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 (MMF) spliced to a sensing element, it comprises a gold-coated single-mode fiber (SMF) 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 (TOC) 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 ( $\lambda_bare$ and $\lambda_Covered$ ) shifts towards larger and shorter wavelengths, respectively. On the other hand, as the level of the sensed sample increases, the deepness of $\lambda_bare$ increases too, but the position of $\lambda_bare$ and $\lambda_Covered$ remains constant for a particular liquid and temperature. We expect that this novel sensor becoming a promising candidate for biomedical, food, and industrial applications.

Temperature measuring is a daily procedure carried out worldwide in practically all environments of human activity, but it takes particular relevance in industrial, scientific, medical, and food processing and production areas. The characteristics and performance of the temperature sensors required for such a large universe of applications have opened the opportunity for a comprehensive range of technologies and architectures capable of fulfilling the sensitivity, resolution, dynamic range, and response time demanded. In this work, a highly sensitive fiber optic temperature sensor based on a double-cavity Fabry-Perot interferometer (DCFPI) is proposed and demonstrated. Taking advantage of the Vernier effect, we demonstrate that it is possible to improve the temperature sensitivity exhibited by the polymer-capped fiber Fabry-Perot interferometer (PCFPI) up to 39.8 nm/°C. The DCFPI is sturdy, reconfigured, and simple to fabricate, consisting of a semi-spherical polymer cap added to the surface of the ferrule of a commercial single-mode fiber connector (SMF FC/PC) placed in front of a mirror at a proper distance. The length of the air cavity (L_air) was adjusted to equal the thickness of the polymer cap (L_pol) plus a distance δ to generate the most convenient Vernier effect spectrum. The DCFPI was packaged in a machined, movable mount that allows the adjustment of the air cavity length easily but also protects the polymer cap and simplifies the manipulation of the sensor head.

Attaching sensitive materials to the tip of an optical fiber has been a successful strategy, extensively used to develop selective and highly sensitive sensors. In this work, we demonstrate tha,t it is possible to enlarge up to 9 times the displacement dynamic range of an extrinsic fiber Fabry-Perot interferometer (EFPI) by adding a semi-spherical polymer cap to the end-face of a single-mode fiber (SMF) FC/PC connector. Using a single modified EFPI and taking advantage of the Vernier effect displacement measurements with subnanometer resolution, it is demonstrated.

A contact stylus profilometer based on an intensity-modulated optical fiber curvature sensor is proposed and demonstrated. The sensor head consists of a multimode-coreless-multimode (MCM) fiber structure, very sensitive curvature, clamped to a metal beam. The 3D printing stylus was glued to the center of the metal beam. The displacement of the stylus, during the surface scan of the 3D printing pieces, bends the metal beam, which in turn, induces a change in the curvature of the MCM producing a decrement of the losses of the characteristic attenuation notch. The profile generated with this simple-to-implement optical fiber sensor was compared with that obtained with a commercial stylus profilometer, observing a very good coincidence between both profiles.

Graphene-based materials are capable of enhancing the refractometric response of prism- and optical fiber-based surface plasmon resonance (SPR) sensors; however, complicated multistep and time-consuming attaching processes could limit their practical applications. Herein, for the first time, we demonstrate the immobilization of graphene oxide (GO) submicrometric sheets onto the surface of a gold-coated single-mode fiber using a coating of fungal self-assembling proteins, the hydrophobins (HFBs), as an adhesive nanolayer. Hetero-core fiber tip SPR structures used in this study, consisting of a mirrored multimode–single-mode fiber structure coated with different thin layers (a chromium layer of 3 nm and a gold layer of 30 nm on top) exhibited a refractive index sensitivity (S_RI) of 1842 nm RIU⁻¹ (RIU: refractive index unit) at a refractive index (RI) of 1.36. Self-assembly of GO over the SPR fiber tip via HFB, offered an enhancement of up to 20% in the S_RI. Moreover, this HFB-GO coating prevented degradation of the Al thin film mirror caused by corrosive salt-water solutions. The process is very simple, harmless, rapid (around 15 min) and scalable, as it is mostly based on one plasma treatment, which can be performed in large chambers and two dip coating steps, in liquid baths. All these features make the use of self-assembled bio/non-bio hybrid coating a green industrial method to improve the performance of SPR fiber biosensors, if compared with traditional chemical methods. Materials applied in this technology, fungal proteins and derivatives of graphite, are sustainable and largely available.

This study proposes the development of a dual-wavelength optical fiber sensor (DWOFS) that integrates two optical fiber structures in a multimode transmission line to measure the refractive index and temperature of a liquid concurrently. One structure is based on a refractive index sensor that utilizes surface plasmon resonance, comprising a 5 mm long single-mode fiber (SMF) section coated with chromium/gold (Cr/Au) films. The secondary structure employs a multimode interferometer with a 29 mm long no-core fiber (NCF) section covered with a thick layer of polydimethylsiloxane (PDMS) to measure temperature. The measurements obtained reveal two distinct drops in the transmission spectrum at approximately 600 nm and 1550 nm, respectively, enabling precise measurement of the two parameters. The sensor demonstrates a high degree of sensitivity to both refractive index and temperature, spanning the visible ($2770.30\mathrm{n}\mathrm{m}/\mathrm{R}\mathrm{I}\mathrm{U}$) and infrared $(-0.178\mathrm{n}\mathrm{m}/°\mathrm{C}$) regions of the spectra, respectively. Furthermore, the thermo-optical coefficient for water $({-0.9928\times 10}^{-4}\mathrm{R}\mathrm{I}\mathrm{U}/°\mathrm{C})$ was estimated. The proposed sensor offers a compact solution for the simultaneous measurement of refractive index and temperature in liquid samples for a variety of applications, including biological, environmental, and healthcare research.