Philippe Jousset, Gilda Currenti, Shane Murphy, Eva Eibl, Corentin Caudron, Lise Retailleau, Christopher Wollin, Sara Klaasen, Takeshi Nishimura, Andreas Fichtner, Zack J. Spica, Arnaud Le Marchand, Charlotte M. Krawczyk

This chapter describes fiber optic sensing methodologies and their applications for understanding volcanic structure and processes. We assess their benefits for volcano monitoring and offer possible solutions to address their challenges. The physical principles at the basis of fiber optic sensing technologies have been known for several decades. These principles are related to various processes involving electro-magnetic interactions of light sent by a laser within glass. Intrinsic physical properties of glass within the optical fiber, when engineered appropriately and interrogated with an adequate light source, enable us to access a number of environmental parameters, such as temperature, strain and rotation over a large frequency range. However, only recently developed instruments have been able to sense these parameters efficiently, either at a point or densely in a distributed way for geophysical and volcanological applications. Rotational sensors allow us to measure the rotational components of the seismic wave field, which have been discarded in the past. Distributed fiber optic sensing provides access to quasi-continuous measurements of temperature, strain and strain rate along km-long fibers with a high spatial resolution (meter) and sampling rate (kHz). We show examples on volcanoes both on land and in submarine environments. We demonstrate that data from optical strainmeters, rotational sensors, distributed fiber optic strain and/or temperature sensing can reveal unknown structural features and processes in volcanoes. These examples testify that fiber optic sensing methodologies owe to be implemented as additional tools for improved volcano monitoring and for volcanic crisis management.