Fundamentals of Optical Fiber Communication

July 1, 2011 - PRLog -- Fiber optic holds an advantage over copper media in its ability to handle high-speed signals over extended distances. Other advantages of fiber optic transmission include:


1. Higher bandwidth. Fiber has higher bandwidth than any alternative available.

2. Immunity from Electromagnetic (EM) Radiation and Lightning. Since optical fiber is made from dielectric (nonconducting) materials, EM radiation does not affect optical fiber.


3. Lighter weight. In real world applications, lighter fiber optic cables routinely replace heavier copper cables


4. Easily upgraded. The limitation of fiber optic systems today, and for years to come, is the electronics and electro-optics used on each end of the fiber.
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Structure of Optical Fiber

The optical fiber is made up of two concentric cylindrical strands of silica surrounded by a plastic coating.

The center most silica strand is the core of the fiber with a refractive index of approximately 1.48. The core of the fiber physically transports most of the optical power.

The core is surrounded by another strand of silica called the cladding. The cladding has a slightly lower refractive index, 1.46 and provides the interface that confines the optical signal to the core.

The outermost layer of the optical fiber is the buffer coating. This thin plastic covering protects the glass from mechanical and environmental damage.

Signal Transport Mechanism

Light is confined within the core of the optical fiber through total internal reflection. To understand the phenomenon of total internal reflection and how it is responsible for the confinement of light in an optical fiber, we will take a look at the ray theory (a ray of light incident on the fiber core).

Light enters the core of the optical fiber and strikes the core/cladding interface at an angle. If this angle is greater than the critical angle, then the ray will reflect back into the core thus experiencing total internal reflection. This ray of light will continue to experience total internal reflection as it encounters core/cladding interfaces while propagating down the fiber.
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Fiber Types

The two fiber types get their names from the way they transmit light.


1. Single Mode Fiber

Single mode fibers have a small core size   that permits only one mode or ray of light to be transmitted. This tiny core requires precision alignment to inject light from the transceiver into the core, significantly driving up transceiver costs. Single mode fibers are designed for systems of moderate to long distance (e.g., metro, access, and long-haul networks).

Beyond 550 meters at 10 Gb/s (or 1 km at 1 Gb/s), it is necessary to utilize single mode fiber. There are new choices for single mode fiber today, so be sure to consider your options. A bend insensitive full spectrum single mode fiber provides more transceiver options, more bandwidth, and is less sensitive to handling of the cables and patch cords than conventional single mode fibers.

Single mode fiber has an advantage of higher capacity/bandwidth and is also much less sensitive to the effects of dispersion than multimode fiber. It is also possible to incorporate wavelength division multiplexing techniques to further increase the transmission capacity of a single mode fiber.

An important aspect of fiber communication speed growth is that the increase results from changing the electronics, not the single mode fibers, at either end of the system. The capacity of a single mode system is limited by the capabilities of the electronics, not of the fiber. One advantage of single mode fibers is that once they are installed, the system’s capacity can be increased as newer, higher-capacity transmission electronics becomes available.
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2. Multimode Fiber

Multimode fibers have larger cores that guide many modes simultaneously. The larger core makes it much easier to capture light from a transceiver, allowing source costs to be kept down. Similarly, multimode connectors cost less than single mode connectors due to the more stringent alignment requirements of single mode fiber. Multimode connections can be easily performed in the field, offering installation flexibility, cost savings, and peace-of-mind.
Large core fiber is attractive due to the ease in which light from a source can be coupled into the fiber, significantly reducing the cost of transmitter design and packaging. Multimode fiber is very sensitive to dispersion, which tends to limit the distance and bandwidth of an optical system.

Enterprise environments present particular network challenges, including limited spaces and tight bends, high connection density, and components that get handled frequently. Multimode fibers are ideally suited for these conditions. And since distances within a premises system rarely approach 550 meters, multimode fiber should be the choice for these applications.

The network designer or end user who specifies multimode fiber for short reach systems still must choose from two types – 50um or 62.5um. Today, 50um laser-optimized multimode (OM3) fiber offers significant bandwidth and reach advantages for most building applications, while preserving the low system cost advantages of 850nm based multimode fiber.

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