Fiber optics technology is among the fastest-growing communications technologies in the world. It is increasingly being used for communications, signaling, and electrical control instead of older-technology hardwired systems.
Basic fiber systems allow the signal to travel anywhere from three to seven miles without repeaters or amplifiers, and very high-performance systems allow the signal to travel as far as 72 to 75 miles without a repeater. The demand for high-speed Internet communications is driving the industry to consider optical fiber as their primary means of transmitting telephone, television, and Internet information.
Understanding some of the terminology and design criteria is essential before even considering the installation requirements for optical fiber cables. An optical fiber cable transfers light as a ray or beam at a certain frequency and wavelength. The light beam must originate from a transmitter and terminate at a receiver.
The light beam normally travels in a straight line and can be directed down a fiber cable, a highly refractive light tube. The tube that carries the light beam or ray can be constructed of many different materials, but is often a very high grade of glass, silica, fused quartz, or plastic.
The fiber cable acts as a wave guide or a travel medium for the light. The center of the cable is called the core. It is surrounded by insulation or cladding, which helps to contain the light beam. It will tend to absorb a very small amount of the light beam or light ray, depending upon the material used for the cladding. The core and cladding is also surrounded by an outside coating or jacket, which helps protect the fiber and cladding from abrasion and external pressure. It also acts as a shock absorber for the fragile interior components.
The core of the cable, where the light actually travels, has a refractive index that has been graded for its ability to transmit light through the fiber with as little dispersion or spreading of the light beam frequency as possible. Since light travels through various mediums at different speeds, each material has a basic index of refraction based upon the velocity that light would travel through the material.
Air has an index (n) of 1.00 with a velocity of light at around 300,000 kilometers per second. Glass has an index (n) of about 1.50 with the velocity of light at around 200,000 kilometers per second. The core of the optical fiber cable has almost the same index of refraction and about the same velocity of light as glass. The cladding usually has a slightly different index to help contain and direct the light beam.
Since both the core and the cladding are essentially composed of totally nonmetallic material, ordinary electromagnetic fields do not affect the signal. Therefore, shielding from the effects of electromagnetic interference (EMI) is not necessary. Some cables have a steel wire incorporated into the interior design of the cable to act as a messenger wire and to help protect the cable where it is installed in a raceway.
Two basic optical fiber types are commonly being installed: singlemode and multi-mode. Each type has a particular use in the communications industry, although there may be some overlap in usage.
A singlemode optical cable may have a core diameter of eight to 10 micrometers (microns), while a multi-mode cable may have a core diameter of between 50 and 1,000 micrometers. A singlemode cable allows a higher bandwidth to be transmitted over a longer distance than a multi-mode cable. The multi-mode cable, however, has a larger core, and will allow more modes of light to travel down it. The bandwidth of a digital system is the number of bits of signal per second that can be transmitted over the cable (expressed in Megahertz or Gigahertz) for a certain distance and a specific amount of time.
The National Electrical Code (NEC) provides installation requirements for optical fiber cables and their raceways in Article 770. Section 770-5 divides these fiber cables into the following three different cable types:
The nonconductive cable. This cable contains no metallic members and does not have any electrically conductive materials within the cable.
The conductive cable. This cable contains noncurrent-carrying conductive members, such as metallic strength members, metallic vapor barriers, and metallic armor or sheath, as well as other fiber cable.
The composite cable. These cables contain optical fibers and current-carrying electrical conductors. The composite cable can also contain noncurrent-carrying conductive members, much the same as the conductive cable previously mentioned. Composite cables are classified as electrical cables in accordance with the type of current-carrying electrical conductors or cables that are contained within the cable.
New cables are now being tested that can carry signals in the 1 Gbps and the 10 Gbps speeds. These cables will help revolutionize the telecommunications industry, but undoubtedly, they are just the beginning. As we learn more and more about this relatively new form of telecommunications, communications speed and system accuracy will only improve.
ODE is a staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at (919) 549-1726 or by e-mail at firstname.lastname@example.org.