New approach for localizing disturbances on optical fiber networks

In this talk, we are presenting a new method for localizing disturbances along an optical fiber cable. In contrast to Distributed Acoustic Sensing (DAS) techniques, our approach uses coherent optical phase measurements of stable low-power CW lasers that are recorded with an accurate time-stamp at each end of an optical fiber. This method is compatible with most types of deployed optical amplifiers, and it should be feasible to make localized strain/disturbance measurements over thousands of kilometers and spanning ocean basins, with around 200 m spatial resolution. When a fiber optic cable experiences vibrations, temperature changes, or other strains, the light passing through it is subject to a phase shift. Since the magnitude of the phase shift is proportional to that of the disturbance, it is possible to reconstruct the original disturbance strain or temperature change based on the phase of the light we observe. The comparison of the arrival time of the phase disturbance at each end relative to the local clock then gives the distance along the fiber of the disturbance relative to the clocks at each end. The accuracy of this localization step is critical for most of the technology’s promising sensing applications, such as undersea seismometry and strain measurements. Using GPS-steered quartz oscillators, absolute timing uncertainties of 10 ns (equivalent to around 5 m in spatial terms) are feasible relative to GPS/UTC time. The limiting factor is instead the ability to accurately extract and compare the arrival times of the two signals, since a temporal resolution on the order of microseconds is required and seismic signals tend to be low-frequency – typically below 10 Hz. Sophisticated time delay estimation (TDE) signal processing techniques are therefore required to determine the difference in time between the onset of each signal. The difficulty is further amplified by the fact that the light going to both detectors is subject to the same background vibrations and broadband noise. Traditional TDE techniques, typically based on a cross correlation, either struggle or falter completely in such environments, where significant common noise exists. We propose instead a new TDE technique, “derivative matching,” which is based on a geometric comparison between the difference of the two time-shifted signals and their averaged derivative. This novel technique requires significantly less time and computational power to perform, and yet we find that it provides at least an order of magnitude of improvement in localization accuracy, while also enabling us for the first time to analyze multiple simultaneous disturbances along a single cable. In a comparison test of 39 samples taken from a noisy 16 km fiber routed through several buildings on the Stanford campus, we were able to localize the vibrational signal produced by a piezoelectric fiber stretcher to within 0.14 ± 0.39 km, while the best traditional technique (“Compensated Time-Shifting Deviation”) had an error of 4.8 ± 8.7 km. These results indicate that derivative matching has the potential to enable CW fiber optic interferometry with disturbance/strain localization at the few-hundred-meter level over fiber links of thousands of kilometers undersea and elsewhere.