  An optical gateway to convert four-level pulse amplitude modulation to
quadrature phase shift keying modulation format having shaping gain was
proposed for flexible intensity to phase mapping which exploits non-uniform
phase noise. The power consumption of the optical modulation format conversion
can save by making a DNN-based decision on the receiver side for the generated
QPSK signal with non-uniform phase noise. A proof-of-principle experiment has
shown that an optically geometric constellation shaped QPSK modulated signals
generated from regular PAM4 signals with Gaussian-distributed noise. The shaped
QPSK signal shows BER and generalized mutual information improvement by 1dB
gain through the use of digital neural network signal recovery.
