 of R are around 1 W/A (Agrawal, 2002; Liu, 2005). The decisions that take place in a receiver are of special importance in all forms of digital communications, and the role of the decision circuit is to interpret the individual bits, deciding whether each one is a 1 or a 0. Bitstream Corruption In a real optical system, the arriving bitstream is corrupted as a result of the trans- mitter laser, the fiber transmission medium, the optical amplifiers, and (on conversion to a Chapter 6 Optical Fiber Transmission photocurrent) the receiver itself. As explained in an early section of the chapter, the emitted pulse stream from the transmitter is less than ideal in three main respects: The pulses exhibit high-frequency fluctuations called "Relative-Intensity Noise" (RIN). The pulses can have unintended chirp. In cases where the laser is biased slightly above threshold, the logical Os are not normally at zero optical power. The fiber medium imposes chromatic and polarization mode dispersion and a number of effects that lead to nonlinear crosstalk, as described in later sections. The optical amplifiers always add noise, as described in an earlier section. Temperature drifting of the fiber, mis- timing at any digital cross-connects through which the signals pass, and interactions between amplifier noise and nonlinear optical effects in the fiber can cause jitter, which is a random- ization of the pulse arrival times. Furthermore, the receiver circuitry imposes thermal noise on the signal and the semiconductor detector adds a "dark current" to the signal, even during the 0 bits. Collectively, these influences combine to ensure that the electrical bitstream at the decision circuits differs significantly from ideal 1s and Os. Before discussing in greater detail BERs and the factors that influence them, we describe a useful means of visualizing bitstreams with various levels of corruption (Kartalopoulos, 2004). Figure 6.12 shows three representative "eye diagrams," as would be displayed on an oscilloscope screen for NRZ bits. 