What is Optical Fiber Cables

Optical Fiber

Optical Fiber Cables transmit data in the form of light. Its contain long thin strands of pure glass, called the fiber, with each strand having a diameter of about 5 microns. Optical fiber cables are generally used in pairs, in which, one of the cables transmits the data while the other receives the data. A typical optical fiber cable consists of the following three components:

Cross-Section of Optical Fiber Cable

Core: The core contains the optical fiber conductor (glass) that transmits light.

Cladding: The core is surrounded by another optical material to prevent any light from escaping the core. The function of cladding is to reflect the light back into the core.

Sheath or Outside Jacket: The core and cladding are covered with a sheath, usually made of plastic, to protect the fiber from damage.

Unlike coaxial cable and twisted-pair cables, which transmit data in the form of electric signals, optical fiber cables transmit data in the form of light. therefore, the data to be transmitted is converted into light by a CODEC (coder and decoder) present at each end of the fiber. the codec converts the data from the computer into light, and the light is then transmitted across the cable with the help of either a Light Emitting Diode (LED) or an Injection Laser Diode (ILD) at the destination computer, a decode receives the light beam and converts it into data. If an LED is used to transmit light, the transmission is called multi-code transmission, whereas and ILD is used for single-mode transmission. These transmissions are detailed in the following sub-topics.

Multi-Mode Transmission

A mode is defined as the angle at which a ray of light enters the core of the optical fiber cable. If the light enters the core at different angles, it is called multi-mode transmission. Multi-Mode Transmission occurs when an LED is used as the light source.

Multi-Mode Vs Single-Mode Transmission

An LED is not a concentrated light source, and, therefore, the rays of light beam disperse after travelling a certain distance through the fiber. However, not all the rays of the light beam disperse. The rays in the centre of the beam do not disperse whereas the rays on the circumference of the light beam disperse and hit the cladding. This effect is similar to the dispersion of light from a flashlight or the headlights of an automobile.

The dispersed light rays hit the cladding of the cable and are reflected back into the core. The dispersion and refection occurs at different angles, or modes, resulting in multi-mode transmission. This dispersion and reflection of light rays does not affect their strength. However, the rays on the circumference of the light beam travel a greater distance than the rays in the center of the beam. therefore, for a given signal, the dispersed light beams arrive later than the un-dispersed ones, a delay which can result in slower transmission rates.

Disadvantage of multi-mode transmission: collision of light beams due to dispersion and reflection. These collisions weaken the signal strength, resulting in attenuation.

Multi-mode transmission is inexpensive because the cost of LED is less than that of an ILD. Its preferred in LAN and in networks that connect computers in a large college or school campus, which may be spread across a few miles. The drawbacks of multi-mode transmission such as delay and attenuation occur only when the distance between the devices is more than a few miles. Therefore, multi-mode transmission is not suited for WAN.

Single-Mode Transmission
An ILD is used to emit a light beam (leaser) that carries data. ILD is an extremely concentrated light source, and therefore, the leaser beams do not disperse when traveling through the fiber. As a result, all the light beams reach the destination at the same time. In addition, the beams do not collide, thereby preventing any attenuation of the light signals.

However, the equipment required for single-mode transmission, such as the grade of fiber used and the ILD, are expensive. Therefore, single-mode transmission is used to connect computers that are part of a LAN backbone or in large WAN spanning across a country or a continent.

Backbone refer to a main network to which other networks are connected for communicating with one another. For example assume that an organisation consists of three departments: Systems Marketing and Finance. The computers in the Marketing and Finance departments may be connected to the System department, which acts as the central hub for connectivity. The computers of the System department can then be considered as the LAN backbone. Optical fiber cables are also used to connect computers that are far apart, in which cases coaxial or twisted-pair cables cannot be used.

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