Hello mohammed,
The question that you are asking has been addressed in multiples researches. It is known, that dispersion, in general, is classified on modal and material (do you have multi-mode or single-mode fibers?). It’s depends on the index profile as well. I am attaching you a link to a well-performed study on your question that gives the overall dispersion equations for polymer fibers. Please, see the link below: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=848400