It is foreseeable that the 100 Gb/s/ $\lambda$ and beyond passive optical network (PON) will be required in future optical access networks to meet the explosive growth of data traffic. The coherent optical systems could be a promising solution for the future beyond 100G PON. Coherent PON using digital subcarrier multiplexing (DSCM) can provide flexible bandwidth allocation to a large number of access subscribers by dividing subcarriers of the DSCM signal into time slots for time-and-frequency division multiple access. When the optical network unit is allocated a new subcarrier, digital signal processing (DSP) should converge fast in the allocated time slot to ensure a low handoff latency for real-time bandwidth allocation. However, the traditional coherent DSP is hard to realize fast convergence due to blind and complex algorithms. In this paper, we design a specific training sequence (TS) structure and propose data-aided DSP to achieve fast convergence for coherent PON. The feasibility of the proposed scheme is experimentally verified in an 8 Gbaud/SC×8 SCs 400 Gb/s-net-rate coherent PON using DSCM with 16 quadrature amplitude modulation. The experimental results show that fast convergence is jointly realized by the proposed TS structure and data-aided DSP using a 416-symbol TS with a 52 ns duration. The receiver sensitivity at the 20% soft-decision forward error correction limit is approximately -27 dBm and an optical power budget of about 35.5 dB is achieved with a booster amplifier.

In this paper, we propose adaptive channel-matched detection (ACMD) to effectively compensate channel distortions for C-band 64-Gbit/s intensity-modulation and direct-detection (IM/DD) optical on-off keying (OOK) system over a 100-km dispersion-uncompensated link. The proposed ACMD can adaptively compensate most of the link distortions based on channel and noise characteristics, which includes a polynomial nonlinear equalizer (PNLE), a decision feedback equalizer (DFE) and maximum likelihood sequence estimation (MLSE). Based on the channel characteristics, PNLE eliminates the linear and nonlinear distortions, while the followed DFE compensates the spectral nulls caused by chromatic dispersion. Finally, based on the noise characteristics, a post filter can whiten the noise for implementing optimal signal detection using MLSE. To the best of our knowledge, we present a record C-band 64-Gbit/s IM/DD optical OOK system over a 100 km dispersion-uncompensated link achieving 7% hard-decision forward error correction limit using only the proposed ACMD at the receiver side.

In this paper, we present a field-trial C-band 72Gbit/s optical on-off keying (OOK) system over 18.8 km dispersion-uncompensated submarine optical cable in the South China Sea. Chromatic dispersion (CD) of 18.8 km submarine optical cable causes four spectral nulls on the 36 GHz 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 residual 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 2.9 dB improvement of 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.

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 cooperating with equalizers to deal with spectral distortions for intensity-modulation and direct-detection (IM/DD) optical transmission systems. The equalizers compensate the spectral distortions caused by limited bandwidth and chromatic dispersion, and the optimized detection decodes the useful bits from equalized symbols with severe colored in-band noise. To more accurately extract the bits, the optimized detection with larger memory length is required. However, the classical optimized detection based on maximum likelihood-sequence estimation (MLSE) requires exponential-growing computational complexity and storage space with the increase of memory length. The fixed-state Log-MAP detection can decrease the computational complexity and storage space from the exponential order to linear order. Therefore, the fixed-state Log-MAP detection can support larger memory length to more accurately extract bits with less logic gate resources and storage resources compared to MLSE. We experimentally verified the fixed-state Log-MAP detection in a C-band 64Gbit/s IM/DD on-off keying optical system over a 100km dispersion-uncompensated link. The fixed-state Log-MAP detection not only improves the receiver sensitivity of 2dB, but also reduces the computational complexity and storage space by three orders of magnitude.

As the 50G optical access gradually matures, it is time to discuss Beyond 50G optical access. According to the evolution rules of optical access standards, the data rate of Beyond 50G optical access may achieve 200 Gb/s. Direct detection faces great challenges for Beyond 50G optical access, which makes coherent detection a potential solution. Similar to 50G optical timing-division-multiple access (TDMA), burst-mode digital signal processing (BM-DSP) is also required for Beyond 50G coherent optical TDMA (CO-TDMA). This paper proposes coherent BM-DSP (Co-BM-DSP) based on approximately 10 ns designed preambles to process the burst signal for 200 G CO-TDMA, which can fast estimate the state of polarization, frequency offset, sampling phase offset, synchronization position, and equalizer coefficients. Meanwhile, for obtaining the equalizer coefficients based on the designed preamble, the channel estimation based on the minimum-mean-square-error criterion is theoretically proven to have a unique solution for ensuring reliability. In conclusion, the proposed Co-BM-DSP based on the designed preambles paves the way for the applications of Beyond 50G CO-TDMA.

Neural networks (NNs) have been successfully applied to channel equalization for optical communications. In optical fiber communications, the linear equalizer and the nonlinear equalizer with traditional structures might be more appropriate than NNs for performing real-time digital signal processing, owing to its much lower computational complexity. However, the optimization algorithms of NNs are useful in many optimization problems. In this paper, we propose and evaluate the tap estimation schemes for the equalizer with traditional structures in optical fiber communications using the optimization algorithms commonly used in the NNs. The experimental results show that adaptive moment estimation algorithm and batch gradient descent method perform well in the tap estimation of equalizer. In conclusion, the optimization algorithms of NNs are useful in the tap estimation of equalizer with traditional structures in optical communications.