The results obtained are for a type of glass. It is absolutely probable that silica used in optical fibers has a similar graph with the same trend but a bit different values. If you believe it is different for other materials, I suggest you introduce the graph.
If we dope the core, then we have changed refractive index by changing the medium. This has nothing to do with the issue of “the change of refraction index with the change of wavelength for the same material”.
I suggest we respectfully do not deviate from the main subject.
Regards