Localization of disturbances along an optical fiber

Long-distance telecommunications fibers have been shown to provide a new method for detecting acoustic disturbances passing through the ground. Passing waves stretch and compress the fiber, leading to a phase shift of light traveling within. In a first proof-of-principle experiment [1], seismic waves from nearby earthquakes were detected by injecting a narrow-linewidth laser into ~100 km long fibers, retroreflecting the beam at the far end, and measuring the time-varying phase shift between the outgoing and returning beams.

One shortcoming of this approach is that it is not sensitive to the location of the disturbance along the fiber. In this work, we demonstrate an improved method that can localize a single-frequency accoustic noise source to within about 5 meters and localize taps on the fiber to within 300 meters. For this first demonstration, two labs (A and B) are connected by a 350 meter long fiber. An ultrastable laser in Lab A is injected into the fiber and sent to Lab B. In Lab B, the arriving beam is split: one part is frequency shifted and retroreflected back to Lab A, while the optical phase of the other part is compared to the optical phase of a second ultrastable laser in Lab B. The phase of the retroflected light returning to Lab A is compared to the phase of the local reference using an optical beat note detector. By analyzing the time delay between phase fluctuations observed in Lab A and in Lab B, we are able to determine the location of the noise source.

The optical phase differences are recorded locally in each lab and only later combined for analysis. To help synchronize the two measurement traces, an amplitude modulation following a pseudrorandom binary sequence is applied to the beam from Lab A as it enters Lab B. This imprints a synchronized 131071-element repeating pattern at a rate of 100 kHz onto both measurements. By computing the cross-correlation between the measured amplitude and a locally-generated copy of the pseudorandom sequence, the relative timing offset between the two traces can be determined with an uncertainty of less than 10 ns with a 50-second averaging time.

[1] Giuseppe Marra et al., Ultrastable laser interferometry for earthquake detection with terrestrial and submarine cables. Science 361, 486-490 (2018).