 quality. The three most widely used quality parameters are derived from The degree of polarization, as measured by an optical polarimeter in front of the detector An analysis of the electrical spectrum of the detected signal at the output of the compensator Electronic monitoring of the eye diagram after the detector Of these options it should be noted that the polarimeter technique is applicable only to optical PMD compensation. Chapter 6 Optical Fiber Transmission Electronic PMD compensation is always performed after the detection process and is possible at 10 Gbit/s and 40 Gbit/s, thanks to the availability of high-speed ASICs. As with electronic chromatic dispersion compensation, discussed in the previous section, the benefit of the electronic approach is in integrating the compensator functionality with the receiver's own electronics, which leads to lower costs. The main categories, as reviewed in Bülow and Lanne (2005), are the Feed-Forward Equalizer (FFE) and the Decision Feedback Equalizer (DFE). However, because of the square-law nature of direct detection, there is a loss of phase and polarization information once the signal has been converted from the optical to the electrical domain. The result, unfortunately, is a nonzero power penalty. For this reason, electronic equalizers do not always provide the low residual optical SNR penalties that can be achieved with their optical counterparts. Nonlinear Optical Transmission When low-power light is guided by an optical fiber over a reasonably short distance, L, it conforms to the requirements of linear optics. The word "linear" refers to the guided-wave equation, which predicts that the accumulated phase change is proportional to the distance traveled according to Out = BL, in which B is the fiber's propagation constant, 2neff 12. Similarly, the input and output powers obey the proportionality Pout(A) = Pin(A) exp(-a L). In reality, these equations are only approximations because they are based on the assump- tion that the optical fiber's properties are ind