Redesigning Fiber Optics From the Inside Out

As the backbone of the internet, optical fiber is already responsible for carrying huge amounts of data around the world. But now, researchers believe that these fibers could double as extremely precise sensors by changing their shape. A team from KTH Royal Institute of Technology and the University of Southampton has developed a new ribbon-like optical fiber that can detect pressure with up to 1,000 times greater sensitivity than conventional circular fibers.

Called high aspect ratio flat fiber (HARFF), the new design replaces the familiar round cross-section with a thin, quadrilateral shape. That geometry isn't just for looks. It changes how the glass bends and deforms, giving researchers another way to tune the fiber for specific sensing applications.

Flat optical fibers aren't entirely new, but previous approaches typically started with round preforms that were machined or collapsed into flatter shapes. Those techniques can limit aspect ratios and make it difficult to precisely control the fiber's final geometry and internal structure.

The new technique instead uses a stack-and-draw fabrication process combined with laser processing. This lets the researchers build internal features, such as air channels, into the fiber while controlling their size and location. HARFF can be manufactured in lengths exceeding 100 meters, with dimensions measured in micrometers for thickness and millimeters for width.

That unusual shape gives the fiber very different mechanical properties depending on the direction in which it is bent. The researchers can also adjust the internal channels to control how the fiber deforms under pressure.

To test their approach, the team produced an 80-micrometer-thick, 655-micrometer-wide microstructured HARFF pressure sensor containing a laser-written Bragg grating-based Fabry-Perot cavity. The resulting sensor achieved pressure resolution on the order of single-digit kilopascals using standard optical interrogation equipment. That's an improvement of up to three orders of magnitude compared with similar sensors made using commercial silica fiber.

The design also performed well when temperature changed. Temperature contributed less than 1% of the pressure response, which could be particularly useful when sensors need to distinguish pressure changes from changing environmental conditions.

One potential application for the technology is embedding HARFF sensors into composite structures, including aircraft and infrastructure, to monitor internal pressure and strain. Their flat geometry could also make them useful inside batteries, where changes caused by electrode expansion, gas formation, or thermal events can provide important information about a cell's condition.

Because the fibers are compatible with commercial optical fiber systems, adopting the technology may not require replacing the entire sensing infrastructure around them. Future work will investigate structural health monitoring and distributed sensing, along with potential applications in microfluidics, planar photonics, high-power laser delivery, and other areas where the round optical fiber may no longer be the best shape for the job.