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September 10th, 2006

On "magical" technology

The University recently spent quite a lot of money buying four strands of "dark" fiber optic cable in a vague ring shape around the state, connecting the three campuses in Chicago, Urbana, and Springfield. To put actual networking services on this cable, you need something that can a) optically drive that much fiber so that the light signal is recoverable at the far end, in some cases hundreds of kilometers away; and b) can make best economic use of those very few strands of very expensive fiber.

The solution is a technology called Dense Wavelength-division Multiplexing or DWDM. This allows us to pack up to 40 channels, of up to 10 Gb/s each, onto a single fiber strand, by sending each one on a slightly different wavelength of light. Because the window of transparency of optical fiber is fairly narrow, the different wavelengths or DWDM channels are packed in pretty tightly, less than a nanometer apart. The physics involved in making all this work just boggles my mind.

You need something to break apart the combined signal with all channels on it into separate signals, one for each channel. Since this is just a matter of picking out different wavelengths of light, this is no more mysterious than a prism or diffraction grating. However, picking out a VERY FAINT optical signal out of an optical fiber, sending it through a diffraction grating, and then jamming the VERY PRECISE AND PACKED TOGETHER individual signals each back into their own optical fibers seems pretty amazing.

You need a way to reamplify or boost the signal every 50 kilometers or so. The economical way to do this is to figure out a way to boost the whole thing, all 40 channels at once, on the fly. This is done with something called an Erbium-doped Fiber Amplifier or EDFA. A "pump laser" raises the electrons in atoms of the Erbium dopant to a higher energy state; the signal photons flying down the fiber then interact with these energized atoms, stimulating them to emit more photons and thus amplifying the signal.

Finally, different wavelengths of light propagate through glass at different speeds (this is why a prism can break white light up into a spectrum). The narrowband pulses may be less than a nanometer in width, but this is still finite, so this chromatic dispersion will, over enough length of fiber, tend to smear out those pulses into miniature spectra. Eventually the pulses run into each other and the original signal is lost. To compensate for this, special glass is made with a chromatic dispersion coefficient opposite that of the "travel" fiber. So after traveling and smearing out over 70km of fiber or so, the signal is amplified and then sent into "dispersion compenstion fiber", 10km or so of it wound neatly on a spool. This reverses the smearing effect and allows the slow wavelengths to catch up with the faster ones, neatly and magically undoing the dispersion effect.

It's all magic. I can explain it, but to understand it fully requires math and physics that are way beyond me.

Several of us spent four days at Movaz, the company that makes the DWDM equipment we will be using on our dark fiber network. It was incredibly useful and incredibly information-packed. So much so that I felt a little dizzy and spaced out at the end of every day. But now I know an awful lot about their product and DWDM theory, and am confident that, even if I don't have to make this stuff WORK, that I can fix it if it breaks, and exploit it to its fullest potential.

We got to work on live equipment in their lab, and it was a fucking blast. I took a few pictures with my phone. Click for Gallery.

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Charley

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