1. Introduction

Since optical fibers can be used to measure a variety of physical parameters directly or indirectly, sensing in optical fibers has been intensively investigated in the past few decades [1]. Fiber optic sensors can be split into two big categories, that is, point and distributed sensors. The former type, such as the fiber Bragg grating (FBG), measures the physical parameters only at a particular location but with relative high resolution and sensitivity. The detectable range of the latter one, such as optical time-domain reflectometry (OTDR) [2], is relatively large (usually the fiber length itself) and continuous but with only moderate resolution and limited sensitivity [3]. However, numerous investigations have been carried out to design a novel sensor with the advantages of both types, but avoiding their disadvantages.

With a moderate spatial resolution in the range of 1 m, distributed Brillouin sensing offers a more cost-effective solution than applying numerous point sensors in long-range sensing and a more accurate result than traditional distributed sensing methods such as OTDR [4]. Due to the recent success in breaking the limits of numerous sensing parameters such as spatial resolution, sensing range, and measurement time, the Brillouin sensing technique has proved its eligibility in a variety of fields. Thus, it has aroused significant interest in many different applications like civil engineering, oil and gas pipelines leakage monitoring, and other infrastructure surveillance tasks.
