Hi Sushank,
I think I understand the problem you are getting at in the original post here. Although what you suggested is possible in OptiSystem (measuring the beam diameters at different propagation lengths), the results will not be very interesting. OptiSystem does not model the interference of modes with different propagation constants, which can cause the spatial profile to change. It also assumes a fiber with no imperfections or bends, which can cause mode coupling. Although I think these effects are more important on smaller length scales. On the other hand, if by transmission link you are interested in free space propagation, then beam broadening will be observed.
If you are interested in these effects, you can use OptiFiber to determine the spatial profile change with propagation distance.