Silicon Photonics-based Active Optical Cables (AOCs) will disrupt data center networking, particularly in rack-to-rack connections spanning up to 100 meters. This technology represents a significant advancement over current Direct Attach Copper (DAC) cables and traditional AOCs. DAC cables, utilizing PAM-4 encoding and Forward Error Correction, achieve 400 Gbps over short distances (less than 3 meters) with 1W per 100 Gbps power efficiency. These cables employ equalization techniques to mitigate high-frequency signal attenuation in copper media. However, their performance degrades rapidly with distance due to insertion loss and crosstalk. Traditional AOCs, based on VCSELs or directly modulated DFB lasers, match the 400 Gbps speed over 100 meters. They typically use parallel optics (for example, 4x100G or 8x50G) and consume 1.5W per 100 Gbps. While offering superior reach and signal integrity compared to DACs, they face scalability challenges for 800 Gbps and 1.6 Tbps interconnects.

Silicon Photonics-based AOCs integrate photonic elements directly onto silicon substrates using CMOS fabrication processes. These devices employ sub-micron silicon waveguides, typically 220nm thick, to guide light with low loss (less than 0.5 dB/cm), precisely controlling mode properties, dispersion, and polarization. On-chip Wavelength Division Multiplexing (WDM) is a key feature, with dense WDM techniques supporting up to 64 wavelengths in O-band (1260-1360 nm) or C-band (1530-1565 nm), with 100 GHz channel spacings, enabling more than 800 Gbps per fiber. The integration capabilities of Silicon Photonics platforms are extensive. They incorporate high-speed modulators, typically Mach-Zehnder or ring resonator-based, with bandwidths exceeding 50 GHz. Germanium-on-Silicon photodetectors boast responsivities greater than 0.8 A/W and bandwidths also surpassing 50 GHz. Wavelength (de)multiplexers using arrayed waveguide gratings or echelle gratings are also integrated, along with optical switches and variable optical attenuators for dynamic power management. Laser integration, a challenging aspect of Silicon Photonics, is addressed through various methods. These include heterogeneous integration of III-V materials like Indium Phosphide on silicon, flip-chip bonding of external laser dies, and ongoing research into monolithic integration of Germanium-on-Silicon lasers.

Advanced packaging techniques are crucial to the performance of these devices. They employ 2.5D and 3D integration with electronic ICs using silicon interposers or through-silicon vias. Precision fiber alignment is achieved using v-grooves or lensed fiber arrays, and the entire assembly is hermetically sealed for environmental protection and thermal management. Many implementations also co-package DSP (Digital Signal Processing) chips, enabling advanced modulation formats like PAM-4 or DMT, and equalization techniques that compensate for optical impairments and extend reach. The performance characteristics of Silicon Photonics-based AOCs are impressive. They offer bandwidths exceeding 800 Gbps per fiber, with roadmaps targeting 1.6 Tbps. Power efficiency is significantly improved, with consumption dropping below 0.5W per 100 Gbps - some designs even approach 0.3W per 100 Gbps. Despite their advanced capabilities, these AOCs maintain compact designs compatible with standard form factors like QSFP-DD and OSFP. They offer reliable operation over distances from 500 meters to 2 kilometers, depending on the specific implementation.

Challenges persist in thermal management, particularly for integrated lasers. Coupling efficiency between on-chip waveguides and optical fibers typically incurs a 1-2 dB loss per coupling, impacting overall system performance. Cost-effective manufacturing at scale remains a key focus for ongoing development. Ongoing research and development are addressing these challenges, with advancements in efficient laser-to-silicon coupling, athermal designs, and automated testing and packaging processes. As the technology matures, we can expect to see AOCs with bandwidth densities exceeding 3.2 Tbps per fiber, power consumption dropping below 0.2W per 100 Gbps, and reaches extending beyond 10 kilometers.

Worth watching:

• Mellanox Technologies (now part of NVIDIA) (USA/Israel): Offers a comprehensive range of high-performance interconnect solutions, including advanced AOCs and DACs for data center applications.
• Cisco (USA): Utilizes Luxtera's silicon photonics technology to develop high-speed optical transceivers and interconnects for next-generation data center networks.
• Broadcom (USA): Develops silicon photonics-based optical interconnects that integrate seamlessly with their switching and routing platforms, offering high-bandwidth density solutions.
• Juniper Networks (USA): Invests in silicon photonics technology to enhance their network infrastructure products, focusing on improving data center interconnect performance.
• Ayar Labs (USA): Specializes in in-package optical I/O solutions using advanced silicon photonics, aiming to replace traditional electrical I/O in computing systems.