Abstract
An optical fiber pH sensor based on a long-period fiber grating was produced by coating the sensing region with two bilayers of polyethylenimine (PEI) and polyacrylic acid (PAA) using the layer-by-layer self-assembly technique. © Optica Publishing Group 2023, © 2023 The Author(s), © 2023 José M. da S. A. Pereira, João P. Mendes, José M. M. de Almeida and Luís C. C. Coelho.

Abstract

Abstract
Due to the increase in energy consumption based on fossil fuels, sustainable alternatives have emerged, and green hydrogen (H-2) is one of them. This fuel is a promising eco-friendly energy source but is highly flammable. Therefore, continuous monitoring is essential, where optical sensors can contribute with a fast and remote sensing capability. In this field, plasmonic sensors have demonstrated high sensitivity, but with the plasmonic band in the visible range and low definition in the infrared. It presents a sensing structure for H-2 sensing composed of inexpensive materials (SiO2 and TiO2) and Pd as a sensitive medium, which supports Tamm Plasmon Resonance. The structure is numerically optimized to obtain a plasmonic band around 1550nm, which was experimentally validated with a sensitivity of 9.5nm in the presence of 4 vol% H2 and a response time of 30 seconds. This work aims to emphasize the advantages of this plasmonic technique for gas sensing at the infrared spectral range, allowing remote sensing.

Abstract
Hydrogen (H-2) is a promising alternative to fossil fuels. However, safety concerns need constant monitoring. Fiber optical sensors have become crucial in this field due to their capability for remote measurements. Traditional plasmonic techniques applied on optical fibers rely on expensive materials, which implies removing the fiber protection, and the optimized bands are outside the infrared spectral range preferred in optical communications. To address these challenges, this work presents an alternative plasmonic structure at the fiber tip of a single-mode fiber. The approach is based on Tamm Plasmon Resonance (TPR), which can be excited at normal incidence with depolarized light. Numerical results indicate that the numerical aperture of the fiber has minimal impact on the TPR band. Experimental results validate the possibility of this approach for H-2 detection, showing a wavelength shift of 8.5nm for 4 vol% H-2 with the TPR band centered around 1565nm. The sensor presents a response time of 29s and a reset time of 27s. These findings open new avenues in the development of plasmonic optical fiber sensors for H-2 sensing, as they enable the possibility of exciting plasmonic modes without removing the fiber's cladding and with simple structures.

Abstract
The fabrication of Mach-Zehnder and Fabry-Perot interferometers in SMF-28e fibers by femtosecond laser direct writing is demonstrated. The feasibility and effectiveness of this technique in fabricating high-sensitivity fiber optic interferometers is highlighted. TiO2 coated Mach-Zehnder interferometers exhibit improved refractive index sensitivity compared to uncoated interferometers, while the dual-cavity intrinsic Fabry-Perot interferometers shows enhanced spectral response and sensitivity for measurement of gas pressure.

Abstract
Concrete structures require precise temperature and humidity monitoring during curing to ensure optimal strength and prevent defects like cracking. A compact optical sensing system was developed using a single fiber that can be embedded directly within the concrete. The system functions as both a temperature and humidity sensor when paired with a spectral interrogation unit operating in the 1500-1600 nm range. Temperature monitoring is achieved through a Fiber Bragg Grating, while humidity sensing is facilitated by a Fabry-Perot interferometer at the fiber tip. The interferometer cavity is formed with a layer of polyvinylpyrrolidone (PVP). Initial air humidity sensor tests showed a significant change in the interference period with RH, demonstrating low hysteresis and high reproducibility. Calibration of one sensor revealed an approximately 3 nm period decrease when RH increased from 55% to 95%, with results suggesting a quadratic relationship between the interference period and RH values.