In this paper, we experimentally compared the performance of bit-loading discrete multi-tone (DMT) and pre-equalized discrete-Fourier-transform spread (DFT-Spread) DMT in intensity modulation with direct detection (IM/DD) system for 2-km optical interconnect for the first time. Pre-equalization is applied to compensate the high-frequency power fading in wideband communication system. However, it remains to be verified whether zero-forcing (ZF) channel estimation based complete pre-equalization can achieve the best performance for system with serious high frequency fading. In an IM/DD system with 120 Gbit/s data rate, we demonstrated that the bit error rate of DFT-Spread DMT signal with optimized quadratic curve fitting based partial pre-equalization outperforms ZF based complete pre-equalization after 2-km standard single mode fiber (SSMF) transmission. What's more, we experimentally compared the performance of pre-equalized DFT-Spread DMT and Chow-Cioffi-Bingham (CCB) algorithm-based bit loading DMT. The results show that the receiver sensitivity of pre-equalized DFT-Spread DMT is improved by 2.5 dB compared with CCB algorithm-based bit-loading DMT at hard decision forward error correction threshold of 3.8 × 10 ^-3 after 2-km SSMF transmission.

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.

The performance of high-speed intensity modulation direct detection (IM-DD) transmissions is severely degraded by the linear inter-symbol interference (ISI), due to both bandwidth-constrained optoelectrical components and the chromatic dispersion (CD). Meanwhile, nonlinear ISI due to intensity modulation and square-law detection can further deteriorate the IM-DD system performance significantly. Here, we first propose three metrics to quantify those impairments in IM-DD transmissions. Then, we compare both the BER performance and computational complexity of different DSP schemes to enable short-reach high-speed IM-DD transmissions with severe bandwidth constraint, using a C-band PAM-4 experimental test-bed with an end-to-end 10-dB bandwidth of 17 GHz. Those DSP schemes include receiver-side feedforward equalization (FFE), receiver-side Volterra filter equalization (VFE), transmitter-side Tomlinson-Harashima precoding (THP) together with receiver-side FFE, transmitter-side THP together with receiver-side VFE, channel-shortening filter (CSF) together with maximum likelihood sequence estimation implemented with linear branch metrics (MLSE-LI-BM), and CSF together with MLSE implemented with nonlinear branch metrics (MLSE-NL-BM). Our experimental results indicate that, only the combination of CSF and MLSE-NL-BM enables a 112 Gbit/s transmission over 5 km fiber at the KP4-FEC threshold, but with an increased computational complexity.

Efficient co-transmission of 10-W power light and optically carried 5G new radio (NR) signal over standard single-mode fiber (SSMF) is experimentally demonstrated, with an optical power transmission efficiency (OPTE) of power light up to 71.8%. This efficiency record is enabled by carefully manipulating the linewidth of the power light and appropriately adjusting the wavelength spacing between the power light and 5G NR optical signal to effectively mitigate the nonlinear effect arising in the SSMF. In the experiment, the optically carried 5G NR 64-level quadrature amplitude modulation orthogonal frequency division multiplexing signal at 1550 nm, with a data rate of 1.5 Gbit/s, is successfully co-propagated with 10-W power light at 1064 nm over 1-km SSMF. The error-vector magnitude (EVM) is 0.48% under a received electrical power of − 25 d B m . In comparison with back-to-back transmission, only slight EVM degradation of 0.02% is observed, showing that the 5G NR optical signal is almost unaffected by the existence of power light. Moreover, the power fluctuation of the collected power light is less than 0.2% over 6 h, while the EVM fluctuation is smaller than 0.01% within 30 min. Our scheme is promising to realize an optically powered remote antenna unit through the existing 5G fronthaul SSMF link.

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, we experimentally demonstrate the transmission of 56 Gbaud four-level pulse amplitude modulation (PAM4) signal over 2-km single mode fiber (SMF) with intensity modulation and direct detection (IM/DD) scheme, while the bit-error-ratio (BER) of the PAM4 signal is under hard-decision forward error correction (HD-FEC) threshold of 3.8 × 10⁻³. Linear pre-equalization is implemented in the transmitter side with a 3-tap finite-impulse-response (FIR) filter to compensate for the intersymbol interference (ISI) induced by system bandwidth limitation. Receiver side equalization is realized with training sequence (TS) based feed-forward equalizer (FFE) with decision-feedback equalizer (DFE). Furthermore, an Adaptive Notch Filter (ANF) is proposed to suppress the digital-to-analog converter (DAC) clock leakage induced narrowband interference for the first time, and the bandwidth of the ANF is optimized to achieve the best BER performance.