We demonstrate a 400G dual-wavelength dual-polarization IM-DD TDM-PON based on optical duobinary modulation with 34 dB back-to-back optical power budget. After 20 km of SSMF we find an optical path penalty below 1 dB.

A 100 Gb/s Intensity Modulated Direct Detection dispersion tolerant downstream PON with 38 dB power budget based on optical duobinary using low-complex equalization is experimentally demonstrated. Optical duobinary transmission using avalanche photodiodes in combination with Electrical Duobinary Detection is shown for the first time. This combination is very tolerant to bandwidth limitations at both the transmitter and the receiver. Results are further improved upon by using a wider bandwidth Semiconductor Optical Amplifier -filter- P-type Intrinsic N-type receiver. Optical duobinary transmission also permits transmission at higher optical powers without incurring a meaningful penalty. The results show the promise of using the Optical Duobinary scheme for future very high speed cost-effective PON systems beyond 50 Gb/s.

We experimentally show feasibility of downstream 200 Gbit/s IM/DD TPDM PON system with >4 dB margin to 29 dB optical power budget based on two 100 Gbit/s polarization channels in a single wavelength window. The system uses SBS suppression to mitigate nonlinear fiber loss and duoternary modulation to overcome bandwidth limitation.

Due to continuously emerging high bandwidth applications, research and standardization of time division multiplexed passive optical networks (TDM-PONs) have focused on increasing the peak bitrate. However, increasing the bitrate while supporting the stringent optical power budget of a PON becomes increasingly challenging because of the larger chromatic dispersion penalties as well as reduced receiver sensitivity when the bitrate of the intensity modulation with direct-detection (IM-DD) based PON is increased. Also, increasing bitrate generally causes higher power consumption, which leads to more challenging thermal designs and misalignment with environmental targets. In this paper we give an overview of flexible concepts that can help achieve the required optical power budget and support reduced power consumption of a future IM-DD based TDM-PON. We demonstrate that a flexible PON can provide an increased overall throughput or an extended reach and power budget with the use of flexible modulation formats, probabilistic and geometric shaping, and flexible rate forward error correction (FEC). Another dimension of flexibility in the form of a configurable optical distribution network (ODN) is described and its merits and challenges are discussed. Flexible concepts based on interleaving of FEC codewords can align signal processing like FEC decoding and the protocol processing closer to the user-rate of an optical network unit (ONU), which leads to reduced power consumption. Flexibility based on multiple channels based on wavelength multiplexing or spatial multiplexing enables optimization of the power consumption to the amount of traffic on the PON. Several flexible concepts have already been adopted in the PON standards. We highlight flexible gain FEC for upstream 50G PON, the transmitter dispersion eye closure (TDEC) metric, and the flexible split-ratio ODN for power saving. Flexible modulation has not been adopted yet in PON standards, but it is expected that flexibility is more and more needed to support the performance, cost effectiveness, and power conservation of future optical access systems.

We detail the TDEC(Q) method and its usage in the context of system level specifications for 50 and 100G PONs. Real-time experimental results obtained with representative PON transmitters are used to analyze and validate interoperability of these PONs.

We assess the status of current generation 25G and 50G time division multiplexed passive optical network (TDM PON) technologies based on leveraging the cost efficiencies of the Ethernet intra-datacenter ecosystem. As a first step towards 100G TDM PON, we predict the real-world impact of a flexible modulation enhancement to 50G PON, whereby four-level pulse amplitude modulation (PAM4) symbols can be transmitted at the same symbol rate as 50 Gb/s PAM2, but only where excess margins permit. We find that sufficient margins are likely to exist to allow for a majority of future 50G PON optical network units to operate at 100 Gb/s PAM4. Next, we look at the options for a 100G PON capable of supporting the full loss budget and reach requirements. There is no technical risk if coherent technology is adopted, but intensity-modulation and direct-detection (IM-DD) will provide lower complexity, lower cost, and lower power dissipation. We evaluate this option and conclude that by following IM-DD Ethernet optics to 100 GBd, single wavelength IM-DD will continue to be feasible for 100G PON and will be a strong contender for the next generation of PON after 50 Gb/s.

