Abstract
This chapter outlines the basics of fibre-optic sensors and sensing systems, with particular emphasis on modern sensing approaches with straightforward implementation. It describes the main interrogation method and also describes sensors based on intensity methods and light polarization change. The chapter discusses simple fibre-optic interferometers, fibre Bragg grating sensors, and simple multiplexing techniques for fibre-optic sensors. Optical fibre sensing system can assemble several sensing unit and principles. However, an important distinction is to understand whether the system is working in transmission or in reflection. Intensity-based sensors typically make use of a LED light source coupled into the sensing fibre and connected to a photodetector. It is common practice to design interrogation systems with multiple channels, including reference channels that measure the power in each part of the optical system, thus compensating power fluctuations and fibre extra losses.