silicon photonics
**Silicon Photonics** is the **technology of fabricating optical components (waveguides, modulators, detectors) on silicon wafers using standard CMOS fabrication processes** — enabling high-bandwidth, low-power optical interconnects that transmit data at the speed of light through on-chip waveguides, addressing the fundamental bandwidth and energy limitations of electrical copper interconnects in data centers and AI accelerator clusters.
**Why Silicon Photonics?**
- **Electrical limit**: Copper interconnects at high data rates (>100 Gbps per lane) suffer from signal loss, crosstalk, and power consumption that scale poorly.
- **Optical advantage**: Photons don't suffer resistive loss, no crosstalk between waveguides, bandwidth scales with wavelength division multiplexing (WDM).
- **Silicon advantage**: Fabricate photonics on same Si wafers → leverage existing CMOS infrastructure.
**Key Components**
| Component | Function | Silicon Implementation |
|-----------|---------|----------------------|
| Waveguide | Guide light on chip | Si strip in SiO₂ cladding (220nm × 500nm) |
| Modulator | Encode data on light | Mach-Zehnder interferometer (MZI) or ring resonator |
| Photodetector | Convert light to electrical signal | Ge-on-Si photodiode (absorbs 1310/1550nm) |
| Laser | Generate light | III-V bonded to Si (InP laser on Si platform) |
| Multiplexer | Combine wavelengths | Arrayed waveguide grating (AWG) or ring filters |
**Data Center Applications**
- **Pluggable Transceivers**: 400G/800G optical modules (QSFP-DD, OSFP) with Si photonics engines.
- **Co-Packaged Optics (CPO)**: Optical engine integrated into switch package — eliminates front-panel transceivers.
- Reduces power: 5-10 pJ/bit (electrical) → 1-3 pJ/bit (optical).
- **AI Interconnect**: NVIDIA ConnectX + optical → GPU-to-GPU communication across racks.
**WDM (Wavelength Division Multiplexing)**
- Multiple wavelengths (colors) of light on single fiber.
- CWDM: 4-8 wavelengths, 20nm spacing.
- DWDM: 32-96+ wavelengths, 0.8nm spacing.
- Each wavelength carries independent data → multiply bandwidth per fiber.
- Example: 8 wavelengths × 100 Gbps/wavelength = 800 Gbps per fiber.
**Major Players**
| Company | Technology | Products |
|---------|-----------|----------|
| Intel | Monolithic Si photonics | 100G-400G transceivers |
| Broadcom | Si photonics engine | CPO for switches |
| Cisco (Acacia) | Si photonics + DSP | Coherent transceivers |
| Marvell | Si photonics PAM4 | Data center optics |
| GlobalFoundries | SiPh foundry (GF Fotonix) | Photonic wafer services |
| TSMC | SiPh process development | Emerging |
**Challenges**
- **Laser integration**: Silicon cannot efficiently emit light — requires bonded III-V lasers or external laser sources.
- **Coupling**: Connecting fibers to on-chip waveguides with low loss (< 1 dB).
- **Thermal sensitivity**: Ring resonator wavelength shifts with temperature → requires active tuning.
Silicon photonics is **the enabling technology for next-generation data center interconnects** — as AI training clusters demand 10-100x more bandwidth between GPUs and switches, optical interconnects fabricated on CMOS-compatible silicon wafers are the only technology path that can deliver the required bandwidth at acceptable power levels.