Pawel Maniewski, Robin Hartley, Timothy Lee, Matthew Whitaker, Bruno Moog, Martynas Beresna, Janice M. Dulieu-Barton, Christopher Holmes
6 min
Traditional optical fibers are axisymmetric, limiting their design space and making them largely insensitive to transverse loads such as hydrostatic pressure. While non-circular flat fibers offer promising sensing capabilities, existing fabrication methods like physical machining or collapsed drawing suffer from poor geometric control, twisting, and low aspect ratios. Furthermore, alternative stack-and-draw techniques have been restricted to low-durability glasses rather than robust fused silica. This paper introduces high aspect-ratio flat fibers (HARFFs), a specialty optical fiber platform featuring a quadrilateral cross-section made from fused silica. By combining laser-based processing with stack-and-draw fabrication, the authors achieve precise control over fiber geometry and microstructure, unlocking enhanced mechanical compliance and multi-parameter sensing.
The manufacturing of HARFFs involves preform preparation via a custom laser-based glass processing system followed by conventional fiber drawing. Flat silica wafers and germanium-doped core rods are cut using a focused CO2 laser, assembled into a stack, and selectively laser-welded along their edges to create airtight internal channels. This laser-based welding avoids the surface-tension-related deformations common in flame-based processing. The resulting preform is then drawn into continuous fibers exceeding 100 meters in length using a high-temperature furnace, while a pressure-feedback system maintains internal channel stability. A protective UV-curable coating is applied online to prevent surface flaws, yielding dimensionally uniform fibers that withstand high proof-test loads.
Because of their planar geometry, HARFFs exhibit unidirectional flexibility, featuring a lateral flexural rigidity comparable to standard circular fibers while supporting much wider cross-sections. When incorporating internal air channels, termed microstructured HARFFs (uHARFFs), external hydrostatic pressure induces asymmetric cross-sectional deformation. This creates an anisotropic strain field and strain-induced birefringence at the optical waveguide located at the center. Finite element modeling demonstrates that optimizing the cross-sectional geometry maximizes this birefringence sensitivity. Empirically, the uHARFF pressure sensors achieve a sensitivity resolution in the single kPa range—up to three orders of magnitude improvement over conventional circular silica fibers—along with a peak-to-valley sensitivity of 7.24 dB/MPa. Furthermore, filling the microstructure with a Sn-alloy demonstrates enhanced temperature sensing capabilities.
The development of HARFFs bridges the gap between the superior optical and chemical properties of fused silica and the design flexibility of non-circular geometries. Because these fibers are fully compatible with standard commercial single-mode fibers and can be spliced seamlessly, they offer immediate utility for real-world deployment. These capabilities open new avenues for high-performance structural health monitoring, conformal biosensing, and multi-parameter measurement in demanding environments.
A planar silica optical fiber platform, characterized by an ultra-high aspect ratio, is introduced, termed high aspect ratio flat fiber (HARFF). Non‑circular fibers offer unique opportunities for sensing, yet their mechanical behavior and fabrication have remained challenging. The HARFF is a specialty optical fiber with a quadrilateral cross-section, which is demonstrated to provide enhanced mechanical compliance and tunable microstructures that enable highly sensitive multiparameter sensing. Using laser-based processing combined with stack-and-draw fabrication, HARFFs are engineered with tailored mechanical properties and microstructure. By exploiting their flat geometry, we demonstrate pressure sensors with sensitivities up to three orders of magnitude higher than those based on conventional circular fibers. Furthermore, using a multi-material microstructure, enhanced temperature sensitivity was demonstrated using a Sn-alloy-filled HARFF design. These findings highlight the potential of HARFFs, opening new avenues for innovation in non-circular high aspect-ratio optical fiber geometries.
Alex: So the pressure physically changes the light's behavior, and you measure that change to figure out how much pressure was applied.
Sam: Precisely. And the flat geometry makes the fiber far more responsive to sideways pressure than a round fiber would be. The paper reports a pressure sensitivity roughly a thousand times greater than what you'd get from a conventional round fiber.
Alex: Does temperature interfere with those readings?
Sam: That's a key concern in any sensing system. If heat and pressure both shift the light pattern, how do you know which one you're actually measuring? The results here are encouraging — the pressure cavity showed a temperature cross-sensitivity of less than one percent. Even if the temperature changes, it barely affects the pressure reading. The two signals stay cleanly separated.
Alex: Can the same fiber also measure temperature directly?
Sam: It can, but with a different internal design. Instead of leaving the microchannel empty, the researchers filled it with a tin-based metal alloy — a metal that melts at a relatively low temperature. When heat is applied, the metal expands much more aggressively than the surrounding glass. That mismatch creates a strong internal squeeze, which produces a sharp, trackable change in how light moves through the fiber. So you're using the same basic principle — light interference — but now the signal is driven by thermal expansion rather than mechanical pressure.
Alex: So the same platform — flat fiber, hollow channels, light interference — works for both pressure and temperature, just with different internal configurations.
Sam: That's the underlying design logic. One geometry for pressure, one for temperature, both built from the same fabrication process.
Alex: What are the main limitations the paper flags?
Sam: The most significant one involves what happens at higher pressure ranges. The interference pattern the sensor reads is cyclical — it repeats itself, like a wave. If the pressure climbs beyond a certain point, the signal can wrap around and become ambiguous. You might not be able to tell whether you're reading a low pressure or a high one, because they look identical in the data.
Alex: So it's like a clock that only has an hour hand — you know it's three o'clock, but you can't tell if it's three in the morning or three in the afternoon.
Sam: That's a useful analogy. The paper acknowledges this as an open engineering challenge. Solving it would likely require either continuously tracking the signal as it evolves, or combining multiple interferometers to resolve the ambiguity.
Alex: How did the fiber hold up over time in testing?
Sam: After a ten-month ageing cycle that included repeated heating and cooling, the fiber showed consistent characteristics with no evidence of progressive degradation. That points to a structurally stable fabrication process — which matters a great deal if you're embedding these sensors inside aircraft components or battery systems that are expected to last for years.
Alex: So where does the field go from here?
Sam: The logical next step is moving from individual point sensors — which measure pressure or temperature at one specific location — to fully distributed sensing along the entire length of a fiber. That would let you monitor a whole structure continuously, not just sample it at fixed spots. Imagine being able to see the stress map of an entire wing, in real time, using a single fiber running through it.
Alex: That's a meaningful expansion of what fiber sensing could do in practice. And it all starts from a relatively simple geometric change — round to flat.
Sam: That's the core insight. The geometry unlocks the sensitivity. The rest of the engineering follows from there. Thanks for listening to ResearchPod.