  A class of circular 64-QAM that combines 'geometric' and 'probabilistic'
shaping aspects is presented. It is compared to square 64-QAM in back-to-back,
single-channel, and WDM transmission experiments. First, for the linear AWGN
channel model, it permits to operate close to the Shannon limits for a wide
range of signal-to-noise ratios. Second, WDM simulations over several hundreds
of kilometers show that the obtained signal-to-noise ratios are equivalent to -
or slightly exceed - those of probabilistic shaped 64-QAM. Third, for real-life
validation purpose, an experimental comparison with unshaped 64-QAM is
performed where 28% distance gains are recorded when using 19 channels at 54.2
GBd. This again is in line - or slightly exceeds - the gains generally obtained
with probabilistic shaping. Depending upon implementation requirements (core
forward-error correcting scheme for example), the investigated modulation
schemes may be key alternatives for next-generation optical systems.
