Raman gains (10-20 dB), this leads to severe multipath interference due to double Rayleigh backscattering (DRBS) of the signal (DRBS is inversely proportional to the square of effective area) and degrades the system performance. In addition, the signal intensity at the span output becomes comparable to that at the input, owing to the difference in effective areas, and the nonlinear degradations increase.

These disadvantages of small-effective-area –D fiber can be avoided by positioning it between two sections of large-effective-area +D fiber. This “symmetric” +D/-D/+D DMF was invented, demonstrated, and characterized by Dr. Vasilyev and his colleagues at Corning Inc. We have shown that in the symmetric DMF, where only a fraction of the total Raman gain is generated in the DRBS-prone -D fiber, the total DRBS power generated in the span is drastically reduced, and DRBS-caused systems degradations diminish. Additionally, with symmetric DMF, the pump intensity in the -D fiber section is high enough to produce noticeable Raman gain early in the span. As a result, the minimum signal power in the symmetric (+D/-D/+D) DMF is higher than that in the classic (+D/-D) DMF or in conventional fiber (see Fig. 1), yielding the best noise performance in the symmetric configuration (see Fig. 2).

Fig. 1. Evolution profiles of a) pump intensity and b) signal power for symmetric and classic DMFs.
Fig. 2. The measured effective lumped noise figure of a backward-pumped distributed Raman amplifier for non-zero-dispersion-shifted fiber (NZDSF), classic DMF, and symmetric DMF. The symmetric DMF shows dramatic (~2.5 dB) noise figure improvement over classic DMF and NZDSF. The improved noise figure of symmetric DMF comes from more uniformly distributed Raman gain.

One of the key trends in the evolution of Raman amplifiers is the progress toward approaching the performance limit of the ideal distributed amplifier which exhibits optimum trade-off between the nonlinearities and noise. For a single-fiber span, such a trade-off is achieved when the gain and loss are balanced at every point in the fiber, making the signal power constant versus distance. In symmetric DMF, the intensity distribution along the span is relatively uniform (and can be improved by combination of forward and backward pumping), and the performance is within a fraction of a dB from the ideal.

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October 3, 2005: ELECTRICAL ENGINEERING PROFESSOR RECEIVES PATENT FOR FIBER CABLE FEATURE