Rayleigh Backscattering (RBS) in optical fiber is a fundamental phenomenon caused by random fluctuations in the index profile along the fiber optic length. Optical reflectometry is the best tool to obtain RBS signals of fiber laser with a distributed way along the fiber and is widely used as a nondestructive measurement at one end of the fiber. With the help of optical reflectometry, RBS signals are used for distributed fiber optic sensing with temperature/strain/vibration information along the fiber. Understanding the mechanisms of RBS provides a powerful technique for static and vibration sensing used for applications such as structural health monitoring and damage assessment analysis. The technology based on RBS signals to extract the environmental perturbation is mature, but researches on obtaining a high spatial resolution together with a long measurement range are still very active, promoting the technology to be used in more industrial applications with strict requirements on these parameters such as monitoring the optical fiber inside the aircraft wings with a spatial resolution of better than 1 mm over several 100 m.

The different kinds of optical reflectometry based on time-domain, frequency-domain, and coherence-domain techniques, like Optical Time Domain Reflectometry (OTDR),Optical Frequency Domain Reflectometry (OFDR), Optical Coherence Domain Reflectometry (OCDR) etc.Distributed Acoustic Sensing (DAS) using Coherent Rayleigh Backscattering in an optical fiber has become a ubiquitous technique for monitoring multiple dynamic events in real time. It has continued to constitute a steadily increasing share of the fiber-optic sensor market, thanks to its interesting applications in many safety, security, and integrity monitoring systems. In this contribution, an overview of the recent advances of research in DAS based on phase-sensitive optical time domain reflectometry (ϕ-OTDR) is provided. Some advanced techniques used to enhance the performance of ϕ-OTDR sensors for measuring backscattering intensity changes through reduction of measurement noise are presented, in addition to methods used to increase the dynamic measurement capacity of ϕ-OTDR schemes beyond conventional limits set by the sensing distance. Recent ϕ-OTDR configurations which significantly enhance the measurement spatial resolution, including those which decouple it from the probing pulse width, are also discussed. Finally, a review of recent advances in more precise quantitative measurement of an external impact based on frequency shift and phase demodulation methods using simple direct detection ϕ-OTDR schemes is given.