Optical Networks - _2_1 Loss and Bandwidth Windows_23

Optical Networks - _2_1 Loss and Bandwidth Windows_23 - 48...

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48 Propagation of Signals in Optical Fiber Finally, the phemomena discussed in this chapter also apply to various compo- nents. Some of these components are designed not to minimize effect on the prop- agating signal but rather to produce some desired interaction. This will provide the underpinnings for understanding the physical limitations of components described in Chapter 3. 2.1 Loss and Bandwidth Windows The loss incurred by propagating down a fiber can be modeled easily as follows: the output power P out at the end of a fiber of length L is related to the input power P in by P out = P in e αL . Here the parameter α represents the fiber attenuation. It is customary to express the loss in units of dB/km; thus a loss of α dB dB/km means that the ratio P out /P in for L = 1 km satisfies 10 log 10 P out P in = − α dB or α dB = ( 10 log 10 e)α 4 . 343 α. The two main loss mechanisms in an optical fiber are material absorption and Rayleigh scattering . Material absorption includes absorption by silica as well as the impurities in the fiber. The material absorption of pure silica is negligible in the entire 0.8–1.6 μ m band that is used for optical communication systems. The reduction of the loss due to material absorption by the impurities in silica has been very important in making optical fiber the remarkable communication medium that it is today. The
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