Better Understanding WDM Technology

Here are some information about the wavelength and wave division multiplexing(WDM), hope this post will help you better understanding WDM technology.

Wavelength
This is where the fun starts. Back in the day, about the best they could do was 850nm (nano-meter) wavelength light--it's visible to the naked eye and red.

These days, a bunch of wavelengths are used. The more common ones you'll see are 850nm on MM, 1310 on MM and SM and 1550 on SM.

Wavelengths are often called "colors" because, well, different colors are different wavelengths. You can't actually see the wavelengths used in fiber optics (except for 850), but they're still called colors.

You'll also see a bunch of other ones...1490 is becoming more common. When we get to WDM, you'll understand more.

Wave Division Multiplexing
This is called WDM. WDM comes in a couple flavors, but basically you use different wavelengths of light and a prism to combine them onto a single strand at one side and another prism to split them back out into different strands at the other. This lets you put more than one connection on the pair, and none of them interfere with each other. They don't even have to be the same speed or anything, and the equipment can be anything you want--so long as you can get the right optics. You will see fiber-channel, Ethernet, and SONET all on the same pair of fiber between buildings sometimes.

You take a massive power loss on passive WDM equipment. You can use active equipment or higher power optics if you need that.

Probably the most common place you see WDM is in bi-directional(BiDi) optics. You have to use them in matched pairs but one end transmits on one frequency and receives on another. The other end does the opposite. There's essentially just a piece of glass in each optic that accomplishes this. They're usually color coded (eg, blue at one end, green at the other). When you're dealing with interconnecting two different vendors equipment, you have to read the optics' specs to find out if they're compatible. Common pairs are 1310/1550, 1310/1490 and 1310/1270. This is normally where you'll see colors on optics other than blue. Sometimes the manufacturers will change the colors on you and you have to be aware of wavelengths--for example, a vendor we used to use went from blue/green to green/yellow at one point. Hard to keep straight, so just read the optic.

40G is 4x10G channels already muxed on the two strands. 100G is 4x25G channels, so you can't do WDM as easily with them. You'll have to find special muxes and optics in that case.

WDM systems are divided into different wavelength patterns, conventional/coarse (CWDM) and dense (DWDM). CWDM systems provide up to 8 channels in the 3rd transmission window (C-Band) of silica fibers around 1,550 nm. DWDM uses the same transmission window but with denser channel spacing. Channel plans vary, but a typical system would use 40 channels at 100 GHz spacing or 80 channels with 50 GHz spacing.

Common DWDM equipment include DWDM multiplexer, DWDM SFP and DWDM SFP+. CWDM equipments include CWDM multiplexer, CWDM SFP and CWDM SFP+. These items are wide used in high-date rate networks.