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Optical Fibre


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Optical Fibre Optical Fibre is a very fine, optically pure glass fibre through which light can be transmitted by multiple internal reflections, thus conveying, an image (in case of bundles of fibres) or information from one end to the other. In simplest terms, an optical fibre is a carrier of light. In an optical fibre, a thin core of transparent material is coaxially surrounded by a material of lower refractive index, known as cladding. Light waves are guided along the core of the fibre by a series of total internal reflections between the core-cladding interface (constituting a mode of transmission). The principle is inherently simple, yet powerful and elegant. By carefully reducing the impurities in the core, losses of light as low as 0.3 decibels per kilometre of fibre (about 7% per km) can be achieved in practical silica-based fibres. The optical fibre has found many practical applications, such as in communications, in medical imaging systems and as a convenient guide of light from high-power lasers for surgical and industrial applications. Optical fibres are increasingly being used to replace copper wire in telephone cables, the messages being coded as pulses of light rather than a fluctuating electric current. Although still relatively expensive to produce and install, optical fibres can carry more data than traditional cables, and are less susceptible to interference.

Optical fibre communication is perhaps the most well-known application of optical fibres. Bangladesh does not yet have any facility for manufacturing optical fibres, but it is increasingly used for telecommunication. For example, bangladesh railways (BR) has installed optical fibre system to meet their telecommunication (including signalling) need. BTTB (Bangladesh Telegraph and Telephone Board) is also using optical fibres in many parts of the telephone network of major cities and towns, and in some local-area networks (LANs). dhaka university, rajshahi' university and shahjalal university of science and technology have optical fibre network backbone, which is used as local area network system of the university for telephone and high-speed date transmission (Internet). In telephone systems, a single fibre can replace hundreds or even thousands of copper circuits. By using optical fibres, much lower cost per voice channel and more reliable and maintenance-free operation can be realized.

Depending upon the transmission mode the fibre could be of two types- multimode fibres and single mode fibres. Multimode fibers are preferred when large optical power is required to be transmitted. Single mode fibres are preferred when larger bandwidth of transmission is the desired option. Again depending on the refractive index profile of the core, fibres could be mainly of two types, viz., step index fibre and graded index fibre.

Previously the entire telecommunication volume of Bangladesh were provided by a number of relatively low-capacity satellite stations and very-small aperture terminals (VSATs). A milestone in overseas connectivity was reached in 2006, when a large-capacity undersea (submarine) optical fibre cable was laid under the bay of bengal to connect Cox's Bazar with the SEA-MEA-WE-4 (Southeast Asia-Middle East-Western Europe 4) Global Fiber Optic Network. SEA-MEA-WE-4 has a total capacity of 1280 Gbits/sec, uses multiple lasers for wavelength-division multiplexing (WDM), and has a combined length of 19,000 km. The submarine cable's allocation for Bangladesh is about 14 Gigabits/sec. After this connection was completed, the overseas connectivity of the country, increased tremendously due to the much larger available bandwidth, the cost of overseas telephone calls have decreased significantly, and larger bandwidth was made available to ISPs so that their clients have a faster speed of internet browsing at a relatively lower cost. A second overseas fiber optic cable for providing backup redundancy is also being considered. [Kazi Monowar Abedin]

See also laser technology.