SPR Fiber Tip Sensor for the Simultaneous Measurement of Refractive Index, Temperature, and Level of a Liquid

This paper provides the design, fabrication, and experimental demonstration of a novel, compact, and simple-to-construct optical fiber-tip sensor for the simultaneous measurement of the refractive index (RI), temperature, and level of liquid samples by means of surface plasmon resonance (SPR) phenomenon. The proposed fiber-tip SPR sensor consists of a multimode fiber spliced to a sensing element, it comprises a gold-coated single-mode fiber tip, whose end face is coated with a metallic mirror. By covering part of the sensing element with a thermo-responsive material, whose thermo-optic coefficient is high, it is possible to measure temperature; whereas the metal-bare section is used to measure the changes in the RI and level of a liquid sample. As the RI and temperature of the liquid sample increase, the position of the resonance wavelengths ({\boldsymbol {\lambda }_{\boldsymbol {bare}}} and {\boldsymbol {\lambda }_{\boldsymbol {Covered}}}) shifts toward larger and shorter wavelengths, respectively. On the other hand, as the level of the sensed sample increases, the deepness of {\boldsymbol {\lambda }_{\boldsymbol {bare}}} increases too, but the position of {\boldsymbol {\lambda }_{\boldsymbol {bare}}} and {\boldsymbol {\lambda }_{\boldsymbol {Covered}}} remains constant for a particular liquid and temperature. We expect that this novel sensor becoming a promising candidate for biomedical, food, and/or industrial applications.