Modeling and preparation of practical optical filters

Abstract

Multilayer bandpass and bandstop filters have been produced using electron beam evaporation. Initially bandstop filter is modeled with non absorbing zinc sulphide (ZnS) and zinc selenide (ZnSe). When the absorption data was incorporated for the said materials significant absorption was observed at shorter wavelengths of the spectral band restricting the practical usage of the filter. ZnS and ZnSe were then replaced by dispersive silicon dioxide (SiO2), tantalum penta oxide (Ta2O5) and titanium dioxide (TiO2) along with their absorption and the filters are optimized to get desired bandpass and bandstop data. Bandpass and bandstop filters with desired performance were experimentally characterized with two combinations SiO2/Ta2O5/glass and SiO2/TiO2/glass. The measured average transmission for both combination (bandpass) in the pass band was about 92% with T<1% in the stop band. Slightly narrow bandwidth was observed for SiO2/TiO2/glass as compared to SiO2/Ta2O5/glass which is attributed to layers densification. Similarly Tavg⩾80% was achieved for two combinations of bandstop filters with T<0.1% in the stop band. The structure and surface morphology of the prepared filters were characterized by X-ray diffraction and scanning electron microscopy. XRD analysis reveals amorphous structure. SEM analysis also reveals that the layers are compact and have good surface quality.