 Distributed Bragg reflector

A distributed Bragg reflector uses a periodic refractive index structure to make successive weak reflections add coherently. Near the design wavelength, constructive wave interference produces strong reflection even when a single interface reflects only a small fraction of incident radiant power.

A quarter-wave Bragg stack alternates layers with optical thickness ${\lambda }_0/4$ at normal incidence. Measuring transmission across wavelength reveals a rejected band rather than a perfectly reflecting surface at every frequency. Such mirrors provide wavelength selection in an optical resonator; their reflection band shifts with incidence angle and material dispersion.