Guided light waves are waves propagating in a waveguide structure, i.e., a spatially inhomogeneous structure which counteracts the beam expansion that would normally be caused by diffraction. The waveguide structure effectively eliminates the divergence of a guided wave, but also modifies other properties such as its wavenumber and chromatic dispersion (→ waveguide dispersion). Therefore, the properties of guided waves deviate in various respects from those of, e.g., plane waves. Such deviations are particularly large in cases where the waveguide dimensions are not much larger than the wavelength.
A particularly important example of guided optical waves is light propagating in optical fibers.