CASIRJ Volume 2 Issue 2 ISSN 2319 – 9202 International Research Journal of Commerce Arts and Science http:www.casirj.com Page 272 Physics of modern telecommunication Vijay Kumar Sharma Department of Physics Shyam Lal College (University of Delhi) Shahdara, Delhi, 110032, India Abstract: Major characteristic of a telecommunication system is unquestionably its information-carrying capacity. Though, capacity is the priority for system users, but the extent of limit is decided by the Shannon- Harley theorem. The capacity is proportional to its bandwidth, which in turn is proportional to the frequency of the carrier. High-speed communication systems use light-as a carrier with the highest frequency among all the practical signals. This demands the use of Guiding media-fibre, high sensitivity detectors, receivers and switches. The explosive growth of Internet traffic, deregulation and the increasing demand of users, therefore, require more bandwidth, which is controlled only by optical networks which can deliver the required capacity. The physics of such optics based telecommunication systems involve some fundamental concepts such as light scattering, superposition of waves, optical excitations of electrons in semi-conducting crystals and in glasses. The art of optical communication using fibre optical cable for information exchange is one of the recurring themes of this paper. Introduction: It is not just the sophisticated technology driven modern era that is using light to communicate. Since, earliest time man is dependent on light – mostly in the form of fire or sunlight. Time to time people tried to use light to deliver some type of information between remote locations. In its visibility and ease of use light makes such communication extremely practical but reliability is another matter. The hitch was the dependence on atmospheric condition, which made direct optical communication so unreliable that it could never be accepted in every day applications. The advent of lasers in the early 1960’s, however, prompted scientists and engineers in optical communication system but without commercial success. Communication technology always tried to use high frequency signal carriers. The shift from radio frequency to microwaves allowed the engineers to increase the system’s information carrying capacity by ten-fold. This success inspired researchers to seek a solution to the problem of a reliable communication by continuing to increase microwave frequency. But at a frequency of more than 100 GHz where microwaves overlap the infrared zone, microwave attenuation in air reaches such a high level that the transmission distance became unacceptably short. The solution to the problem appeared in making use of wave- guide structure to transmit ultra high frequency (UHF) electromagnetic waves. These wave guides in the form of steel tubes rectangular in cross-section with opening at each ends have been used for years in radar and other UHF systems for delivering and distributing micro waves over very short distances of the order of several feet. In late 1960’s and early 1970’s scientists and engineers at Bell laboratories achieved a significant progress in designing wave guides for long distance systems with a very impressive characteristics: 238000 voice channels per unit. But the wave-guides were still the same old open-ended steel tubes about several centimetres of inner diameters circular in cross-section. These were also unsuitable from cost installation, maintenance and other practical standpoints. The idea of “ increasing the carrier frequency even higher than the highest micro waves (mw) frequency” was