Optical Fiber Communications with CD-ROM by Gerd Keiser, Keiser

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The third edition of this popular text and reference book presents the fundamental principles for understanding and applying optical fiber technology to sophisticated modern telecommunication systems.

Optical-fiber-based telecommunication networks have become a major information-transmission-system, with high capacity links encircling the globe in both terrestrial and undersea installations. Numerous passive and active optical devices within these links perform complex transmission and networking functions in the optical domain, such as signal amplification, restoration, routing, and switching. Along with the need to understand the functions of these devices comes the necessity to measure both component and network performance, and to model and stimulate the complex behavior of reliable high-capacity networks.

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Table of Contents

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Chapter 1: Overview of Optical Fiber Communications

Ever since ancient times, one of the principal needs of people has been to communicate. This need created interest in devising communication systems for sending messages from one distant place to another. Many forms of communication systems have appeared over the years. The basic motivations behind each new form were either to improve the transmission fidelity, to increase the data rate so that more information could be sent, or to increase the transmission distance between relay stations. Before the nineteenth century, all communication systems operated at a very low information rate and involved only optical or acoustical means, such as signal lamps or horns. One of the earliest known optical transmission links, for example, was the use of a fire signal by the Greeks in the eighth century B.C. for sending alarms, calls for help, or announcements of certain events.

The invention of the telegraph by Samuel F. B. Morse in 1838 ushered in a new epoch in communications-the era of electrical communications.' In the ensuing years, an increasingly larger portion of the electromagnetic spectrum, shown in Fig. 1-1, was utilized for conveying information from one place to other. The reason for this trend is that, in electrical systems, the data are usually transferred over the communication channel by superimposing the information onto a sinusoidally varying electromagnetic wave, which is known as the carrier. At the destination, the information is removed from the carrier wave and processed as desired. Since the amount of information that can be transmitted is directly related to the frequency range over which the carrieroperates, increasing the carrier frequency theoretically increases the available transmission bandwidth and, consequently, provides a larger information capacity. Thus, the trend in electrical communication system developments was to employ progressively higher frequencies (shorter wavelengths), which offer corresponding increases in bandwidth or information capacity. This activity led to the birth of radio, television, radar, and microwave links.

Another important portion of the electromagnetic spectrum encompasses the optical region shown in Fig. 1-1. In contrast to electrical communications, transmission of information in an optical format is carried out not by frequency modulation of the carrier, but by varying the intensity of the optical power. Similar to the radio-frequency spectrum, two classes of transmission medium can be used: an atmospheric channel or a guided-wave channel. In optical systems it is customary to specify the band of interest in terms of wavelength, instead of in terms of frequency as used in the radio region. However, with the advent of high-speed multiple-wavelength systems in the mid-1990s, the output of optical sources started to be specified in terms of optical frequency. The reason for this is that in optical sources such as mode-locked semiconductor lasers, it is easier to control the frequency of the output light, rather than the wavelength, in order to tune the device to different emission regions. Of course, the different optical frequencies v are related to the wavelengths lambda through the fundamental equation c =vlambda. Thus, for example, a 1552.5-nm wavelength light signal has a frequency of 193.1 THz (193.1 X 10 ¹² Hz). The optical spectrum ranges from about 50 nm (ultraviolet) to about 100 gm (far infrared), the visible region being the 400-to-700-nm band. This book addresses optical fiber communications, which operate in the 800-to-1600nm wavelength band.

1.1 Basic Network Information Rates

The period of the 1990s saw a burgeoning demand on communication-network assets for services such as database queries and updates, home shopping, video- on-demand, remote education, telemedicine, and videoconferencing. This demand was fueled by the rapid proliferation of personal computers (PCs), coupled with a phenomenal increase in their storage capacity and processing capabilities, the widespread availability of the Internet, and an extensive choice of remotely accessible programs and information databases. To handle the ever increasing demand for high-bandwidth services from users ranging from homebased PCs to large businesses and research organizations, telecommunication companies worldwide are using light waves traveling within optical fibers as the dominant transmission system. This optical transmission medium consists of hairthin glass fibers that guide the light signal over long distances.

