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.
