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To improve the reproducibility of passive alignment, large-core single-mode waveguides are demonstrated, which can be connected to thermally expanded core fibers with increased alignment tolerance. It is shown that polymer waveguides with a core dimension of 25 ×25 μm2 and an index contrast less than 0.001 can satisfy single-mode condition. As a novel functional device incorporating the large-core waveguide, variable optical attenuators (VOAs) are designed and fabricated. For the fabrication of the thick core structure, a soft molding process is developed. Due to the small index contrast of the waveguide, efficient attenuation is expected for small electrical power consumption, which is confirmed by three-dimensional beam propagation method. From the fabricated VOA, more than 20 dB of attenuation is obtained by applying 20 mW.