Optical Networks: A Practical Perspective - Part 28. 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. | 240 Modulation and Demodulation Binary data o 0 NRZ format RZ format Figure On-off keying modulation of binary digital data. encoded by the presence of a light pulse in the bit interval or by turning a light source laser or LED on. A 0 bit is encoded ideally by the absence of a light pulse in the bit interval or by turning a light source off. The bit interval is the interval of time available for the transmission of a single bit. For example at a bit rate of 1 Gb s the bit interval is 1 ns. As we saw in Section we can either directly modulate the light source by turning it on or off or use an external modulator in front of the source to perform the same function. Using an external modulator results in less chirp and thus less of a penalty due to dispersion and is the preferred approach for high-speed transmission over long distances. Signal Formats The OOK modulation scheme can use many different signal formats. The most common signal formats are non-return-to-zero NRZ and return-to-zero RZ . These formats are illustrated in Figure . In the NRZ format the pulse for a 1 bit occupies the entire bit interval and no pulse is used for a 0 bit. If there are two successive Is the pulse occupies two successive bit intervals. In the RZ format the pulse for a 1 bit occupies only a fraction of the bit interval and no pulse is used for a 0 bit. In electronic digital communication the RZ format has meant that the pulse occupies exactly half the bit period. However in optical communication the term RZ is used in a broader sense to describe the use of pulses of duration shorter than the bit period. Thus there are several variations of the RZ format. In some of them the pulse occupies a substantial fraction say 30 of the bit interval. The term RZ without any qualification usually refers to such systems. If in addition the pulses are chirped they are also sometimes termed dispersion-managed DM solitons. In other RZ systems the pulse occupies only a small fraction
