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Transoceanic Fiber Cable: a Luminary Discovery

March 24, 2011 - 0 Comments

By Steven Shepard, Contributing Columnist

This story is a bit more technical that what I’ve previously shared. That said, I’ll link to some definitions for you non-technical readers — I promise, this one is going to be worth the extra effort. However, a bit of technology is required in the telling — so please bear with me.

Let’s step back in time. The first transoceanic cables used copper wire as the conductor that carried signals between continents. Unfortunately, the technology at the time was such that the cables were extremely bandwidth-limited and could therefore support a very small number of simultaneous conversations.

Furthermore, the physics of metallic transmission dictated that the transmitted signals would decay over distance, making it necessary to amplify and/or regenerate the transmitted signal periodically. This was costly, and required additional circuitry to filter electromagnetic interference and increase the signal level every few thousand feet.

Now, think about this for a moment: it’s one thing to install repeaters or regenerators in a terrestrial circuit; every 6,000 feet or so — just hang the device on a nearby telephone pole.

But when deployed on the ocean floor, they have to be installed as part of the cable, made waterproof, and powered by what has to be an industrial-strength power feed alongside the transmission strands.

Fiber-Optics Transformed Undersea Cables

Needless to say, when optical fiber arrived on the scene, long distance communications companies celebrated. Whereas a copper facility must be “cleaned-up” (a process sometimes called ‘grooming’) every 6,000 feet or so, an optical signal decays much more slowly, and only needs grooming every 80 kilometers or so.

But the grooming process requires what is called an Optical-Electrical-Optical  (O-E-O) conversion, in which the optical signal is converted to electrical in the repeater, cleaned-up and amplified, and then converted back to an optical signal for transmission across the next 80-kilometer link in the optical cable chain.

This is much more efficient than the pure copper solution, but it still requires a low-voltage, very high current (1,000 amps) electrical feed to operate the O-E-O repeaters.

What if we could do this with a simpler infrastructure?

The Power of Excitement (the electrical variety)

Enter the Erbium-Doped Fiber Amplifier (EDFA). This is where the very technology part begins. Erbium is a rare earth element that, like all elements, becomes electrically excited when it is struck by energy at the right level. When certain elements (like erbium) are energized, their outermost electrons start vibrating, and somewhere along the way they jump up to the next-highest energy level.

When the energizing force is removed, the electrons fall back to their original energy level, and in the process also spit out a photon at a specific wavelength.

Many optical systems transmit their signals at 1550 nanometers, the wavelength of the data-carrying signal. Don’t ask; it’s for a specific reason, but it’s not important for our story. But consider this fact. It turns out that erbium has a rather intriguing property. When it is energized by a signal of the proper wavelength, it gets excited; it’s electrons jump up to the next orbital energy level; and when they drop back down again, they give off photons at — wait for it — 1550 nanometers. So what, you say?

Well, think about it. Erbium gives off light at precisely the wavelength of transmitted optical data signals, which means that we can “pump” 1550 nanometer photons into the fiber, thus amplifying the transmitted signal. Is that cool, or what? The fiber itself becomes an amplifier!

So how do we do this? It’s pretty simple, actually. Every 80 kilometers or so, we cut into the cable and insert a coil of what is called (don’t get excited, now) erbium-doped fiber. This is fiber that has had erbium atoms embedded in the silica matrix at the time the fiber is manufactured.

The coil of fiber, which is the EDFA I mentioned earlier, is powered by a small laser that pumps light into the erbium-doped coil at its excitation wavelength. As the data signal flows through the erbium amplifier, it is amplified enough to carry it along for a very great distance, thus dramatically reducing the need to perform an O-E-O grooming exercise.

Call it geeky, but I think it’s pretty darn cool that a very clever scientific luminary (pun intended) — likely somewhere in the depths of Bell Labs — figured out this creative and yet practical solution to an otherwise incredibly complex problem.

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