Optics for AI Data Centers: Lenses, Wavelengths, and Coupling Components Powering 800G-to-1.6T Interconnects

As AI GPU clusters scale into the hundreds of thousands of accelerators, copper simply can’t move data far or fast enough. Optics for AI data centers — from pluggable 800G modules to co-packaged silicon photonics — have become the backbone of modern GPU interconnect fabrics. This article breaks down the wavelengths, standards, and precision optical components (lenses, GRIN couplers, metalenses) driving this transition, and what it means for engineers sourcing optics today.

Key Takeaways

• 1.6T transceivers succeed 800G using eight 200G PAM4 electrical lanes but are not backward-compatible with 800G ports.
• IEEE 802.3dj draft specifies four 1.6T PHY types (KR8, CR8, DR8, DR8-2) but no FR/LR wavelength-multiplexed PMD — duplex-fiber 1.6T optics remain MSA-defined.
• Wavelength choice (850nm, 1310nm, 1550nm) is driven by reach, cost, and dispersion trade-offs, not just raw speed.
• Co-packaged optics (CPO) embeds lenses and photonic ICs directly on the switch package, cutting electrical trace loss and power draw.
• Metalenses enable wafer-level optical coupling in next-generation CPO designs, while GRIN lenses remain the workhorse discrete coupling optic inside pluggable modules.

---

Why AI Data Centers Are Running Out of Electrical Bandwidth

GPU clusters demand ever-higher east-west bandwidth between accelerators, switches, and storage. Electrical signaling over copper degrades sharply with distance and frequency, and direct attach copper (DAC) cabling is only practical for around 2 meters — active copper cable (ACC) extends this modestly before optics become unavoidable.

This is why analysts such as LightCounting project the datacom optical component market to exceed $16 billion in 2025, driven largely by 400G and 800G shipment growth. 800G transceiver shipments alone are expected to double year-on-year in 2025, with 1.6T units entering production for hyperscale and NVIDIA-class AI infrastructure.

From 800G to 1.6T: How Optical Transceiver Speeds Are Scaling

A 1.6T optical transceiver is a pluggable module carrying 1.6 terabits per second of aggregate throughput across eight electrical lanes of 200 Gb/s-class PAM4 signaling — the direct successor to 800G. IEEE P802.3dj’s draft standard defines the electrical and short-reach PHYs:

Note: There is no standardized 1.6T FR or LR wavelength-multiplexed PMD in 802.3dj. Duplex-fiber 1.6T optics on the market today — such as 2xFR4 configurations covering 2 km on four fibers at roughly 23.5 W typical power — are MSA-based, built from proven 800G silicon photonics engines.

Signal quality tightens as speed rises: TDECQ limits shrink from 4.5 dB (100G) to 3.4 dB (400G) to 3.0 dB (800G), demanding better transmitter optics and coupling precision at every generation.

Choosing the Right Wavelength: 850nm, 1310nm, and 1550nm Explained

Wavelength selection balances reach, cost, and dispersion:

850nm VCSEL transceivers are substantially cheaper than single-mode equivalents, making them the default for intra-DC GPU links. OM4 fiber is rated for 550 m at legacy 10GbE, but that reach shrinks substantially at the PAM4 lane rates used in modern AI clusters — typically 50–100 m at 100G-per-lane rates (802.3db) — so multimode’s real advantage in AI racks is density and cost, not long reach.

1310nm offers a zero-dispersion benefit and is standard for campus and metro-access links, with FR4/CWDM4-class modules built for 2 km and LR4-class modules extending to roughly 10 km. 1550nm DWDM packs 40–120 channels in the C- and L-bands for maximum capacity, while CWDM spans 8–18 wavelengths across 1270–1610nm for moderate-capacity links. ITU-T recommends fiber attenuation under 0.4 dB/km at 1310nm and 0.3 dB/km at 1550nm, figures that inform link-budget planning across both CWDM and DWDM data center optics deployments.

---

Co-Packaged Optics: Moving Lenses and Lasers Onto the Switch ASIC

Co-packaged optics (CPO) eliminates the discrete pluggable transceiver and DSP by embedding electronic-to-optical translation directly onto the switch ASIC package. Shorter electrical traces reduce the need for high-power drivers, repeaters, and retimers.

At GTC 2025, NVIDIA unveiled Spectrum-X and Quantum-X silicon photonics switches. Quantum-X Photonics and Spectrum-X Photonics use co-packaged 1.6T ports, marking a shift toward on-package optical interconnect for AI GPU clusters.

In an NVIDIA CPO design built on TSMC’s COUPE platform, an electronic IC (7nm FinFET CMOS) bonds to a photonic IC. Light enters from the top through a lens and grating coupler, with a metallic backside reflector cutting loss — achieving roughly 1 dB of loss in the ~1300–1320nm band.

