Decision threshold placement for optically preamplified receivers is studied, with particular emphasis on the implications for timing sensitivity. These receivers can require an optimum threshold that is depressed significantly below the midrange of the vertical eye opening; by incorporating a new signal-shaping filter design into the receiver, improved tolerance to alignment jitter and slightly better sensitivity are secured, compared with the widely adopted near-raised-cosine receivers. This is of special significance for multigigabit systems, for which even a small timing error can represent a substantial proportion of a signal time slot. Accordingly, results are presented relating to systems operating in the 1 Gbit/s-20 Gbit/s range.