ution is stimulated by the high transmitted power, this is no more true and a further sensitivity penalty is induced by this phenomenon. In a first approximation, we can divide polarization fluctuations between fast nonlinear and slow linear. Due to the fact that the linear polarization tracker has a small bandwidth, it passes through the fast part of polarization fluctuations. Kerr-induced polarization fluctuations, depending on the transmitted message, can be seen as a random process inducing random time depolarization of the received field [65]. As far as the induced depolarization degree is small, we can imagine that it is passed through by the polarization tracker SO that practically only the linear fluctuations are tracked and the nonlinear depolarized field appears to fluctuate randomly after multipli- cation by the inverse of the Jones matrix and have to be dealt as signal dependent noise. Calling POL the nonlinear depolarization degree, which is the reciprocal of the polariza- tion degree defined in Equation 9.34, the received optical field can be written as (9.80) where s(t) is a randomly varying unitary vector determined by nonlinear polarization evolu- tion that has a bandwidth larger than Rs a and y are the Jones matrix parameters (along with an absolute phase here set to zero) that the polarization tracker has to estimate Even if the correlation between the depolarized and the polarized part of the field is not zero, a simple way to evaluate the impact of nonlinear polarization evolution is to consider the depolarized terms if it was a noise with a colored spectrum. In particular, the expressions of the photocurrents after polarization recovery becomes (9.81) Next Generation Transmission Systems Electrical signal to noise ratio (dB) 0.00025 0.0025 0.025 FIGURE 9.47 SN penalty due to depolarization in the approximation of small impair- Ideal electrical signal to noise ratio (dB) ment for M-4QAM. The 4 X 4 matrix A is defined stating from the quadratures of M+s(t). Due to its definition, A is a unitar