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We demonstrate low-loss Si wire waveguides with good thermal stability and thermooptic (TO) switches made from them. The propagation losses of the fabricated waveguides were around 2 dB/cm and the coupling losses to an external fiber were 0.4 dB. These values were achieved by improving the fabrication process and introducing spot-size converters made from inorganic materials. Using these waveguides, we made two types of TO switches, a 1×1 switch with a multimode interference (MMI) branch and a 2×2 switch with a 3-dB directional coupler. Both switches exhibited a high extinction ratio of more than 30 dB at a low electric power of 30 mW and a switching time shorter than 200 µs.

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