Cross-Layer Analysis of Thermal Tuning Stalls in Wafer-Scale Optical Interconnects for LLM MoE Training

Abstract

Mixture-of-experts (MoE) training depends heavily on all-to-all communication, which wafer-scale optical interconnects with dense wavelength-division multiplexing can serve. Their microring resonators rely on thermo-optic tuning, while MoE compute bursts swing the photonic-layer temperature by about 10 K within tens of milliseconds. This paper quantifies the communication stall by coupling packet-level network simulation, transient thermal simulation of a 3D-stacked GPU and photonic die, and a ring detuning criterion, and by feeding the stall back into the network timeline until both agree. Including this feedback, a tracking loop at the measured 5 nm/s lengthens the training iteration of Mixtral 8x7B and LLaMA-MoE 6.7B by factors of about 1.16 and 1.25 at full model depth. Measured H100 die temperatures match the modeled swing of 300 ms bursts within 15%. A loop slewing eight times faster, a heater driven at each kernel launch, or a dummy load near 75% of peak power removes the stall. An athermalized lithium-niobate ring with a non-volatile ferroelectric setpoint removes it with no holding power or fast loop, which makes it suitable for wafer-scale optical interconnects.