Dispersion Question ?

Hi All!

The Unit of Dispersion at each wavelength is: ps/nm/km.
I think ps is pico second, nm is nano metre, km is kilo metre. But i'm not sure!
Please tell what is ps,nm and km and physic meaning of ps/nm/km.

Thank for concern!

i know ps is per second but i'm not sure about others,like you!

what is the source of this description？

light travel in optic fiber.

ps/(nm km) (picoseconds per nanometer wavelength change and kilometer propagation distance).

from:Encyclopedia of Laser Physics and Technology - group velocity dispersion