You normally think of fiber optic as something used in network cables. However, scientists employ dedicated fibers to detect earthquakes. In simple terms, they fire a laser down the fiber and watch reflections caused by imperfections. When vibrations hit the cable, it changes the defects, which show up in the return pattern. However, with the right techniques, those vibrations could just as easily be from people speaking near the cable.
If you are alarmed, there’s good news and bad news. The good news is that the technique seems to be limited to coils of fiber that are not buried, and you have to be within about 5 meters of the fiber. The bad news is that there is plenty of dark cable all over the place. Besides, if researchers can do this successfully, you would imagine three-letter agencies around the world could do it even better.
There have been several recent papers about the same topic. Of course, you can also read laser bounces from windows. Noisy keyboards can also give you away.
Title image from [Compare Fibre] via Unsplash.
The USA has this type of system on the Mexican border and pulled from subs and ships. It can not only be buried, but the exact points (plural) can be selected to listen from. The advanced versions can act like a phased-array microphone.
About 35 years ago I was working on a sub. Layout out fiber optic was done with specification such that bend radius could not be smaller than X to prevent light leaking through the insulation that could be used to listen I to the signal.
The local water utility here embeds fiber sensors in all the new main water supply pipe they lay, continuously monitoring for acoustic disturbances. The pipe is concrete with steel wire reinforcement. When (not if) a steel strand breaks, the fibers can localize the ping to within centimeters.
The idea is that once they record enough pings from a segment of pipe, they pre-emptively do an orderly repair or replace before it catastrophically ruptures and becomes an emergency.
But it’s a very different kind of fiber — not the communications-grade stuff that’s littered all over the place. This is Fiber Bragg Grating “FBG” fiber. This sensing fiber has deliberate sensing “defect” structures fabricated into it to enable high sensitivity detection and localization. It’s purpose-made for the task and would give much higher performance than the incidental fiber acoustic sensitivity described in the article.
Optical Bragg Grating fiber is fairly cool stuff.
Mostly comes from aerospace research, used to measure wing deflection. Latest use is in medical equipment that’s inserted into the body as use of digital positioning for tools. The fibers, I assume when clustered, can produce a 3D measurement of flex and bends. OBG is also single-ended, which is why it’s additionally useful in the applications it goes into, diode and sensor exist at the same end.
I’m excited to mess around with the stuff for some projects, once there’s a material source that’ll cater to parts vendors, all of the industrial stuff is likely a five figure transaction for something like a ten spool order and that’s being conservative. I’m surprised robotics hasn’t picked up on it considering it can be used as a 3D multi-point (technically continuous) position sensor. I’m curious how finite the grating can be before it starts producing errors, especially at higher production scale.
We are operating an optical clock link of a few kilometers going through the city. The active fiber noise cancellation works in principle but still we regularly picks up metro announcements. We’ve never been able to localize the pickup point, tho
Are there countermeasures, such as burying fiber into sound absorbing material, or intentionally coupling it with an audio noise source?
I think one of the most amazing technologies of this flavor is the “ring gyro” seen on almost every large commercial aircraft in use today.
https://www.cryptomuseum.com/covert/bugs/thing/