Researchers at Sweden’s KTH Royal Institute of Technology and the UK’s University of Southampton have developed HARFF, a flat ribbon-shaped optical fiber whose full name is High Aspect Ratio Flat Fiber. It responds to pressure 1,000 times more sensitively than standard round fiber. The team has already pulled more than 120 meters of it from a single preform, proving the design can scale.
The trick is in the shape. Instead of flattening a finished round fiber, engineers build the preform as a flat ribbon from the start, with an aspect ratio of up to 20:1. That makes the internal channels, air cavities, and waveguide position independent variables. Engineers can now decide exactly where the glass bends and where it stays rigid during the draw.
For the pressure sensor, the team added two elongated air channels inside the ribbon. External force deforms the channels unevenly, creating asymmetric stress in the glass. Light then refracts differently depending on its polarization direction. Two matching fiber Bragg gratings built into the ribbon form an optical cavity, and its interference pattern shifts proportionally to the applied pressure.
In tests at 0.40 MPa, the fiber reached a sensitivity of 31.6 radians per megapascal — three orders of magnitude higher than an equivalent round fiber. Temperature barely interferes with the readings: cross-sensitivity stays under 1% of the pressure signal.
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To measure temperature, engineers filled HARFF’s internal channel with a tin alloy whose thermal expansion coefficient is 46 times higher than the silica the fiber is made of. As the alloy heats up, it expands far more than the surrounding glass and presses on it from inside. That creates the same kind of mechanical stress — and the same shift in optical properties — as the external pressure in the sensor design above.
“This isn’t just a differently shaped fiber — it’s a new design space for optical fibers,” said KTH researcher Pavlo Manevskyi.
For now, HARFF remains a lab prototype, with optical losses of about 0.16 dB/m — still higher than commercial fiber. The researchers want to improve control over the fiber’s shape during drawing and cut those losses further. Longer term, they see such fiber sensors embedded in drone wings, bridges, industrial structures, and batteries.
