Fiber-optic vibration sensing ‒ II: Intrinsic sensing with scattered or transmitted light and their seismological applications

The intrinsic vibration sensing of optical fiber measures the dynamic strain (i.e., vibration) along with the fiber by repeatedly emitting detection lasers at one end of the fiber, receiving scattered or transmitted light at the same or another end, and demodulating its changes. The sensing system takes the fiber as sensors and has the advantages of being flexible and straightforward to build, and being easy to operate and maintain. It can be used in harsh environments with fluid, high temperature, high pressure, or strong electromagnetic interference. As a result, it can significantly expand the monitorable area while also achieving relatively low-cost and long-term continuous monitoring. Furthermore, many redundant fiber resources already present in fiber optic cables of cities, intercity, and across oceans around the world. Utilizing these fiber optic resources can be quickly converted to vibration monitoring networks, significantly improving vibration monitoring capabilities in these areas.Today, a single instrument can achieve intensive distributed monitoring along with the nearly 100 kilometers fiber with meter-level intervals based on vibration sensing of scattered light. At the same time, sensing based on transmission light lacks distributed observation capability but can detect strong vibration along optical fibers tens of thousands of kilometers long. This paper describes the fundamental principles, application scenarios and prospects of these two types of fiber-optic vibration sensing.