The definition of Modal Index seems to be consistent and well defined. Modal index is the propagation constant of the mode divided by the free-space wavenumber, k ( k = 2 Pi / wavelength ). Since Modal Index is the quotient of two wave numbers it is dimensionless, like refractive index. Effective Index is sometimes used to mean Modal Index, but that is a bad choice of words because Effective Index is a term used for other things too. Effective Index sometimes means a number that will be useful in characterizing a material when a 2D simulation will be used to approximate propagation on a 3D structure. That is why I always use the term Modal Index when referring to a number that represents the propagation constant of a mode. Less confusing/ ambiguous. As for how Modal Index is calculated in OptiFDTD, well, that is a long story. You can find it in Optiwave OptiMode documentation.