  In this paper, we present a field-trial C-band 72Gbit/s optical on-off keying
(OOK) system over 18.8km dispersion-uncompensated submarine optical cable in
the South China Sea. Chromatic dispersion (CD) of 18.8km submarine optical
cable causes four spectral nulls on the 36GHz bandwidth of 72Gbit/s OOK signal,
which is the main obstacle for achieving an acceptable bit-error-rate (BER)
performance. Decision feedback equalizer (DFE) is effective to compensate for
the spectral nulls. However, DFE has a serious defect of burst-error
propagation when the burst errors emerge due to the unstable submarine
environment. Weighted DFE (WDFE) can be used to mitigate the burst-error
propagation, but it cannot fully compensate for the spectral nulls because only
a part of feedback symbols is directly decided. Fortunately, maximum likelihood
sequence estimation (MLSE) can be added after the WDFE to simultaneously
eliminate the resisting spectral distortions and implement optimal detection.
Compared to the joint DFE and MLSE algorithm, the joint WDFE and MLSE algorithm
can effectively suppress the burst-error propagation to obtain a maximum 2.9dB
improvement of $\boldsymbol{Q}$ factor and eliminate the phenomenon of BER
floor. In conclusion, the joint WDFE and MLSE algorithm can solve the
burst-error propagation for the field-trial fiber-optic communications.
