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Unformatted text preview: 30 Introduction to Optical Networks and ( ) dB = 30 dB . In this context, a signal being attenuated by a factor of 1000 would equivalently undergo a 30 dB loss. A signal being amplified by a factor of 1000 would equivalently have a 30 dB gain. We usually measure loss in optical fiber in units of dB/km. So, for example, a light signal traveling through 120 km of fiber with a loss of 0.25 dB/km would be attenuated by 30 dB. 1.8 Network Evolution We conclude this chapter by outlining the trends and factors that have shaped the evolution of optical fiber transmission systems and networks. Figure 1.13 gives an overview. The history of optical fiber transmission has been all about how to transmit data at the highest capacity over the longest possible distance and is remarkable for its rapid progress. Equally remarkable is the fact that researchers have successfully overcome numerous obstacles along this path, many of which when first discovered appeared to impede further increases in capacity and transmission distance. The net result of this is that capacity continues to grow in the network, while the cost per bit transmitted per kilometer continues to get lower and lower, to a point where it has become practical for carriers to price circuits independently of the distance. We will introduce various types of fiber propagation impairments as well as optical components in this section. These will be covered in depth in Chapters 2, 3, and 5. 1.8.1 Early DaysMultimode Fiber Early experiments in the mid-1960s demonstrated that information encoded in light signals could be transmitted over a glass fiber waveguide. A waveguide provides a medium that can guide the light signal, enabling it to stay focused for a reasonable distance without being scattered. This allows the signal to be received at the other end with sufficient strength so that the information can be decoded. These early experiments proved that optical transmission over fiber was feasible. An optical fiber is a very thin cylindrical glass waveguide consisting of two parts: an inner core material and an outer cladding material. The core and cladding are designed so as to keep the light signals guided inside the fiber, allowing the light signal to be transmitted for reasonably long distances before the signal degrades in quality. 1.8 Network Evolution 31 Transmitter Receiver MLM laser 1.3 m m l P Single-mode fiber Transmitter Receiver LED l P Regenerator Multimode fiber (a) Transmitter Receiver SLM laser 1.55 m m l P (b) Transmitter Transmitter Transmitter Receiver Receiver Receiver SLM laser l P Optical amplifier (c) (d) l 1 l 2 l 3 l 1 l 2 l 3 WDM multiplexer WDM demultiplexer Figure 1.13 Evolution of optical fiber transmission systems. (a) An early system using LEDs over multimode fiber. (b) A system using MLM lasers over single-mode fiber in the 1.3 m band to overcome intermodal dispersion in multimode fiber. (c) A later system using the 1.55 m band for lower loss, and using SLM lasers to overcome chromatic dispersion limits. (d) A current-generationlower loss, and using SLM lasers to overcome chromatic dispersion limits....
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- Spring '09