Chapter 11: Linear Fiber-Optic Signal Transportation

11.1 Introduction

Although coaxial cable distribution systems are used for the last mile delivery of broadband services to subscribers, other technologies are frequently used to cover most of the physical distance between headend and tap. One reason for this is that the interrelationship between bandwidth, signal levels, number of cascaded devices, noise, and distortion (discussed in Chapter 10) limits the size of purely coaxial systems. Second, large coaxial systems are best suited to delivery of a common spectrum of signals to all parts of the network, whereas many services such as data communications and telephony require a bidirectional bandwidth allocation to every customer. In order to provide this capacity in a network of attainable bandwidth, the network must be segmented, and this requires the use of small coaxial sections each connected by separate communications links to the headend.

In modern networks, these links almost always use linear fiber optics for at least the segment that interfaces with the coaxial network (though Chapter 12 will discuss alternatives). Fiber strands have a loss that, at the optical wavelengths used by cable television systems, is approximately two orders of magnitude lower than the best coaxial cables. Furthermore, the noise and distortion added by typical 5 15-mile-long fiber-optic links is less than that generated in comparable length coaxial trunk line sections.

This chapter will begin with a brief introduction to some principles of optics that are important to fiber transmission of light. Then optical fibers will be examined in detail, particularly the interaction between the fiber and the signals passing through it. Passive optical devices, comparable with their coaxial counterparts, will be treated next. Active devices will include transmitters, amplifiers, and receivers. Finally, the total performance of optical links will be examined with end-to-end performance calculations of noise and distortion.

11.2 Optical Basics

Light waves are just much higher frequency RF waves about 300,000 times the highest frequency used in coaxial cable systems and 20,000 times the microwave frequencies sometimes used to relay cable television signals. Optical wavelengths are very short (remember that l= c/ f in a vacuum). For instance, visible yellow light has a frequency of about 5.5 x 10 ¹⁴ Hz, and thus a wavelength of only 0.00000055 meter, more easily expressed as 550 nm (one nanometer is one billionth of a meter), considerably less than 1/10,000 of an inch.

At these frequencies, electromagnetic waves behave in ways that are sometimes counterintuitive. For one thing, physicists have long understood that the flow of energy is...