In this paper, we employ artificial neural networks (ANNs) to optimize joint probabilistic shaping (PS) and geometric shaping (GS) for a realistic 50G IM/DD passive optical network (PON) link. Apart from being able to find a generalized mutual information (GMI)-maximizing modulation for channel conditions unseen at the training phase, the compatibility of the ANN training with Monte Carlo simulation also enables us to use a more complicated channel model that more closely resembles a real PON system where fiber dispersion, bandwidth limitation and digital signal processing (DSP) are present. The forward error correction (FEC) requirement that must be satisfied in an actual implementation is imposed on the learned modulation by including a normalized GMI (NGMI) penalty term in the loss function. The proposed scheme is demonstrated with simulations. Results show that the ANN can achieve similar performance compared to a case-by-case optimization while also being capable of generalizing to a wide range of received optical power (ROP) from −30 dBm to −18 dBm and/or a broad range of fiber distance from 0 km to 20 km. About 0.1-bits/symbol GMI improvement is attained compared to uniform modulation.

Adeep neural network based equalizer is proposed to mitigate the intersymbol interference observed in next generation high speed passive optical network (PON) links. The DNN based equalizer is shown to outperform the best known conventional equalizer, the maximum likelihood sequence estimator (MLSE) both in back-back and through fiber experiments. To reduce the hardware complexity of DNN based equalizer for PON systems, we investigate the use of embedded parallelization within a DNN structure having multiple symbol outputs from one DNN. We further investigate using a classification output stage with cross entropy cost to perform joint decision on multiple symbol outputs and demonstrated that the sensitivity gain of such scheme over regression output. To understand the complexity of hardware implementation, the fixed-point DNN based equalizers are developed and implemented in FPGA. The impact of fixed-point resolution on the receiver sensitivity and hardware resource utilization in FPGA implementation is analyzed and reported in detail. We show that a reduction of over 40% in LUTs (look up table) utilization is possible by reducing the DNN's weight resolution from 8-bit to 4-bit while incurring a small penalty in receiver sensitivity.

Joint probabilistic and geometric shaping is considered for flexible PON. Optimal modulation is found through ANN, which generalizes to various ROPs and fiber lengths while taking dispersion, limited bandwidth, and receiver-side DSP into account. © 2022 The Author(s)

The application of pairwise probabilistic shaping combined with geometric shaping is investigated for a pulse-amplitude-modulation-based intensity modulated and direct detection (IMDD) system. With probabilistically shaped signals, we experimentally demonstrate the flexible information rate beyond 100 Gb/s in a practical passive optical network (PON) link. Similar to the G.hsp 50 Gb/s PON standard, the PON link assumes the existence of a semiconductor optical amplifier (SOA), which boosts the transmitter power to achieve a high link budget. While the optimal signal distributions for the IMDD link with a SOA tend to be unipolar distributions, we consider a practical methodology of accommodating forward error correction (FEC) parity bits using pairwise probabilistic distribution. Since pairwise signaling causes performance degradation when the optimal signal would be strongly shaped, we propose to combine geometric shaping (GS) with probabilistic shaping (PS) to overcome the shortcomings of the pairwise signaling. We present experimental demonstrations of the various achievable rate measurements using PS + GS modulations with flexible FEC assumption. We also propose a novel descrambling-based decoding technique to evaluate the post-FEC performance based on transmission data that is agnostic of the FEC code. The post-FEC performances are then presented using a fixed and practical low-density parity check code.

FLCS-PON is a concept of a flexible passive optical network (PON), where the modulation and coding parameters of the downstream signal, consisting of modulation order, code rate, and probabilistic shaping entropy, can be adjusted on a timeslot basis to opportunistically achieve mean net bitrate increase. The system harnesses unused margins of transceivers and optical distribution networks. In this work we describe the architecture of FLCS-PON, which is designed to leverage the 50G-PON ecosystem. We provide updated and accurate performance metrics for FLCS-PON low-density parity check (LDPC) forward error correction (FEC) codes. We then report on the operator trial of a FLCS-PON system prototype carried out jointly with Vodafone and demonstrate maximum achievable net bitrates as a function of the optical path loss for five different transmitter configurations (non-return-to-zero, PAM-4, and probabilistically shaped PAM-4 with three different entropy values). On the receiver side we test four different configurations: FFE23+DFE5 or FFE16+DFE1 equalizers, each followed by either a hard- or a soft-input LDPC decoder. Finally, we consider two study cases on mean bitrates achievable with FLCS-PON over deployed optical distribution networks (ODNs). We map net bitrates obtained during the operator trial to Vodafone’s actual ODN optical path loss probability distribution or an ensemble of simulated ODNs. We show that in the majority of cases, FLCS-PON can provide a significant improvement of mean net bitrate. Further, FLCS-PON also enables extension of nominal power budget classes, beyond guaranteed conventional ODN loss, making it possible to provision links that might otherwise be unsupported by conventional PON.

