High aspect-ratio quadrilateral flexible silica optical fibers for enhanced multi-parameter sensing.

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.