Optical interconnection through stacked silicon foundry complementary metal-oxide-semiconductor (CMOS) circuitry has been demonstrated at a data rate of over 40 Mb/s with an open eye diagram. The system consists of a 0.8-μm transmitter and receiver realized in foundry digital CMOS. The use of digital CMOS enables on-chip integration with more complex digital systems, such as a microprocessor. Two layers of these circuits were integrated with thin-film InP-based light emitting diodes and metal-semiconductor-metal photodetectors operating at 1.3 μm (to which the silicon is transparent) to enable vertical optical through-Si communication between the stacked silicon circuits.