A technology of co-packaged optics, which is mounting photonics integrated circuits and electronic integrated circuits on the same board, is essential to meet the demands of high-capacity transmission in data centers. In this respect, polymer optical waveguides have attracted much attention because of mechanical stability, excellent processability, and compatibility with electronic circuits. To achieve high-capacity optical transmission, we are developing a new package substrate, which we call active optical package (AOP) substrate, as a solution of co-packaged optics. The AOP substrate consists of a conventional organic package substrate such as glass-epoxy substrates, on which silicon photonics dies are embedded and SMF connectors are mounted. In the AOP, the silicon photonics inputs/outputs (I/Os) and the optical connectors are connected by 3D optical wiring technology that has a function of converting the pitch size. The 3D optical wiring is realized with a pair of micro-mirrors and a single-mode polymer waveguide.