Design and experimental approach of optical reflection filters with graded refractive index profiles

A novel optical multilayer filter with graded refractive index profiles using a quarterwave stack was designed. This filter exhibits a high reflectance band, a sharp cutoff and a wide transmittance region as the result of sidelobe suppression. A 31 layer TiO2/SiO2 high reflectance filter with a graded refractive index profile was fabricated by helicon plasma sputtering on BK7 and Si (100) substrates. The deposition system was operated under ambient gas pressure of 1.8×10−1 Pa at room temperature. The measured transmittance of the spectrum was in good accord with calculated results. Transmission electron microscopic observations confirmed the expected microstructure of the filter.