Table 1-1 gives examples of information rates for some typical voice, video, and data services. To send these services from one user to another, network providers combine the signals from many different users and send the aggregate signal over a single transmission line. This scheme is known as time-division multiplexing (TDM). Here, N independent information streams, each running at a data rate of R b/s, are interleaved electrically into a single information stream operating at a higher rate of N x R b/s. To get a detailed perspective of this, let us look at the multiplexing schemes used in telecommunications.

Early applications of fiber optic transmission links were largely for trunking of telephone lines. These were digital links consisting of time-division-multiplexed 64-kb/s voice channels. Figure 1-2 shows the digital transmission hierarchy used in the North American telephone network. The fundamental building block is a 1.544-Mb/s transmission rate known as a T1 rate. It is formed by the timedivision multiplexing of 24 voice channels, each digitized at a 64-kb/s rate. Framing bits are added along with these voice channels to yield the 1.544-Mb/s bit stream. At any level, a signal at the designated input rate is multiplexed with other input signals at the same rate.

The system is not restricted to multiplexing of voice signals. For example, at the T1 level, any 64-kb/s digital signal of the appropriate format could be transmitted as one of the 24 input channels shown in Fig. 1-2. As noted in Fig.1-2 and Table 1-2, the multiplexed rates are designated as T1 (1.544 Mb/s), T2 (6.312 Mb/s), T3 (44.736 Mb/s), and T4 (274.176 Mb/s). Similar hierarchies using different bit-rate levels are employed in Europe and Japan, as Table 1-2 shows. The correct nomenclature used to describe generic digital systems is DS1, DS2, and DS3, where DS stands for digital system. In the strict sense, "DSx" refers to the framing format and other interface specifications, whereas "Tx" refers to the transmission medium over which the DSx signal is sent. However, the two designations are often used interchangeably.

With the advent of high-capacity fiber optic transmission lines in the 1980s, service providers established a standard signal format called synchronous optical network (SONET) in North America and synchronous digital hierarchy (SDH) in other parts of the world. These standards define a synchronous frame structure for sending multiplexed digital traffic over optical fiber trunk lines. The basic building block and first level of the SONET signal hierarchy is called the synchronous transport signal-level I (STS-1), which has a bit rate of 51.84 Mb/s. Higherrate SONET signals are obtained by byte-interleaving N STS-1 frames, which are then scrambled and converted to an optical carrier-level N (OC-N) signal. Thus, the OC-N signal will have a line rate exactly N times that of an OC-1 signal. For SDH systems, the fundamental building block is the 155.52-Mb/s synchronous transport module-level I (STM-1). Again, higher-rate information streams are generated by synchronously multiplexing N different STM-1 signals to form the STM-N signal. Table 1-3 shows commonly used SDH and SONET signal levels. Chapter 12 presents more details on this subject.

Beyond the use of fiber optics for telephone trunking lies an enormous world of both analog and digital applications. For example, by putting information in an asynchronous transfer mode (ATM) format, it is possible to transmit simultaneously both narrowband and broadband communication services, such as telephone, videoconferencing, video entertainment, digital imaging, and data on a single subscriber line. In particular, digital imaging is rapidly becoming a major information resource within the Internet. This service can require tremendous bandwidth, which will be a challenge for transmission-link providers. Another key concept is the use of fiber optics for the integrated services digital network (ISDN). The ISDN scheme encompasses the ability of a digital communication network to handle voice (telephone), facsimile, data, videotex, telemetry, and broadcast audio and video services. Transmission rates for these concepts vary from 155 Mb/s (SONET OC-3) for localized applications to around 10 Gb/s for high-capacity backbone trunks (SONET OC-192)....