Download Complete Fiber Optics User’s Guide
In addition to fiber optics technical advantages, the cost of materials for Fiber optics is becoming more attractive because the cost of copper wire has risen substantially recent years.
A significant benefit of fiber-optic transmission is the capability to transport signals long distances. Basic systems are capable of sending signals up to 5 km over multimode fiber and up to 80 km over single mode without repeaters. Most modern fiber-optic systems transport information digitally. A digital fiber-optic system can be repeated or regenerated virtually indef- initely. An electro-optical repeater or an erbium doped fiber amplifier (EDFA) can be used to regenerate or amplify the optical signal.
As discussed in previous sections, fiber has a bandwidth of more than 70 GHz using typical off-the-shelf fiber-optic transport equipment. Theoretically, hundreds, even thousands, of video and audio signals can be transported over a single fiber. This is achieved by using a combination of time-division multiplexing (TDM) and optical multiplexing. Fiber-optic transport equipment is readily available to transport more than
8 video and 32 audio channels per wavelength. Off-the-shelf coarse wave-division multiplexing CWDM equipment easily provides up to 18 wavelengths. This combination of equipment provides up to 144 video and 576 audio channels, as shown in Figure 6.10-9.
Fiber-optic cable is very small in diameter and size when compared to copper. A single strand of fiber-optic cable is about 3 mm. A video coaxial cable is typically much larger. Fiber cable facilitates higher capacity in building conduits. There is often limited space in existing building conduits for infrastructure expansion. In mobile and field productions for sports and news events, fiber is often the cable of choice due to space limitations in a mobile and electronic news-gathering vehicle.
A fiber-optic cable is substantially lighter in weight than copper cable. A single core PVC-jacketed fiber weighs about 25 pounds per kilometer; RG-6 copper coaxial cable may be three to four times as much.
A signal traveling on a copper cable is susceptible to electromagnetic interference. In many applications it is unavoidable to have to route cabling near power substations; heating, ventilating, and air-conditioning (HVAC) equipment; and other industrial sources of interference. A signal traveling as photons in an optical fiber is immune to such interference. The photons traveling down a fiber cable are immune to the effects of electromagnetic interference. In military applications, fiber systems are immune to an electromagnetic pulse (EMP) generated by a nuclear explosion in the Earth’s atmosphere. Fiber-optic equipment is used in command and control bunkers to isolate facilities and systems from EMP interference. A fiber-optic signal does not radiate any interference or noise.
FIGURE 6.10-9 Time-division and optical multiplexing equipment offers substantial capacity for carrying video and audio signals.
Ease of Installation
One of the myths regarding fiber is that it is difficult to install and maintain. This may have been true in the early days, but now it is as simple to terminate an optical fiber with a connector as it is to install a BNC connector on coax. Fiber-optic termination kits are now available that require no epoxy and special pol- ishing. Simple cable stripping tools are used, similar to those used for copper coax, to prepare the fiber for ter- mination. Epoxy-free connectors are available to terminate both multimode and single-mode fiber optic cable. The connectors are already prepolished. No polishing equipment is needed.
Over the years as fiber-optic communications have grown and changed, there have been many different types of connectors. Today there are four common connector types that are used in most fiber-optic applications (Figure 6.10-10).
The first is the ST connector (Figure 6.10-10(a)). It is a bayonet-style connector similar to a coaxial BNC connector, and is available for single-mode and multimode applications.
The next style is the FC connector (Figure 6.10-10(b)). This connector has a threaded screw–type receptacle. It is similar to an RF-type connector, and is only used for single-mode applications.
The telecommunications industry standardized on the SC connector (Figure 6.10-10(c)). It is a square snap-in–type connector and has gained popularity in the video and computer networking industries. Telecommunications and networking applications typically require two fibers: one for transmitted data and one for received data traffic. Since SC-type connectors were popular in these types of applications,two SC connectors were required. As the size of fiber equipment reduced and the density of fiber-optic input/outputs (I/Os) increased, a small alternative to the SC connector was required. This led to the LC connector as shown in Figure 6.10-10(d). An LC is approximately half the size of an SC connector. It is rectangular in shape and has a locking clip.
FIGURE 6.10-10 Fiber-optic communications connector types.
Ease of Splicing
Another myth is the repair or maintenance of a broken or cut fiber. The cost of fusion splicing equipment has come down significantly. The fusion splicer is a small portable device that is easily carried in the field.
A fusion splice is easy to perform. First, the fiber is stripped and prepared using simple tools. The fiber is then placed in the fusion-splicing machine. An LCD screen shows the device automatically aligning the fibers. With the press of a button a fusion arc is generated to splice the fibers together. The fusion splicer even tests the connection when complete.
There is now an even simpler way to splice a fiber in the field—mechanical splicing. A mechanical splice consists of a small device that is used to splice a fiber. It is about 2 inches long by 1/2 inch wide. The process involves first stripping the fiber-optic cable and then inserting the ends into the splicing unit with mating gel. A key is used to close and clamp the unit shut. The mechanical splice gives fiber installers the ability to splice and repair with inexpensive equipment in areas where no electrical power is available.
Radiation and Security
Fiber-optic transport is a secure means of communications. Since a fiber-optic cable emits or radiates no RF energy, it is impossible to passively listen or to tap into a fiber-optic circuit. The only way to tap into a fiber-optic cable is to physically cut the cable. An eaves-dropper would have to cut the fiber and install a splitter to tap into the fiber-optic link.The cut in the fiber and the inserted splitter can be detected by fiber-optic test equipment.
Fiber-optic cable is immune to most environmental conditions. Fiber-optic cable is capable of tolerating temperature extremes. Unlike copper cable, fiber is immune to moisture. Fiber is available with jacketing that is resistant to nuclear radiation. Many fiber-optic systems are used for the inspection of nuclear reactors. Many military applications require fiber-optic equipment and cable to have resistance to radiation.