Fibre Optic

Specialist optical fibre

An optical fiber (or fibre) is a glass or plastic fiber designed to guide light along its length by total internal reflection.

Optical fibers are widely used in fiber-optic communication, which permits digital data transmission over longer distances and at higher data rates than electronic communication.

The operating principle of optical fibers applies to a number of variants including multi-mode optical fibers and single-mode optical fibers.

Fibre definitions:

Loose Tube (L/T)
This is the protective tube surrounding one or more fibres and is usually found in cables used for outdoor installations. The protective tube loosely surrounds the optical fibre cores and is often filled with protective gel.

Tight Buffered
A protective coating (usually 900 µm) that is used to "buffer" the primary coating of fibres. Provides strength, durability, easy handling and termination.

Armoured
The inner jacket of the cable is overlaid with a protective armouring. Two main types are Steel Wire Armoured (SWA) and Corrugated Steel Tape armoured (CST or STA). The armouring gives the best resistance against rodent attack or mechanical damage and provides high tensile strength and crush resistance.

Multimode
Multimode fibre carries multiple light rays or modes concurrently, each at a slightly different reflection angle within the fibre core. Multimode fibre has a larger core than single mode and is typically either 50µ (Micron) or 62.5µ (Micron) in diameter. Multimode fibre transmission is used for relatively short distances because the modes tend to disperse over longer lengths. The maximum distances of the cable depends on the application the cable is being used for. To run Gigabit over Multimode, a guideline of 550 Metres maximum for 50/125 cable and 220 Metres maximum for 62.5/125 cable should be used. For longer distances, Singlemode fibre is used. The most common application for Multimode is LAN's(Local Area Network).

Singlemode
Singlemode fibre is designed for the transmission of a single ray or mode of light as a carrier over long-distance signal transmission. Singlemode fibre has a much smaller core than multimode fibre, between 8µ and 10µ (micron). Singlemode cable can carry Gigabit upto 5,000 Metres. Singlemode provides greater bandwidth than multimode.

Strength Member
Part of optic cable that increases the cable's tensile strength and bears the load. It can be made of Kevlar, fiberglass filament, or steel strands.

Micron (µm)
One micrometer or one millionth of a meter. Used to express the dimensions of fibre optic cores.

Fibre optics versus copper wire:

In an optical system. Signals are transmitted in the form of photons ( light ) which have no electrical charge and, therefore, cannot be affected by the electric fields as experienced in high voltage environments or during a lightening discharge. Similarly, high magnetic fields from motors, machinery, transformers etc., have no effect on optical transmission. There are no crosstalk problems as the small leakage of flux which may occur at the fibre boundary interface is retained by the opaque primary jacket ensuring that optical signals cannot interfere with each other when fibres are in close proximity. This factor also guarantees security of transmission, for the signal is unable to be externally detected throughout the length of the fibre.

Features:

Freedom from Electro-Magnetic Interference
Freedom from Crosstalk
Security of Transmission
Elimination of Sparking and Fire Hazards
Electrical Isolation
Absence of Ground Loops
Low Weight Coupled with High Strength
Increased Bandwidth and Lower Transmission Losses than in Coaxial Cables at High Frequencies

The inert nature of optical fibres means they can tolerate most kinds of weather and be immersed in many fluids. Their low weight and small size is useful in many applications. Complete electrical isolation is a distinct benefit, giving more freedom in the design of the transmitter and receiver and ensuring the elimination of ground loop problems. However, by far the major advantage is in bandwidth. In either coaxial or parallel wire cable the bandwidth varies inversely as the square of the length, while in fibre optic cables it varies inversely as the length, only.