Nonlinear distortion from a directly modulated laser (DML) is one of the major limiting factors to enhance the transmission capacity beyond 10 Gb/s for an intensity modulation direct-detection (IM/DD) optical access network. In this letter, we propose and demonstrate a low-complexity nonlinear equalizer (NLE) based on a machine-learning algorithm called artificial neural network (ANN). Experimental results for a DML-based 20 Gb/s signal transmission over an 18km SMF-28e fiber at 1310-nm employing pulse amplitude modulation (PAM)-4 confirm that the proposed ANN-NLE equalizer can increase the channel capacity and significantly reduce the impact of nonlinear penalties.

We experimentally demonstrate 4×75 Gbit/s optically amplified 4-level pulse amplitude modulation (PAM-4) transmission based on an external Mach-Zehnder modulator (MZM) and electro-absorption modulator (EAM) for monolithically integrable intensity modulation and direct detection wavelength division multiplexed passive optical networks (WDM-PONs). The effects of semiconductor optical amplifier (SOA)-induced nonlinear distortions during single- and multi-wavelength amplification in the optical distribution networks are investigated for various channel spacings through experiments and simulations. A machine learning-based nonlinear equalizer termed as a recurrent neural network (RNN) is proposed to compensate for the nonlinear impairments. Finally, by employing a T-spaced RNN in conjunction with a traditional feed-forward equalizer (FFE), we achieve link budgets in excess of 31 dB and 28 dB on every WDM channel at the hard-decision forward error correction (HD-FEC) limit of 3.8×10-3 for MZM and EAM-based WDM-PONs, respectively, after 4×75 Gbit/s PAM-4 transmissions at 1550 nm with a 100 GHz channel spacing over 25 km feeder and 1 km distribution single-mode fiber links.

This work presents the use of a directly modulated laser (DML) in time- and wavelength division multiplexed passive optical networks (TWDM-PONs), in which an EDFA-based booster amplification and an SOA-based pre-amplification are utilized to improve the optical power budget. We experimentally demonstrate the C-band optically amplified data transmissions and compare the performance of the non-return-to-zero on-off keying (NRZ-OOK), four and eight-level pulse amplitude modulation (PAM-4/PAM-8) using a 10-G class DML. With optical amplification, the uplink and downlink power budgets of about 28 dB and 35 dB, respectively, are achieved for the PAM-4 54-Gbit/s signals at the hard-decision forward error correction (HD-FEC) limit of 3.8x10 ^-3 after transmissions over a 25-km single-mode fiber link. To mitigate waveform distortions caused by the limited bandwidth, nonlinear dynamics, memory effects, and strong laser frequency chirp of the DML, especially during multi-level signal modulations, artificial neural network-based machine learning equalizers and low-complexity Volterra nonlinear equalizers are applied, which operate at the signal baud rate. The bit error ratio performance in conjunction with an enhanced power budget through the use of a low-complexity nonlinear equalizer can justify the validity of using a DML in the next-generation PONs.

We investigate the performance of $1.3~\mu $ m directly modulated multiple-quantum-well strained AlGaInAs distributed feedback laser in an optically amplified wavelength-division multiplexed (WDM) system for next-generation passive optical networks. We experimentally demonstrate $2\times 53$ Gbit/s PAM-4 transmissions over 20 km of a single-mode fiber employing two 16-GHz bandwidth-limited directly-modulated lasers and compare the performance of quantum-well and quantum-dot semiconductor optical amplifiers (SOAs) in amplifying DWDM links. We successfully obtained the total power budget of 34 dB for each WDM channel using a quantum-dot SOA and 30-tap T-spaced feed-forward equalizer.

We experimentally demonstrate the downstream transmission of 112.5 Gbit/s pulse amplitude modulated (PAM) signals in the O-band for future time-division multiplexed long-reach passive optical networks (LR-PONs). For the first time, this work demonstrates the use of a commercial quantum-well semiconductor optical amplifier (QW-SOA) in the remote node (RN) of a 100G-class PON to extend the transmission distance between the optical line terminal and the optical network unit. Besides, we discuss the performance of a QW-SOA in amplifying uniform and probabilistically shaped (PS) continuous-mode downstream PAM signals, such as PAM-4, PAM-8, and PS-PAM-8, and also discuss its potential during upstream burst-mode operations. Considering the hard-decision low-density parity-check bit error ratio limit of 1 × 10^− 2, the optical power budgets of 32.85 dB and 31.3 dB are achieved for the downstream PAM-4 and PS-PAM-8 signals, respectively, using a truncated T-spaced Volterra nonlinear equalizer after 112.5 Gbit/s SOA-amplified transmissions over a 50 km single-mode fiber, satisfying the class N2 power budget requirement of 31 dB for PONs. To the best of our knowledge, this is the highest power budget ever reported for 100G-class LR-PON with 50 km optical reach employing a commercial QW-SOA in the RN of a PON.

High-capacity data transmission for intra- and inter-data center links is essential to handle the massive increase in data traffic. To address the insatiable demands, IEEE has been working on creating specifications for 800 GbE (IEEE 802.3df) and 1.6 TbE (802.3dj) using a multi-lane distribution method by aggregating 100 Gbps and beyond lanes. Optical frequency comb sources based on semiconductor mode-locked laser (MLL) are cost- and energy-efficient optical sources that can enable multi-lane transmission systems. Here, we demonstrate 14×100 Gbps PAM4 signals and 10×100 Gbps probabilistically shaped PAM8 signals over 10 km SMF with an O-band InAs/InGaAs quantum dot mode-locked laser (QD-MLL) consuming only ∼284 fJ/bit. We also successfully demonstrated 128 Gbps/λ OOK signal transmission with four lines from the QD-MLL spaced at 160 GHz, with a single booster amplifier for multichannel amplification. We show that the performance of the ≥1 Tbps and 512 Gbps WDM signals is within the standard FEC limits with preamplifier and booster semiconductor optical amplifiers, respectively.