wn as "Nonreturn to Zero" (NRZ) and "Return to Zero" (RZ), both of which are illustrated in Figure 6.1. In the ideal case, light is transmitted for the entirety of each 1 bit in NRZ modulation. In contrast, it is present for only a fraction of the duration of the 1 bits in RZ modulation. The percentage of the bitslot that is occupied by a pulse is called the "duty cycle," and common values are between 25 and 50 percent. (NRZ modulation can be regarded as RZ with a 100 percent duty cycle.) For most of this chapter we refer to NRZ usage, but a later section will address RZ and other modulation formats. The datastream in Figure 6.1 is shown as perfect rectangles. In reality, the RZ pulses are close to Gaussian in the time domain and the NRZ pulses are often modeled as super-Gaussians (Norimatsu and Yamamoto, 2001), as shown in the figure inserts. For medium- and high-datarate operation, the output from an unmodulated laser of wavelength a should be continuous wave (cw), single-longitudinal mode, and narrow bandwidth. When modulated, the wave experiences spectral broadening to frequency band- width Av, and in wavelength space it is given by equation 6.1. Approximately Super-Gaussian: exp(-t2p/t2p) Approximately Gaussian: exp(-t2/t2) Fig. 6.1 Bitstreams: (top) NRZ; (bottom) RZ. The actual pulse shapes can often be modeled by the mathematical forms in the inserts (p is a positive integer and T is a constant with the dimensions of time). Chapter 6 Optical Fiber Transmission A2=(22/c).Av (6.1) The magnitude of Av depends on several influences, which include the bitrate, the temporal profile of the pulses, the nature of the digital data sequence, and the modula- tion format used. In general, by Fourier transform considerations (Bracewell, 1986), short pulses have broad bandwidths; long pulses, narrow bandwidths. Therefore, the bandwidth increases with the bitrate. Figure 6.1 shows that RZ signaling uses shorter pulses than NRZ signaling and SO results in a greater Av. The influence of the modulation format is considered f