Research and Development of Optical Waveguides and Elements Using Anodically Oxidized Alumina Thin Film

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We have studied the optical Properties of anodized alumina films and demonstrated both theoretically and experimentally that they are promising materials for small and efficient optical elements:1. We have fabricated the anodized alumina films of 30-100 mum thicknesses m u by anodic oxidation of aluminum substrates. The optical loss becomes lowest when the aluminum is anodized with an electric current of 50 mA/cm^2 in the sulfuric acid of 20- 30 %, and the loss is reduced further by annealing the film at 200-300 ﾟC.2. We have examined the alumina films by two-bean interference and found that the refractive index decreases as the distance from the substrate increases. This phenomenon becomes more eminent in a film of longer anodization time. It has also been found that the water-sealed alumina film has birefringence as large as 4 X 10^<-3> at the wavelength of 0.63 mum.3. We have fabricated a micropolarizer by electroplating nickel into the cylindrical pores in the alumina film. An extinction ratio of - 30 dB has been obtained at the wavelength of 1.3 mum. Polarizing function has also been found in the alumina film anodized with a large electric current, where there are residual aluminum columns that are not oxidized in the anodization process.4. We have calculated theoretically the optical loss and refractive index of the anodized alumina film. The theory explains the experimental results well and gives a guideline for designing novel optical elements.