IRA FLATOW: If you think back to the summer of 2021, perhaps you’ll recall that lots of folks in the Eastern US were hearing the sounds of cicadas.

[CICADAS BUZZING]

It was the emergence of the Brood x cicadas, an event that occurs once every 17 years. But it turns out it wasn’t just something you could hear or see flitting across the lawn or crunching against the car windshield. No, that emergence was something that could be detected using fiber optic cables.

How does that work? Joining me to talk about it is Dr. Sarper Ozharar. He’s a researcher, who studies optical networking and sensing at NEC Labs in Princeton, New Jersey, and is one of the authors of a report published this week in the Journal of Insect Science, all about sensing the cicada emergence with fiber optic cables.

Welcome to Science Friday.

SARPER OZHARAR: Hi, Ira. Thanks for having me.

IRA FLATOW: You’re welcome. The Journal of Insect Science doesn’t seem like the normal place for electronics researchers to be publishing in.

SARPER OZHARAR: Yes, exactly.

IRA FLATOW: What were you trying to do there?

SARPER OZHARAR: So at NEC Labs, we are working for the smart city, safe city applications of fiber optic sensors. And we have a testbed here in Princeton, New Jersey, that consists of some utility poles, employing real communication fibers like the ones you will see outside. It’s just the regular communication systems. And there, we do some smart city application tests– like traffic monitoring or emergency vehicles– like siren detection and siren monitoring, things like that.

And we realized one day, we are receiving some interesting signals from the testbed. And there’s a distinct frequency, quite strong, everywhere on the cables. So we were wondering what it was. And it’s a short walk from our lab. So when we checked it out, we realized it was the cicadas.

IRA FLATOW: No kidding. How can an optical fiber detect a sound? I mean, we think of there’s light running through it, right?

SARPER OZHARAR: Yes. So a good analogy is you can think of it as like a radar. Many people are familiar with the concept of radar.

IRA FLATOW: Right.

SARPER OZHARAR: You send a pulse– an electromagnetic pulse– and it hits a target and it returns back to you. And you just time it. How many seconds does it take to get back to you? And you know the speed of the electromagnetic wave so you can calculate where your target is.

So we do something similar. But instead of sending an electromagnetic wave in the air, we send an optical pulse– a short optical pulse– along the fiber. And we just time it for the return. But one main difference is, in the radar example, you have one target. But in the fiber optic cable, we have a target everywhere along the fiber.

So we get a return signal from every 1 meter or something. So because of those non-uniformities in the fiber, we always get a return signal from almost all locations. So what happens is, when there’s any kind of sound source, we hear it through the vibrations hitting our ears through the air, but also the ground also vibrates. Even when we are talking right now, the desk also vibrates. And if there’s a fiber nearby, the fiber also vibrates.

But in that fiber, because of that vibration, they couple into a change in the refractive index of the fiber– a very, very, very small effect. So it doesn’t affect regular communication, but it modulates the optical pulse ever so slightly.

And then we also discussed the return signal. It’s a very, very weak signal. And on top of that very weak signal, there is a very weak modulation.

IRA FLATOW: So you’re using the fiber specifically to detect the sounds– you’re saying gunshots, traffic, et cetera. And by, oh, serendipity, you picked up the cicadas.

SARPER OZHARAR: Exactly. So it was a bit surprising at first. But then, when you see those cicadas in the Princeton area, they’re quite loud and then they’re kind of everywhere. So it was a nice surprise actually. So we were trying to do some localization and traffic-related experiments in the testbed. And then, voila, you have these other frequencies that should not be there.

IRA FLATOW: And did the other frequencies that should not be there– were those the right frequencies that told you, hey, these are cicadas?

SARPER OZHARAR: Yes. We had to confirm them, of course, for the scientific process. So we checked out the testbed. We did some other comparisons. And we realized, yeah, it was indeed the cicadas.

IRA FLATOW: Wow. You know, cicadas, they talk about how they make sounds that’s relevant to what the ambient temperature is, right? Were you able to tell something about the temperature based on the buzz?

