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Fiber

Fiber cable comes in two varieties. multi-mode and single-mode. Multi-mode is used within buildings and data centers, single-mode fiber is what is used for long haul and FTTH applications.

A fiber cable is a bundle of anywhere from 1 to 576 individual fiber strands encased in various protective sheathing. At a minimum, the cable provides protection against water intrusion, but can also have armor plating (for direct burial applications), or have integrated messenger wire (for stringing between telephone poles). In addition, there is usually a strength member inside the cable to give the cable rigidity and protection for the fibers. Fiber cable is shipped on reels and you can buy unbroken lengths up to 20km in some cases. Cable sizes come in increments of 12 fibers, so you'd have 12, 24, 36, 48, then it might jump to 96, then 144, 192, 288 and 576 fiber count cables.

Single-mode fiber can be constructed as loose tube or ribbon. Loose tube bundles 12 fibers into a buffer tube. Ribbon fiber also groups fibers into groups of 12, but does so glued side by side. This allows a ribbon of 12 fibers to be fusion spliced together in one pass rather than having to splice individual fibers. However ribbon fiber has several disadvantages including very stiff cables and lack of flexibility in installs, resulting in it typically being used only for long haul city to city networks.

Here's a typical loose tube fiber cable. You can see various outer protective sheathing and a strength member in the middle. Inside, there are numerous colored buffer tubes, and inside each buffer tube are 12 fibers. Each fiber strand is composed of a (again) colored protective sheath which holds the really thin 9 µm glass core surrounded by a 125 µm cladding. When you strip the protective sheath away from the core/cladding, as you do when fusion splicing, you can barely see the core/cladding with the naked eye.

The extremely small diameter of each fiber is why you can have a 288 fiber cable that is only 20 mm in diameter (0.8 inches).

The optical fiber itself can be made to different specifications. Very long haul fiber has a slightly different requirement set than a FTTH network. The fiber spec that most FTTH networks use is ITU-T G.652D. This is widely available and inexpensive. Note that these fibers are bend sensitive - you typically must not bend the individual fibers less than a 3 cm radius. That is why splice closures have internal plastic structures to route fibers so that they can never be bent more than that.

Depending on the exact electronics you will be using in your FTTH network, a G.652D fiber will be able to carry signals at least 20 km (for GPON networks), and up to 40 km (for point to point ethernet networks).

There is one other type of fiber you would use in a FTTH network, and that is generically known as bend insensitive fiber. These fiber cables have more optical loss than a backbone fiber, but they are very supple and can be installed inside a house or patch panel without worrying about bend radiuses.

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