Sol-Gel Derived Polyvinylpyrrolidone/Silicon Oxide Composite Materials and Novel Fabrication Technique for Channel Waveg
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SLAB WAVEGUIDES Preparation of PVP/SiO9 Sol-gel technology has been studied by many people, and now we can refer to comprehensive text books. (11-12) We followed a conventional sol-gel process to make thin films. Tetraethyl orthosilicate (TEOS) was mixed with water with acid catalyst (hydrochloric acid) to make a sol-gel precursor. PVP dissolved with a proper solvent was mixed with this solution. The precursor was then spin-coated onto a glass plate or a glass plate with a cladding layer. After the heat treatment under argon, the refractive index and the thickness of the slab waveguide were measured by the prism coupling technique. Optical propagation loss was evaluated by looking at the intensity decay along a streak line with a CCD camera.(13 ) Results and discussion
Prior to the impregnation of PVP into the sol-gel materials, the characteristics of the pure PVP were evaluated. The results suggested that the PVP gradually becomes insoluble to ethanol when it is heated at 180 'C or higher because of crosslinking; however, in this temperature range, degradation of PVP takes place as well, resulting in losing optical transparency at visible wavelengths. Sol-gel derived SiO2/PVP films with various PVP ratios were prepared at different temperatures and evaluated for refractive index and optical propagation loss change. The results of refractive index measurement showed that refractive index increases as the baking temperature increases because of crosslinking of PVP and densification of SiO2. Sol-gel processed SiO2 is known to be densified by heat. (10) The results of the optical propagation loss change are shown in Fig. 1. All the PVP/SiO2 waveguides had low optical propagation losses which are less than 1 dB/cm up to 225 *C. In contrast, the pure PVP waveguide had over 20 dB/cm of loss at this temperature. This suggests that the incorporation of SiO2 prevents the decomposition of PVP. This is because SiO2 provides a relatively inert environment for PVP. The decomposition reaction is usually a radical chain reaction. Inert SiO2 molecules stop the propagation of the chain reaction 20
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100% PVP, 633nm - 50% PVP, 633nm --- I-- 60% PVP,633nm 70% PVP,633nm ---A 0---
50% PVP, 812nm 60% PVP, 812nm 70%PVP, 812nm
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Baked temperature (*C) Fig. 1. Optical propagation loss at 633 nm and 812 nm of PVP/SiO2 waveguide baked at different temperatures for 5 minutes.
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in PVP, and yield better optical transparency. The less the PVP concentration, the lower the optical propagation loss in the raised loss regions. PVP/SiO2 material system produced cracked film when PVP concentration was 40 % or less. Therefore, 50 weight % PVP/SiO2 was found to be the best in optical transparency in this series of materials. 50% PVP/SiO2 waveguide was then evaluated for temporal thermal stability, at 110 'C for 1300 hours. The results showed no noticeable index change after an initial slight increase as shown in Fig. 2. The samples at the room temper
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