  Intensity-modulation and direct-detection (IM/DD) transmission is widely
adopted for high-speed optical transmission scenarios due to its
cost-effectiveness and simplicity. However, as the data rate increases, the
fiber chromatic dispersion (CD) would induce a serious power fading effect, and
direct detection could generate inter-symbol interference (ISI). Moreover, the
ISI becomes more severe with the increase of fiber length, thereby highly
restricting the transmission distance of IM/DD systems. This paper proposes a
dual-tap optical-digital feedforward equalization (DT-ODFE) scheme, which could
effectively compensate for CD-induced power fading while maintaining low cost
and simplicity. A theoretical channel response is formulated for IM/DD
transmission, incorporating a dual-tap optical equalizer, and the theoretical
analysis reveals that for an IM/DD transmission using 1371nm over 10km standard
single-mode fiber (SSMF), frequency notch is removed from 33.7GHz to 46GHz.
Simulation results show that the DT- ODFE achieves an SNR gain of 2.3dB over
IM/DD systems with symbol-space feedforward equalizer (FFE) alone. As the fiber
length increases to 15 km, DT- ODFE performs well, while FFE, decision-feedback
equalizer (DFE) and Volterra nonlinear equalizers (VNLE) all fail to compensate
for the power fading and the 7% hard-decision FEC limit is not satisfied. For
200 Gb/s/$\lambda$ PAM-4 over 15km SSMF, results show that the signal-to-noise
ratio (SNR) of the proposed DT- ODFE with optimal coefficients satisfies the 7%
hard-decision FEC limit, which uncovers the great potential of the DT- ODFE for
high-speed IM/DD systems in LR/FR scenarios.
