In the Fiber Profile dialog box (Design Fiber), select one of the Regions, then
1) in the Function combo box, select “user function”
2) click the “Define” button to the right of the Profile combo box.
In the User DefinedFunction dialog box, write your function. See the OptiFiber manual, Appendix A – Script Language Reference for the syntax.
3) click Display to check you got it right.
4) close the box, you should see your funtion in the Fiber Profile now. It will be approximated in “steps”, the number entered in that field in the Fiber Profile dialog box