  We numerically and experimentally investigate the laser phase noise tolerance
of probabilistically shaped (PS) and uniformly shaped (US) quadrature amplitude
modulation (QAM) signals. In the simulations, we compare PS-64QAM to US-16QAM,
PS-256QAM to US-64QAM, and PS-1024QAM to US-256QAM under the same information
rate (IR). We confirm that a sufficient shaping gain is observed with narrow
linewidth lasers, whereas degradation of the shaping gain is clearly observed
when large phase noise and high order modulation formats are assumed. In our
experiments, we compare polarization-division-multiplexed (PDM) 16-GBd
PS-1024QAM and US-256QAM under the same IR using lasers with 0.1-kHz and 40-kHz
linewidths. For carrier phase recovery (CPR), we employ a pilot-assisted
digital phase locked loop. Results reveal that PS-1024QAM achieves high
performance with the 0.1 kHz-laser or > 5% pilot ratio, whereas US-256QAM
outperforms PS-1024QAM when lasers with 40-kHz linewidth and < 5% pilot ratio
are used. We also evaluate the pilot ratio dependency of the required optical
signal-to-noise ratio at the forward error correction limit and the achievable
information rate. Additionally, we compare the performance of two types of CPR
updating schemes: updating phase estimation at only the pilot symbol or at all
symbols.
