Fiber-Optic Refractometer Based on a Reflective Aspheric Prism Rendering Adjustable Sensitivity

This study proposes a miniaturized refractometer with adjustable sensitivity that incorporates a reflective prism linked to a pair of precision-aligned fibers. An incident beam emanating from a single-mode fiber (SMF) undergoes Fresnel reflection at the interface between a slanted sensing facet of the prism and the aqueous solution under test and is properly steered via an aspheric mirror facet of the prism toward a multi-mode fiber (MMF). The refractive index (RI) of the solution can be accurately measured by observing the optical power. The sensitivity and sensing range with respect to the RI may be flexibly adjusted by tailoring the wedge angle of the prism. The proposed refractometer has been designed through rigorous simulations then constructed by integrating an appropriate prism in polycarbonate to both an SMF and MMF imbedded in a silicon chip. The embodied refractometric sensor was practically applied to inspect an aqueous sucrose solution, whose RI was varied from 1.3166 to 1.4029 for λ = 1550 nm. As for the major achieved performance, the sensitivity ranged up to 54.6 per refractive index unit (RIU) while the resolution was as small as $9.95\times 10^{\hbox{--} 6}$ RIU. The sucrose solution could be efficiently in situ monitored to provide a minimum detectable RI change of ~0.0001.