  We examine pulse amplitude modulation (PAM) for intensity modulation and
direct detection systems. Using a straight-forward, mixed noise model, we
optimize the constellations with an autoencoder-based neural network (NN), an
improve required signal-to-noise ratio of 4 dB for amplified spontaneous
emission (ASE)-limited PAM4 and PAM8, without increasing system complexity.
Performance can also be improved in O-band wavelength division multiplexing
system with semiconductor optical amplifier amplification and chromatic
dispersion. We show via simulation that for such a system operating at 53
Gbaud, we can extend the reach of PAM4 by 10-25 km with an optimized
constellation and a NN decoder. We present an experimental validation of 4 dB
improvement of an ASE-limited PAM4 at 60 Gbaud using an optimized constellation
and a NN decoder.
