  In this paper, achievable information rates (AIR) for fiber optical
communications are discussed. It is shown that AIRs such as the mutual
information and generalized mutual information are good design metrics for
coded optical systems. The theoretical predictions of AIRs are compared to the
performance of modern codes including low-parity density check (LDPC) and polar
codes. Two different computation methods for these AIRs are also discussed:
Monte-Carlo integration and Gauss-Hermite quadrature. Closed-form ready-to-use
approximations for such computations are provided for arbitrary constellations
and the multidimensional AWGN channel. The computation of AIRs in optical
experiments and simulations is also discussed.
