In all silica-based optical fibers, minimum material dispersion occurs naturally at a wavelength of approximately 1300 nm.
The engineering tradeoff is a slight increase in the minimum attenuation coefficient.
This type of refractive index change versus wavelength due to different geometry is called waveguide dispersion.
After reaching a certain peak power within the pulse the non-linear refractive index starts to play an important role leading to frequency generation processes like self-phase modulation (SPM), modulational instability, soliton generation and soliton fission, cross phase modulation (XPM) and others.
All these processes generate new frequency components, meaning that input light with narrow bandwidth expands into a wide range of new colours, through a process called supercontinuum generation.