Fiber for broadcast

A few simple guidelines toward making your first fiber network
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Building your first fiber network, whether for broadcast or computer applications, can be a bit daunting. However, there is no reason for the task to make you apprehensive. A few simple guidelines will make the task enjoyable and a true learning experience.

There are several options open to you as you build your new fiber network. While decisions about connector type and fiber mode may already have been made for you by the equipment manufacturer, you can still select the type of cable you will use, number of strands, brand of connector and whether you will use ducting or other protective cable raceways. All of these decisions will contribute to the quality and maintainability of your fiber installation.

Choosing a fiber-optic cable

If you are a product engineer, there are several choices you need to make regarding fiber diameter, mode, laser frequency and so on.

If you are an end user, the equipment designer will have made these choices for you. Matching the cable to its intended use is the next step.

Generally, cable is divided into two groups: multi-fiber cable, which is optimized for outdoor use between buildings or other exterior applications, and cable designed for indoor distribution. There are other cables available for intercity and long-haul applications, but the broadcaster typically does not install and maintain this type of fiber-optic network.


Figure 1. Belden M97112, single-fiber indoor-use cable. Click here to see an enlarged diagram.

Comparing cross sections of indoor and outdoor cables can tell us a lot about their construction and intended use.

Figure 1 shows a single-fiber cable suitable for indoor use. Note that it contains a single-fiber strand with a tight buffer surrounding it. The result is a robust cable that can be terminated directly without any special considerations. A broadcaster may choose to employ a cable like this as a jumper cable between a desktop graphics device and a wall plate.


Figure 2. Belden M9A029, multi-fiber indoor-use cable. Click here to see an enlarged diagram.

Figure 2 illustrates a multi-fiber indoor- use cable. This design contains two or more tight buffer cables surrounded by a common outer jacket. An important difference between this cable and a cable intended for outdoor use is that the outer jacket can be stripped away to reveal two fiber cables that are ready to be terminated. A cable such as this one may be larger in a multi-fiber configuration than its outdoor counterpart, but it will not require a breakout kit for termination. (More on breakout kits later.)


Figure 3. Belden M9A840, multi-fiber outdoor-use cable. Click here to see an enlarged diagram.

Figure 3 is a cross section of a cable designed for outdoor use. Note that there are several significant differences between this cable and the one in Figure 2. The first difference is that the cable contains several fibers separated by a loose buffer tube. The tube slides over the fibers, lubricated by a gel. Several fibers are contained in each buffer tube. Second, note that the individual fiber strands are not surrounded by a protective jacket. This saves space in the cable but means that you must use a breakout kit to terminate the cable. Third, the cable incorporates a water-blocking agent. Finally, note that the jacket on the cable is much thicker than the jacket used on indoor cable. It is treated to resist water and ultraviolet radiation.

Usually when going from outdoors to indoors, a transition of some sort is required. This involves breaking out the loose-tube individual fiber strands and connecting them to a panel. The other side of the panel is connected using indoor cable.


Figure 4. Fiber-optic breakout kit. Click here to see an enlarged diagram.

Figure 4 shows a typical breakout kit. The kit converts the loose-tube multi-fiber cable to several tight buffer individual fiber cables that can be terminated with the appropriate connector. The tight buffer cables are much more rugged and will withstand the environment of a broadcast facility much better than the individual strands of a loose-tube cable.

Connector selection

If you are concerned about terminating fiber cables yourself, there is good news: Terminating cables is much easier than it used to be.

If you ever tried terminating fiber cables in the past, you probably still have nightmares about epoxy ovens and sanding little figure-eights while repeating some magic incarnation. Well, all that has changed. 3M, Siecor and other companies now make crimp connectors and splices for fiber that allow you to terminate fiber easily and at a low cost. (See Figure 5.)


Figure 5. Fiber mechanical splice. Click here to see an enlarged diagram.

If you were not all that great at making splices in the past, you can probably do better with these. Your losses will be 1/2dB to 1dB per connector.

Gone are the days of the old bi-conic fiber connectors where alignment was a real problem. The FC connectors always mate correctly with little problem. (See Figure 6.) With a little practice, you should be able to terminate a fiber in under a minute.


Figure 6. Ceramic FC connector Click here to see an enlarged diagram.

Ducting

If you ever have the opportunity, take a tour of a large fiber installation. When you look at the equipment racks, one of the first things you notice is a large number of ducts running all over the building. Fiber-optic ducts are specially designed to carry fiber-optic cables and protect them from damage. They have carefully designed curves and openings that protect the cable from stress and excessive bending that could degrade the performance of the cable or possibly break it. Your decision to use ducting will be based on the amount of fiber you are installing and whether the fiber is concentrated in a central area.

Guidelines for building fiber

Here are some guidelines to consider as you build your fiber network:

  • Use the right cable. Broadcast installations are not typical telco or cable system installations; our needs are specific. For example, using a loose- tube cable when a tight buffer cable is required can cause maintenance headaches down the road.
  • Use breakout kits and termination panels to avoid cable damage and down time.
  • Keep clean. Dirt is a serious problem in fiber installations. If you are building a network, but the termination equipment is not yet installed, be sure to use dust caps to cover the ends of the cables.
  • Avoid unnecessary transitions. Each transition introduces from 1/2dB to 1dB of loss per connector. If you are in an indoor/outdoor application, investigate some of the hybrid products available that will keep you from adding a transition at each building entry point.
  • Run more fiber than you need. The labor cost of installing fiber is much more expensive than the fiber itself. Figure out how many fibers you need and then multiply the number by three. I used to say multiply by two, but experience has proven that this is inadequate.
  • Run multiple fiber types. If your equipment uses multi-mode fiber, pull in some single-mode fiber along with it. Use of multi-mode fiber is on the decline. You may avoid a costly cable pull later if you install cable now.
  • Talk to your equipment manufacturer. You can learn a lot about fiber optic technology from these experts.

Brad Gilmer is president of Gilmer & Associates, executive director of the AAF Association and executive director of the Video Services Forum.