Jehan Fonseca Do Nascimento

This paper presents an experimental evaluation of a hetero-core MNM fiber-optic sensor designed for monitoring electrolyte density in lead-acid batteries. The study focuses on two key aspects that affect sensor performance: electrolyte contamination and temperature variation. Spectral analysis reveals that although the sensor's sensitivity slightly decreases with contamination, it remains effective and suitable for long-term applications. Furthermore, experiments were conducted to assess how temperature influences the optical response at low (1100 g/L) and high (1290 g/L) electrolyte concentrations. Results indicate that electrolyte density is temperature-dependent, and significant shifts in optical response were observed with thermal variation. Therefore, incorporating temperature monitoring and compensation is essential to ensure accurate and reliable density measurements. These findings support the feasibility of the proposed sensor for real-time applications, provided proper temperature correction is implemented.

This work presents the analytical modeling of an optical sensor based on Surface Plasmon Resonance (SPR) employing a D-shaped single-mode optical fiber coated with aluminum. The model incorporates the effects of fiber curvature, enabling assessment of how geometry impacts sensor sensitivity. The analysis, grounded in the transfer matrix method and Fresnel formalism, demonstrates that minor curvatures lead to significant variations in the relative transmitted optical power and shift the resonance point, making it a useful tool for precision sensing. The results indicate that the proposed sensor is capable of operating with high sensitivity within a refractive index range of 1.4600 to 1.4700, enabling applications in the monitoring of liquids such as vegetable oils and glycerin, commonly found in the food and personal care industries for safety and quality control. Furthermore, the sensor's capability to detect mechanical deformations was demonstrated through the correlation between curvature and variations in the effective refractive index. The proposed solution stands out for its simple construction, low cost, and potential for integration into compact and selective optoelectronic systems. This study emphasizes an analytical formulation that highlights how fiber curvature can be used as a fine-tuning parameter for sensitivity and selectivity. This theoretical basis establishes the groundwork for future numerical and experimental validations.

Optical sensors that can measure multiple parameters is a promising area of research. This can also be attributed to the fact that monitoring two or more measurands at the same time is crucial in embedded systems, especially when they are interconnected, as is the case of temperature and corrosion, or temperature and refractive index. In this work, an optical fiber sensor based on Raman scattering in single-mode optical fibers to simultaneously monitor temperature, refractive index and corrosion is proposed and experimentally demonstrated. This sensor consists of a distributed temperature detection system based on Raman scattering with additional sensor heads: for multipoint corrosion monitoring and multipoint refractive index variation measurement. The multiparameter sensor was evaluated for solutions with refractive indices in the range of 1.3370 – 1.3840 RIU (Refractive Index Unit), for temperatures between 30 °C and 100 °C, and for aluminum thin-film corrosion in acid solution. The experimental results show that the sensor presents sensitivities of 0.5 dB/10 ^-3 RIU to refractive index variations, of 0.43 dB/nm for corrosion, and of 0.243 dB/°C for temperature, with resolution of 2 °C. The sensor system can also provide the corrosion rate and operate in fiber links several kilometers long.

In this work, we demonstrate refractive index (RI) sensors based on the cascade of hetero-core structures using multimode and no-core optical fibers in sequence. The sensor device is engineered to have resonances at different wavelengths using different sensing region lengths. The device fabrication involves simple fiber cleaving and fusion splicing. For the experiments, the two sensor regions are exposed to liquids with different RIs. For the hetero-core fiber insertion length of 45 mm, the transmission valley is centered at 1082.5 nm with 15.1 nm full width at half-maximum (FWHM) for an external medium RI of 1.3370. Additionally, it shifts 4.1 nm towards longer wavelengths as the RI of the external medium increases to 1.3840. For the 30 mm long hetero-core structure, the valley is centered at 1599.7 nm with 23.3 nm FWHM for an external medium RI of 1.3370, which shifts 7.4 nm as the RI increases to 1.3840. The sensor sensitivities are up to − 476 d B / r e f r a c t i v e index unit (RIU) and 270 nm/RIU. The resolution of the devices is estimated to be 2 × 10 − 5 R I U .

This paper presents the evaluation of the influence of surface roughness on the performance of a refractive index sensor based on a D-shaped optical fiber. In addition, the manufacturing process of D-shaped sections on standard silica single-mode optical fibers with different rms roughness values is described. The surface roughness morphology generated by the abrasive process is measured by processing microscopic images using the Gwyddion software. A model for the surface roughness, using the third order Weierstrass fractal function, is proposed and simulated with COMSOL Multiphysics. Experimental and simulation results are presented. The results show that the surface roughness increases the device insertion loss, as expected, but contributes positively to the sensitivity of the refractive index sensor, reaching sensitivity levels of 148.87 dB/RIU with a resolution of 10-4 RIU. The good agreement between the computational and experimental results is highlighted. Due to the good figures of merit obtained, the optical sensor has potential of application in biochemical sensing.

Distributed temperature sensors have been proposed for a wide variety of scientific and industrial applications and represent an important technology for modern infrastructures. Due to the range of potential applications, distributed fiber optic sensors based on spontaneous Raman scattering have been studied both theoretically and experimentally. This paper presents the experimental evaluation of a distributed fiber optic temperature sensor based on spontaneous Raman scattering over a wide temperature range, from -196 °C to +400 °C. The sensor is implemented using a commercial OTDR, a standard EDFA amplifier and a standard optical fiber link. Besides, a special type of optical fiber for high temperature operation, featuring metallic coating, is also assessed as the sensor element. The integrity of the fibers following the exposure to the extreme temperatures is analyzed. The results provide values for the sensitivity, accuracy and resolution of the sensor in the wide range of temperatures.