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primarysourced Photonics sector Coherent
COHR
~6 min read · 1,307 words ·updated 2026-04-29 · confidence 61%

Silicon photonics

Silicon photonics (SiPh) leverages CMOS-compatible silicon-on-insulator (SOI) processes to build passive and active optical components — waveguides, splitters, polarization controllers, modulators, and germanium photodetectors — on standard 200 mm or 300 mm wafers. The economic argument is compelling: SOI fabs amortize CMOS-foundry depreciation, and integration density on silicon outpaces InP for complex optical functions like polarization-diverse coherent receivers and large modulator arrays.

The fundamental limitation: silicon does not emit light (indirect bandgap). Silicon photonics needs an external light source — almost always an InP DFB or CW laser butt-coupled or hybrid-bonded into the SiPh PIC. This is exactly where Coherent’s vertical integration story converges: the same InP fab that produces EMLs (InP EML process) produces the high-power CW lasers that feed SiPh PICs.

Coherent’s in-house SiPh process

Coherent runs an in-house silicon-photonics fabrication process — historically modest volume relative to dedicated foundries (TSMC, Tower, GlobalFoundries) but operationally important for vertically integrated transceiver builds. Public detail on the process node is limited ⚠ — Coherent does not disclose wafer size, transistor pitch, or specific design-rule details. What is documented:

  • PN-junction Mach-Zehnder modulator — Coherent has a proprietary 400G pure-silicon PN-junction MZM design (OFC 2026 PR 2026-03-17) ✓
  • In-house heterogeneous integration — Coherent attaches its own InP CW lasers to its own SiPh PICs to assemble modulator subsystems
  • Application-specific PICs — Coherent designs and fabricates SiPh PICs for both client-side (1.6T-DR8 transceivers) and coherent-DWDM (800G ZR transceiver receivers)

The in-house SiPh capability complements rather than replaces Coherent’s InP modulator capability. The choice between InP EML and SiPh modulator at any given line rate is a cost-power-performance tradeoff:

TechnologyOptical performancePowerCost at volumeBest fit
InP EML (single-chip)very highlowhigh100G/200G/lane single-mode datacom
SiPh + InP CWmoderate–highmoderatelowest at scale200G+ /lane large-array, CPO, coherent
InP IQ modulatorvery highmoderatehighcoherent-DWDM transponder
Polymer EO modulator (LWLG)very high (research)very lowTBD ⚠future ≥400G/lane

For large-array applications (CPO with 32×200G or 16×400G optical engines), SiPh’s integration density wins decisively over discrete InP EMLs. For high-power-budget single-mode pluggables, InP EML often wins on raw transmitter performance.

OFC 2026 — 400G/lane SiPh demonstration

The OFC 2026 conference (Los Angeles, March 17–21, 2026) was Coherent’s major 400G/lane SiPh public unveiling. Key demonstrations:

1. Coherent in-house 400G PN-junction MZM

Per Coherent’s 2026-03-17 press release: “For emerging 3.2T transceivers, Coherent demonstrated 400G/lane PAM4 optical links with both a 400G Differential EML as well as a silicon photonics PIC implementation based on Coherent’s 400G pure silicon PN junction Mach-Zehnder Modulator” ✓ (OFC 2026 next-gen pluggable PR).

This is significant because pure silicon (not lithium niobate, not thin-film LiNbO3, not polymer) at 400G/lane has been historically considered bandwidth-limited by silicon’s plasma-dispersion-effect modulation efficiency. Coherent’s PN-junction design — likely employing depletion-mode push-pull traveling-wave geometry with optimized doping profiles — pushes the silicon platform to the 400G/lane line rate at production-relevant insertion-loss and drive-voltage budgets.

2. Tower Semiconductor partnership — 420 Gbps PAM4

A separate OFC 2026 demonstration paired Coherent’s InP CW high-power laser with Tower Semiconductor’s PH18 silicon-photonics platform to show 420 Gb/s PAM4 with a clear open eye (Converge Digest) ✓. This is a foundry-process variant — Tower’s PH18 is a third-party SiPh foundry tier — and the demonstration validates that Coherent’s CW laser is compatible across both in-house and merchant SiPh fabs.

