The effect of nonlinearities can be reduced by designing a fiber with a large effective area. Non-Zero Dispersion Shifted fibers (NZ-DSF) have a small value of the chromatic dispersion in the 1550nm band to minimize the effects of chromatic dispersion. Unfortunately, such fibers also have a smaller effective area.
Recently, an NZ-DSF with a larger effective area – over 70 μm2 – has been developed by both Corning (LEAF) and Lucent (TrueWave XL). This compares to about 50 μm2 for a typical NZ-DSF and 85 μm2 for standard single mode fiber. These fibers thus achieve a better trade-off between chromatic dispersion and nonlinearities than normal NZ-DSFs.
However, the disadvantage is that these fibers have a larger chromatic dispersion slope – about 0.11 ps/nm-km2 compared to about 0.07 ps/nm-km2 for other NZ-DSFs, and about 0.05 ps/nm-km2 for reduced slope fiber. Another trade-off is that a large effective area also reduces the efficiency of distributed Raman amplification.
A typical refractive index profile of LEAF is shown in the following figure.
The core region consists of three parts. In the innermost part, the refractive index has a triangular variation. In the annular (middle) part, the refractive index is equal to that of the cladding. This is surrounded by the outermost part of the core, which is an annular region of higher refractive index. The middle part of the core, being a region of lower refractive index, does not confine the power, and thus the power gets distributed over a large area. This reduces the peak power in the core and increases the effective area of the fiber. The following figure shows the distribution of power in the cores of DSF and LEAF.