Design and synthesis of A33-laver narrow bandpass filter with graded refractive index profiles

A novel bandpass filter consisting of graded refractive index profiles and a three-cavity stack was designed. It exhibits good optical characteristics, i.e., high transmittance (>99%) at the designated wavelength of 1550±25 nm and high reflectance (>99%) in wavelength regions of 1400±50 nm and 1780±100 nm. This designed 33-layer TiO₂-SiO₂ film with graded refractive index profiles was prepared by helicon plasma sputtering. The measured transmittance spectrum exhibited good agreement with the designed spectrum. The microstructure of this multilayer film was investigated using electron microscopy.