Wat is Chromatic dispersion, dispersion compensation?

Chromatic dispersion is the spreading of a light pulse in a fibre because its different wavelengths travel at slightly different speeds, measured in picoseconds of spread per nanometre of spectral width per kilometre. Dispersion compensation undoes it, either optically, with a length of fibre of opposite dispersion (dispersion-compensating fibre) in each span, or electronically, by signal processing in a coherent receiver.