Words very may have flown over an undersea cable before reaching your eyeball. Hundreds of thousands of miles of fiber optic fiber traverse the world’s oceans, shutting down emails, Netflix shows, and news articles as packets of light. And, scientifically speaking, that boy has a story to tell — not so much about what happens on land, but what happens in the depths.
Diary last week ScienceResearchers describe how they use a 3,600-mile cable line that extends between Halifax, Canada, and Southport, in the United Kingdom, to detect storms, tides and earthquakes. Because the cable is on the seafloor, such disturbances create small but measurable disturbances in the fiber, which alter the speed of light over the Atlantic Ocean. These changes allow reading of the location of an earthquake or other disturbance.
This technique – a form of interference sensing – is similar to another system that is growing in popularity among researchers: distributed acoustic sensing, or DAS. Here, scientists fire a laser through underground (but unused) telecommunications fiber optic cables and analyze what is bouncing back. If a vehicle or a person go through and disturb the cable, that vibration scatters some of the light back to its source. Measuring the time it takes for scattered light to travel gives an idea of the size of the object passing through the head. The researchers also laid cables around Mount Etna, an active volcano in Italy, and used DAS to follow its rumble.
This new technique exploits a device in the underground cable called a repeater. (It looks like a slightly bulge in the line.) In the map below, they are indicated by yellow dots. “Normally, every 60 to 80 kilometers you need an optical amplifier, which basically takes the incident light and amplifies it,” said Giuseppe Marra, metrologist at the Kingdom’s National Physics Laboratory. He and the lead author of the new paper said. “So they pass through to the next interval, and then there’s another amplifier, and you just keep doing that to get to the other side.”
Courtesy of Giuseppe Marra
Each repeater enhances the signal to ensure that it reaches its destination without attenuation. So Marra and his colleagues were able to send their own signal over the cable and analyze what it looked like as it reached each repeater. Unlike DAS, they do not attempt to parse the perturbation that reflects a small amount of light back to its source, but rather the frequency of the light reaching the repeaters. “In the absence of disturbances, we get a stable signal: The frequency we received is the same as the frequency we sent,” says Marra. But if there is a disturbance, that frequency will change.
