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A communication system is designed to transmit and receive information. The information to be sent is generally superimposed by modulation onto an electromagnetic wave called a carrier. It is then transmitted to a receiver where the information is recovered by demodulation. Various types of communication systems are often designated by the location of the carrier frequency, or wavelength, in the electromagnetic spectrum (see Fig. 7.1). The longest wavelengths correspond to radio frequency (RF) waves that extend from hundreds of kilometers down to less than a millimeter. Along with the standard broadcast bands, microwaves and millimeter waves are also included in the RF range. In a laser communication system, commonly called lasercom, the carrier frequency is selected from the optical spectrum, typically on the order of 10 14 Hz . In this chapter we focus our attention primarily on lasercom systems, but much of the material is applicable beyond the optical regime. Compared with conventional RF systems, there are several significant advantages offered by lasercom systems that are simple consequences of the short wavelengths (high frequencies) associated with optical waves. Among these advantages are the following: - smaller antenna (telescope) - smaller size and weight of the components - power concentration in a very narrow beam (a more secure channel) - potential increase in modulation bandwidth The last advantage is particularly important because the amount of information transmitted by a communication system is directly related to the bandwidth of the modulated carrier, which is usually some fraction of the carrier frequency itself. Hence, increasing the carrier frequency has the potential to increase the information capacity of the system (higher data rates).
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