Lab: flat fiber reaches up to 1,000× the pressure sensitivity of a comparable circular fiber

Inside a pressure vessel, a ribbon of glass was tested up to 0.40 MPa. The flat optical fiber produced up to 1,000 times greater pressure sensitivity than a comparable circular design, according to researchers at Sweden’s KTH Royal Institute of Technology and the UK’s University of Southampton.

Optical fiber is not only a glass thread for carrying data. Stretching, pressure, or heat can alter its physical and optical properties, and engineers can infer the force by tracking changes in light. Conventional cylindrical fiber is particularly useful for forces along its length, but responds less strongly to transverse forces and hydrostatic pressure around it. That limits the sensitivity available where pressure is the quantity to measure.

The team built its High Aspect Ratio Flat Fiber, or HARFF, as a flat structure from the preform stage rather than flattening a finished round fiber. The resulting geometry can approach 20:1 in aspect ratio. Two elongated air channels make the structure deliberately asymmetric: pressure deforms the ribbon, changes its birefringence—the difference in how two light polarizations travel through the glass—and shifts the interference pattern between two identical fiber Bragg gratings.

The pressure sensor showed a peak-to-valley response of 7.24 dB/MPa and, with another analysis method, sensitivity of up to 31.6 radians/MPa. Temperature cross-sensitivity was less than 1% of the pressure sensitivity. The same platform also sensed heat after one channel was filled with a tin-based alloy: silica’s thermal expansion coefficient is roughly 0.5 × 10⁻⁶/K, versus around 23 × 10⁻⁶/K for the alloy, so heating creates stress in the surrounding glass.

So what changes, concretely? Engineers gain a tunable sensing platform in which width, thickness, wall thickness, internal channels, waveguide position, and channel-filling materials can be adjusted to concentrate stress where it produces the strongest optical signal. More than 120 m was drawn from a single experimental preform, demonstrating that the unusual geometry can be manufactured. Pawel Maniewski, a KTH researcher, called it “a new design space for optical fiber.” For now, the evidence is a laboratory proof of concept, not a sensor demonstrated in deployed conditions.