ABSTRACT
The term rugate is derived from biology where the meaning is essentially that of corrugated. It was introduced to describe a structure exhibiting a regular cyclic variation of refractive index resembling a sine or cosine wave. Such structures have the property of reflecting a narrow spectral region and transmitting all others. They exhibit properties similar to a quarter-wave stack but without the higher-order reflection bands. Thus, they are notch filters and particularly useful in removing bright spectral lines from weaker continua. Many of their applications involve laser sources, and they are especially relevant in the field of laser protection. This strict original definition of rugate has been relaxed somewhat, so that the term is also sometimes used for any layer system in which there is a deliberate attempt to induce an inhomogeneity whether or not it is of a cyclic kind.
