Résonance, cohérence, interférence: 3 notions qui éclairent internet | Sébastien Bigo | TEDxSaclay
URL: https://www.youtube.com/watch?v=TKKiWZEGIvw Video ID: TKKiWZEGIvw ============================================================ Translator: eric vautier Reviewer: Moroni Ramos My standing in front of you results from my once having misplaced a worked exam paper in a file binder. When I was a teenager, I used to fix everything around the house, and put together electronic boards. Putting it succinctly, my professional carrier was all sorted: I would be an electronic engineer. The entrance exam day arrived for the flagship school not very far from here in France's silicon valley. Electronics test, very heavily weighted. I return home, confident, I open my binder, I find a copy there, the section at the sixth tab, one so poorly written I thought it must be a draft, one I had not gone back over. I failed the entrance exam for my dream school by 1 point out of 500. On this day, my professional career changed direction. I would not be an electronic engineer, but an optics engineer. And today, I'm going to talk to you about light and resonance, and we're going to play around with them. We'll begin with - If you think of resonance, the word can refer to a signal that echoes back to you. Here yo see two everyday objects that you recognize: a flashlight, and a laser pointer. These two objects are two sources of light. They behave very differently, and yet at the heart of these two objects, there's the same generator of light that we call a diode. This diode sends light in all directions. That’s why they're used for lighting. The rays propagate in straight lines. Imagine inserting this diode inside a box, a box made up of two mirrors facing each other. The rays propagate in all directions, leave the box except when they strike an obstacle. When they hit an obstacle, they behave as if in a game of ping-pong. I don't know if you know this game, but it's probably the only game parents know better than children. Anyway, the thing that's important to me is that the only rays that can go back and forth multiple times are those that transverse on a path perpendicular to the two mirrors. And it's in that way that you obtain a light output composed of single rays sharply aligned in one direction. In the same way it is said that an argument made up of several ideas is coherent if the ideas go parallel in the same direction, scientists and engineers also call light like that "coherent." Why is this useful? Well, you may know that our French silicon valley, where we are, will, at the end of this year, be the first area 100% connected by fiber. This, this is a fiber. This is the fiber that comes to your home. And you can easily imagine that it’s much easier to inject light when you have a highly directional beam than when you have a flashlight because this is a very fine glass tube, extremely narrow. And it carries your emails, your videos, all your data, by transmitting light in the form of ones and zeros: when there is light, it's a "one"; when there isn't, it’s a "zero." So, light is useful, but why is an optical fiber so interesting? Because it’s a transparent medium. No surprise, it's glass! It has an attenuation of only 4%. This means that if you send 100 light particles, 96 will arrive after traveling 1 kilometer. That's fantastic. But, if they go 2 kilometers, the attenuation is 0.96 times 0.96. After 3 kilometers, 0.96 times 0.96 times 0.96. If you want to cross the ocean, it will be 0.96 to the power of 6,000. If you have a calculator that has the capacity to do the calculation - not all have - you will find that it's 0.000000... 107 zeros. To give you an idea of what such an attenuation represents, let’s suppose that I want to receive a single photon, a single particle of light, after propagation over 6,000 kilometers. Well, it would be necessary to inject into the fiber billions of billions of times the energy of the universe. You might argue that I do not know exactly the energy of the universe. Does that matter? The fiber will have melted long before! And yet, you can carry information to the other side of the Atlantic Ocean and converse with people in the United States, for example. How do we do it? Once again, it's resonance that comes to our rescue. A different resonance, one where we say two vibrations resonate in agreement with one another. Suppose the light is in accord with a vibration in the material of the fiber, one which can convert to light. This is what can happen. Sir, have you ever heard of erbium? No... Madam? You neither. It is not a material found in DIY stores and yet it is essential in today's Internet. It is a material, which, introduced into the glass in small quantities, provides the property to vibrate, to store the vibration, provided by an intense light source. We say it becomes excited. Then, the propagated signal that has become feebler will be able to take up energy that the erbium returns to it. Thereby, one photon becomes two photons, two become four, and so on, until the initial amplitude of the signal is recovered. If you insert such a component, each just a few meters long, every 50 or 100 kilometers, you reach your destination easily. Now, you're going to ask how much energy is required to transmit - taking an extreme case - all the data sent