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Chapter 1:
Optical Communications
1.1 Introduction Light has been used for communication since signal fires were first used to send messages. Paul Revere's lanterns, manually operated signal lanterns on ships, and signal flares are other examples of early optical communication systems. A revolution in technology occurred in 1880 when Alexander Graham Bell's photophone was used to send intensity modulated sunlight over a distance of a few hundred feet. From the late 1880's to the early 1960's low-capacity short-range links represented the state-of-the-art in optical communication. The 1960's invention of the laser, the 1970's development of low-loss optical fiber, the 1980's demonstration of long-lived semiconductor laser diodes, and the 1990's development of practical optical amplifiers have ushered in a new era for optical communication. It is now possible for optical fiber systems to cross oceans and continents, while free-space systems provide high data-rate communication links between satellites at geosynchronous distances. Optical communications, in combination with microwave and wireless technologies, are enabling the construction of high-capacity networks with global connectivity. The most common wavelengths used for optical communication fall between 0.83 and 1.55 microns. Other wavelengths are also used but this range encompasses the most popular applications. A wavelength of 1 micron corresponds to a frequency of 300 THz (300,000 GHz). This is a considerably higher frequency than those associated with conventional radio, microwave, or millimeter-wave communication systems. A high center frequency implies that extremely high-speed modulations should, in principle, be possible. A one-percent bandwidth, easily achieved in conventional microwave systems, would imply over a THz modulation bandwidth for optical carriers. This is far more modulation bandwidth than a microwave carrier can provide. Although wide bandwidth is important, it is not the only reason optical communications technology is causing such revolutionary changes in our ability to transfer information. To gain a broader understanding, we need to examine the advantages light has in both guided and unguided modes of transmission. Optical fiber is an exceedingly low-loss propagating medium. Losses can approach 0.15 dB/km and, with modest amounts of optical amplification, multi-Gbps data streams can be transmitted over tens of thousands of kilometers without electronic regeneration. Conventional electronic systems are orders of magnitude less capable of providing such raw communications transport capacity. A good quality optical fiber loses half of the optical power in the propagating field in 15–20 km. In comparison, conventional coaxial cables will lose half the electrical power in just a few hundred meters! In addition to significantly lower loss, packaging and volume benefits are also realized when fiber is used. Low-loss coaxial cables frequently have diameters of a centimeter or more. An optical fiber has a diameter of a few hundred microns. This substantial reduction in cross-sectional area is particularly significant when fiber is used in crowded conduits. In free-space systems, optical communication again has advantages.
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