Hello Taban,
It is possible to design a fiber in OptiFDTD in principle. The question is what are the dimensions of this fiber that you wish to design and what exactly are you designing for.
With an FDTD based product you have to ensure that you have a minimum of 10 grid points per wavelength within the highest refractive index material. If you are talking fibers with diameters of 125 um and up then you are looking at a large mesh and my run into issues having sufficient resources on your simulation computer.
We do offer products, OptiFiber and OptiBPM, that are likely more appropriate for fiber design. Can you provide more information regarding what you are trying to do?
Scott