Chapter 2.13.5 - Dispersion Slope and Dispersion Compensation

2.13.5   Dispersion Slope and Dispersion Compensation

Chromatic dispersion is compensated for using a dispersion-compensating fiber
(DCF). A DCF is a fiber that causes dispersion opposite to conventional single-
mode fibers. For instance, if SMF has positive dispersion over a specific range of
wavelengths, DCF has negative dispersion over the same range.

Typically, the core of DCF is heavily doped with germanium and the cladding
with fluoride, and with a proper index profile, a negative dispersion is achieved. In
addition, the DCFs core has an effective aperture three to four times greater and a
refractive index about five times that of standard SMF, but the DCF attenuation co-
efficient is twice that of standard SMF. Thus, if standard SMF and DCF fiber are
alternated over the complete length, the dispersion induced by the SMF is undone by
the DCF (Figure 2.15).

However, SMF and DCF are not spectrally balanced and if there is more than
one optical channel in a fiber, as in DWDM, all optical channels are not compensated
to the same degree, which results in a residual dispersion. The variation of residual
dispersion is known as dispersion slope mismatch (Figure 2.16).