Fiber optics
Fiber optics....................... The internet, cell phones, fax machines and pagers are a way of life in modern society. All these technologies rely on lasers and fiber optics. Faced with the aforementioned fundamental shortfalls of a satellite-based system, real interest in optical communication was aroused with the invention of the laser in early 1960's. Proposals for using optical fibers to avoid degradation of the optical signal while propagating through the atmosatmosphere were made almost simultaneously in 1966. Early systems exhibited high attenuation (1000 dB/km). Today, less than 40 years on, attenuation of less than 0.2 dB/km is easily achieved for a carrier wavelength of 1.55μm
Thus the majority of the transmission and receiver systems are geared for operation at this wavelength. Unlike some of its predecessors, fiber optics technology has many unrivaled advantages, some of which are
listed below:1. Enormous potential bandwidth: the optical carrier frequency in the range 1013 to 1014Hz offers the potential for a fiber information carrying capacity that is many orders of magnitude in excess of that obtained using copper cable or wideband radio systems. This enables fibers to simultaneously carry voice, data, image and video signals.
2. Small size and weight: an optical fiber is often no wider than the diameter of a human hair; thus even after applying protective layers, they are far smaller and much lighter than corresponding copper cables. This is a tremendous boon to alleviating duct congestion in cities.
3. Immunity to interference and cross talk: they form a dielectric and are therefore free from electromagnetic interference.
4. Signal security: as light from a fiber does not radiate significantly, a transmitted optical signal cannot be obtained non-invasively, thus ensuring a high degree of signal security.
5. Low transmission loss: with losses as low as 0.2 dB/km, this feature alone has become a major advantage of optical fiber as extremely wide repeater spacings (70 to 100km) may be used in long-haul communication links. This in turn reduces both system cost and complexity.
6. System reliability and ease of maintenance: due to the low loss property, system reliability is generally enhanced in comparison to conventional electrical conductor systems. Furthermore, reliability of optical components have predictedlifetimes of 20 to 30 years. Combined, these factors tend to reduce maintenance time and costs.
There are three major applications of fiber optic telecommunications - long haul backbone networks (1.55μm); metro area networks (1.3μm) and local area optical networks (0.85μm). Domestic intercity systems based on optical fibers have now been widely implemented. These use digital transmission with pulse rates ranging from a few hundred Mbit/s to about 2Gbit/s. With the usage of single mode fibers since 1984, repeater spacing of up-to 40km or more is achieved. Furthermore, with rapid progress in time, the distinction between local, intra-city and intercity systems is blurring.
