While conventional silicon photonic (SiP) waveguides achieve high data rates with low loss, they can only be placed on top of the chip surfaces horizontally. In this article, we present a design approach for multimode chip-to-chip interconnects with similar low-loss and high-bandwidth density properties as SiP waveguides. The state-of-the-art fabrication of optical through silicon via (OTSV) requires conventional back end of line (BEOL) processing with an additional step: metal coating lowers the loss of air-filled TSVs. Alternatively, the TSV is converted into a polymer waveguide by filling of low-loss polymer (Ormocore) to achieve <0.1 dB optical power loss over 380 μm distance. A 3D multimode optical interconnect is measured at 40 Gbit/s with the bit error rate (BER) <10-12 at 1550 nm wavelength. The proposed solution supplies a new path for 3D integration of optical waveguides capable of delivering high-speed data within 3D stacked chips.