How is dispersion caused in an optical fibre?

In optical fibres, there are three types of dispersion, chromatic, modal, and material. Chromatic dispersion results from the spectral width of the emitter, which determines the number of different wavelengths that are emitted from the LED or laser. The smaller the spectral width, the fewer the number of wavelengths that are emitted and because longer wavelengths travel faster than shorter wavelengths, these wavelengths will arrive at the end of the fibre ahead of the shorter ones, spreading out the signal – causing chromatic dispersion. Modal dispersion deals with the path or mode of each light ray. The modes that enter at sharp angles, called high-order modes, take much longer to travel through the fibre than the low-order modes and therefore contribute to modal dispersion. Finally, material dispersion is caused by the wavelength dependence of the refractive index on the fibre core material, while the waveguide dispersion occurs due to dependence of the mode propagation constant on the fibre parameters such as, core radius, and difference between refractive indexes in the fibre core and fibre cladding, and the signal wavelength.