Building optical circuits — waveguides, modulators, splitters, detectors — in silicon using semiconductor fabrication.
What Is Silicon Photonics?
Silicon photonics uses semiconductor manufacturing to build optical components on a silicon wafer. Light is guided through etched waveguides instead of fibre, modulated by structures built into the silicon, split and combined on-chip, and detected by germanium photodiodes grown on the same substrate.
The appeal is the same one that made integrated circuits win: once a function can be printed lithographically, density goes up and per-unit cost goes down with volume. A silicon photonic die can carry eight or sixteen optical lanes in the space a single discrete assembly used to occupy.
How the Pieces Work
Modulation is usually done with a Mach-Zehnder interferometer or a micro-ring resonator. An applied voltage changes the refractive index of a silicon waveguide slightly, which changes the optical path length, which turns into an intensity change at the output. Ring resonators are smaller and lower power; Mach-Zehnder structures are larger but far less temperature-sensitive, which is why they dominate in products.
Detection uses germanium, which absorbs at telecom wavelengths where silicon is transparent. Growing germanium on silicon is now a routine process step, and it is the reason the receive side integrates cleanly while the transmit side does not.
The Laser Problem
The one thing silicon cannot do is emit light — it is an indirect-bandgap material. Every silicon photonic transceiver therefore has a laser attached: either a small indium phosphide die bonded to the silicon, or an external laser source feeding light in through fibre.
The external approach is the one co-packaged designs favour, because it keeps the hottest, least reliable component off a package that is already thermally busy. It also means a laser failure is a laser replacement rather than an engine replacement — which matters a great deal when the engine is soldered to a switch ASIC.
Where You Meet It
Silicon photonics is already inside a large share of shipping 400G and 800G single-mode parallel modules, usually without being advertised on the label. It is a manufacturing choice more than a user-visible feature at the pluggable level.
Where it becomes strategically visible is co-packaged optics, which is essentially not buildable without it.
Silicon photonics
| Fabrication | CMOS-style silicon wafer processing |
|---|---|
| Waveguides | etched silicon, sub-micron cross-section |
| Modulation | Mach-Zehnder interferometer or micro-ring resonator |
| Detection | germanium photodiodes grown on the same die |
| Light source | indium phosphide — bonded or external |
| Main advantage | integration density and wafer-scale cost |
| Enables | CPO, NPO and high-lane-count parallel modules |
Related Terms
CPO, NPO, EML, DFB lasers.