Optical and non-optical characterization of Nb2O5–SiO2 compositional graded-index layers and rugate structures

The deposition of graded-index layers and rugate structures was performed by coevaporation of silicon dioxide as the low index material and niobium pentoxide as the high index material. To obtain information about the composition depth profile of the films, we used cross-sectional transmission electron microscopy to supplement deposition rate data recorded by two independent crystal quartz monitors during film preparation. The concentration depth profile was transformed to a refractive index profile using the effective medium approximation. The thus obtained refractive index profiles turned out to represent efficient initial approximations for re-engineering purposes.