Linearity-Enhanced Refractometric Sensor Utilizing Ultra-High Numerical Aperture Fiber Combined With a Plastic Prism

Compact optical sensors are deemed to be of significant importance in industrial applications owing to their high performance and cost-effectiveness. In this study, a linearity-enhanced refractometric sensor was developed by employing an ultra-high numerical aperture (UHNA) fiber tethered to a plastic prism. The proposed refractometer can appropriately alter the intensity profile of incident light over the sensing area associated with the prism by adopting UHNA fibers with different mode profiles as the lead-in fiber. This provides an adjustable sensing regime that yields a linear response with a coefficient of determination (R
²
) over 0.99. The fabricated sensor achieved a sensitivity of 13.6 per refractive index unit (RIU) and a high resolution of 5.80 × 10
^-5
RIU for refractive indices ranging from 1.3166 to 1.3826 at λ = 1550 nm. Further, the proposed sensor was successfully applied to inspect the refractive index of a sample of coolant used in metalworking equipment, indicating that the proposed refractometric sensor can facilitate real-time monitoring of industrial production processes.