Microstructured optical fibers are a more complex analog to conventional core/clad optical fibers, still governed by total internal reflection, but possessing additional regions of higher or lower refractive index that typically run parallel to the central core longitudinally down the fiber. These engineered index regions, which normally are periodic in cross-section about the fiber, help control the electromagnetic field of the propagating mode and, therefore, can tailor the fiber’s mode-field area, dispersion, nonlinearities, and other performance-related attributes.
In this work by Kostecki, et al., the microstructure of the fiber is an “exposed-core” in which the central light-guiding core is directly accessible to the outside world. As such, they are especially useful for sensing where gasses or liquids can come into contract with the core and interact in a measurable way with the propagating light. While such structures have previously been made using soft glasses, often made by an extrusion method, these fibers are fabricated from pure silica, the mainstay of telecomm and laser/amplifier fibers. And to paraphrase a colleague: “if you can do it in silica; do it in silica…” these words ring true given silica’s high strength, very low loss, industrial acceptance, and market incumbency.
However, as fiber fabricators know, such fibers also fall into that unenviable category of “easier said than done.” Keeping a periodic array of holes open in photonic crystal fibers during the draw can be tricky. Keeping the holes intact when one side of the preform is missing and the resultant regions between the holes (called ‘webs’ or ‘struts’) is only about 1 micrometer in diameter can be even trickier. One added insight into this dynamism during the draw can be seen in Figures 1(a) and 1(b). Slicing out a section of the glass periphery, as shown in Figure 1(a) is not unreasonable as long as the preform is sufficiently large in order to withstand the stresses of machining (note that the width of the removed slide is less than the diameter of the hole neighboring the core). The fact that this gap is now much wider in Figure 1(b), a testament to surface tension, provides a greater sense for how fiber geometry can chance during the draw adding a degree of complexity and variability that needs to be understood and accounted for in the draw process.
As with any new work, improvements are needed and will come with continued progress. In this case, contaminants buildup on the exposed surface and micro-fractures deteriorate the fiber’s transmission. However, the transition of this fiber-based sensing technology from exotic glasses to silica is an important one and one that could mark the greater potential for commercial implementation.
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