Optical Communications, 2nd Edition

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THE OPTICAL COMMUNICATION SYSTEM

The objective of any communication system is the transfer of information from one point to another. This information transfer is accomplished most often by superimposing (modulating) the information onto an electromagnetic wave (carrier). The modulated carrier is then transmitted (propagated) to the destination, where the electromagnetic wave is received and the information recovered (demodulated). Such systems are often designated by the location of the carrier frequency in the electromagnetic spectrum (Fig. 1.1). In radio systems, the electromagnetic carrier wave is selected with a frequency from the radio frequency (RF) portion of the spectrum. Microwave or millimeter systems have carrier frequencies from those portions of the spectrum. In an optical communication system, the carrier is selected from the optical region, which includes the infrared, visible, and ultraviolet frequencies.

The principal advantages in communicating at optical frequencies are (1) the potential increase in modulation bandwidth, (2) the ability to concentrate power in extremely narrow beams, and (3) the significant reduction in component sizes. In any communication system, the amount of information transmitted is directly related to the bandwidth (frequency extent) of the modulated carrier, which is generally limited to a fixed portion of the carrier frequency itself. Thus, increasing the carrier frequency theoretically increases the available transmission bandwidth, ...