Thunderbolt interconnect – comparing optical and copper approaches

Thunderbolt Interconnect Technology has adopted both copper and optical cables. One single cable can support two bi-directional signal lanes at a data rate of 2×20.625 Gb/s, which is driven by demand of 4K video. Future 8K video and Virtual Reality (VR) will push the bandwidth requirement even higher. Form factor, link distance, usage model and heat dissipation are all placed into design consideration. Key technologies developed to enable this high data rate consumer cable are discussed, such as robust copper and optical cables, miniature optical engine, thermal design and 2×20.625Gb/s low power integrated circuits for VCSEL-based optical link. Copper and optical interconnect technologies are compared from the perspective of cost, power, form factor, and scalability. Total power consumption is 146mW for each 25.78Gb/s optical link, which gives 5.69mW/Gb/s. Among the commercially available optical ICs we evaluated at 25.78 Gb/s, this work has lowest power consumption and smallest die area in industry.