ECTC 2026 | KYOCERA | Development of a High-Efficiency, Wide-Temperature-Range Optical Coupling Structure for CPO Modules

2 days ago

4 min read

At ECTC 2026, KYOCERA tackled a very practical CPO module problem: how to make the optical coupling between silicon photonics and a polymer waveguide low-loss, stable over a wide temperature range, and able to dissipate heat at the same time. Two key decisions: first, mount the silicon photonics chip "face-down" so the laser's heat dissipates straight downward — the EIC runs 5.5°C cooler and the laser 15.3°C cooler, with SNR improved by 90% and jitter reduced by 20%. Second, make the coupling mirror curved (0.8 mm radius of curvature, 41° angle), cutting coupling loss from 2.80 dB with a flat mirror to 1.22 dB, while a wrap-around core structure extends alignment tolerance to ±5 µm (<1 dB). The finished module runs error-free at 32 Gb/s/lane across the full 0–70°C range. In one sentence: low loss doesn't come from one parameter — it comes from "face-down cooling + curved mirror + wide-tolerance core" working together.

1. Paper background: CPO module coupling needs not just low loss, but temperature robustness

This paper from KYOCERA (Kyoto, Japan) was presented at the 2026 IEEE 76th ECTC. One key to package-level progress in CPO is the optical coupling interface between silicon photonics (SiPh) devices and polymer waveguides. KYOCERA uses an all-in-one SiPh device integrating the laser, driver and TIA (5×5×0.8 mm, 4 lanes each for Tx/Rx, 1310 nm, built-in Fabry-Perot LD, multimode fiber I/O), aiming to make this coupling high-efficiency, wide-temperature and manufacturable.

For context on polymer waveguides as an optical redistribution layer, see Technical Paper Analysis | Polymer Waveguides Withstand +20 dBm for Six Hours; and making fiber connectors detachable and placed at the module edge is a hidden gating factor for CPO volume production — see The Detachable Fiber Battle.

Schematic of the optical coupling structure between the SiPh device and the polymer waveguide (including the path-turning mirror). Source: KYOCERA, ECTC 2026 - Fig. 5

2. The core question: the whole paper in one sentence

The problem this paper sets out to solve: how to design the optical coupling between silicon photonics and a polymer waveguide so that it simultaneously achieves low loss, wide-temperature stability and wide alignment tolerance, while also dissipating the laser's heat.

The answer is three things stacked together: face-down cooling, a curved mirror, and a wrap-around wide-tolerance core.

3. Key figures, one by one

3.1 This figure shows "the power of face-down cooling"

This figure (Fig. 2, Fig. 3) compares face-up and face-down configurations. Face-up, the fiber array (FA) sits on top of the laser and blocks heat dissipation; face-down lets heat flow straight down from the chip. Result: EIC 5.5°C cooler, laser (LD) 15.3°C cooler; at high frequency, face-down improves SNR by 90% and reduces timing jitter by 20%. A lower laser temperature also lowers threshold current and raises slope efficiency — improving reliability, power and link margin all at once.

3.2 This figure shows "a curved mirror cutting coupling loss in half"

This set of figures (Fig. 6, Table II) covers mirror geometry. The laser output has a divergence angle, so to couple efficiently into the polymer waveguide core the mirror needs curvature. Ray-tracing optimization shows the lowest coupling loss at a 0.8 mm radius of curvature and a 41° mirror angle. Measured comparison: flat mirror (41°, zero curvature) coupling loss of 2.80 dB vs. only 1.22 dB for the optimized curved mirror, almost identical to simulation (1.23 dB). Cutting coupling loss by more than half is a real improvement to the link budget.

Table II: flat vs. curved mirror coupling loss — curved mirror at 1.22 dB (measured) beats the flat mirror's 2.80 dB. KYOCERA, ECTC 2026 - Table II

3.3 This figure shows "alignment tolerance stays wide across temperature"

This set of figures (Fig. 7, Fig. 9) covers tolerance. The grating coupler's emission angle shifts with temperature, causing the center of the tolerance map to drift toward +Y; but KYOCERA's wrap-around core structure (a larger effective input aperture) absorbs it. Measured at −40/25/70/105°C, the tolerance for <1 dB excess loss is ±5 µm at every temperature (in both X and Y). FDTD also shows that even as the emission angle shifts with temperature, worst-case insertion loss at 0°C only reaches 2.04 dB.

4. Technical highlights

The first highlight is that the face-down configuration makes "cooling" part of the coupling design. Traditionally, placing the fiber on top of the laser sacrifices heat dissipation; KYOCERA reverses this, gaining a lower laser temperature, better high-frequency signals and larger link margin from face-down mounting — one configuration choice drives multiple metrics.

The second highlight is the curved mirror + wrap-around core coupling design: the curved mirror compensates for divergence and halves the loss, while the wrap-around core enlarges the input aperture, extending alignment tolerance to ±5 µm and resisting temperature drift. The finished module is error-free at 32 Gb/s/lane across 0–70°C, with eyes open at every temperature.

5. Industry link: how far from volume production? Who benefits?

Distance: this paper's maturity leans toward "module-level validation" — optimized design, measured coupling loss, wide-temperature tolerance, and error-free transmission across the full temperature range (including loopback). It's engineering aimed at data center and HPC volume production, and KYOCERA itself is a packaging and module supplier.

Beneficiaries: most directly, CPO module and packaging suppliers (KYOCERA itself), turning SiPh-to-polymer-waveguide coupling into a manufacturable, temperature-robust module; next, the all-in-one SiPh device and multimode fiber ecosystem. The sober note: 32 Gb/s/lane is conservative relative to the leading edge (100+ Gbaud), and the multimode 1310 nm route serves a different market segment from the single-mode high-speed route — this solution's sweet spot is mid-speed, wide-temperature, manufacturable applications, not chasing the highest data rate.

6. Conclusion

The one sentence to remember from this paper: low-loss coupling in CPO modules doesn't come from a single parameter, but from doing three things together — "face-down cooling + curved mirror + wide-tolerance core." KYOCERA cut coupling loss to 1.22 dB, extended tolerance to ±5 µm, and achieved error-free 32 Gb/s/lane across 0–70°C. For anyone tracking CPO packaging, the takeaway: to evaluate a coupling module, look at cooling configuration, mirror geometry, core tolerance and temperature stability together, not just one coupling-loss number.

References

Shuhei Sudo, Megumi Oishi, Misa Takahashi, Shogo Enomoto, Kono Sasaki, Tomoyuki Akahoshi, "Development of a High-Efficiency, Wide-Temperature-Range Optical Coupling Structure for CPO Modules," 2026 IEEE 76th ECTC. KYOCERA Corporation, Kyoto, Japan.