Wavelength division multiplexing (WDM) allows multiple data channels to travel simultaneously over a single fiber optic cable by using different light wavelengths. This content covers bidirectional WDM, coarse WDM, and dense WDM, including how they differ in channel capacity, cost, and typical deployment distance.
Wavelength Division Multiplexing
We have a multiplexer and a demultiplexer. A multiplexer will take in several channels on one side, put it across the media, and then break it back down on the other side into several channels. So that's a multiplexer, or a mux/demux.
We can do the same thing with light. We can do division multiplexing with different wavelengths, so there is a wavelength division multiplexer, and we're going to talk about different ways that we can do that. Essentially we can shine different lights into one side of this and then break it back out on the other side. It's similar to if you have two lights that cross each other: they don't interfere with each other, and as long as we can break them out on the other end, then we can successfully determine the signalling on the other end.
One example of this is bidirectional wavelength division multiplexing, or BWDM. In this case we can have a laser pointing in this way and the fiber optics routes it in that direction, but we can also go the other direction as well. There's nothing that's colliding in the middle or going to interfere in the middle; they just pass through each other. We just need a receiver and a transmitter both on each side, and then we've got bidirectional wavelength division multiplexing.
Another way that we can do this is we can shoot different wavelengths in one side, combine them, and as long as we can separate them out on the other side, then we can do the same thing with this division multiplexing. Here's an example: I've shined different colors down here — blue, yellow, green, red — and then I can separate that back out with maybe some sort of prism on the other side, and then be able to interpret the data on the other side. So this is a way to use fiber optics to get those four different colors through here and onto the other side. We call this coarse wavelength division multiplexing.
Now, coarse wavelength, because there is a dense wavelength as well. Dense wavelength division multiplexing is essentially the same thing as coarse wavelength division multiplexing, but the difference between coarse and dense is that with coarse we're taking a wide range that has a lot of degree of separation between those wavelengths, and we're shining it in one side. Versus dense wavelength, where our colors and our wavelengths can be much tighter. So it's much more dense, and we actually have many more options and can fit much more down this pipeline, down this fiber optics line. With dense wavelength, just think that we're taking much smaller spectrums of wavelengths and we're shining it down this fiber optics.
Just a really quick overview between coarse and dense: coarse has fewer channels, because we're taking much bigger spectrums here, so we only have four or eight, or possibly we could fit 16, different channels into there. Versus the dense wave, where we have more like 40 or 80 channels, or possibly we could fit 160 channels down the dense wave.
The coarse wavelength is a little cheaper to implement, and you don't need as powerful equipment to operate, and so typically we'll see that with shorter distances. But with dense wave, we want to capitalise on these lines that are going these long distances, so to do that we'll want to use dense wavelength. So we tend to see the dense wavelength division multiplexing being used on those.
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