A low-loss, compact, and highly sensitive optical fiber curvature sensor is presented. The device consists of two identical low-loss fused fiber tapers in tandem separated by a distance L. When the optical fiber is kept straight and fixed, no interference pattern appears in the transmitted spectrum. However, when the device is bent, the symmetry of the straight taper is lost and the first taper couples light into the cladding modes. In the second taper, a fraction of the total light guided by the cladding modes will be coupled back to the fundamental mode, producing an interference pattern in the transmitted spectrum. As the fiber device is bent, visibility of the interference fringes grows, reaching values close to 1. The dynamic range of the device can be tailored by the proper selection of taper diameter and separation between tapers. The effects of temperature and refractive index of the external medium on the response of the curvature sensor is also discussed.

We report on a functional optical microfiber mode interferometer and its applications for absolute, temperature-insensitive refractive index sensing. A standard optical fiber was tapered down to 10 μm. The central part of the taper, i.e., the microfiber, is connected to the untapered regions with two identical abrupt transitions. The transmission spectrum of our device exhibited a sinusoidal pattern due to the beating between modes. In our interferometer the period of the pattern—an absolute parameter—depends strongly on the surrounding refractive index but it is insensitive to temperature changes. The period, hence the external index, can be accurately measured by taking the fast Fourier transform (FFT) of the detected interference pattern. The measuring refractive index range of the device here proposed goes from 1.33 to 1.428 and the maximum resolution is on the order of $3.7\times {10}^{-6}$.

An in-fiber Mach-Zehnder Interferometer (MZI), based on hollow core fiber (HCF), for measuring curvature is fabricated and experimentally demonstrated. The sensing part was fabricated by splicing a section of HCF between two sections of multimode fiber (MMF), and different HCF lengths were investigated in order to achieve the highest curvature sensitivity. These devices were tested in a transmission configuration using lead-in and lead-out single mode fibers (SMF). The sensor was attached to a steel sheet by using the polymer Polydimethylsiloxane (PDMS) for accurate control of the applied curvature. The modal analysis was carried out using a commercial software based on finite element method (FEM) and, by incorporating some of the experimental data, it was feasible to determine the two dominant modes that interfere in this sensor. The devices were characterized by measuring the fringe contrast variations due to the curvature changes. The interferometer fabricated with a HCF 2.5 mm in length HCF showed the highest curvature sensitivity, -17.28 ${\pm}$ 2.30 dB/m $^{-1}$ , in a range from 1.84 m $^{-1}$ to 2.94 m $^{-1}$ . Moreover, the sensor that exhibited a better performance was fabricated with a HCF length of 1 mm, combining the most extensive curvature range (from 0.95 m $^{-1}$ to 2.68 m $^{-1}$ ) and an adequate sensitivity (-11.80 ${\pm}$ 1.30 dB/m $^{-1}$ ). The analysis of the interferometric signal of this device in Fourier domain, allows us to establish a one to one relationship between the contrast and the curvature in a broader range (from 0 m $^{-1}$ to 2.94 m $^{-1}$ ). Moreover, the fringe contrast showed a very low dependency on temperature (from 30°C to 90 °C), showing that this device was not affected by temperate fluctuation.

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.

A simple to fabricate, cost-effective, robust, and highly accurate fiber optic temperature sensor is proposed and experimentally demonstrated. The sensor head consists of an in-fiber Fabry-Perot interferometer (FFPI) embedded in a mold made of a polyester resin. FFPI, which in general present good strain sensitivity and low temperature sensitivity, was assembled by splicing a microsegment of capillary fiber (CF) of 14.66 μm between two single-mode fibers (SMFs). The reflected spectrum of the FFPI exhibited a single interference dip within a span of 80 nm. The dimensions of the resin mold can be modified by changing the ambient temperature, which in turn changes the length of the FFPI air-cavity. The dip of the reflected spectrum is shifted towards shorter (larger) wavelengths when external temperature is decreased (increased). The large free spectral range (FSR) allowed to monitor the temperature changes by either a wavelength or an intensity interrogation scheme. Using a laser and an optical power meter it was possible to demonstrate a real-time temperature sensor with a sensitivity of -60.79 nW/°C and a resolution of 0.5 °C, in the range of 10-30 °C. We believe that the practicality of this real-time sensor can be very attractive for environmental sensing application.

We demonstrate the capability of an air cavity Fabry-Perot interferometer (FPI), built with a tapered lead-in fiber tip, to measure three parameters simultaneously, distance, group refractive index and thickness of transparent samples introduced in the cavity. Tapering the lead-in fiber enhances the light coupling back efficiency, therefore is possible to enlarge the air cavity without a significant deterioration of the fringe visibility. Fourier transformation, used to analyze the reflected optical spectrum of our FPI, simplify the calculus to determine the position, thickness and refractive index. Samples made of 7 different glasses; fused silica, BK7, BalF5, SF2, BaF51, SF15, and glass slides were used to test our FPI. Each sample was measured nine times and the results for position, thickness and refractive index showed differences of ± 0.7%, ± 0.1%, and ± 0.16% respectively. The evolution of thickness and refractive index of a block of polydimethylsiloxane (PDMS) elastomer due to temperature changes in the range of 25°C to 90°C were also measured. The coefficients of the thermal expansion and thermo-optic estimated were α = 4.71x10⁻⁴/°C and dn/dT = −4.66 x10⁻⁴ RIU/°C, respectively.