Flat, Ribbon-Shaped Optical Fibers Outsense Round Ones by Up to 1,000 Times

Researchers at Sweden's KTH Royal Institute of Technology and the UK's University of Southampton have built a flat, ribbon-shaped silica optical fiber that responds far more strongly to pressure and temperature than the round fibers used in conventional sensors. The new platform, called High Aspect Ratio Flat Fiber (HARFF), achieved up to 1,000 times greater pressure sensitivity in one comparison, and the team has already drawn more than 120 meters (394 feet) of it from a single experimental preform.

Optical fiber sensors work by tracking how light passing through the glass changes when the fiber is stretched, pressed or heated. Conventional fiber is cylindrical, a shape that responds well to forces along its length but poorly to pressure from the sides. The researchers built HARFF as a flat structure from the start, with an aspect ratio approaching 20:1, so its width, thickness and air channels can be tuned to control how it flexes.

For the pressure sensor, two elongated air channels running through the fiber cause it to deform asymmetrically under pressure, shifting a light property called birefringence that the team tracked using two fiber Bragg gratings written into the glass. Tested inside a pressure vessel up to 0.40 megapascals, the sensor reached a pressure sensitivity of up to 31.6 radians per megapascal — up to three orders of magnitude higher than a comparable round fiber with an elliptical core, the researchers reported.

A second version, with one internal channel filled with a tin-based alloy — whose thermal expansion coefficient of about 23 × 10⁻⁶ per kelvin is far higher than silica's roughly 0.5 × 10⁻⁶ per kelvin — turned the same platform into a temperature sensor, with temperature cross-sensitivity below 1% of the pressure reading. "This is not just a different looking fiber, it is a new design space for optical fiber," said KTH researcher Pawel Maniewski. "By changing the geometry, we can make the glass itself much more responsive to the physical world around it."