Fiber Optic Overview:
Fiber Optic cable employs photons for the transmission of digital signals. A fiber optic cable consists of a strand of pure glass a little larger than a human hair. Photons pass through the glass with negligible resistance. The glass is so clear that, according to Michael Coden of Codenoll Technologies Corporation (a major fiber vendor), "a 3-mile-thick fiber optic window would give you the same view as a 1/8-inch-thick plate glass window." The optic core of fiber optic cable is pure silicon dioxide. It makes for good tricks. You can wrap it around yourself, then shine a light in one end and see that light on the other end. Copper cable, on the other hand, is subject to problems with attenuation, capacitance, and crosstalk.
Fiber optic cable is resistant to electromagnetic interference and generates no radiation of its own. This last point is important in locations where high levels of security must be maintained. Copper wire radiates energy that can be monitored. In contrast, taps in fiber optic cable are easily detected. fiber optic cable also extends to much longer distances than copper cable.
Information is transmitted through fiber optic cable by pulsing laser light. The electronic 1s and 0s of computers are converted to optically coded 1s and 0s. A light-emitting diode on one end of the cable then flashes those signals down the cable. At the other end, a simple photodetector collects the light and converts it back to electrical signals for transmission over copper cable networks.
Figure 1-A illustrates the fiber optic cable structure. The core is the transparent glass (or plastic) component of the cable. Light shines through it from one end to the other. The cladding, which is a glass sheath that surrounds the core, is a key component. Like a mirror, it reflects light back into the core. As light passes through the cable, its rays bounce off the cladding in different ways as shown in Figure 1-B.
FIGURE 1-A. fiber optic cable structures
FIGURE 1-B. Light dispersion in fiber optic cable
A "dopant" is added to the core to actually make it less pure than the cladding. This changes the way the core transmits light. Because the cladding has different light properties than the core, it tends to keep the light within the core. Because of these properties, fiber optic cable can be bent around corners and can be extended over distances of up to 100 miles.
A typical laser transmitter can be pulsed billions of times per second. In addition, a single strand of glass can carry light in a number of wavelengths (colors), meaning that the data-carrying capacity of fiber optic cable is potentially thousands of times greater than copper cable.
The types of fiber optic cable are listed here:
- Plastic cable, which works only over a few meters, is inexpensive and works with inexpensive components.
- Plastic-coated silica cable offers better performance than plastic cable at a little more cost.
- Single-index monomode fiber cable is used to span extremely long distances. The core is small and provides high bandwidth at long distances. Lasers are used to generate the light signal for single-mode cable. This cable is the most expensive and hardest to handle, but it has the highest bandwidths and distance ratings.
- Step-index multimode cable has a relatively large diameter core with high dispersion characteristics. The cable is designed for the LAN environment and light is typically generated with a LED (light-emitting diode).
- Graded-index multimode cable has multiple layers of glass that contain dispersions enough to provide increases in cable distances.
Cable specifications list the core and cladding diameters as fractional numbers. For example, the minimum recommended cable type for FDDI (Fiber Distributed Data Interface) is 62.5/125 micron multimode fiber optic cable. That means the core is 62.5 microns and the core with surrounding cladding is a total of 125 microns.
- The core specifications for step-index and graded-index multimode cables range from 50 to 1,000 microns.
- The cladding diameter for step mode cables ranges from 125 to 1,050 microns.
- The core diameter for single-mode step cable is 4 to 10 microns, and the cladding diameter is from 75 to 125 microns.
Optical Cable Types:
The following cable types are available from Optical Cable in Roanoke, Virginia. They are listed here as a representative sample of the cable types available from various manufacturers.
- A-Series simplex and duplex interconnect cables are flexible, resilient, and ideal for patch cords and jumpers.
- 12-fiber standard B-series breakout cable has up to 156 fibers. It is designed for direct terminations with connectors in local area networks.
- 12-fiber standard D-series distribution cable has up to 156 fibers. It is compact in design and ideal for longer trunking distances. The cable is designed for direct termination with connectors within patch panels.
- 36-fiber DB-series subgrouping cable has up to 1,000 fibers. It is designed for high fiber count packaging with easy direct termination. Plenum-rated (firesafe) cable is also available.
- 12-fiber D-series S-type plenum cable has 12 fibers and is designed for plenum areas.
- 4-fiber standard B-series plenum breakout cable has up to 102 fibers and is designed for plenum areas.
- D-series distribution armored cable has 156 fibers and is designed to be "rodent-proof" in direct-burial environments.
- M-series aerial cable has up to 48 fibers and a stainless steel or all-dielectric messenger or self-supporting round cable for outside plant aerial installations.