Silicon Photonics in Optical Transceivers: Why the Substrate Shift Matters

For decades, optical transceivers were built on indium phosphide (InP) or gallium arsenide (GaAs) — specialized III-V semiconductor platforms that are expensive, hard to scale, and require dedicated fabrication lines. Silicon photonics changes this by building optical components on standard CMOS silicon wafers. The implications for cost, volume, and integration reach far beyond the transceiver itself.

What Silicon Photonics Replaces

A traditional optical transceiver contains discrete components: lasers, modulators, photodetectors, and a multiplexer — each fabricated on its own substrate and assembled into a module. Silicon photonics integrates modulators, waveguides, multiplexers, and photodetectors onto a single silicon die using the same 300mm wafer processes that produce billions of CPU and GPU chips.

The components built on silicon:

• Mach-Zehnder modulators (MZM) and micro-ring modulators. Encode electrical data onto the optical carrier at 100G+ PAM4 per lane.
• Arrayed waveguide gratings (AWG). Multiplex and demultiplex wavelengths directly on-chip, eliminating external MUX components.
• Germanium photodetectors. Monolithically integrated germanium-on-silicon receivers that convert light back to electrical current.
• Spot-size converters and edge couplers. Bridge the on-chip silicon waveguide to the external single-mode fiber with sub-2 dB loss.

The one component silicon cannot do: the laser light source itself. Silicon is an indirect-bandgap material — it cannot emit light efficiently. So silicon photonics transceivers still use an external III-V laser (typically InP), either attached to the silicon die or co-packaged alongside it. Research into heterogeneous integration — bonding InP lasers directly onto silicon — is closing this gap.

Silicon Photonics vs Traditional Optics

Why It Matters for 800G and Beyond

Volume scalability. A single 300mm CMOS wafer can produce tens of thousands of silicon photonic chips. This is the same wafer-scale economics that drove CPU costs down for four decades — now applied to optics. For 800G DR8 modules that need 8 modulators, 8 photodetectors, and an 8-channel MUX per module, silicon photonics turns an expensive assembly problem into a single-die solution.

Power efficiency. Silicon waveguides have lower capacitance than III-V equivalents, reducing the power needed to drive modulators. In a 32-port 800G switch where every watt matters, silicon photonics modules typically draw 1–3 W less per port than equivalent InP-based designs — a 50–100 W saving per switch.

CPO readiness. Co-packaged optics — where optical engines sit directly on the switch substrate — is only viable with silicon photonics. InP dies cannot scale to the density and yield required for CPO. Every major CPO demonstration from Intel, NVIDIA, Broadcom, and Ayar Labs is built on silicon photonics.

The laser gap is closing: Heterogeneous integration — bonding InP laser dies directly onto silicon wafers — has moved from research to early production. Intel 800G DR8 silicon photonics module already integrates the laser on-package. When on-die lasers become production-ready, silicon photonics transceivers will have zero III-V components, completing the shift to all-silicon optical engines.

APEX Group 800G DR8 and FR4 modules are built on silicon photonics platforms, selected for power efficiency, manufacturing scalability, and readiness for the CPO architectures coming in 2027+. Combined with DWDM MUX/DEMUX and EDFA amplification, they deliver an optical layer built on the same substrate economics that transformed the semiconductor industry.