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>> The Background In recent years there has been a lot of work on dispersion-compensating fibers (DCFs), which are being used extensively for upgrading the installed 1310nm optimized optical fiber links for operation at 1550nm. In the following two sections, we will discuss the basic principle behind dispersion compensation, and the characteristics of dispersion compensating fibers (DCFs).   >> What is Dispersion Compensation Let’s look at a pulse (with spectral width of Δλ0) which is propagating through a fiber characterized by the propagation constant β. The spectral width Δλ0 could be due to either the finite spectral width of the

>> The Birefringence of Single Mode Fiber Circular core fibers whose axes are straight are not birefringent – that is, the two orthogonally polarized LP01 mode have the same effective indices. Bending such a fiber introduces stresses in the fiber and makes the fiber linearly birefringent with the fast and slow axes in the plane and perpendicular to the plane of the loop, respectively. The bending-induced birefringence of a single mode silica fiber is given by: where nex and ney represent the effective indices of the LP01 modes polarized in the plane and perpendicular to the plane of the bend,

Chromatic dispersion is the term given to the phenomenon by which different spectral components of a pulse travel at different velocities. To understand the effect of chromatic dispersion, we must understand the significance of the propagation constant β. We will restrict our discussion to single mode fiber since in the case of multimode fiber, the effects of intermodal dispersion usually overshadow those of chromatic dispersion. So the propagation constant β in our discussions will be that associated with the fundamental mode of the fiber. Chromatic dispersion arises for two reasons. The first reason is that the refractive index of silica,

>> Effective Length Le The nonlinear interactions in optical fibers depends on the transmission length and the cross-sectional area of the fiber. The longer the link length, the more the interaction and the worse the effect of nonlinearity. However, as the signal propagates along the link, its power decreases because of fiber attenuation. Thus, most of the nonlinear effects occur early in the fiber span and diminish as the signal propagates. Modeling this effect can be quite complicated, but in practice, a simple model that assumes that the power is constant over a certain effective length Le has proved to

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