Baba

Devices for Telecommunications 409

Devices for Telecommunications
M Sugawara and N Hara, University of Tokyo, Meguro-ku, Japan
& 2005, Elsevier Ltd. All Rights Reserved.

Introduction
The recent global spread of telecommunication networks is due to the advent of optical-fiber and wireless communication technologies. These technologies continue to develop to meet the explosive demand fordata traffic on the Internet. In a ubiquitous society, where anyone can access the Internet anywhere at any time, diverse modes of services via various types of devices for connecting to the network are right across the corner. The optical-fiber communication system basically consists of optical-fiber cables, light transmitters, amplifiers, and light receivers as illustrated in Figure 1a. Lighttransmitters convert electrical signals to optical signals with different wavelengths for different services. Optical signals propagate through the optical fiber to receivers at destinations, being amplified in the case of long fiber span over several tens of kilometers. The receivers reconvert the optical signals to electrical signals for data transmission to be accomplished. The multiplexing technologythat uses many

wavelengths in the same fiber increases the total transmission bandwidth, and is called wavelength division multiplexing (WDM). In 1996, the total bandwidth of 1.1 Tb s À 1 was accomplished for a 150-km transmission by using 55 wavelengths with a spacing of 0.6 nm at 20 Gb s À 1 per wavelength. Wireless communication systems provide a variety of services such as cellular phones,wireless local area networks (WLAN), electric toll collection system (ETC), satellite communication, fixed wireless access (FWA), and automotive radar as shown in Figure 1b. Their progress is due to the development of high-performance transmitters as well as low-noise receivers in the frequency range of 1–100 GHz. A variety of high-speed transistors with output power of 0.01– 500 W based on compoundand silicon semiconductors play a significant role in this development. This article explains structures and operation principles of major optical and electrical devices for telecommunications.

Optical Fiber
Optical fibers are transporting media of high-speed optical signals. Fibers have diverse properties superior to copper cables such as immunity to electromagnetic fields, anti-corrosion, lowtransmission loss of less than 0.2 dB km À 1, and extremely broad bandwidth of 40 THz per single fiber. Figure 2 shows (1) the structure of an optical fiber and (2) the internal light propagation. The fiber consists of the core, the clad surrounding the core, and resinous coating. The fiber cable used in long-haul communication consists of a bundle of optical fibers. A fiber is made by vertically drawing acylindrical preform made of ultrapure silica in which dopants such as
Resinous coating Clad

Light transmitter
1

Light receiver Amplifier Optical fiber
1

2

2

3

Multiplexer

Demultiplexer

3

(a) 1000 100 Output power (W) 10 1 0.1 0.01 (b)
Cellular terminal DBS Cellular base station

Core
Satellite communication ETC WLAN

(a)
FWA

Propagating light

Automotiveradar

1

10 Frequency (GHz)

100 (b)

Core Clad
Figure 2 (a) Structure of optical fiber. (b) Internal light propagation.

Figure 1 (a) WDM optical-fiber communication system. (b) Requirements for transistors in wireless communication system.

410 Devices for Telecommunications

GeO2 and B2O3 are added in a controlled manner to adjust the refractive index profile of the fiber. Therefractive index is increased by GeO2 and decreased by B2O3. The diameter of the clad is 125 mm. The refractive index of the core is slightly larger than that of the clad, so that light entering the core propagates through the optical fiber by repeating the total reflection at the core–clad boundary. There are two types of fibers with different core diameters for different applications. Fibers with a...