Biosensing and physical sensing methodologies based on optical fiber gratings

Fiber gratings have long been utilized as optical sensors, particularly for measuring chemical, biochemical, and physical parameters. These sensors typically operate by detecting changes in the external refractive index or by monitoring physical parameters such as strain, temperature, and pressure. Fiber gratings can be categorized into two main types based on the grating period and their distinct optical characteristics: (i) long-period gratings (LPGs), with grating periods of hundreds of micrometers, couple guided light from the fiber core to the cladding, facilitating interaction with the surrounding environment, and are primarily used for chemical and biochemical sensing; (ii) fiber Bragg gratings (FBGs), with grating periods of hundreds of nanometers, reflect light within the core's fundamental mode and are mainly employed for physical parameter sensing.

This work presents two practical applications of these methodologies. First, we characterize an LPG optical sensor inscribed in an optical fiber, enhanced with a 40 nm gold layer. The gold coating offers several benefits, such as ease of functionalization and the potential for multiphysics sensing, including its use as an electrode for electrical measurements. The fiber sensor is integrated into a thermostated microfluidic system. Our experimental results indicate that the sensor's sensitivity, with the gold coating, is comparable to a similar sensor without the coating. However, the gold layer introduces significant opportunities for integrating electrical and optical sensing capabilities.

The second application involves the design and implementation of a remote sensing apparatus for monitoring deformation and temperature in large structures, such as bridges. This system employs two FBGs: one for strain and one for tilt sensing. The FBG interrogation is managed by a single-board computer, and custom software is developed for data acquisition, analysis, and remote monitoring. The system is designed for low power consumption, with provisions for battery power and solar charging, ensuring energy efficiency and sustainability.

Acknowledgements:
We acknowledge the support of the European Union by the Next Generation EU project PRIN2022 – 2022JRKETK_PE7 - Versatile hybrid in-fiBer Optical-electrocHemical systEMs for wIdely Applicable bioseNsing – BOHEMIAN, and the support of CNR DIITET Project FOE 2022 STRIVE “INOSTRI”.