  In this paper, an optimized detection based on log-maximum a posteriori
estimation with the fixed number of surviving states (fixed-state Log-MAP) is
proposed to cooperate with equalizers to deal with the spectral distortions
caused by limited bandwidth and chromatic dispersion for intensity-modulation
and direct-detection (IM/DD) optical systems. The equalizers compensates the
spectral distortions and optimized detection decodes the useful bits from the
noise. For accurately extracting the bits from more serious noise, the
optimized detection with larger memory length is required. However, the
classical optimized detection such as maximum likelihood-sequence estimation
(MLSE) requires exponential-growing computational complexity and storage with
the increasing memory length. The fixed-state Log-MAP detection can decrease
the computational complexity and storage from the exponential order to linear
order. Therefore, the fixed-state Log-MAP detection can compensate more
distortions compared to MLSE under the same hardware condition. We
experimentally verify the fixed-state Log-MAP detection in a C-band 64 Gbit/s
IM/DD on-off keying optical system over a 100 km dispersion-uncompensated link.
Under the same hardware condition, the fixed-state Log-MAP detection has a 2 dB
improvement of receiver sensitivity compared to MLSE. In conclusion, the
fixed-state Log-MAP detection shows the potential for practical IM/DD optical
systems.
