Designing fiber optic assemblies that perform reliably at cryogenic temperatures — including environments as cold as 20 millikelvin — presents engineering challenges that most commercial off-the-shelf solutions are not built to address. This article covers the key considerations engineers need to understand when specifying fiber optic cables for cryogenic quantum systems.
Most fiber optic assemblies are designed to operate across commercial temperature ranges, typically -40°C to +85°C. Cryogenic quantum systems operate far below this range — superconducting qubit systems typically use dilution refrigerators that cool stages to 4 Kelvin (-269°C) and millikelvin temperatures. The extreme thermal contraction that occurs as assemblies cool from room temperature to these levels creates significant mechanical stress on every element of the assembly.
Thermal contraction mismatch: Different materials contract at different rates as temperature drops. The fiber, coating, ferrule, and connector body all expand and contract differently. If these mismatches are not accounted for in the assembly design, stress at the fiber-ferrule interface can increase insertion loss or damage the fiber.
Coating selection: Standard acrylate fiber coatings become brittle at cryogenic temperatures. Polyimide-coated fibers are often preferred for cryogenic applications because of their superior thermal stability and resistance to delamination under thermal cycling.
Connector adhesives: Epoxies used to fix fibers in ferrules must maintain bond integrity through repeated thermal cycling from room temperature to cryogenic temperatures. Standard connector epoxies may crack or de-bond.
Thermal conductivity: Metal components in fiber assemblies conduct heat. In dilution refrigerator systems, heat load at each cold stage is carefully managed. Assemblies with excessive metallic content can degrade system cooling performance.
Vacuum compatibility: Many cryogenic quantum systems operate in vacuum. Fiber assemblies entering the system must use vacuum-compatible materials and may require hermetic feedthroughs.
Impact ES–Ventura, formerly Coastal Connections, approaches cryogenic fiber assembly design with careful attention to each of these challenges:
We select fiber coatings, ferrule materials, and adhesive systems specifically for cryogenic thermal cycling performance. Where polyimide coating is required, we have the capability to strip and terminate polyimide-coated fibers — a process that requires specialized tooling and technique.
Our precision polishing capabilities allow fiber end-faces to be prepared to the exacting tolerances required for low-loss performance, which must be maintained even after thermal cycling. We also manufacture multi-channel vacuum feedthroughs — including versions with multiple PM fibers in a single feedthrough — for systems that require hermetic sealing.
All assemblies for cryogenic applications are designed with thermal budget considerations in mind. We work with customers to minimize metallic content where low thermal conductivity is required, and to specify appropriate cabling materials.
Testing is essential for cryogenic assemblies. Impact ES–Ventura performs in-house thermal cycling from -65°C to 150°C, and works with Experior Labs — a local laboratory specializing in fiber optic environmental testing — for more extreme thermal environments.
Post-cycling optical testing verifies that insertion loss, return loss, and polarization extinction ratio (where applicable) remain within specification after thermal stress. This testing discipline is what gives our quantum computing customers confidence in the long-term reliability of our assemblies in their hardware.
Cryogenic fiber optic assemblies require purpose-built design — not commercial off-the-shelf products adapted for a demanding environment. Every material, coating, and adhesive choice matters at millikelvin temperatures.
Don't adapt commercial assemblies to cryogenic environments—design for them from day one.
