A novel optical interposer with optical interconnections is proposed for integrating three-dimensional (3D) LSI chips on this interposer. Vertical-cavity surface-emitting laser diode (VCSEL) chips and photo diode (PD) chips are buried in the optical interposer with polymeric optical waveguides. The VCSEL is 0.25mm in width, 0.35mm in length, and 0.15mm in height. We realize precise passive alignment between the optical waveguides and the VCSEL/PD chips using two-step alignment processes consisting of cavity-assisted positioning and the subsequent surface-tension-powered self-assembly with a molten solder. In addition, we demonstrate the basic operation of the buried VCSEL chips in the optical interposer through tapered through-silicon vias (TSVs). The tapered TSVs are successfully formed by copper electroplating and are 64 mm in top width, 34 mm in bottom width, and 168 μm in length.