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This work presents the implementation of a thermo-refractometer, which integrates the measurement of both refractive index and temperature in a single optical fiber structure. To this purpose, a lossy mode resonance (LMR)-based refractometer is obtained by means of the deposition of a titanium dioxide (TiO2) thin film onto a side-polished (D-shaped) single mode fiber. Measurement and subsequent temperature compensation are achieved by means of a fiber Bragg grating (FBG) inscribed in the core of the D-shaped region. The LMR wavelength shift is monitored in transmission while the FBG (FBG peak at 1533 nm) displacement is observed in reflection. The LMR is sensitive to both the surrounding refractive index (SRI), with a sensitivity of 3725.2 nm/RIU in the 1.3324-1.3479 range, and the temperature (- 0.186 nm/°C); while the FBG is only affected by the temperature (32.6 pm/°C in the 25°C - 45°C range). With these values, it is possible to recover the SRI and temperature variations from the wavelength shifts of the LMR and the FBG, constituting a thermo-refractometer, where it is suppressed the effect of the temperature over the refractometer operation, which could cause errors in the fourth or even third decimal of the measured SRI value. Ministerio de Ciencia, Innovación y Universidades (FPU18/03087, PID2019-106231RB-I00 TEC); Universidade Tecnológica Federal do Paraná; Conselho Nacional de Desenvolvimento Científico e Tecnológico.

The mode transition phenomenon is experimentally demonstrated in tilted fiber Bragg gratings (TFBG) through the deposition of an indium tin oxide (ITO) thin film employing a DC sputtering machine.

The temperature dependence of core mode resonance has been thoroughly studied in fiber Bragg gratings (FBGs), but it is not the case for cladding mode resonances in multi-resonance gratings such as tilted FBGs (TFBGs). In this work, the temperature sensitivity of ultraviolet written TFBGs in SMF-28 fibers is assessed, demonstrating in the first, to the best of our knowledge, place that a single gauge factor K T = 6.25 ⋅ 10 − 6 ± 0.02 ⋅ 10 − 6 ∘ C − 1 can be employed to characterize the response to temperature of the resonances over the full spectrum in the 10°–50°C range. Then, a simulation model is obtained, enabling to predict TFBG spectra in the 10°–50°C range with high accuracy. This requires a calibration of the core index and dispersion of the TFBG measured in air at 25°C, and determination of the glass refractive index thermo-optic coefficient ( d n / d T = 8.46 ⋅ 10 − 6 ± 0.1 ⋅ 10 − 6 ∘ C − 1 , common to both core and cladding glasses), leading to a mean error on the wavelength position of resonances between 1 and 3 pm. This mean error can be further reduced (less than 1 pm) by considering a linear dependence with temperature of d n / d T . Therefore, this model will enable to completely remove the temperature-induced shifts of all resonances in TFBG sensing applications and measure with great accuracy the variables of interest by using the scaled averages of groups of resonances instead of (less accurate) individual shifts.

7 pages, 6 figures, 2 tables., We describe a new gold-silver complex based on 2,2-bipyridine, whose formula is {Au2Ag2(C6F5)4[(C5H4N)-(C5H4N)]2}n, used to detect volatile organic compounds (VOCs) such as ethanol, methanol, and acetic acid. This organometallic material is presented in the form of bright yellow powder, and suffers a change in its optical properties when it is exposed to VOCs. A new fiber optic sensor is presented based on the properties of a new vapochromic material. The sensor works in a reflection configuration and consists of an optic fiber pigtail (core diameter is 200 µm and cladding is 230 µm) on the cleaved end of which the vapochromic material, previously mixed with a commercially available solvent, Liquicoat®, is deposited by using the dip-coating technique. Using an optical source and a photodetector, it is possible to detect and quantify the change in reflected intensity-modulated signal when the sensor is exposed to VOC inside a sealed chamber. This behavior can be related to the VOC concentration. The study of the sensor's response is made at a specific wavelength for different VOC concentrations. Limits of detection of 2.16, 1.73, and 3.73 mg/L of vapors of ethanol, methanol, and acetic acid vapors, respectively, are attained., The authors gratefully acknowledge the financial support of the FPU Grant of the Ministry of Education, Culture and Sport (reference AP2002-1033), Projects MAT 2003- 09074-C02-01, CICyT TIC2003-00909, and BQU2002- 04090-C02-02 of the Ministry of Science and Technology, and to Consejería de Medio Ambiente de la Junta de Andalucía and Gobierno de Navarra.