OPTICAL FIBRE CABLE

An optical fiber cable is a cable containing one or more optical fibers. The optical fiber elements are typically individually coated with plastic layers and contained in a protective tube suitable for the environment where the cable will be deployed.

DESIGN:Optical fiber consists of a core and a cladding layer, selected for total internal reflection due to the difference in the refractive index between the two. In practical fibers, the cladding is usually coated with a layer of acrylate polymer or polyimide. This coating protects the fiber from damage but does not contribute to its optical waveguide properties.

A critical concern in outdoor cabling is to protect the fiber from contamination by water. This is accomplished by use of solid barriers such as copper tubes, and water-repellent jelly or water-absorbing powder surrounding the fiber.

Finally, the cable may be armored to protect it from environmental hazards, such as construction work or gnawing animals. Undersea cables are more heavily armored in their near-shore portions to protect them from boat anchors, fishing gear, and even sharks, which may be attracted to the electrical power that is carried to power amplifiers or repeaters in the cable.

CABLE TYPES: OFC: Optical fiber, conductive OFN: Optical fiber, nonconductive OFCG: Optical fiber, conductive, general use OFNG: Optical fiber, nonconductive, general use OFCP: Optical fiber, conductive, plenum

OFNP: Optical fiber, nonconductive, plenum OFCR: Optical fiber, conductive, riser OFNR: Optical fiber, nonconductive, riser OPGW: Optical fiber composite overhead ground wire ADSS: All-Dielectric Self-Supporting

DATA TRANSMISSION:Data transmission fiber optics, simply put, is the sending and receiving of data from point-to-point via a network, thus the fundamental function of all fiber systems from small to large. Data transmission requirements range from very simple cables connecting servers or storage arrays inside a network or telecommunications system, to large multi-fiber distribution cables supporting intra-building connectivity and beyond.

For smaller, localized data transmission applications, a multitude of products are available to move data from place to place. Primarily multimode, these applications use single fibers to move multiple signals over distances, usually less than 300 meters. Depending on the particular application or system requirement, data transmission cabling can take many forms from basic simplex (SX) or duplex (DX) cable assemblies to ribbon fiber distribution cables, and various combinations of customized products.

In larger data transmission applications, data transmission can be multimode, single mode, or a combination of the two, depending on bandwidth and transmission distance requirements. These applications generally use a higher volume or longer lengths of cabling, or in some case both, supporting data centers, building-to-building, campuses, and carrier network communications.

PRINCIPLE OF OPERATION:

An optical fiber is a cylindrical dielectric waveguide (nonconducting waveguide) that transmits light along its axis, by the process of total internal reflection. The fiber consists of a core surrounded by a cladding layer, both of which are made of dielectric materials. To confine the optical signal in the core, the refractive index of the core must be greater than that of the cladding.

Index of refractionThe index of refraction is a way of measuring the speed of light in a material. Light travels fastest in a vacuum, such as outer space. The speed of light in a vacuum is about 300,000 kilometers (186,000 miles) per second. Index of refraction is calculated by dividing the speed of light in a vacuum by the speed of light in some other medium. The index of refraction of a vacuum is therefore 1, by definition. The typical value for the cladding of an optical fiber is 1.52. The core value is typically 1.62. The larger the index of refraction, the slower light travels in that medium.

Total internal reflectionWhen light traveling in an optically dense medium hits a boundary at a steep angle (larger than the critical angle for the boundary), the light will be completely reflected. This is called total internal reflection. This effect is used in optical fibers to confine light in the core. Light travels through the fiber core, bouncing back and forth off the boundary between the core and cladding. Because the light must strike the boundary with an angle greater than the critical angle, only light that enters the fiber within a certain range of angles can travel down the fiber without leaking out. This range of angles is called the acceptance cone of the fiber. The size of this acceptance cone is a function of the refractive index difference between the fiber's core and cladding.

Multi-mode fiberFiber with large core diameter (greater than 10 micrometers) may be analyzed by geometrical optics. Such fiber is called multi-mode fiber, from the electromagnetic analysis. In a step-index multi-mode fiber, rays of light are guided along the fiber core by total internal reflection. Rays that meet the core-cladding boundary at a high angle greater than the critical angle for this boundary, are completely reflected. The critical angle (minimum angle for total internal reflection) is determined by the difference in index of refraction between the core and cladding materials. Rays that meet the boundary at a low angle are refracted from the core into the cladding, and do not convey light and hence information along the fiber.

Single-mode fiberFiber with a core diameter less than about ten times the wavelength of the propagating light cannot be modeled using geometric optics.The electromagnetic analysis may also be required to understand behaviors such as speckle that occur when coherent light propagates in multi-mode fiber. As an optical waveguide, the fiber supports one or more confined transverse modes by which light can propagate along the fiber. Fiber supporting only one mode is called single-mode or mono-mode fiber. The most common type of single-mode fiber has a core diameter of 8–10 micrometers and is designed for use in the near infrared. The mode structure depends on the wavelength of the light used, so that this fiber actually supports a small number of additional modes at visible wavelengths. Multi-mode fiber, by comparison, is manufactured with core diameters as small as 50 micrometers and as large as hundreds of micrometers.

Special-purpose fiberSome special-purpose optical fiber is constructed with a non-cylindrical core and/or cladding layer, usually with an elliptical or rectangular cross-section. These include polarization-maintaining fiber and fiber designed to suppress whispering gallery mode propagation.

Disadvantages of Optical fibres:Price - Even though the raw material for making optical fibres, sand, is abundant and cheap, optical fibres are still more expensive per metre than copper. Although, one fibre can carry many more signals than a single copper cable and the large transmission distances mean that fewer expensive repeaters are required.Fragility - Optical fibres are more fragile than electrical wires.Affected by chemicals - The glass can be affected by various chemicals including hydrogen gas (a problem in underwater cables.)Opaqueness - Despite extensive military use it is known that most fibres become opaque when exposed to radiation.Requires special skills - Optical fibres cannot be joined together as a easily as copper cable and requires additional training of personnel and expensive precision splicing and measurement equipment.

Advantages Of Fiber Optics Immunity to Electromagnetic Interference Data Security Non Conductive Cables Eliminating Spark Hazards Ease Of Installation High Bandwidth Over Long Distances

Uses of Optical Fibres

Until the optical fibre network was developed, telephone calls were mainly sent as electrical signals along copper wire cables. As demand for the systems to carry more telephone calls increased, simple copper wires did not have the capacity, known as bandwidth, to carry the amount of information required.

Systems using coaxial cables like TV aerial leads were used but as the need for more bandwidth grew, these systems became more and more expensive especially over long distances when more signal regenerators were needed. As demand increases and higher frequency signals are carried, eventually the electronic circuits in the regenerators just cannot cope.

Optical fibres offer huge communication capacity. A single fibre can carry the conversations of every man, woman and child on the face of this planet, at the same time, twice over. The latest generations of optical transmission systems are beginning to exploit a significant part of this huge capacity, to satisfy the rapidly growing demand for data communications and the Internet.  

The main advantages of using optical fibres in the communications industry are:

 - A much greater amount of information can be carried on an optical fibre compared to a copper cable.

- In all cables some of the energy is lost as the signal goes along the cable. The signal then needs to be boosted using regenerators. For copper cable systems these are required every 2 to 3km but with optical fibre systems they are only needed every 50km.

- Unlike copper cables, optical fibres do not experience any electrical interference. Neither will they cause sparks so they can be used in explosive environments such as oil refineries or gas pumping stations.

- For equal capacity, optical fibres are cheaper and thinner than copper cables which makes them easier to install and maintain.