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.
Distributed fiber-optic sensors have become ubiquitous in the measurement of changes in environmental parameters induced by multiple events over long distances. They have interesting applications in a number of safety and integrity monitoring applications including, among others, transportation, oil and gas, and process control systems . While some monitoring scenarios involve measurement of slowly changing parameters such as temperature or static strain, a number of natural phenomena that need to be monitored in many other scenarios involve fast changing parameters such as dynamic strain or vibration which require fast and accurate measurement systems operating in real time. Consequently, advanced distributed fiber-optic sensing schemes for such dynamic measurements have recently attracted significant attention. One possible technique to implement such systems is based on stimulated Brillouin scattering . Although a number of investigations have been made in this area to enhance the speed of the measurement over short distances with high spatial resolution, measurement at long distances is inherently slow . On the other hand, distributed dynamic fiber-optic sensors based on coherent Rayleigh scattering offer interesting alternatives for fast distributed measurements over long distances. Recently, distributed acoustic sensing (DAS), which is the use of an optical fiber to measure vibrations over an extended region, has become a ubiquitous technique . Recent survey indicates that the DAS market has been growing steadily in the past few years and is expected to surpass $2 bn by the year 2025 . It has a number of interesting applications including monitoring the safety of assets such as railway infrastructures and oil and gas pipelines.