A low-complexity flexible forward error correction scheme based on different shortening and puncturing of the standard G.hsp 50G PON LDPC mother code to achieve enhanced throughput and robustness in upstream PON is motivated and presented. © 2022 The Author(s)

The use of coherent techniques for TDM-PON including its link budget and complexity is discussed. NRZ modulation with EML in two polarizations combined with coherent detection is experimentally exemplified as a candidate for cost-effective solution. © 2021 The Authors

DFE induced error correlation and mutual information (MI) degradation penalize the 50G PON LDPC code soft-input decoding performance. Bit-interleaving across multiple codewords mitigates the correlated error penalty, but not the MI degradation penalty.

The pairwise probabilistic combined with geometric shaping is applied to IMDD system. With shaped PAM signals, the flexible information rate beyond 100Gb/s is experimentally demonstrated for the first time in a practical PON link.

This paper provides an overview of transceiver technologies to be used for current and next-generation passive optical networks (PONs). The uninterrupted scaling of PONs to higher bitrates in a cost-effective way to meet future bandwidth demands will drive the need for continuous improvement in PON transceiver technologies. In this paper we try to analyze the requirements needed and the impact on this for next-generation transceiver technologies based on past and present PON transceiver designs.

We demonstrate concepts and results of a field trial for a flexible-rate passive optical network (FLCS-PON), which delivers bitrates up to 100 Gbit/s and allows for adaptations in the transmission method to match the users' channel conditions and optimize throughput. FLCS-PON builds on top of the hardware ecosystem that will be developed for ITU-T 50 Gbit/s PON and employs three new ingredients: optical network unit (ONU) grouping, flexible modulation format, and flexible forward error correction (FEC) code rate. Together, these techniques take advantage of the optical distribution network (ODN) statistics to realize a system capable of more than twofold throughput increase compared to the upcoming 50 Gbit/s PON, but still able to support a full array of deployed fiber edge cases, which are problematic for legacy PONs. In this paper we explain the concepts behind enabling techniques of FLCS-PON. We then report on a field trial over a deployed fiber infrastructure, using a system consisting of one FLCS-PON OLT and two ONUs. We report both pre- and post-forward-error-correction (post-FEC) performance of our system, demonstrating achievable net bitrate over an operator's fiber infrastructure. We realize a downlink transmission at double the speed of ITU-T 50 Gbit/s PON for ONUs exhibiting lower optical path loss (OPL), while simultaneously continue to support ONUs at high OPLs. We additionally realize a record-high 31.5 dB loss budget for 100 Gbit/s transmission using a direct-detection ONU with an optical preamplifier.

We introduce probabilistic shaping and soft-input FEC to our 100-Gbit/s flexible PON concept, FLCS-PON, beside varying the modulation format and FEC code shortening/puncturing. These features improve granularity of bitrate adjustment for various channel conditions in downstream direction. We demonstrate our prototype in an operator trial.

DSP techniques are presented for hybrid modulated TDM-PON. FFE/DFE and various CDRs are tested with measured 50G/100G NRZ/PAM4 hybrid modulated data with 25G commercial devices. The measured data are further processed offline for the worst-case ISI to analyze DSP tolerance to chirp and dispersion effect.

Performance of Deep Neural Network receiver equalization is investigated after LDPC FEC. It is shown that DNN equalization results in a lower level of error clustering and higher performing equalization relative to more traditional equalization for bandwidth-limited and dispersion-limited channels for 50G PON.