Silicon modulators hit an RC bottleneck at 50–60 GHz. Nokia Bell Labs flip-chip integrated a TFLN modulator with a commercial driver IC to build the first CPO transmitter of its kind: Vπ·L 2.3 V·cm, EO bandwidth >45 GHz. Coupling loss and heat are the next hurdles.

Silicon modulators are stuck at 3,000–5,000 µm and 60 GHz by the diffraction limit, short of the 3.2T AI needs. Marvell's plasmonic modulator shrinks to ~10 µm, reaches 1 THz, and can be added at the back end of existing SiPh processes. It's a key piece for extending SiPh beyond 3.2T — but volume yield is the real test.