Long-range static and dynamic distributed sensing

In the past few decades, Brillouin-based distributed temperature and strain sensors have attracted the attention of both the academic and industrial sectors. In particular, B-OTDA systems have been lately a trendy study area due to their numerous and well proofed applications, such as in monitoring large civil structures, energy transportation or environmental applications. One of the main areas of research in B-OTDA sensors focuses on the extension of the measuring range. To do so, it becomes necessary to increase the signal to noise ratio (SNR) of the sensor, in particular considering the reduced signal contrast at the far end of the fiber. Several techniques have been applied in order to achieve this goal, such as Raman amplification and pulse coding. Here, we review some of these techniques (particularly Raman amplification) and their application for range extension of these sensors beyond the 100 km range. In addition, we discuss routes to perform dynamic measurements over such ultra-long spans.