Quality Magazine

Quality Communication of Optical Information

May 4, 2011

In this blog, we will briefly discuss the epochal paper that provided the basis for how quality communication can be obtained in the transmission of signals (including optical) in noisy channels in demanding situations where signal integrity can be critical to practical use.

The paper, “A Mathematical Theory of Communication,” by Claude Shannon was published in 1948 in the Bell System Technical Journal. Shannon’s paper “instantaneously created the field of information theory with applications in engineering and computer science.” The director of natural sciences for the Rockefeller Foundation (now Rockefeller University) reportedly told his president that “Shannon had done for communication theory what Gibbs did for Physical Chemistry.”

For Shannon, communication was purely a matter of sending a message over a noisy channel so that someone else could recover it.

This dense mathematical paper has provided the basis of a methodology that has been successfully applied for more than 60 years. The message was redefined: the information in the message could be quantified in binary digits. The actual meaning of the message was considered irrelevant to the engineer.

Shannon’s Theorem
Shannon's law says that “accurate transmission of information is possible in a communication system with a high level of noise. Even in the noisiest system, errors can be reliably corrected and accurate information transmitted, provided that the transmission is sufficiently redundant.”

A recent popular book by James Gleick, “The Information,” looks at the history of information, and indicates “its key moment can be pinpointed to 1948. . . when … Shannon published a paper called ‘A Mathematical Theory of Communication’ … For Shannon, communication was purely a matter of sending a message over a noisy channel so that someone else could recover it. ” “It is the founding document for the modern science of information.”

Coding Example
For example, laser scanning of optical discs (CDs, DVDs, etc.) can be compromised by the presence of scratches and dust that can alter the light being detected, thus causing pulses to be skipped or distorted. Error correction techniques, which are a consequence of Shannon’s Theorem, that automatically add coded bits to the transmitted digital signals, can provide the means to detect and correct for any lost or damaged information by decoding after photo-detection. The insertion of redundant information is called coding.