Fiber array unit (FAU)

A fiber array unit (FAU) is a precision component that holds and aligns multiple optical fibers in a fixed array so they couple light efficiently into a silicon photonics chip. FAUs are a critical building block for co-packaged optics powering AI data center networks.

What a fiber array unit is, in plain terms

A **fiber array unit (FAU)** is a small but extremely precise component that bundles several optical fibers side by side and locks them into a fixed, evenly spaced row. Think of it as a connector block that takes many hair-thin glass fibers and lines them up so their light-carrying cores hit exactly the right spots on a chip. The physical heart of an FAU is usually a **V-groove substrate** — a piece of glass or silicon with precisely etched grooves, each holding one fiber. The fibers are dropped into the grooves, fixed with adhesive, and the end face is polished. Because the grooves are made with semiconductor-grade etching, the fibers sit at known spacings (a common pitch is 127 µm) with sub-micron accuracy.

How it works: coupling light into a chip

A modern **silicon photonics** chip routes light through on-chip waveguides far thinner than a human hair. The challenge is getting light from a standard optical fiber into those waveguides without losing much of it. That hand-off is exactly what the FAU manages. There are two main coupling methods at the FAU-to-chip interface. **Edge coupling** sends light into the side of the chip and tends to be a one-dimensional row of fibers. **Grating coupling** sends light down into the chip's surface and can support a two-dimensional layout — multiple rows of fibers — for higher channel density. Whichever method is used, the FAU must align each fiber core to its waveguide within a fraction of a micron, because even a tiny misalignment causes significant **insertion loss** (signal lost at the junction).

Why FAUs matter for AI and data centers

AI training and inference clusters move enormous amounts of data between GPUs and switches, and electrical wiring is hitting hard limits on speed, distance, and power. The industry's answer is **co-packaged optics (CPO)**: putting the optical engine right next to (or on) the switch or accelerator package, so signals convert to light as close to the chip as possible. CPO only works if you can reliably attach many fibers to each photonic engine. That is the FAU's job. Per SemiAnalysis's CPO analysis, early CPO designs use **high-channel-count FAUs** that are genuinely hard to build: they need many fibers, often a mix of standard single-mode and polarization-maintaining fibers, sometimes integrated micro-optics, and very tight alignment tolerances. As channel counts climb, the FAU becomes one of the trickiest and most valuable parts of the assembly.

Where the FAU sits in the photonics supply chain

An FAU is a mid-stream component in the optical interconnect stack. Upstream are the **photonic integrated circuit (PIC)** foundries and the fiber, ferrule, and V-groove makers. The FAU is then assembled and **fiber-attached** to the PIC during optical packaging. Downstream, that packaged optical engine goes into a pluggable transceiver or a co-packaged switch or GPU module. Nvidia's published CPO supply chain breaks this into many specialized roles — laser sources, external laser modules, the FAU itself, FAU alignment tools, FAU assembly, shuffle boxes, MPO connectors, MT ferrules, fibers, and electro-optical testing — which shows how the FAU is one link in a long, precision-driven chain rather than a commodity.

Who the key players are

FAU and fiber-attach capability spans component specialists and large packaging houses. **Corning** offers ultra-compact FAUs aimed squarely at CPO and next-gen switch optics, and companies such as **Orbray**, **AGC**, **PHIX**, **Neptec**, **ACON Optics**, and **Focuslight** supply FAUs, V-grooves, or related micro-optics. Two names that show up in tracked equity portfolios are worth noting as secondary context. **Fiber Optic Communications (FOCI), Taipei Exchange ticker 3363**, integrates components into finished FAUs and engineers the FAU-to-optical-engine connection. **Fabrinet (NYSE: FN)**, a leading optical packaging and precision manufacturing house, has partnered with **GlobalFoundries** to enable high-fiber-count, passively aligned fiber arrays for silicon photonics — positioning it in the FAU assembly and fiber-attach part of the chain.

What is changing now

The big shift is the move from pluggable optics to **co-packaged optics in volume**. In 2025 Nvidia introduced Spectrum-X and Quantum-X Photonics switches with CPO, and Broadcom advanced its Tomahawk CPO line — both relying on TSMC's **COUPE** photonic-engine platform, which includes FAU co-design support. That ramp is pushing FAU requirements toward higher fiber counts, mixed fiber types, and tighter tolerances, while driving more automation. Test and alignment vendors and AI-assisted, real-time alignment systems are being deployed to lift yield and throughput. The net effect: FAUs are evolving from a niche telecom part into a strategic, high-volume building block for AI infrastructure.

Frequently asked

What does FAU stand for?

FAU stands for fiber array unit. It is a precision assembly that holds multiple optical fibers in a fixed, accurately spaced row, typically in etched V-grooves on a glass or silicon substrate, so they can couple light into a photonic chip.

What is the difference between an FAU and a regular fiber connector?

A standard connector usually mates one or a few fibers to another fiber or port. An FAU aligns many fibers with sub-micron precision specifically to a photonic chip's waveguides, where even tiny misalignment causes large signal loss. It is a chip-level optical interface, not just a cable connector.

Why are FAUs important for co-packaged optics?

Co-packaged optics (CPO) puts the optical engine next to the switch or GPU to save power and boost bandwidth. CPO requires attaching many fibers to each photonic engine reliably, and the FAU is the component that does that alignment, making it one of the most critical and difficult parts of a CPO assembly.

What is the difference between edge coupling and grating coupling in an FAU?

Edge coupling injects light into the side of the chip and is generally a single row of fibers. Grating coupling sends light into the chip surface and can support multiple rows of fibers, enabling higher channel density per optical engine. Each has different loss, alignment, and density trade-offs.

Which companies make fiber array units?

Makers and assemblers include Corning, Orbray, AGC, PHIX, Neptec, ACON Optics, and Focuslight. In tracked equity portfolios, Fiber Optic Communications (FOCI, ticker 3363) integrates finished FAUs and Fabrinet (FN) provides FAU assembly and fiber-attach for silicon photonics, partnering with GlobalFoundries.

Related companies

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