  Analytic expressions are derived for the Wiener filter (WF), also known as
the linear minimum mean square error (LMMSE) estimator, for an
intensity-modulation/direct-detection (IM/DD) short-haul fiber-optic
communication system. The link is purely dispersive and the nonlinear
square-law detector (SLD) operates at the thermal noise limit. The achievable
rates of geometrically shaped PAM constellations are substantially increased by
taking the SLD into account as compared to a WF that ignores the SLD.
