Are you asking about single mode EDF or MM EDF?
The EDF models used in OptiSystem are described in the datasheet that you may access through the help button in the component properties.
There is no specific number used in the simulation for core refractive index after adding the Er ions, as the models don’t require explicit value of that value. Although, other parameters in the model such as Er core radius, Er ion density, ion distribution, etc. affect the new refractive index. You may search the literature for references that measure the refractive index of EDF.