An ultracompact, cost-effective, and highly accurate fiber optic temperature sensor is proposed and demonstrated. The sensing head consists of Fabry-Perot microcavity formed by an internal mirror made of a thin titanium dioxide (TiO₂) film and a microscopic segment of single-mode fiber covered with Poly(dimethylsiloxane) (PDMS). Due to the high thermo-optic coefficient of PDMS the reflectance of the fiber-PDMS interface varies strongly with temperature which in turn modifies the amplitude of the interference pattern. To quantify the changes of the latter we monitored the visibility of the interference pattern and analyzed it by means of the fast Fourier transform. Our sensor exhibits linear response, high sensitivity, and response time of 14 seconds. We believe that the microscopic dimensions along with the performance of the sensor here presented makes it appealing for sensing temperature in PDMS microfluidic circuits or in biological applications.

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.

A simple-to-fabricate and highly sensitive optical fiber temperature sensor is presented. The device consists of an optical microfiber mode interferometer embedded in a polymer with high thermo-optic coefficient which is also used as a packaging material. Temperature modifies the refractive index of the polymer, hence the phase difference between the interfering modes, which results in a prominent wavelength shift of the interference pattern. The sensor was tested in the 20 °C-48 °C range. The temperature sensitivity was found to be 3101.8 pm/°C. The simplicity and the performance of our device make it attractive for several applications.

A compact, simple-to-fabricate, low-cost, and highly sensitive optical fiber temperature sensor based on surface plasmon resonance (SPR) is reported. The sensor consists of a core mismatch fiber structure fabricated by splicing a small piece of single-mode fiber (SMF) between two multimode fibers (MMF). SPR is generated when evanescent field interacts with the gold layer deposited over the SMF cladding. Then, the sensor was embedded in polydimethylsiloxane (PDMS), which acts as a temperature to refractive index transducer. Due to PDMS high thermooptic coefficient, the SPR dip underwent a noticeable wavelength shift when a variation of temperature occurred. The device was tested in the 20-60 °C range showing a linear response and a sensitivity of 2.60 nm/°C. This sensor is appealing for temperature monitoring in microfluidic devices made of PDMS due to its high performance and simply fabrication process.

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.