SARPER OZHARAR: Yes. Again, we are also monitoring the weather conditions of the testbed. We can also detect rain or wind or other kinds of effects with our fibers as well. So we were checking the frequency of these cicada calls. And then we realized there’s a relation with the temperature. They kind of follow the temperature, but it wasn’t like a one-on-one mapping. But there’s a tendency that they’re affected by it.

IRA FLATOW: This is Science Friday, from WNYC Studios.

This all sounds really fascinating. But my question is, why would you want to do this rather than just stick a microphone out the window or something?

SARPER OZHARAR: The main advantage of fiber sensing is you just use a piece of fiber. It’s a piece of glass that is kilometers long. And you don’t need to have any kind of power on the field. You don’t need to have any kind of electronics. Just you lay a piece of glass, and everything– all your power source, electronics, computation– is just on the end of the fiber.

So that’s the main advantage. Because if you want to put vibration sensors for a kilometers-long distance, you need to put separate cables for those sensors. You need to get data from them. So each of them should have their own power source or batteries. Each of them should have their own communication cables, et cetera. It’s a mess. But in our case, just one fiber, one device, thousands of sensors. And the cable is already there.

IRA FLATOW: What other kinds of things do you think you can detect using these fiber optic cables?

SARPER OZHARAR: Some major things we do is, again, as I mentioned, real-time traffic monitoring and accident reporting. Like, if there’s a car crash or anything like that, a big vibration. Another one is gunshot detection and localization. One other thing is infrastructure health monitoring.

IRA FLATOW: Really?

SARPER OZHARAR: If a tree falls on one of the cables, we can just see it, localize it. Or if there’s damage on the utility poles, or we can even use these fibers actually for measuring, for example, the health of a bridge because it’s like a vibration sensor. And we can look at how the bridge is responding as cars are passing by. And if there are some unwanted frequencies starting to emerge, we can say, oh, there’s some kind of a problem with the bridge. So it needs to be repaired.

IRA FLATOW: Do you think, once you’re done testing it, we might see fiber optic cables strung about all around the countryside– maybe even on a bridge, let’s say– as something that’s being used to test or to predict?

SARPER OZHARAR: Yes, definitely. And the thing is actually, currently, right now, it is expected that there is 4 billion kilometers of fiber worldwide. It’s already everywhere. And it’s going to increase more and more with the 5G and 6G because the demand is too high. Copper cable is not enough, we all know. So we need more fiber more than ever.

And the good thing is you can use the same fiber that you use for communication– you can use it also for sensing. So you don’t need a specific or a separate fiber cable for sensing applications. You can already use the readily available ones.

IRA FLATOW: It sounds like there’s a business here.

SARPER OZHARAR: Yes.

[LAUGHTER]

That is correct.

IRA FLATOW: And I would think the military would be eating this stuff up right now– about remote sensing.

SARPER OZHARAR: Yes. So it has some military applications as well, especially the border security. Because you can take this fiber, just put it underground in an area that you want to protect, and if someone is getting closer to the fiber– these steps of the people walking or some animals– you can also detect those.

IRA FLATOW: Now that we’ve been talking about picking up vibrations, I can see listener mail coming in– can someone use this to eavesdrop on me?

SARPER OZHARAR: Well, that is a very good question, actually. And the answer is no. No need to worry about that. Even for the cicadas, it’s not an easy thing to do– to decode that thing. And also, currently, the only thing we can detect is really loud events that will be shaking a lot of the fiber.

IRA FLATOW: So I would have to be really screaming in my conversation for the fiber to detect it?

SARPER OZHARAR: Yes, scream right next to it. Get your fiber on your hand. Just scream on it. And then maybe we can.

IRA FLATOW: I’m going to try that tonight, Dr. Ozharar. Thank you for taking time to be with us today. Very fascinating.

SARPER OZHARAR: Sure. You’re very welcome.

IRA FLATOW: Dr. Sarper Ozharar is a researcher, who studies optical networking and sensing, at NEC Labs in famous Princeton, New Jersey.

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