Fiber-optic Cables and the Nature of Light

One of the most significant technological breakthroughs in data transmission has been the creation of the fiber-optic cable. It allows data rate of up to 1600Gbs using WDM. Currently, data rates and bandwidth utilization over fiber-optic cable are limited not by the medium but by the signal generation and reception technology available. The fiber-optic cable transmits electromagnetic signals known as visible light.

A fiber-optic cable has a cylindrical shape and consists of three concentric sections: the core, the cladding and the jacket. The core is made up of dense plastic or glass and it is here that the light travels. The cladding is made up of less dense material which reflects the light signals back to the core instead of absorbing it. The jacket surrounds one or more cladded cores to protect against environmental dangers.

Propagation Modes

Before we discuss propagation modes, we need to discuss how light travels through a medium. Light travels through a straight line as long as it is moving through a single uniform substance. When a light enters another substance it changes direction. If the angle of incidence (the angle the light makes with the line perpendicular to the interface between the two substances) is less than the critical angle, the ray refracts to the substance of lower density. If the angle of incidence is equal to the critical angle, the light bends along the interface. If the angle is greater than the critical angle, the ray reflects and travels again in the denser substance.

Multimode

Multimode fiber-optics transmits multiple light signals. How the beam travel depends on the density composition of the core.

Multimode step-index

The core is evenly dense. The light travels straight until it reaches the interface of the core and cladding. The abrupt change causes it to be reflected back to the core. Step-index refers to the abrupt change of direction of the light. There are multiple paths with varying lengths, therefore varying transmission time which limits the data rate. This is the least expensive propagation mode and its commonly used for local area networks.

Multimode graded-index.

The core is densest at the center and the density decreases gradually towards the edge of the core. This varying density allows the light to bend. This allows focusing the signals more efficiently than step-index. This propagation mode is moderately expensive compared to the other two modes and is commonly used for telephone lines.

Single-mode

Single mode uses step-index fiber and a highly focused light source that limits the light beam to a small range of angles close to the horizontal. The core itself has a small diameter and lower density making the path of the light almost horizontal. The propagation time for all signals are almost equal. Delays are negligible. This is the most expensive propagation mode for fiber-optics commonly used for long distance telecom lines.

Fiber Sizes

Optical fibers are defined by the ration of the diameter of their core to the diameter of their cladding.

Type	Core	Cladding	Mode
50/125	50	125	Multimode, graded-index
62.5/125	62.5	125	Multimode, graded-index
100/125	100	125	Multimode, graded-index
7/125	7	125	Single-mode

Connectors

Subscriber Connectors (SC) is used in cable TV. The Straight-Tip (ST) is used for networking devices. It is more reliable than SC. MT-RJ connector has the same size as the RJ45.

Advantages

1. Very high bandwidth and data rate
2. Less signal attenuation
3. Immunity to electromagnetic interference
4. Resistance to corrosion
5. Light weight
6. Difficult to tap

Disadvantages

1. Installation/maintenance
2. Unidirectional
3. Cost