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FIBER OPTIC CABLE

Fiber cable comes in two forms: multimode and single-mode. A mode in optical transmission is a ray of light entering the core at a particular angle. Modes can therefore be thought of as bundles of light rays of the same wavelength entering the fiber at a specific angle.

Single-mode and multimode fiber have differences that range from structural to use in structured cabling systems. Single-mode fiber is capable of higher bandwidth and greater cable run distances (up to 3000 meters) than multimode fiber (up to 2000 meters only). Because of these characteristics, single-mode fiber is often used for interbuilding connectivity or WANs (for example telephone company switch-to-switch connection). Multimode fiber is more commonly used in LAN backbones within buildings.

Multimode fiber uses LEDs as the light source, while single-mode fiber generally uses laser light sources. Furthermore, single-mode fiber is typically more expensive than multimode. The reason is because the almost hair-size glass fiber in single-mode is more fragile and it needs added protection (coating and buffering materials) to make it manageable.
The much smaller and more refined fiber core in single-mode fiber, although it entails more manufacturing costs, is the reason single-mode has a considerably much higher bandwidth and cable run distances than multimode fiber.

Multimode
Multimode fiber allows multiple modes of light to propagate through the fiber-optic core, as compared to single-mode fiber, which only allows one mode. Multiple modes of light propagating through fiber might travel different distances, depending on their entry angles. This causes them to arrive at the destination (receiving end of the cable) at slightly different times, a phenomenon called "modal dispersion". Multimode uses a type of glass, called graded index glass, which has a lower index of refraction towards the outer edge of the core. This causes the light to slow down when passing through the center of the core and accelerate when passing through the outer areas of the core, ensuring that all modes of light reach the end at approximately the same time.

A standard multimode fiber-optic cable (the most common brand of fiber-optic cable) uses an optical fiber with a 62.5-micron core and 125-micron cladding diameter. This is commonly designated as 62.5/125 optical fibers. Because the diameter of the cladding is considerably larger than the wavelength of the light being transmitted, the light bounces around (reflects) inside the core as it is propagated along the transmission line.

Multimode fiber uses LEDs as the light-generating device. LEDs are cheaper to build, require somewhat less safety concerns, and are effective for shorter distances than the lasers used in single-mode. Multimode (62.5/125) can carry data over distances of up to 2000 meters (6,560 ft.). It is mainly used in LAN applications including backbone cabling.

Single-mode
Single-mode fiber uses only one mode of light to propagate through the fiber-optic core. In single-mode fiber-optic cabling, the core is considerably smaller (8 to 10 microns) in diameter. A 9/125 optical fiber indicates that the core fiber has a diameter of 9 microns and the surrounding cladding is 125 microns in diameter.

The core in single-mode fiber is only approximately 10 times larger than the wavelength of the light it is carrying. This leaves very little room for the light to bounce around. As a result the data carrying light pulses in single-mode fiber are essentially transmitted in a straight line through the core.

Typically single-mode uses a laser light source, which is more expensive to produce, requires higher levels of safety awareness, and can transmit data further than multimode. Single-mode (such as a 9/125) can carry data up to 3000 meters (9,840 ft.) according to the existing standard (note that the standard in this case may not reflect the physical limitation). Single-mode is often used in exterior segments and to connect buildings in larger campus environments.

There are several advantages that have lead to the ever-increasing development and implementation of fiber-optic cable systems. Compared to copper, fiber-optic is proving to superior in the following categories:

  • Electromagnetic immunity including non-conductivity
  • Security considerations
  • Decreased attenuation and increased transmission distance
  • Increased bandwidth potential
  • Small diameter and weight
  • Long term economics

Since fiber-optics use light to transmit a signal, it is not subject to EMI, RFI, or voltage surges. This can be important when laying cables near sources of these forces like motors, fans, some light sources (sodium vapor, mercury vapor, neon, and florescent), pumps, transformers, power lines, and so on. In some factory or industrial environments, these factors can be so great as to make any other communication media virtually worthless. Since fiber does not use electrical impulses and therefore cannot produce or transmit electric sparks, it becomes the logical solution for passing through flammable environments like paint rooms, solvents facilities, or even fuel tanks. Furthermore, the non-conductivity nature of fiber-optic makes it great choice for areas of high lightning-strike incidence and even running through liquids, such as running under the oceans. Finally, a fiber-optic connection avoids the problem of differing ground potentials and eliminates the danger ground loops pose to personnel and equipment. Fiber in effect isolates devices connected to either end of it, making it a good choice where completely separate systems are linked together, such as two LANs in different buildings.

Unlike metallic-based systems, the use of light in optical fiber makes it impossible to remotely detect the signal being transmitted within the cable. Signals sent on copper wires can be intercepted by devices placed in close proximity to the cable. The only way to tap a fiber circuit is to actually access the optical fiber itself, which requires intervention that is easily detectable by security surveillance. As a result, fiber is usually the choice of cable used by governments, banks, and other organizations with major security concerns.

An optical repeater is used to boost the light pulse in a fiber-optic cable. The advantage of optical fiber is that it performs better with respect to attenuation. Fiber-optic cable needs fewer boosting devices than copper cable. Long, continuous segment lengths of fiber-optic cable also provide advantages for manufacturers, installers, and end-users.

Currently, the fiber circuits used in trunk connections between cities and countries carry information at up to 2.5 gigabits per second (Gbps). This is enough to carry 40,000 telephone conversations or 250 television channels. Industry experts predict larger bandwidths than this as light frequency separation becomes available. Private communication systems are already using much higher bandwidths.

Compared to copper, optical fiber is relatively small in diameter and much lighter in weight. These characteristics have made it desirable as intra-floor conduits and wiring duct space has become increasing plugged with expanded copper cable installation. It is even becoming commonplace to install new fiber cabling within existing duct systems to replace many copper circuits and free up much needed duct space.

  • A 1 cm, 24-strand fiber cable operating at 140 Mbps carries the same number of voice channels as a 7.5 cm 900-pair copper cable.
  • One kilometer of this 24-strand fiber cable weighs approx. 60 kg while the 900-pair copper cable weighs approx. 7250 kg.
  • One single strand of single-mode fiber can now carry up to 5 million phone calls simultaneously.

While increased demand for optical fiber has brought the prices down to be more competitive with copper, it is still true that new fiber installations cost more than copper installations. This imbalance shifts more towards copper when considering extending existing copper networks. In the short term it is often less expensive to continue using copper cabling for covering expanded communication needs. By simply adding more wire to an existing system, expanded needs can be covered. Since transmitters, converters, optical repeaters, and a variety of connecting hardware will be needed; the initial cost of changing over to fiber can be quite expensive.

The most often identified disadvantages of fiber-optics include:

  • Higher initial cost than copper
  • Fiber can be less forgiving of abuse than copper cable
  • Fiber connectors are less forgiving of abuse than copper connectors
  • It takes a higher level of training and skill to terminate fiber
  • The installation tools and meters are still more expensive

 


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