  Probabilistic constellation shaping (PCS) has been widely applied to
amplified coherent optical transmissions owing to its shaping gain over the
uniform signaling and fine-grained rate adaptation to the underlying fiber
channel condition. These merits stimulate the study of applying PCS to
short-reach applications dominated by intensity modulation (IM) direct
detection (DD) systems. As commercial IM-DD systems typically do not employ
optical amplification to save the cost and power consumption, they are no
longer subject to an average power constraint (APC) but a peak power constraint
(PPC), which poses unique challenges to take full advantages of PCS. This paper
provides a comprehensive investigation of PCS in IM-DD systems without optical
amplifiers. In particular, we reveal that if the transmitter enhances the
peak-to-average power ratio of the signal, a PPC system can be partially or
even fully converted to an APC system in which the classical PCS offers its
merits. The findings are verified through an IM-DD experiment using 4- and
8-ary pulse amplitude modulations.
