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Optical Fiber Cables

In the world of voice and data communications, one phrase pops up again and again: optical fiber.

No wonder.  The most important characteristics of almost all new communications system is bandwidth - the amount of signal you can get from point A to point B.  And there is nothing that can compare with optical fiber bandwidth capacity.

Consider this:  While twisted-pair conductors can carry a few megabits per second (Mbps), coaxial cables go up to 1 gigabits per second (Gbps), and satellites can carry a few GB's, fiber's limit is not even known!  (Systems in operation today have bandwidth up to 400 Gbps.)

Furthermore, thanks to fiber, the price of bandwidth is going down, just like the price of processors has dropped in the past 10 years.


There are two primary types of optical fibers in widespread use today - multi-mode and single-mode.

Simply put, multi-mode fibers are those with multiple pathways through which light travels.  Within the core of these cables are several hundred layers of glass, each with a lower index of refraction as you move outward from the center.  Since light travels faster in the glass with lower indexes of refraction, the light waves refracted to the outside of the fiber are speeded up to match those traveling in the center.

If that explanation is too complicated, don't worry.  The most important thing to know about multi-mode fibers is that they are fine for distance of up to about a kilometer and slower bandwidths.  That makes them suitable for data networks within buildings or between nearby buildings.

Single-mode fibers, on the other hand, are considered the more appropriate choice when running cable longer than a kilometer, when putting through a signal of more than 2 or 3 Gbps, or when the system must be "future-proof."

The reason single-mode fibers have such a great capacity lies in their design.  The core of a single-mode fiber cable is extremely small - usually between 8- to 9-micron (compared with 62.5-micorn in the typical multi-mode fiber cable) - and only one light wave at a time can be transmitted down the core.  Because of quantum mechanical effects, the light traveling in the very narrow core stays together in packets, rather than bouncing around the core of the fiber.

While this enables single-mode fibers to handle far more signal over far greater distance than their multi-mode cousins, single-mode fibers are considered to be a bit more difficult to terminate than multi-mode fibers.  Plus, single-mode fibers require lasers as their light sources, which are more expensive than the LEDs that are typically used with multi-mode fibers.


You might think that optical fibers are quite fragile - they're made of glass, after all.  But optical fibers are a lot stronger than fine glassware, and once they're enclosed in protective cables, they can be even tougher than copper communication cables.

To achieve this strength, all optical fibers are designed in three concentric layers.  At the center is the core, the central piece of glass that carries the light.  Surrounding the core is the cladding, a second layer of glass, which keeps the light from escaping the core.  And around both of these lies the buffer, an outer layer of plastic, which provides the real protection and strength.

There are two basic types of buffers in use today: tight buffers and loose tubes.

Tight buffers consist of a thin layer of plastic extruded over the individual fibers, much like the insulation placed on copper wires (except a lot smaller).  Tight buffer configurations are generally used with indoor cables, though they can also be found on some outdoor cables.

Loose tubes, meanwhile, are used in most outdoor cables.  In a loose-tube setup, one or more fiber (usually up to 12) are installed inside of a plastic tube that resembles a drinking straw, and the assembly is then bound into the cable.  Frequently, these tubes are filled with a water-block gel (usually and descriptively called "icky-pick" on the job site), which prevents water from entering the cable.

The loose tube assembly provides particularly good protection for the fibers, but such cables are messy to terminate and, because the fibers have a tendency to fall out of the vertical cables, should never be used as risers.

For added strength during pulling, most fiber cables also contain a strength member.  Except for very short, easy runs (like 20 feet, between two outlets in the same room), every time you pull a fiber cable, you must attach the pull string of the fish tape to the strength member.  If you do not, you stand a good chance of ruining the cable.

With layers cables, strength members may be made of a steel rod, steel wires, or fiberglass/epoxy rods.  Most strength members, however, consist of a layer of Kevlar yarn in the cable.  (Warning: use special scissors to cut the Kevlar.  This is the stuff they make bullet-proof vests out of, and cutting it with your lineman's pliers will ruin them quickly.)


Beyond these basic configurations, optical fiber cables can take many forms.  Here's look at some of the most common:

Simplex and zip cord cables consist of one or two fibers that have been tight buffered and Kevlar reinforced and jacketed.  Such cables are used mostly for patch-cord and backplane applications.

Tightpack cables, a.k.a. distribution cables, are made up of several tight buffered under the same jacket with Kevlar reinforcement.  These cables are small in size, but because their fibers are not individually reinforced, they need to be terminated inside a patch panel or junction box.  They are generally used for short, dry conduit runs as well as riser and plenum applications.

Breakout cables are made of several simplex units, cabled together to produce a strong, rugged design suitable for conduit runs and riser and plenum applications.  Because each fiber is individually reinforced, this type of cable allows for a strong termination to connectors and can be brought directly to a computer backplane.  Breakout cables are larger and more expensive then tightpack cables.

Loose tube cables are considered ideal for outside plant trunking applications.  Depending on the actual construction, they can be used in conduits, strung overhead, or buried directly into the ground.

Composite cables are those that contain a number of copper conductors (jacketed and sheathed depending on the applications) in the same cable assembly as optical fibers.  Be careful of the terminology here.  Prior to the 1993 NEC, such cable were known as hybrid cables, but today that term typically refers to cables that contain only optical fibers - both multi-mode and single-mode fibers.