S parameters for a straight length of fibre are only phases. This is because in each mode during propagation there is only delay, no loss and no coupling. The phase delay in each mode is the modal index times the length of the fibre times wavenumber. OptiFiber calculates modal index of each mode, so the S parameters are available from OptiFiber simulations.
The number of modes is determined by the fibre design and the wavelength. Nothing else. a few experiments with OptiFiber will show you that.