Devices for Telecommunications
M Sugawara and N Hara, University of Tokyo, Meguro-ku, Japan
& 2005, Elsevier Ltd. All Rights Reserved.
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 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 receiver Amplifier Optical fiber
(a) 1000 100 Output power (W) 10 1 0.1 0.01 (b)
Cellular terminal DBS Cellular base station
Satellite communication ETC WLAN
10 Frequency (GHz)
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...