High-aspect-ratio flat fiber for optofluidic lab-in-a-fiber sensing and bend-tunable refractive index enhancement

A lab-in-a-fiber sensor platform is demonstrated, enabling simultaneous optical analysis and controlled microfluidic transport in a single monolithic device. It consists of a high-aspect-ratio flat fiber (HARFF) that integrates a mechanically compliant elliptical fused silica capillary and a Ge-doped single-mode waveguide. The waveguide, partially exposed to the capillary, provides evanescent field interaction with analytes introduced into the central microfluidic channel. The high-aspect-ratio microfluidic channel confines side wall effects, producing a flattened laminar flow that supports stable real-time imaging. Alongside particle-flow monitoring, the device exhibits a tunable refractive-index sensitivity. Through in-plane flexure, a more than three-fold sensitivity enhancement was demonstrated at a 50 mm bending radius over a straight fiber. This hybrid fiber sensor design represents cost-effective production of a compact and versatile platform for lab-in-fiber applications, combining optical interrogation and microfluidic functionality.