8 Dispersion compensators for fibre optic paths
8.2 Dispersion Compensation

## Dispersion Compensating Fibres

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).

Sample design (cross-section) of mature DCFs.

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.

Dispersion in broadband DCFs.

## Fibre Bragg gratings

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).

### Disadvantage

- It operates at specific wavelength. One can tune the grating (its period), but for many DWDM channels, one grating is not enough. Many channels = many gratings.
- The grating can be tuned, but wavelength division multiplex DWDM contains many channels, one grating is not enough
- Many wavelengths = many gratings

## Compensation schemes

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.

Dispersion compensation schemes for optical networks.

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.