Flat optical fiber shows superior sensing capabilities

Scientists have been presenting evidence that the Earth is round for well over 2,000 years. Yet flat-Earthers remain unconvinced, with some willing to go to the end of the Earth to prove their point. They just haven't been able to find them. Pun very much intended. Earth aside, scientists have just developed a technology where flat is a clear winner over round: optical fiber sensors.

Researchers at Sweden's KTH Royal Institute of Technology and the UK's University of Southampton have developed a flat ribbon-like silica optical fiber that can be engineered to respond far more strongly to physical forces than conventional cylindrical fibers, making it a far better sensor. Dubbed High Aspect Ratio Flat Fiber (HARFF), the new platform achieved up to 1,000 times greater pressure sensitivity in one comparison, while another version filled with metal demonstrated enhanced temperature sensing. The team has already drawn more than 120 m (394 ft) of the unusual fiber from a single experimental preform, demonstrating its manufacturability.

Optical fiber is best known as the glass thread carrying data around the planet as pulses of light. However, it is also used as a remarkably capable sensor. External forces such as stretching, pressure, or temperature can slightly alter the glass's physical and optical properties, making these forces measurable by tracking changes in the light passing through the optical fiber.

A good, albeit crude analogy is a garden hose with water flowing through it. When you squeeze a cross section of the hose, water flows out more strongly. Engineers can figure exactly how much pressure you applied by measuring that change in flow. The same principle applies to optical fiber sensing, just with much greater sensitivity. By sending light through the fiber and measuring how that light changes, engineers can work backward to determine what happened to the fiber.

One common approach uses fiber Bragg gratings, microscopic periodic structures written into the fiber that reflect particular wavelengths of light. When the fiber stretches or compresses, the reflected wavelength changes, effectively giving engineers an optical ruler for strain. Conventional silica fiber is usually cylindrical, a geometry that works particularly well for measuring forces acting along the fiber. However, a solid glass cylinder is much less responsive to transverse forces and hydrostatic pressure acting around it. This significantly limits its sensitivity, the very characteristic needed in a sensing technology.

To address this issue, the researchers changed the glass shape. Instead of starting with a round fiber and trying to squash it flat, the team builds HARFF as a flat structure from the preform stage, producing solid structures with aspect ratios approaching 20:1. The resulting flat ribbon geometry completely changes how the fiber responds to forces. Think of the difference between trying to bend a round rod and a thin ruler. The flatter geometry makes the ruler much more responsive to the bending forces.

With HARFF, engineers can tune the fiber's width, thickness, wall thickness, and internal channels separately, effectively designing where the glass will flex and where stresses will concentrate.

For the pressure sensor, the team created two elongated air channels running through the fiber. When pressure is applied, this deliberately asymmetric structure deforms to produce different amounts of strain in different directions around the optical waveguide. This changes a property called birefringence, where two perpendicular polarizations of light traveling through the same glass experience slightly different refractive indices.

The researchers then wrote two identical fiber Bragg gratings into the waveguide, forming a tiny optical cavity between them. Light traveling through this cavity produces a characteristic interference pattern. When pressure deforms the glass, its birefringence changes, which measurably shifts that interference pattern.

In simple English: pressure deforms the ribbon, deformation changes the optical properties of the glass, and the changing light tells you how much pressure was applied.

The proof-of-concept sensor was placed inside a pressure vessel and tested up to 0.40 MPa. The researchers measured a peak-to-valley response of 7.24 dB/MPa and, using another analysis method, a pressure sensitivity of up to 31.6 radians/MPa. Temperature cross-sensitivity was less than 1% of the pressure sensitivity, helping the researchers distinguish pressure changes from temperature changes.

The researchers report up to three orders of magnitude higher sensitivity than a comparable circular-cross-section optical fiber containing an elliptical core. They also claim up to a two-order-of-magnitude improvement over other silica fiber designs that had themselves been optimized for hydrostatic pressure sensing.

In addition to pressure, the team demonstrated that the same basic glass platform could also be made sensitive to temperature. For its temperature sensor, the researchers filled one of HARFF's internal channels with a tin-based alloy. Silica has a thermal expansion coefficient of roughly 0.5 × 10⁻⁶/K, while the alloy sits around 23 × 10⁻⁶/K. Heating the pair causes the metal to try to expand far more than the surrounding glass will allow. That mismatch generates stress in the silica, altering its birefringence and, once again, giving the researchers an optical signal they can measure.

Beyond creating much more capable sensors, the researchers have turned the fiber's geometry and internal architecture into another engineering variable.

“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.”

By changing the width, thickness, air channels, waveguide positions, or materials filling those channels, the same basic manufacturing platform could potentially be optimized for different kinds of sensing. A fiber embedded through a composite aircraft wing, for example, could monitor loads and strain without requiring a web of conventional electronic sensors. Similar fibers could find their way into drones, bridges, industrial structures and batteries, where internal temperature or pressure changes may provide early warning that something is going wrong. Optical fibers also have the advantages of being lightweight, compact and immune to electromagnetic interference.

Naturally, a sizable gap remains between an impressive laboratory fiber and an aircraft developing a glass nervous system. The current HARFF remains a proof of concept. Its measured optical propagation loss was around 0.16 dB/m, which the researchers attribute primarily to the experimental core geometry and the silica used rather than an inherent limitation of flat fiber. Splicing to conventional fiber also produced losses typically below 3 dB, leaving plenty of room for optimization.

The researchers now want to improve shape control, reduce optical losses, and take HARFF beyond proof-of-concept sensors into actual systems. KTH points to intelligent drones, advanced composites and safer energy technologies as some of the targets. As a reminder, their work is about fiber-optic sensing, not telecom data transmission, where conventional round fiber remains the standard.

The study was published in Nature Communications.

Source: KTH