Up until a couple of years ago, one never thought about choosing the fiber in the fiber optic cable you bought. If you were buying cable for outside plant applications like telephone or CATV networks, you chose single-mode fiber. If you were installing cable in a building for LANs or security systems—what we call premises applications, you chose multimode fiber. If you were planning for the future, your building or campus cable might include both multimode and single-mode fibers.

Today, there are lots more choices for fiber. Your choice depends on what you plan to use the fiber for now and what you think you might use it for in the future. We’ll concentrate on multimode fiber for the premises applications, which is relevant to most contractors’ work.

Outside plant

Practically all outside plant (OSP) cable uses single-mode fiber, for its higher bandwidth and longer distance capability. Until recently, the most popular fiber was designed for use at 1,310nm. But with today’s dense wavelength division multiplexing (DWDM) systems, the fiber must be optimized for use in the 1,550nm range, where DWDM and fiber amplifiers work best. The demand for this fiber is so great that volume deliveries are quoted with delivery times of about one year! If you plan to use much single-mode fiber, you had better order it now!

Premises applications

Until Gigabit Ethernet (GbE) was developed, all multimode fiber was optimized for fiber distributed data interface (FDDI), a 100 Mbps network that used light-emitting diode (LED) sources on 62.5 fiber. But GbE needed more bandwidth than was available from LED sources, which run out of steam at about 300 Mbps, so it was designed around lasers. GbE standards allow two types of lasers, an 850nm vertical cavity, surface-emitting laser (VCSEL) or a more normal 1,310nm telecommunications laser used in single-mode networks.

Fiber manufacturers have begun to offer special fibers that are optimized for lasers, both in the standard 62.5/125 micron size and a 50/125 micron size. Much has been written about the choice between these two fibers, both pro and con. Choosing between them can be difficult, but it really depends on whether the fiber being installed is likely to be used for networks at speeds of gigabits per second or beyond. If you do not plan on using the fiber for networks of over 100 Mbps, higher performance fiber is a waste of money.

LEDs and lasers work differently with multimode fiber. LEDs launch light with a wide beam of light, “overfilling” the core of the fiber, while lasers have a very narrow beam, “underfilling” the core. This difference in how they launch light into the fiber means the fiber performs differently with each type of source.

In multimode fiber, light travels in many different rays or modes of light in the core of the fiber. LEDs launch more light into the higher order modes in the fiber, that is the rays traveling more toward the outside of the core of the fiber, than the laser. The higher-order modes have the highest attenuation and lowest bandwidth, so the fiber will have lower loss and higher bandwidth with the laser source.

The fiber specifications, which are measured under “overfill” conditions appropriate for an LED, may not accurately predict fiber’s performance with a laser, since the higher order modes tested in a overfill launch (OFL) do not get used by lasers. These higher-order modes are the hardest to compensate for and cause much dispersion for LEDs. Thus a laser will normally have higher bandwidth than an LED in the same fiber.

To more appropriately characterize fiber bandwidth for lasers, the Electronic Industries Alliance/Telecommunications Industry Association (EIA/TIA) fiber committee has a new fiber optic test procedure (FOTP-204) that uses a narrower launch, called restricted modal launch (RML), which more closely approximates the laser launch into multimode fiber. Once we in the industry get some experience with RML, we can better decide if 50/125 fiber is really better than 62.5/125 fiber with lasers for GbE.

If you know the fiber optic cable plant you are installing will be used for gigabit networks, choose a fiber that has been characterized for use with lasers. It will still work for LEDs, if the current use is for slower networks, but an upgrade to higher speeds is planned. The choice between 50/125 and 62.5/125 fiber should be made on two issues: cost and compatibility with current cabling.

* Cost issues are unclear at this time. Both 50/125 and laser grade 62.5/125 fiber cost more than FDDI grade 62.5/125 fiber. The quantity you need and the type of cable you use will determine your costs.

* Compatibility is a big issue. You must make certain that any LED-based networks you plan to use will operate with the 50/125 fiber, and you cannot mix and match the two fibers. If you need to install more fiber in a network, understand that 50/125 and 62.5/125 fiber cannot be mixed. The larger core of the 62.5/125 fiber is carrying light in a larger size core, which will overfill the core of 50/125 fiber. The lowest excess loss we have seen with laser sources is about 2 dB, or over half the loss available on a GbE link!

If you decide to install 50/125 in a network where 62.5/125 fiber already exists, find a way to identify the cables with the two fibers positively. You can accomplish this by ordering patchcords with special colors or colored labels.

HAYES, a frequent contributor to Electrical Contractor, is president of Fotec in Medford, Mass. He can be reached at jeh@fotec.com.