Stacking optics into the chip in 3D wins electrically but pays across the board in thermal management. imec's paper quantifies 2.5D vs 3D CPO thermal crosstalk one number at a time: 8 K vs 60 K, a 13x spatial gradient, a 60x temporal gradient. All 18 figures explained.

KYOCERA breaks low-loss coupling for CPO modules into three parts: face-down cooling, a curved mirror and a wide-tolerance core. Coupling loss halved to 1.22 dB, ±5 µm tolerance, error-free from 0 to 70°C. Why can't you judge a coupling module by a single loss number?