Over 130 years ago, in 1888, doctors first used glass rods to illuminate inside the body. Ninety years ago, in 1930, the first imaging based on a glass rod bundle was demonstrated. Ultimately, such imaging bundles were miniaturized thanks to advances in fiber optics and have found considerable practical use in medical endoscopy and in a variety of sensor systems. When used at mid-infrared (MIR) wavelengths, such bundles can be used for thermal imaging of engines, reactors, and people, to name just a few applications. However, the MIR spectral region imposes quite a few issues for thermal imaging bundles, particularly with respect to the materials from which they can be made. They must possess good IR transparency, exhibit sufficient stability to be drawn (potentially multiple times) into suitable low loss fiber, and the individual core regions, which comprise each pixel, must be clad with a material with similar thermal properties to be co-drawn, yet with a significant index difference to promote strong image fidelity. No small (pun intended) task, but the one taken in the paper by Ventura et al., by a unique team of international scholars with backgrounds in advanced optical fiber modeling, fabrication, and measurement (University of Southampton, UK), chemistry and materials engineering (Politecnico di Milano, Italy), and applied physics, mathematics, materials science, and nanotechnology (University of Pardubice, Czech Republic). With this collective expertise, the work is extremely thorough and thoughtful—in many ways an exemplar of the thoroughness to which journal articles should be written in this day when almost anything can find a publisher regardless of quality or detail—and represents a significant advancement in mid-IR (potentially long wave IR) imaging fiber bundles. More specifically, Ventura and team combined specific IR-transparent chalcogenide glasses in the Ge-As-Se-Te material system with low index fluoropolymer claddings to create flexible coherent fiber bundles with 1200 pixels down to a pixel size of only 13 μm. Sharp images of objects at temperatures as low as 80°C were measured through a bent bundle over 1 meter in length. Inter-pixel cross-talk as low as -13.7 dB/m was measured, which is critical for image fidelity. These results are a direct consequence of the interdisciplinary nature of the team and their respective expertise. With further optimization, the team believes that both transmission losses and mode/pixel interactions could be further reduced. We look forward to "seeing" those results!
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