Fibre attach

Notes

Photonic packaging often costs more than the photonic die itself, largely because of alignment-limited fibre attach.

Overview

Fibre attach is the assembly step that lines up an optical fibre, or a block holding a row of fibres, with the light inputs and outputs of a photonic chip or module and then fixes it permanently in place. Because the light-carrying cores and on-chip waveguides are tiny, the parts must be held to sub-micron accuracy while the bond sets, and the joint must keep that alignment for years. It is often a limiting step for throughput and yield when making optical transceivers and co-packaged optics.

How it works

There are two broad ways to align. In passive alignment, the fibre is positioned by mechanical features or by vision against reference marks, without light passing through it. In active alignment, light is typically sent through the fibre and chip while the coupled power is measured, and a precision stage moves the fibre in several axes until the signal is best. Machines commonly offer both modes.

Once aligned, the fibre is fixed. UV-curable epoxy is widely used, and laser welding and soldering are the main alternatives. Fibre arrays, in which many fibres are held at a fixed spacing in one block, let all the channels of a photonic chip be attached in one operation.

The joint then has to survive the product's life. Optical fibre, silicon and package materials expand by different amounts with temperature, so heating and cooling can pull the fibre off its optimum and reduce coupled power. In co-packaged optics the problem is sharper, because the fibre assembly sits close to a hot processor and the cooling hardware must not press on it. Automated align-and-attach machines are used to raise throughput and repeatability.

Examples

• Attaching a fibre array to a silicon photonic transceiver chip with UV-curable epoxy after active alignment.
• Pigtailing a laser module by laser welding its fibre in place for long-term stability.
• An automated machine aligning fibre arrays over the waveguides of a photonic chip in high-volume production.
• Designing the fibre routing and cooling of a co-packaged optics switch so that thermal stress does not degrade coupling.

Why it matters

Optical loss and assembly cost in a photonic product depend heavily on how well and how quickly fibres can be attached, so fibre attach is a recurring concern for module makers moving to high volume. Optical assembly and alignment engineers develop the process, packaging engineers design the fibre interface into the package, and test engineers measure the coupling loss that results.

Related terms

• Edge coupler
• Grating coupler
• Optical engine
• Optical transceiver

Where it appears on the Hub

Used on 8 pages of the Hub. In each group, the pages that mention it most come first.