DCFs (Dispersion Compensating Fibres) are specific for low negative dispersion parameter ~ -100 ps/nm/km, even to ~ -10000 ps/nm/km (there are many papers reporting even wider range of theoretical values), which is to compensate accumulating dispersion. DCFs differ from standard SMFs, as far as we talk about its geometry and material composition. The most mature DCFs are those based on Photonic Crystal Fibres (PCF).
Some DCFs are designed to operate at a specific wavelength, their wavelength evolution of dispersion is parabolic with one dispersion minimum and two zero dispersion wavelengths (ZDW).
DCFs for DWDM systems have to be able to compensate dispersion in all channels a once. Broadband DCF has strong negative dispersion parameter available at all telecommunication windows, at which fibres are transparent. Dispersion upon wavelength of such DCF copies the reverse slope of a standard fibre to be compensated over broad spectrum of operating wavelengths.
Another possibility is the use of fibre Bragg grating – FBG in the fibre along its core (attention: we do NOT deal with microstructured Bragg Fibre).
Different compensation options are possible: pre or post compensation. To decide, which option is suitable for a given network, it is recommended to perform numerical simulation of a specific network, providing the answer to this question.
While performing dispersion compensation, one should pay attention not to achieve exactly zero dispersion. On one hand zero dispersion means no pulse spreading, but on the other hand it leads to a nonlinear phenomenon known as FWM – Four Wave Mixing. Zero dispersion is one of a few conditions to originate it.