hi zulkarnain,
i want to mention here that the cauchy eqation for refractive index have not that much significance in optical fiber communication..
there is one more equation called sellmier equation which shows the sameand is most widely used equation for dispersion of materials, for convience i am attaching few links for equations for refractive index.
https://en.wikipedia.org/wiki/Sellmeier_equation
http://scienceworld.wolfram.com/physics/CauchysFormula.html
http://link.springer.com/referenceworkentry/10.1007%2F978-1-4419-6247-8_2057
you can also see video lecture on youtube for cauchy equation for refractive index.
hope i answer your question zulkarnain
with regards