Optical Networks: A Practical Perspective - Part 29. This book describes a revolution within a revolution, the opening up of the capacity of the now-familiar optical fiber to carry more messages, handle a wider variety of transmission types, and provide improved reliabilities and ease of use. In many places where fiber has been installed simply as a better form of copper, even the gigabit capacities that result have not proved adequate to keep up with the demand. The inborn human voracity for more and more bandwidth, plus the growing realization that there are other flexibilities to be had by imaginative use of the fiber, have led people. | 250 Modulation and Demodulation Figure Block diagram showing the various functions involved in a receiver. additional noise which increases as the transmitted power is increased. Therefore they in turn impose additional limits on channel capacity. Recent work to quantify the spectral efficiency taking into account mostly cross-phase modulation Sta99 MS00 shows that the achievable efficiencies are of the order of 3-5 b s Hz. Other nonlinearities such as four-wave mixing and Raman scattering may place further limitations. At the same time we are seeing techniques to reduce the effects of these nonlinearities. Another way to increase the channel capacity is by reducing the noise level in the system. The noise figure in today s amplifiers is limited primarily by random spontaneous emission and these are already close to theoretically achievable limits. Advances in quantum mechanics GlaOO may ultimately succeed in reducing these noise limits. Demodulation The modulated signals are transmitted over the optical fiber where they undergo attenuation and dispersion have noise added to them from optical amplifiers and sustain a variety of other impairments that we will discuss in Chapter 5. At the receiver the transmitted data must be recovered with an acceptable bit error rate BER . The required BER for high-speed optical communication systems today is in the range of 10 9 to 10 15 with a typical value of 10-12. A BER of 10-12 corresponds to one allowed bit error for every terabit of data transmitted on average. Recovering the transmitted data involves a number of steps which we will discuss in this section. Our focus will be on the demodulation of OOK signals. Figure shows the block diagram of a receiver. The optical signal is first converted to an electrical current by a photodetector. This electrical current is quite weak and thus we use a front-end amplifier to amplify it. The photodetector and front-end amplifier were discussed in Sections and .
