Rugate filters are optical thin film interference structures with sinusoidal refractive index profiles. Two-wavelength reflection filters have been fabricated by codeposition of SiO(2) and TiO(2). Composition modulation was monitored and controlled using quartz crystal rate controllers. The resulting filters exhibited two well-defined stopbands. Microscopic examination revealed that the structure is glasslike without pronounced thin film microstructure.

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The properties of metal fluoride films (MgF2 CeF3, LaF3 CaF2 and SrF2) deposited using cw laser and electron-beam evaporation were compared. The differences in film properties were subtle, but important. Films obtained by laser evaporation have lower losses in the UV (185–250 nm) and slightly better crystallinity than the films deposited by e-beam evaporation. Other optical and mechanical properties were independent of preparation method. Differences in the film properties were tentatively attributed to the higher proportion of dissociated species in the évaporant plume of the laser-evaporated materials.

Sinusoidal refractive index profile filters (rugates) are ideally monolithic structures, having no interfaces between dissimilar materials. This is expected to avoid the stress and delamination problems which often arise in thick multilayer thin film filters. The sinewave refractive index profile demonstrates a significant suppression of higher frequency reflective sidebands. Furthermore, multiple, independently selectable reflection bands can be obtained by a superposition of sinewaves of appropriate period. This is demonstrated in Fig.1.