Refractive index sensor based on the natural roughness of a directly fabricated D-shape fiber for biological and environmental monitoring purposes

• Refractive index sensing of analytes, using D-shape fiber without any modification. • Novel type of self-made D-shape fiber with uniform cross section along its length. • Transmitted intensity based sensing of liquids exposing 4 cm sensing region. • High sensitivity due to few mode regim affected by scattering at the flat surface. • Water invariant transmission enabled the concentration sensing of H2O-IPA mixture. We present a novel fiber optics sensor approach using an intentionally designed silica fiber that requires no additional processing after fabrication. The fiber, drawn from a D-shaped preform, maintains a D-shape cross-section along its entire length, ensuring high uniformity of geometrical parameters such as core diameter and the flattened surface distance from the core. The primary advantage of our approach is its unique ability to sense analytes at arbitrary distances from the fiber end and at any interaction length using the evanescent wave effect. We demonstrate the sensing performance by exposing a short fiber section (4 cm) to a liquid analyte in its pure form without using any signal-enhancing interlayer. Importantly, the output signal remains unchanged when water is applied as the medium, while exposure to an alcoholic medium significantly increases transmission. We further analyze the refractive index sensitivity of the technique by varying the water-isopropyl alcohol mixture ratio at a 1030 nm wavelength in a few-mode propagation regime. The results, interpreted in terms of the elimination of Rayleigh scattering losses of the higher-order modes by index matching between the fiber and the sensing medium, underscore the potential of our approach for water contamination sensing in both biological and environmental applications with distributed sensing capability, thereby addressing a critical need in the field.