AI Optical Communication Faces Key Transformation! Experts: NPO Expected to Commercialize Before CPO, Glass Bridge Emerges as New Focus for High-Density Interconnect

As artificial intelligence (AI) drives rapidly increasing demand for high-speed optical communication, optical packaging technology has become a focal point in the semiconductor industry. Nomura Securities recently hosted an AI expert conference, inviting specialists in advanced optoelectronic packaging to analyze the current state of technologies such as Glass Bridge, Near-Packaged Optics (NPO), Co-Packaged Optics (CPO), and glass substrates.

Experts believe that Glass Bridge will become one of the key technologies for high-density optical interconnects. Regarding commercialization timelines, NPO is expected to enter large-scale markets earlier than CPO.

Experts point out that glass materials possess excellent transmittance in the visible and near-infrared spectrum, making them highly suitable for optical communication applications.

The primary function of Glass Bridge is to serve as an intermediary structure between photonic integrated circuits (PICs) and optical fibers, adjusting the mode field mismatch through waveguide design to improve optical signal coupling efficiency.

Currently, there are two main manufacturing methods for Glass Bridge. The most mature is ion-exchange technology, which forms waveguide structures through ion exchange. This method has been developed for many years and is the approach adopted by Corning (GLW-US). The other method uses laser modification processes, but currently suffers from low efficiency and higher optical loss.

However, for Glass Bridge to achieve widespread adoption, several technical hurdles must be overcome, including reducing optical loss, improving process consistency, and meeting extremely high coupling precision and deformation control requirements.

Regarding whether Glass Bridge will replace Fiber Array Units (FAUs), experts believe the answer is no.

Glass Bridge acts more like a bridging component between photonic chips and FAUs, responsible for mode field matching while gradually converting the highly dense optical routing on the chip side into a configuration better suited for fiber connection. Thus, the two technologies will complement each other rather than replace one another.

As AI servers continue to demand higher bandwidth, the value of Glass Bridge will be most evident in high-density optical communication applications.

On optical coupling methods, two main technical approaches currently exist: edge coupling and surface grating coupling.

Experts state that edge coupling offers higher optical coupling efficiency and broader optical bandwidth, making it particularly suitable for high-bandwidth applications like wavelength division multiplexing (WDM). As a result, many new designs still prioritize edge coupling.

In contrast, surface grating coupling has narrower optical bandwidth and relatively limited coupling precision.

However, experts note that NVIDIA (NVDA-US) chose surface coupling in its CPO architecture, primarily due to considerations of manufacturing consistency over single-channel efficiency.

For example, in a 6.4Tbps CPO system, assuming 200Gbps per channel, 32 optical fibers must be integrated, requiring extremely high consistency in optical loss across all channels.

Edge coupling uses a one-dimensional arrangement, making it difficult to maintain uniform performance across many channels. Surface coupling, on the other hand, enables 2D array design, offering superior control over overall consistency.

Glass substrates become key in 2.5D packaging — Glass Core Substrate still awaits yield breakthrough

Beyond Glass Bridge, glass materials are gradually extending into advanced packaging fields.

Experts note that in 2.5D packaging, the extensive use of molding compounds can lead to package warpage. Glass substrates, with their high rigidity, can effectively suppress deformation and enhance packaging stability.

As for the highly anticipated Glass Core Substrate (GCS), despite advantages such as reduced electrical loss, improved warpage control, and support for higher bandwidth, it remains some distance from true commercialization.

One of the biggest challenges is the low yield in Through-Glass Via (TGV) processes, primarily due to the fragility of glass and high processing difficulty.

Currently, Intel (INTC-US) is seen as a key driver of GCS technology, with several South Korean companies actively investing in R&D. However, the market still lacks comprehensive reliability verification data.

Meanwhile, Chinese PCB manufacturers have begun providing GCS samples, and BOE (000725-CN) is collaborating with Corning to develop related technologies. Overall, the field remains in its early development stages.

Regarding optical packaging technology roadmaps, experts believe that NPO offers advantages in both low power consumption and high bandwidth while retaining pluggable electrical interfaces, allowing greater flexibility in equipment maintenance and system upgrades.

In contrast, CPO offers higher integration but comes with greater difficulty in repair and deployment. Therefore, NPO is expected to enter mass production and commercialization first, while CPO will need to wait for further maturation of the supply chain.