ptical) dispersion accommodation and compensation techniques. Transmitter EDC, depicted in Figure 6.25, uses an electro-optic modulator that control- lably exerts both amplitude and phase modulation on the signal in response to commands from Digital Signal Processing (DSP) electronics. Ideally, it should be able to achieve any phase variation, but this is not always the case in some device designs. The aim is to provide simultaneous pre-emphasis and prechirping of the signal in anticipation of the chromatic dispersion that it is going to encounter during its transit in the system fiber. In this way, the arriving pulses should be confined as closely as possible to their bitslots. The modulator shown in Figure 6.25 is a dual-parallel Mach-Zehnder design, which can be based on lithium niobate technology. It can access all phase states but is an expensive device with an insertion loss of 10 to 15 dB. An alternative configuration, called the "dual- drive Mach-Zehnder modulator" is lower cost and imposes a loss of only 3 to 6 dB, but it Electronic Transmission Digital- Input Analogue In-Phase System Converter Drive Voltage Digital Signal Mach-Zehnder Processor Modulator (ASIC) Laser Quadrature Drive Digital- Voltage Analogue Converter Driver Amplifier In-Phase Drive Electrode Optical Voltage Waveguide Laser Modulated Input Output Earth Quadrature Drive Phase Shift Voltage Fig. 6.25 Transmitter-end electronic dispersion compensation. (a) Block diagram of the main components. (b) Detail of one design of the electro-optic modulator configuration, known as the dual- parallel Mach-Zehnder modulator. Chapter 6 Optical Fiber Transmission cannot explore the full combination of phase and amplitude states (McGhan, 2006). Never- theless, it is a good compromise, especially for use with NRZ signals. The DSP electronics shown in Figure 6.25 include the key chromatic-dispersion- compensating filter, together with coding, linear pulse shaping, and nonlinear filtering. Until recently, the design of digital-to-analogue converters was e