Sec 15.2 - 498 The Silicon Web: Physics for the Internet...

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498 The Silicon Web: Physics for the Internet Age such as on-demand video like YouTube. Our goal in this chapter is to understand why optical f ber permits such huge data rates. The answer to this question is not in the high speed oF the light itselF. In Fact, light waves in f ber travel about 50% slower than do radio waves in air. The answer lies in the concept oF bandwidth and the Frequencies oF the waves used in each case, as we shall see. In Chapter 8, we discussed analog and digital communication. We treated the basics oF analog radio, including the modulation oF carrier waves to carry inFormation. We also discussed frequency multiplexing , which allows a single physical medium to carry many communication channels . We discussed digital sampling oF an analog signal to change it to a binary Form suitable For transmission across a digital commu- nication channel. Digital communication requires use oF a chosen protocol —a set oF rules For interpreting a list oF binary numbers. We discussed the idea oF the bandwidth oF a communication channel, which is the range oF Frequencies allocated to each channel’s broadcasting. ±or example, each AM station is permitted to use a small range (called a band) oF radio Frequencies covering about 10 kHz (e.g., 1275–1285 kHz), so its bandwidth is 10 kHz. IF a channel has band- width equal to B , the shortest radio or light pulse that can be transmitted in that channel has time duration oF about 1/ B . ±or example, a bandwidth oF 10 kHz corresponds to a pulse duration oF about 1/10 kHz = 10 3 seconds (sec), or 1 millisecond (msec). In this chapter, we will learn about some oF the optical hardware used to make the large bandwidth oF light accessible For f ber-optic communication systems. We will see that using light pulses instead oF radio signals oFFers a huge increase in data rate in digital systems, because the bandwidth oF optical systems is Far greater than in radio systems. The bandwidth oF a medium is determined by its physical properties, so this leads us back to the physics oF waves. In Chapter 13 we studied how light travels in glass opti- cal f bers. The key concepts are refraction oF light and total internal reF ection . This chapter brings together many concepts and discussions that were introduced in earlier chapters, and there are many reFerences to those earlier chapters. Please return to these earlier sections and review them or read them For the f rst time. This is an opportunity to see the connections between the many topics we have discussed in this book. 15.2 OVERVIEW OF FIBER-OPTICAL COMMUNICATION SYSTEMS BeFore discussing data transmission in f ber-optical systems, let us review communication systems in general. Figure 15.1 shows the three main elements oF any communication system: The transmitter , which converts inFormation to a physical Form suitable For transmitting.
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This note was uploaded on 09/29/2011 for the course PHYS 222 taught by Professor Wade during the Spring '09 term at Edmonds Community College.

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Sec 15.2 - 498 The Silicon Web: Physics for the Internet...

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