phase velocity depends on the optical frequency or wavelength
results from: freq.-dependent refractive index, but also from waveguide dispersion
as a result of chrom. dispersion: refraction angles at optical surfaces can be freq.-dependent -> leads to angular dispersion. Exploited in spectrometers with prism paris or pairs of diffraction gratings.
-> also possible to obtain freq.-dependent path lengths, can act like chromatic dispersion; prism pairs and pairs of diffraction gratings can be used for dispersion compensation
-> angular dispersion: also related to phenomenon of pulse front tilt.
results from: different propagation characteristics of higher-order transverse modes in waveguides, e.g. multimode fibers
can severly limit the possible data rate of a system for optical fiber communications based on multimode fibers