itively chirped pulses can be overcome by using a CW laser followed by an external modulator, such as a semiconductor electro- absorption modulator (Beck Mason, 2002) or a lithium niobate Mach-Zehnder modulator (Madabhushi, 2002). Unfortunately, such devices do not fully extinguish the light in the 0 bitslots; for this reason they are said to operate with a finite "extinction ratio." (As defined in a later section, an infinite extinction ratio is obtained when there is no optical energy in the Os.) Nevertheless, in long-span and high-datarate communications, pulses that are not fully extinguished are usually preferable to those with significant positive chirp, which is why such applications mostly use external modulators. The output from a laser can be nonideal in a number of respects that influence the sys- tem's overall performance. Short-haul and lower-datarate communications can be very cost sensitive and therefore operators often use cheap multilongitudinal-mode lasers with direct modulation (Liu, 2005). The resulting transmitted bandwidths can be very large, but because Single-Channel Transmission such network categories do not exert significant propagation impairments (as discussed throughout most of the chapter), the quality of the received signals is within acceptable bounds. A variation on the same theme occurs when the intention is to provide very narrow- band light prior to external modulation but the laser exhibits residual optical side modes that deteriorate the quality of the received signals, especially after traveling many kilo- meters in a nonzero-dispersion fiber. Another effect is that the output from any laser oscillator is never completely constant in time because of an effect called "Relative Inten- sity Noise" (RIN), which is in the form of small-scale and random-intensity fluctuations on the output (Agrawal, 2002; Liu, 2005). The consequence for the system is a reduction in the optical signal-to-noise ratio (SNR) at the receiver, as will be discussed in a later section. Synchronous Digita