from Europe to recipients in the United States in one hour. Well, it will take roughly the energy contained in a dozen flashlights like this one. No more. You see that resonance is very useful. Now, let’s consider another very different form of resonance. Etymologically, a "resonance" is a sounding again, a recurring vibration. I would like us to do a test together. I would like you to imagine me a pop star, the pop star you came to see, and I would like you to applaud me. (Applause) (Clapping in unison) Thank you. (Clapping in unison) Thank you - I didn't deserve so much. (Clapping subsides) I didn't deserve so much. Did you sense the moment when - the time when you were clapping at random and the vibrations had no relation to one another, and then when you all clapped synchronously in rhythm with each other? You became aware of the growing volume, amplification? That's what resonance is about. It also works for light. If you could use a microscope to look extremely closely at a ray of light, you would see a wave, hence a vibration. If we are interested in how vibrations combine, we find that when they are uncoordinated, we obtain a vibration with an amplitude that increases only very slightly with the number of rays. On the other hand, if you have the vibrations align peak-to-peak with each other, you get the sort of amplification that you all experienced by applauding simultaneously. It's this effect that allows us to obtain a coherent addition of vibrations. You followed me, I'd explained clearly, and you came to realize that you all needed to clap in time with one another. When we talk about light and frequency, it's color that determines the number of vibration, the frequency of vibrations. If you want all the vibrations to align peak-to-peak, there must be absolutely only one color. Because if you combine all colors, the combination of the vibrations can only be incoherent. In what way is this useful to us? I could have used the laser pointer and reminded you how one can produce such an intense beam. The directivity of the beam is not our only concern: we also make sure that every round-trip that light makes inside that box is always in line with all the previous ones. As that can only occur when there's just one color, the box also acts as a filter. In other words, light that was initially white - and white is not a color, but the sum of all colors - well, of all the colors there are, only one can come out. All the colors that are not resonant become eliminated. That’s why you can have a laser giving an extremely pure green light. If I had chosen a box of a different size, I would have gotten another color. This is very practical in an optical telecommunication system, where multiplying the number of colors multiplies the number of recipients by the same amount. The good news is that it works both ways. Once we have multiple colors, we need to extract the colors. We can use the same box. If we have 100 colors, we will need 100 boxes of different sizes, each to extract a different color. So we’ve seen that resonance can be used to build a complete telecommunication system ... and this is where the story of the forgotten paper comes back in. Ten years ago, we were a group of eight researchers, a few kilometers from here, looking for a solution to keep up with the extraordinary rise in data traffic through optical fibers. And we had an idea. What you might usually expect to get when combining two beams is a beam that is twice as bright. That is what we do to get the resonance effect. But what if summing the amplitudes of these two beams results in an amplitude lower than the original ones, we, in fact, subtract them, as we'd say in math, but to which we give the fancy term "destructive addition." In fact, it's another way of saying that we allow them to interfere. Interference is when you can bring two beams together and produce the absence of a beam, that is, an absence of light. This is another way of coding information. It’s no longer just a matter of switching the laser beam on and off, but of playing with another method, another property of light, controlling its vibrational state. The technique that we have developed has become an industry standard. It has increased the system capacity by a factor of 10 worldwide. Today, it represents a 12-billion-euro turnover in equipment installed annually worldwide. You've even used it yourselves. I'm thinking particularly of those I lost when I spoke about video games earlier, and who, since I mentioned them, have been trawling the Internet to find out what I was talking about. Is this the end of the marriage between optics and coherence? Probably not. Imagine a world where fiber could read its acoustic environment, be sensitive to what's going on around it, for example, an excavator passing by. The fiber would be informed of what is going on before it was cut. Or cars passing through our region could be counted in order to regulate traffic. This kind of resonance is the kind we are looking to use in the future. You understand now that I almost missed this exciting adventure. It was very near thing. Just that one minute of distraction when I misplaced an exam example. That one minute whereby a science less familiar to me opened itself to me. What if that one minute, for you, was now and in this room? Thank you. (Applause)