The Flat Fiber Revolution: How a Ribbon of Glass Became 1,000 Times More Sensitive Than Standard Optical Fibers

Optical fibers have long been prized for their immunity to electromagnetic interference, light weight, and ability to operate in harsh environments. But in one critical respect, they have lagged behind electronic sensors: raw sensitivity. A new design from researchers at KTH Royal Institute of Technology in Stockholm and the University of Southampton now shatters that limitation.

Published in Nature Communications (doi: 10.1038/s41467-026-72416-6), the team introduces a high aspect-ratio flat fiber (HARFF) — a ribbon-like silica optical fiber that functions as an ultra-sensitive multiparameter sensor .

Pressure Sensitivity: 1,000x Higher Than Standard Fibers

The most striking result from the study is the HARFF's pressure sensitivity. In lab demonstrations, the fiber proved 1,000 times more sensitive than standard circular optical fiber designs .

Why such a dramatic improvement? The flat geometry produces orders-of-magnitude higher strain-induced birefringence under hydrostatic loading compared to conventional round fibers — a finding confirmed by finite element modeling . The rectangular cross-section also inherently mitigates the buckling and twisting that plague circular fibers under mechanical stress .

A Completely New Way to Fabricate Flat Fibers

Rather than flattening a round fiber after drawing — a common but imprecise approach — the team invented a purpose-built fabrication method. They stacked silica plates into a flat preform and used laser-based glass welding to seal the sides, creating internal microstructures such as air channels .

That preform was then drawn on a conventional fiber draw tower with controlled pressurization of the microstructures to maintain shape and wall thickness . Finally, femtosecond laser writing (λ=515 nm, 200 fs pulses) inscribed fiber Bragg gratings and rectangular waveguides directly inside the flat fiber .

This preform-based approach preserves the internal geometry with far greater fidelity than post-draw flattening, enabling reproducible fabrication of the high-aspect-ratio profile.

Beyond Pressure: Tin-Alloy Temperature Sensing and More

The HARFF platform is inherently compatible with fiber Bragg grating (FBG) sensing for strain, temperature, and pressure measurements . But the team demonstrated a particularly clever enhancement for temperature sensitivity: they filled part of the internal microstructure with a tin-based (Sn-alloy) metal. This multi-material design significantly boosted the fiber's temperature sensitivity .

Together, the pressure and temperature sensing capabilities make the HARFF a true multiparameter sensor in a single, compact glass ribbon.

Where Will This Fiber Be Used?

Because optical fibers are immune to electromagnetic interference, lightweight, and operable in harsh environments, the HARFF platform offers a versatile upgrade over both conventional round optical fibers and electronic sensors . The researchers cite application areas across six domains :

• Aerospace: Embedded into composite materials to sense internal strain and pressure in aircraft and drones.
• Energy systems: Monitoring of structural health and environmental conditions in power generation and distribution.
• Infrastructure: Continuous strain and pressure monitoring in bridges, buildings, and civil structures.
• Biomedical devices: Compact, EMI-immune sensors for medical instrumentation.
• Battery monitoring: Detecting pressure and temperature changes inside battery cells as early signs of abnormal behavior.
• Intelligent drones: Lightweight, multiplexed sensing for structural health and flight dynamics.

The Bottom Line

The HARFF represents a fundamental rethinking of what an optical fiber can be. By abandoning the circular cross-section that has defined fiber optics for decades, researchers unlocked a 1,000-fold improvement in pressure sensitivity and added multiparameter capability — all while using conventional draw towers and well-understood silica materials. The result is a sensor platform ready for real-world deployment in everything from bridges to batteries.