Effective Area

All nonlinear effects are dependent upon the intensity of the electromagnetic field in the medium. However, it is the total optical power entering and leaving the fibre that is usually measured. Some method is required for converting between the two when comparing theoretical and experimental results.
The field in a single mode fibre is not evenly distributed or even fully contained within the core. It is larger at the fibre axis than near the core cladding interface and extends into the cladding to a degree depending on the actual refractive index profile. The effective area parameter has been defined for the purposes of calculating nonlinear effects. The larger the effective area or (MFD), the lower the power density and so the lower impact of non linear effects, and the smaller the effective area or (MFD), the higher the power density which leads to higher impact of the non linear effects.
The effective area is sometimes included in the description of the fiber name, such as Corning`s LEAF (Large Effective Area Fiber).