The strategic read: Coherent has optionality on SiPh fab supply — its own internal capacity for proprietary designs plus access to Tower’s external foundry capacity for cost-sensitive or scaling-volume designs.

3. CPO demonstrations at OFC 2026

In addition to pluggable-transceiver SiPh, Coherent showed multiple co-packaged optics (CPO) implementations at OFC 2026 (CPO press release 2026-03-17) ✓:

CPO demoSpecificationSource-laser approach
6.4T (32×200G) socketed CPOsilicon-photonics-basedExternal Laser Source (ELS) module powered by Coherent’s high-power InP CW lasers
Multimode socketed CPOhigh-speed VCSEL-basedCoherent’s 200G VCSELs
InP modulator on silicon at 400Ghybrid InP-on-SiCoherent vertical integration

See CPO roadmap for the broader Coherent CPO position.

How SiPh complements Coherent’s InP source lasers

The integration story Coherent tells investors is that SiPh and InP are complements, not substitutes:

  1. Source generation: InP DFB / CW laser (high optical power, narrow linewidth, controlled wavelength) — Sherman 6-inch InP fab
  2. Modulation: SiPh PIC (high-density modulator array, polarization-diverse coherent receivers) — Coherent in-house SiPh fab
  3. Detection: Germanium-on-silicon photodiodes (CMOS-compatible, integrated on the SiPh PIC) — Coherent in-house SiPh fab

This division of labor mirrors what most leading transceiver houses now build, but Coherent is the only merchant supplier with all three layers in-house at scale ✓. Lumentum acquired NeoPhotonics in 2022 to add SiPh capability but does not run SiPh at the same vertical depth.

Comparison: Coherent in-house SiPh vs merchant SiPh foundries

Foundry tierWafer sizeProcess maturityEnd-customer fitCross-link
Coherent in-houseunspecified ⚠proprietaryCoherent transceivers + CPOthis page
Tower Semiconductor (PH18)200 mmmature merchantMulti-customer; Coherent uses for collaborative demosTSEM
GlobalFoundries Fotonix (45SPCLO/9WG/etc.)300 mmmerchant; CMOS-alignedHigh-volume CMOS-photonic integrationGFS
TSMC SiPh200 mm / 300 mmmaturingCustom large-volume programs—
Intel Foundry SiPh300 mminternal-historicalLimited merchant access—

The strategic question for Coherent is: at what scale does it pay to keep SiPh in-house vs migrating to Tower or GFS for capacity? The OFC 2026 dual-demonstration (in-house design and Tower-foundried design) suggests Coherent is hedging — proprietary IP on internal fab, capacity-scaling demonstrations on external foundry.

Complementary path: external light source modules for CPO

For co-packaged optics, the SiPh PIC sits next to the switch ASIC inside the package and the laser sits outside the ASIC package in a thermally-controlled External Laser Source (ELS) module. This architecture splits responsibilities:

  • Inside the package: SiPh PIC (Coherent in-house or Tower-fabricated) + grating couplers + waveguides → connects to the switch ASIC
  • Outside the package: ELS module — InP CW high-power lasers (Coherent and Lumentum both shipping)

Coherent’s role spans both halves; Lumentum’s role is concentrated on the ELS / CW laser side. See CPO roadmap for the NVIDIA Spectrum-X / Quantum-X Photonics architecture context.

400 mW CW laser sampling

In late 2025 Coherent began sampling 400 mW CW lasers for CPO and SiPh applications, with seven different customers evaluating the technology ◐ — a delivery milestone that prepositions Coherent for late-2020s ELS module design-ins. The 400 mW power class targets fan-out architectures where one laser feeds multiple modulator engines, which is the predominant CPO source-laser topology.

Cross-tenant context

  • Tower Semiconductor (TSEM) — PH18 SiPh foundry; Coherent’s external-foundry demo partner; structurally interesting comparison given Coherent has its own SiPh
  • GlobalFoundries Fotonix (GFS) — alternative merchant SiPh foundry tier
  • Lumentum (LITE) — partial SiPh capability via NeoPhotonics acquisition; structurally narrower vertical integration
  • LWLG — electro-optic polymer modulator on silicon-photonic PIC; alternative path to ≥400G/lane modulation that does not require Coherent’s PN-junction approach
  • MRVL — DSP supplier into SiPh-based transceivers

Sources