Hi,
I want to add that similar to Mode Field Diameter(MFD), the effective area of the fundamental mode is a measure of the area over which the energy in the electric field is distributed.
effective area in a single mode optical fiber determines how much energy the core can can carry without causing non-linear type signal losses. This parameter is important for DWDM applications.
Whilst the effective area characteristics are frozen at the time of manufacture and as such are fixed for the life of the fiber, it is a parameter that is influenced by various properties within the fiber that do vary along its length, so it is useful to measure the effective area even within a cable manufacturing environment to ensure the consistency of the product.
Hope it helps.
Thanks