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Fiber Optics, Part 2

Let’s continue our discussion about the use of Fiber Optic (FO) cable in production and broadcast studios. In last week’s column we looked at basic optical power calculations and cable construction. Now let’s look more closely at types of FO cable.

First, by clarification when we discuss FO cable, we’re usually talking about the optic aspects of cable, not its physical housing characteristics. Those characterizes are usually called; zip cord, distribution, loose tube and breakout. Those names refer to how the fibers are packaged.

Choosing the right fiber

Fiber, like copper, has multiple versions, or technology, each optimized for certain types of applications. Fiber types may be summarized as multimode, with primary sub categories of step-index multimode and graded-index multimode and single mode. These classifications are based primarily on how light is transmitted through the fiber.

Multimode step-index FO cable, shown in See Figure 1, has the highest attenuation and, hence, the lowest bandwidth and typical the lowest cost. This type cable is appropriate for both shorter lengths and lower-speed signals. The actual core fiber of multimode step-index fiber is about the size of a human hair, ranging from 50 to 62.5 microns. For fiber cables, this is a relatively large size. Because of this larger size, light is dispersed into many “modes” or paths, with the light rays bouncing (refracting) from side to side as it traverses the length of the cable.

Recall from Part 1, light that enters the cladding is lost. So, any beam with a high incidence of refraction is sent into the absorbent cladding. Light with a lower angle of refraction reflects from the cladding and continues down the cable. An analogy would be firing a shotgun down a conduit. While lots of pellets enter the conduit, many bounce around inside and never exit. What few that do exit represent the available light passed through a multimode step-index fiber.

Because of cost, multimode-step index fiber is widely implemented as plastic optical fiber (POF) for consumer electronics, entertainment systems and automobile applications. A POF cable has about the same performance capability as that of CAT 5 UTP and the cost is similar.

Now let’s look at multimode, graded-index cable. See Figure 2. This all-glass multimode FO is commonly found in LANs and security applications. The graded-index glass core provides hundreds of times more bandwidth than POF cable and with lower attenuation. Common multimode, graded-index cables are classified by the FO glass core diameter, bandwidth and overall cable diameter.

The name “graded-Index” nomenclature comes from the construction of the core. By applying variations in the construction of the core, a manufacturer creates a changing refractive index value that is “graded”, or changes, from the center to the outer edge of the core. At the most edge of the fiber, the refractive index matches that of the cladding. This construction as the effect of slowing down the light rays by different amounts, based on distance from the core’s center, thereby reducing overall path length differences. Graded-index fiber supports both longer distances and higher bandwidth.

The outside diameter of this these glass fibers is 125 microns (about .0005in) or slightly larger than a human hair. The a multimode graded-index core has a diameter of either 62.5 or 50 microns. The industry’s naming convention is 62.5/125 or 50/125 fiber. The 62.5/125 fiber cable is most commonly known as OM1 Fiber. It works well with LED transmit sources and is usable for signals with up to 200Mb/s bandwidths, such as Fast Ethernet.

There are two common FO cables in the 50/125 class. The first is the older, and slower, OM2 fiber. It operates up to 1Gbps. An OM3 fiber is optimized for laser transmitters and provides capability up to 10GBps. While the OM3 fiber costs more, it is upgradeable to even faster speeds. Naturally, actual bandwidth is length dependent.

Graded-index FO cables have greater bandwidth capacities than step-index fibers. A 600 MHz graded index fiber can transmit a 20 MHz modulation signal as far as 30 km. This cable’s low cost and high bandwidth make it the a good choice for LAN applications.

A graded-index multimode cable works over longer distances than a step-index cable and is the most common type multimode cable used today.

For very high-speed links, those operating above 10Gb/s or those links greater that 1km, many applications rely on single-mode fiber. This type FO cable, shown in Figure 3, provides low attenuation, and almost unlimited bandwidth. However, because of the much higher costs of the required laser transmitters, these links are typically too expensive for short links or lower-bandwidth applications. A single-mode fiber relies on a single beam passing through the center of the glass. Think of single-mode transmission as a rifle shot, whatever gets transmitted, arrives at the other end. No bouncing off the side boundaries.

A couple of key manufacturing differences for this type FO cable. First, it uses only one material for the core and one for the cladding. Most importantly, the size of the glass fiber is shrunk to less than 9 microns. This fiber is so small it requires a microscope to see it. Using a single focused beam, there is virtually no intermodal dispersion, resulting in an operational bandwidth that exceeds 100GHz over long distances.

Fiber or copper?

When considering fiber cable, engineers often begin by asking about cost. One study by Pearson Technologies and the Fiber Optics LAN Section of the TIA found “In many cases, deploying multimode fiber cable throughout the network is significantly less expensive than installing new grades of UTP copper cable.”

A word of caution. In the video space, not all equipment vendors agree on the best type of cable to use in broadcast and production applications. One vendor recommends single mode because it handles more bandwidth and is easily upgradeable from LED to laser transmitters. But, an equally qualified manufacturer may recommend you rely on multimode cable because of the lower cost and easier installation. This decision is not a minor one and should not be taken lightly (sic).

While the benefits of fiber are many, video engineers have a lower comfort factor with fiber, primarily because they have less experience with it. In many, if not most TV plants, fiber remains an unknown, but that is changing. Next, we’ll look at connectors.

Check out Fiber Optics, Part 1, if you missed it.

Editor’s note: Figures courtesy Lynx Technik