Fibre Optic Cabling
Information about Optical Fibre Cabling.
An Optical Fibre uses glass (or plastic) threads to transmit data. A fibre optic cable consists of a bundle of glass threads, each of which is capable of transmitting messages modulated onto light waves.
Fiber optics has several advantages over traditional metal communications lines:
- Fibre optic cables have a much greater bandwidth than metal cables. This means that they can carry more data.
- Fibre optic cables are less susceptible than metal cables to interference.
- Fibre optic cables are much thinner and lighter than metal wires.
- Data can be transmitted digitally (the natural form for computer data) rather than analogically.
The optical fibre can be used as a medium for telecommunication and networking because it is flexible and can be bundled as cables. Although optical fibre can be made out of transparent plastic, glass, or a combination of the two, the fibre used in long-distance telecommunications applications are always glass, because of the lower optical attenuation.
Both multi-mode and single-mode fibre are used in communications, with multi-mode fiber used mostly for short distances (up to 500 m), and single-mode fibre used for longer distance links.
Fibre Optic Cables
For indoor applications, the jacketed optical fibre is generally enclosed, with a bundle of flexible fibrous polymer strength members like Aramid (e.g. Twaron or Kevlar), in a lightweight plastic cover to form a simple cable. Each end of the cable may be terminated with a specialized optical fibre connector to allow it to be easily connected and disconnected from transmitting and receiving equipment.
For use in more strenuous environments, a much more robust cable construction is required. In loose-tube construction the fibre is laid helically into semi-rigid tubes, allowing the cable to stretch without stretching the fiber itself. This protects the fiber from tension during laying and due to temperature changes. Alternatively the optical fibre may be embedded in a heavy polymer jacket, commonly called "tight buffer" construction. These fiber units are commonly bundled with additional steel strength members, again with a helical twist to allow for stretching.
Another critical concern in cabling is to protect the fiber from contamination by water, because its component hydrogen (hydronium) and hydroxyl ions can diffuse into the fibre, reducing the optical fibre strength and increasing the optical attenuation. Water is kept out of the cable by use of solid barriers such as copper tubes, water-repellant jelly, or more recently water absorbing powder, surrounding the fiber.
The cable may be armored to protect it from environmental hazards, such as construction work or animals.
Termination and Splicing
Optical fibers may be connected to each other by connectors or by splicing, that is, joining two optical fibres together to form a continuous optical waveguide. There are two splicing method, fusion splicing and mechanical splicing.
Fusion splicing is done with a specialized instrument that typically operates as follows: The two cable ends are fastened inside a splice enclosure that will protect the splices, and the fibre ends are stripped of their protective polymer coating (as well as the more sturdy outer jacket, if present). The ends are cleaved (cut) with a precision cleaver to make them perpendicular, and are placed into special holders in the splicer. The splicer generates a larger spark that raises the temperature above the melting point of the glass, fusing the ends together permanently. The location and energy of the spark is carefully controlled so that the molten core and cladding don't mix, and this minimizes optical loss.
Mechanical optical fibre splices are designed to be quicker and easier to install, but there is still the need for stripping, careful cleaning and precision cleaving. The fiber ends are aligned and held together by a precision-made sleeve, often using a clear gel (index matching gel) that enhances the transmission of light across the joint. All splicing techniques involve the use of an enclosure into which the splice is placed for protection afterward.
Optical fibres are terminated in connectors so that the fibre end is held at the end face precisely and securely. A fibre optic connector is basically a rigid cylindrical barrel surrounded by a sleeve that holds the barrel in its mating socket. It can be push and click, turn and latch, or threaded. A typical connector is installed by preparing the fiber end and inserting it into the rear of the connector body. Quick set glue is usually used so the fiber is held securely, and a strain relief is secured to the rear. Once the glue has set, the end is polished to a mirror finish.
