division multiplexing and optical amplifi- ers. It then goes on to discuss transmission impairments and methods to mitigate them and ways to optimize system performance. Chapters 7, 8, and 9 provide an introduction to radio transmission, covering the basic theory of electromagnetic waves, propagation of radio waves, and antenna design. 5 Optical Fibers Takis Hadjifotiou Telecommunications Consultant Introduction Optical fiber communications have come a long way since Kao and Hockman (then at the Standard Telecommunications Laboratories in England) published their pioneering paper on communication over a piece of fiber using light (Kao & Hockman, 1966). It took Robert Maurer, Donald Keck, and Peter Schultz (then at Corning Glass Works-USA) four years to reach the loss of 20 dB/km that Kao and Hockman had considered as the target for loss, and the rest, as they say, is history. For those interested in the evolution of optical communica- tions, Hecht (1999) will provide a clear picture of the fiber revolution and its ramifications. The purpose of this chapter is to provide an outline of the performance of fibers that are fabricated today and that provide the pathway for the transmission of light for communications. The research literature on optical fibers is vast, and by virtue of necessity the references at the end of this chapter are limited to a few books that cover the topics outlined here. The Structure and Physics of an Optical Fiber The optical fibers used in communications have a very simple structure. They consist of two sections: the glass core and the cladding layer (Figure 5.1). The core is a cylindrical structure, and the cladding is a cylinder without a core. Core and cladding have different refractive indices, with the core having a refractive index, N1, which is slightly higher than that of the cladding, N2. It is this difference in refractive indices that enables the fiber to guide the light. Because of this guiding property, the fiber is also referred to as an "optical waveguide." As a minimum the