Bragg reflector

A distributed Bragg reflector (DBR) is a reflector used in waveguides, such as optical fibers. It is a structure formed from multiple layers of alternating materials with varying refractive index, or by periodic variation of some characteristic (such as height) of a dielectric waveguide, resulting in periodic variation in the effective refractive index in the guide. Each layer boundary causes a partial reflection of an optical wave. For waves whose vacuum wavelength is close to four times the optical thickness of the layers, the many reflections combine with constructive interference, and the layers act as a high-quality reflector. The range of wavelengths that are reflected is called the photonic stopband. Within this range of wavelengths, light is "forbidden" to propagate in the structure.

The DBR's reflectivity, ${\displaystyle R}$, for intensity is approximately given by

where ${\displaystyle n_{o},\ n_{1},\ n_{2}}$ and ${\displaystyle n_{s}\,}$ are the respective refractive indices of the originating medium, the two alternating materials, and the terminating medium (i.e. backing or substrate); and ${\displaystyle N}$ is the number of repeated pairs of low/high refractive index material.

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