question in FDTD modelling

Dear friends:
I am interested in modelling PCF using FDTD. I have an important question.
I want to calculate the effective index using FDTD program. A lot of papers say that we must take fst fourier transform to the output of the FDTD program then getting effective index. Could any one explain to me how to get effective index from FDTD program

yes but I usually analysis and simulate PCF whit optiFDTD software for further information mail me

The FDTD technique is not an efficient method if you just want to calculate waveguide modes and effective indices. You should look into finite element packages like FEMLab or even coding your own finite-difference method.

-Tip