Advances in distributed fiber optic acoustic sensing technology based on linear distribution of phase change

In recent years, phase optical time domain reflectometer (Φ-OTDR) with distributed acoustic sensing capabilities has received widespread attention. On the one hand, it has been successfully applied in various fields based on the level of existing common devices. On the other hand, Φ-OTDR based on the level of existing common devices continue to be the subject of applied research in terms of structure, algorithm and son on. At the same time, research aimed at non-common and future device levels is also emerging. The author of this article has carried out a series of work using high-end models of common lasers, and this article will focus on reviewing these advancements. In addition to the well-known linear characteristic between physical phase signal and measurement result, it reiterates the linear distribution of phase change along the fiber under the condition of ultra-stable frequency laser. This feature has been used to accurately reconstruct single-point and double-point vibration events, with the fitting degree of the reconstructed sinusoidal signals reaching or nearly 1 during parameter fitting. To overcome the limitation of phase unwrapping algorithm in single-frequency light based Φ-OTDR, dual-frequency light has been introduced, and phase signals have been resolved by the methods of employing auxiliary phase resolution and phase inconsistency elimination, successfully breaking through the limitation and expanding the measurement range. Although these studies were conducted with high-end commercial lasers and experimentally validated under short-term ultra-stable frequency conditions, they lay the research foundation for scenarios where laser frequencies are stable over the long term in practical applications.