This dissertation presents a fibre-optic sensing approach to provide continuous measurements of CO₂ concentration at discrete points under typical conditions of geological CO₂ storage. Carbon capture and storage is considered to have potential for a large-scale reduction in CO₂ emissions in a relatively short period of time while other solutions to replace fossil fuels are being investigated. One significant drawback of carbon capture and storage is the possibility of long-term CO₂ leakage. Therefore, the development of reliable technology for monitoring, verification, and accounting of geological CO₂ storage is critical to fulfill safety regulations and achieve public acceptance. The major limitations of current technology include relatively low resolutions, high costs, and the lack of continuous monitoring for long periods of time.

To address these limitations, two types of fibre-optic sensors are investigated, namely long period gratings and Mach-Zehnder interferometers. The sensing principle for CO₂ detection is based on the sensitivity of these sensors to the refractive index of the medium that surrounds the fibre. Fibre-optic sensors are attractive for downhole applications due to the possibility of fabricating inexpensive high resolution devices that are able to operate in harsh environments over long periods of time.

This dissertation focuses on increasing the refractive index sensitivity of long period gratings and Mach-Zehnder interferometers by applying coatings that have a high refractive index. The dip-coating method is used to coat long period gratings with polystyrene, and the sensitivity at low refractive indices is increased by tuning coating thickness. The results show that long period gratings coated with polystyrene are able to detect CO₂ in gaseous and aqueous media. This work reports the first measurement of CO₂ dissolution in water at high pressure with a fibre-optic sensor.

Additionally, atomic layer deposition is investigated to coat long period gratings and Mach-Zehnder interferometers with hafnium oxide. The study of this coating technique aims to address the main limitation of the dip-coating method:​ the challenge to achieve precise control over coating thickness. The results show that atomic layer deposition is suitable to maximize the sensitivity of long period gratings and Mach-Zehnder interferometers at a target refractive index.