To cope with the explosive bandwidth demand, significant progress has been made in the ITU-T standardization sector to define a higher-speed passive optical network (PON) with a 50 Gb/s line rate. Recently, 50 G PON has become mature gradually, which means it is time to discuss beyond 50 G PON. For ensuring an acceptable optical power budget, beyond 50 G PON will potentially use coherent technologies, which can simultaneously promote the applications of flexible multiple access such as time/frequency-domain multiple access (TFDMA). In this paper, we will introduce the architectures, algorithms, and demonstrations for TFDMA-based coherent PON. The system architectures based on an ultra-simple coherent transceiver and specific signal spectra are designed to reduce the cost of ONUs greatly. Meanwhile, continuous-mode and burst-mode digital signal processing (DSP) algorithms are proposed for dealing with downstream and upstream signals, respectively. Based on the architectures and algorithms, we experimentally demonstrate the first real-time TFDMA-based coherent PON where 50 Gb/(s × subcarrier) continuous downstream signal and 12.5 Gb/(s × subcarrier) burst upstream signal are processed in real time. Owing to the independent subcarrier processing, it has the potential to support peak line rates of downstream 200 Gb/s (i.e. four subcarriers) and upstream 100 Gb/s (i.e. eight subcarriers). Significantly, the proposed architectures and algorithms have been sufficiently verified by real-time demonstrations. In conclusion, the proposed technologies for the coherent PON make it more possible to be applied in the future beyond 50 G PON.

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.

Adaptive moment estimation (Adam) is a popular optimization method to estimate large-scale parameters in neural networks. This paper proposes the first use of Adam algorithm to fast and stably converge large-scale tap coefficients of polynomial nonlinear equalizer (PNLE) for 129-Gbit/s PAM8-based optical interconnects. PNLE is one of simplified Volterra nonlinear equalizer for making a trade-off between complexity and performance. Different from serial least-mean square (LMS) adaptive algorithm, Adam algorithm is a parallel processing algorithm, which can obtain globally optimal tap coefficients without being trapped in locally optimal tap coefficients. Timing error is one of the main obstacles to the PAM systems with high baud rate and high modulation order. Owing to parallel processing and global optimization, Adam algorithm has much better performance on resisting the timing error, which can achieve faster, more-stable and lower-MSE convergence compared to LMS adaptive algorithm. In conclusion, Adam algorithm shows great potential for converging the tap coefficients of PNLE in PAM8-based optical interconnects.

In this paper, three basic intensity-modulated and direct-detected (IM/DD) fast orthogonal frequency division multiplexing (FOFDM) schemes are compared, including Flip-FOFDM, asymmetrically clipped optical FOFDM (ACO-FOFDM) and DC-offset FOFDM (DCO-FOFDM). The modulation/demodulation processing of FOFDM employs the discrete cosine transform. Compared with conventional OFDM based on discrete Fourier transform, FOFDM exhibits lower computational complexity because of its simple one-dimensional modulation and real-valued operation. To the best of our knowledge, Flip-FOFDM is first reported. We analyze the key parameters of three basic IM/DD FOFDM schemes in detail, including spectral efficiency, computational complexity, bit-error-ratio performance, and power efficiency. We categorize ACO-FOFDM and Flip-FOFDM in U-FOFDM. The spectral efficiency of DCO-FOFDM is twice that of U-FOFDM. However, due to the large DC bias of DCO-FOFDM, DCO-FOFDM is less efficient than U-FOFDM in terms of optical power when the bit rate/normalized bandwidth is less than 5, but DCO-FOFDM becomes more efficient than U-FOFDM when the bit rate/normalized bandwidth is greater or equal to 5. FOFDM shows great potential for application in IM/DD systems.

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.