Optical interposers represent an advanced packaging solution that integrates optical interconnects within a separate substrate, facilitating high-bandwidth, low-latency communication between chips. This technology bridges the gap between silicon photonics and traditional electrical interconnects, offering a flexible platform for heterogeneous integration of electronic and photonic components. Current state-of-the-art optical interposers achieve data rates comparable to silicon photonics, with implementations demonstrating up to 100 Gbps per channel. Research efforts are pushing towards 200 Gbps and beyond, leveraging advanced modulation formats and coherent detection schemes. The interposer substrate, often made of silicon or glass, incorporates waveguides, gratings, and other passive optical components, while active devices like lasers and photodetectors are typically attached as separate chips.

Optical interposers support wavelength division multiplexing (WDM), enabling aggregate bandwidths in the terabit-per-second range. Dense WDM systems implemented on optical interposers have demonstrated up to 64 channels, similar to silicon photonics solutions. However, optical interposers offer advantages in terms of thermal management and laser integration, as the separate substrate allows for more design flexibility and potentially better heat dissipation. The primary advantage of optical interposers lies in their ability to decouple the optimization of optical and electronic components. This separation allows for the use of best-in-class technologies for both domains, potentially leading to better overall system performance. Optical interposers can achieve energy efficiencies similar to silicon photonics, reaching sub-picojoule per bit levels for data transmission.

Thermal management in optical interposers can be less challenging compared to monolithic silicon photonics solutions. The separate substrate allows for better thermal isolation between optical and electronic components, reducing the impact of temperature fluctuations on optical performance. This can be particularly beneficial for maintaining the stability of wavelength-sensitive devices like lasers and filters in WDM systems. Integration of laser sources, a significant challenge in silicon photonics, can be somewhat simplified with optical interposers. The separate substrate allows for more straightforward integration of III-V lasers through techniques like flip-chip bonding or transfer printing. This can potentially improve yield and reliability compared to monolithic integration approaches. Coupling efficiency between optical fibers and interposer waveguides remains a crucial consideration. Optical interposers can leverage advanced coupling techniques, such as 3D polymer waveguides or integrated lensed structures, potentially offering improved coupling efficiency and alignment tolerance compared to silicon photonics solutions. Polarization management in optical interposers can be more flexible than in silicon photonics. The interposer substrate can be designed to support polarization-maintaining waveguides or incorporate polarization diversity schemes more easily, potentially simplifying the design of polarization-sensitive systems. Manufacturing complexity and cost remain significant challenges for optical interposers. While they can leverage some existing packaging technologies, the integration of optical components adds complexity to the manufacturing process. However, the decoupling of electronic and optical fabrication may offer advantages in terms of yield management and process optimization compared to monolithic silicon photonics approaches.

In comparison to through-silicon via (TSV) technology, optical interposers offer the advantage of optical communication, potentially providing higher bandwidth and lower power consumption over longer distances. However, TSVs excel in achieving extremely high bandwidth density for short-reach interconnects, with some implementations demonstrating over 2 TB/s/mm² of interconnect bandwidth. TSVs also benefit from a more mature manufacturing ecosystem and may be more cost-effective for certain applications.

Worth watching:

• POET Technologies (Canada): Their Optical Interposer Platform integrates electronic and photonic devices using a unique hybrid approach that utilizes III-V materials on a silicon substrate. They provide a platform for high-speed data communication and focus on applications such as data centers, telecommunications, and AI, offering a commercially viable path for the production of optical interposers.
• TeraXion (Canada): develops components for photonic integration, including technologies closely related to optical interposers. They work on high-speed modulators, wavelength multiplexers, and active photonic components, which are essential for the assembly of optical interposers. Their expertise in integration and packaging makes them a key player in this niche, especially for optical systems needing robust, high-bandwidth interconnect solutions.
• GlobalFoundries (US): Developing silicon photonics solutions and is active in advanced packaging technologies, including the use of optical interposers. They offer foundry services that cater to customers needing integrated electronic and photonic solutions, making use of their expertise in optical interposer substrates like silicon and glass. Their capabilities include integrating multiple optical and electronic components, which aligns with the demands of creating scalable, high-performance optical interposer solutions.