The Precision Optics Inside the Module: Lenses, Coupling, and Metalenses

Whether pluggable or co-packaged, every optical link depends on precision coupling optics:

• Optical coupling lenses align laser output into fiber or waveguide with minimal insertion loss.
• GRIN lens fiber coupling: graded-index rod lenses are a well-established, discrete bulk-optic approach for collimating and focusing light between fiber and photonic chip inside pluggable modules — polished and assembled individually rather than fabricated at wafer scale.
• Silicon photonics lens elements, in general industry practice, couple light between lasers, modulators, and waveguides on-chip, complementing grating couplers at the fiber-to-chip interface.
• Metalens optical interconnect structures are a different manufacturing paradigm: ultrathin, wafer-fabricated metasurfaces that replace bulk lens stacks. TSMC’s COUPE platform is among the first to use a metalens for wafer-level coupling between light-emitting arrays and silicon photonic chips in CPO.

As data rates climb and CPO adoption grows, sourcing decisions increasingly span both paradigms — discrete GRIN and aspheric lenses for pluggable modules, and wafer-scale metalens structures for co-packaged designs.

Standards and Form Factors Buyers Should Know

• IEEE 802.3dj: defines 1.6T electrical and short-reach PHYs (KR8, CR8, DR8, DR8-2); no standardized FR/LR PMD.
• SFP MSA / IEEE 802.3z / ITU-T G.695 / FC-PI v2.0: govern SFP/CWDM modules spanning 850nm, 1310nm, 1550nm, and 1470–1610nm CWDM bands.
• Backward compatibility: 1.6T modules generally cannot plug into 800G ports due to differing electrical lane speeds.
• DR8 reach: standard 500 m over single-mode fiber; extended-reach variants achieve 1–2 km.

What This Means for Optics Sourcing in AI Infrastructure

Buyers evaluating optics for AI data centers must weigh reach, wavelength, power budget, and standards compliance simultaneously — and increasingly, whether pluggable or co-packaged architectures fit their roadmap. Google has demonstrated roughly 40% power savings through optical circuit switching, underscoring how much efficiency is on the table as CPO and next-generation coupling optics mature.

---

Key Specifications

Wavelength Optics is developing wafer-level micro-lens arrays and metasurface structures on silicon and fused silica at its micro-nano optics center, and manufactures AR coatings across the 850nm, 1310nm, and 1550nm windows plus molded glass aspheres for fiber-module and test-bench use. Teams prototyping coupling optics for photonic packaging can engage WOE’s engineering team at the development stage.

As data rates climb toward 1.6T and co-packaged designs, the optics challenge extends well beyond the coupling lens itself. Wavelength Optics supports teams building next-generation interconnect hardware with the precision optics around the module: coated molded glass aspheres (1050–1700nm AR) for laser and fiber collimation in development and test setups, custom AR coatings and silicon or fused silica windows for photonic packaging, and laser-processing optics for the welding, marking, and cutting lines that assemble transceivers at volume. For R&D teams exploring new coupling architectures, our semiconductor and micro-nano optics center offers prototype-scale fabrication of wafer-level micro-lens arrays and metasurface structures on silicon and fused silica.

Ready to discuss precision lens design or custom micro-optic components for your optical interconnect project? Contact us to talk to WOE’s engineering team.

---

Frequently Asked Questions

What is the difference between an 800G optical transceiver and a 1.6T optical transceiver?

A 1.6T optical transceiver doubles throughput versus 800G by using eight 200 Gb/s-class PAM4 electrical lanes instead of 800G’s lane configuration, and the two are generally not interchangeable in the same port due to differing electrical lane speeds.

Is there a standardized 1.6T FR4 or ER4 wavelength-multiplexed module?

No. IEEE 802.3dj defines only KR8, CR8, DR8, and DR8-2 PHYs for 1.6T; wavelength-multiplexed duplex-fiber options like 2xFR4 are multi-source-agreement (MSA) designs built on 800G-proven silicon photonics engines.

Why do data centers use 850nm instead of 1310nm or 1550nm for most links?

850nm VCSEL transceivers are substantially cheaper than single-mode equivalents and pair with OM4 multimode fiber, making them the dominant, cost-effective choice for short-reach, high-density intra-data-center links, while 1310nm and 1550nm serve longer backbone and metro-access reaches.

What role do lenses play in co-packaged optics (CPO)?

In CPO designs, light enters the photonic IC through a lens and grating coupler, and platforms like TSMC’s COUPE use a metalens — an ultrathin, wafer-fabricated metasurface — for wafer-level coupling between light-emitting arrays and silicon photonic chips, reducing loss compared with bulk optics.

Does IEEE 802.3dj cover copper as well as optical 1.6T interconnects?

Yes. Alongside the optical DR8 and DR8-2 PHYs, 802.3dj defines 1.6TBASE-KR8 for backplane and 1.6TBASE-CR8 for twinax copper, plus 1.6TAUI-8/16 electrical interfaces, covering both copper and optical AI GPU cluster interconnect needs.

---

Sources

• vitextech.com/blogs/blog/1-6t-optical-for-ai-data-centers…
• cisco.com/c/dam/en/us/products/interfaces-modules/transce…
• asteraix.com/blog/optical-transceivers-complete-guide
• resources.l-p.com/knowledge-center/sfp-wavelengths-850nm-…
• mapyourtech.com/common-optical-wavelengths-850nm-1310nm-1…