Distributed acoustic sensing (DAS)
Using a standard optical fiber as a continuous line of vibration sensors, read by phase-sensitive OTDR: every few meters along tens of kilometers of fiber, the Rayleigh backscatter records local strain as it changes in time.
Distributed acoustic sensing turns an unmodified fiber into thousands of vibration sensors. An interrogator at one end launches short pulses of highly coherent light and records the Rayleigh backscatter as the pulse travels, exactly as in OTDR, with the difference that the light is coherent. The backscatter from each section of fiber is the sum of contributions from many frozen-in scatterers, a random but fixed interference pattern. When the fiber is stretched, the optical path through that section changes and so does the phase of its backscatter. By measuring the phase difference between two points separated by a gauge length, typically a few to tens of meters, and repeating the measurement with each pulse, the interrogator reads the strain along the whole fiber as a function of time. The measurement is often called phase-sensitive or coherent OTDR.
Pulse timing sets the two main specifications. The spatial resolution is for a pulse of duration : a 100 ns pulse in fiber with resolves 10.2 m. Only one pulse can be in the fiber at a time, or the backscatter from two would overlap, so the repetition rate is limited by the round trip. For 50 km of fiber the round trip is 490 µs, the pulse rate at most 2.04 kHz, and the highest acoustic frequency that can be sampled without aliasing 1.02 kHz; for 10 km those figures are 5.1 kHz of bandwidth. Range and bandwidth trade directly against each other.
The signal is weak and noisy. Only a small fraction of the pulse is scattered back into the guided mode, and the launch power cannot be raised far before fiber nonlinearity distorts the pulse. The laser's frequency noise appears in the measurement exactly like strain, so interrogators use lasers with linewidths of a few kilohertz or less and detect with coherent receivers. Because the backscatter is a random interference pattern, some points along the fiber return almost no light and give no usable phase at a given moment; interrogators counter this fading by probing with several optical frequencies at once. Fibers with enhanced backscatter, or with weak gratings written at intervals, raise the signal where the installation allows special fiber.
The quantity measured is the strain (or strain rate) along the fiber axis, so the sensitivity depends on direction: a wave arriving broadside to the fiber produces little axial strain. Within that limit, a single interrogator covers distances no array of point sensors could, which suits oil and gas wells, pipelines, perimeter security, railways and traffic monitoring. The same technique applied to unused telecom fibers, including submarine cables, has made DAS a seismology instrument, recording earthquakes and ocean waves along tens of kilometers of seafloor. Brillouin and Raman distributed sensors measure static strain and temperature on the same fibers; DAS is the dynamic counterpart.
References: A. H. Hartog, An Introduction to Distributed Optical Fibre Sensors (CRC Press, 2017); N. J. Lindsey, T. C. Dawe, J. B. Ajo-Franklin, Science 366, 1103 (2019); Z. Zhan, Seismol. Res. Lett. 91, 1 (2020).