PAM4: Doubling Spectral Efficiency

800G universally adopts PAM4 (4-Level Pulse Amplitude Modulation), abandoning legacy NRZ. PAM4 transmits 2 bits per symbol, doubling spectral efficiency and enabling higher data rates at manageable baud rates.

However, tighter signal spacing increases sensitivity to Signal-to-Noise Ratio (SNR), demanding advancements in lasers, drivers, and TIAs.

Two Dominant Lane Architectures

• 8x100G: Ideal for Short-Reach (SR) multimode. Leveraging mature VCSEL arrays and OM4/OM5 fibers, it offers low cost, low power, and high compatibility (QSFP-DD/OSFP).
• 4x200G: Targets Mid-Reach (FR) and DR (500m-1km) single-mode. Utilizes EML or Silicon Photonics (SiPh). The challenge lies in signal integrity and thermal management at 200G per lane.

Mapping Reach to Technology

• SR8 (<100m): Multimode, VCSEL arrays, cost-sensitive DC.
• FR8 (500m): Single-mode, EML lasers, campus interconnect.
• DR8 (1km): Single-mode, SiPh or TFLN modulators, HPC clusters.

Trend: As the industry moves toward 1.6T, the 4x200G architecture will dominate. Silicon Photonics offers superior integration and CMOS compatibility.