Overview of high-speed TDM-PON beyond 50 Gbps per wavelength using digital signal processing [Invited Tutorial]

The recent evolution of passive optical network standards and related research activities for physical layer solutions that achieve bit rates well above 10 Gbps per wavelength (lambda) is discussed. We show that the advancement toward 50, 100, and 200 Gbps/lambda will certainly require a strong introduction of advanced digital signal processing (DSP) technologies for linear, and maybe nonlinear, equalization and for forward error correction. We start by reviewing in detail the current standardization activities in the International Telecommunication Union and the Institute of Electrical and Electronics Engineers, and then we present a comparison of the DSP approaches for traditional direct detection solutions and for future coherent detection approaches. (c) 2022 Optica Publishing Grou

Nanoscale III-V Semiconductor Photodetectors for High-Speed Optical Communications

Nanophotonics involves the study of the behavior of light on nanometer scale. Modern nanoscale semiconductor photodetectors are important building blocks for high-speed optical communications. In this chapter, we review the state-of-the-art 2.5G, 10G, and 25G avalanche photodiodes (APDs) that are available in commercial applications. We discuss the key device parameters, including avalanche breakdown voltage, dark current, temperature dependence, bandwidth, and sensitivity. We also present reliability analysis on wear-out degradation and optical/electrical overload stress. We discuss the reliability challenges of nanoscale photodetectors associated with device miniaturization for the future. The reliability aspects in terms of high electric field, Joule heating, and geometry inhomogeneity are highlighted

Key Signal Processing Technologies for High-speed Passive Optical Networks

With emerging technologies such as high-definition video, virtual reality, and cloud computing, bandwidth demand in the access networks is ever-increasing. Passive optical network (PON) has become a promising architecture thanks to its low cost and easy management. IEEE and ITU-T standard organizations have been standardizing the next-generation PON, targeting on increasing the single-channel capacity from 10 Gb/s to 25, 50, and 100 Gb/s as the solution to address the dramatic increase of bandwidth demand. However, since the access network is extremely cost-sensitive, many research problems imposed in the physical layer of PON need to be addressed in a cost-efficient way, which is the primary focus of this thesis. Utilizing the low-cost 10G optics to build up high-speed PON systems is a promising approach, where signal processing techniques are key of importance. Two categories of signal processing techniques have been extensively investigated, namely optical signal processing (OSP) and digital signal processing (DSP). Dispersion-supported equalization (DSE) as a novel OSP scheme is proposed to achieve bit-rate enhancement from 10 Gb/s to 25 Gb/s based on 10G class of optics. Thanks to the bandwidth improved by DSE, the non-return-zero on-off keying which is the simplest modulation format is able to be adopted in the PON system without complex modulation or DSP. Meanwhile, OSP is also proposed to work together with DSP enabling 50G PON while simplifying the DSP complexity. Using both DSE and simple feed-forward equalizer is able to support 50 Gb/s PAM-4 transmission with 10G optics. For C-band 50 Gb/s transmission, injection locking techniques as another OSP approach is proposed to compress the directly modulated laser chirp and increase system bandwidth in the optical domain where a doubled capacity from 25 Gb/s to 50 Gb/s over 20 km fiber can be built on top of 10G optics. For DSP, we investigated the advantages of neural network (NN) on the mitigation of the time-varying nonlinear semiconductor optical amplifier pattern effect. In order to reduce the expense caused by the high computation complexity of NN, a pre- equalizer is introduced at the central office that allows cost sharing for all connected access users. In order to push the PON system line rate to 100 Gb/s, a joint nonlinear Tomlinson- Harashima precoding-Volterra algorithm is proposed to compensate for both linear and nonlinear distortions where 100 Gb/s PAM-4 transmission over 20 km fiber with 15 GHz system bandwidth can be achieved

Towards Higher Speed Next Generation Passive Optical Networks

L'abstract è presente nell'allegato / the abstract is in the attachmen

Integrated Optical-Wireless Interface and Detection

This chapter elaborates on the beneficial aspects and hardware implementations of incorporating ultradense WDM-PONs (UDWDM-PONs) with hybrid optical-wireless fronthaul links and fiber to the home applications. Simulation results on the synthesis of a low-cost and low-energy consumption optoelectronic unit within the future 5G base stations (BS) are presented. In addition, an advanced neural network is investigated capable of compensating for the linear and nonlinear effects induced by semiconductor optical amplifiers (SOA)

Next generation technologies for 100 Gb/s PON systems

The worldwide explosion of Internet traffic demand is driving the research for innovative solutions in many aspects of the telecommunication world. In access systems, passive optical networks (PONs) are becoming the preferred solution towards which most providers are migrating thanks to the unrivalled bandwidth they can offer. PON systems with a capacity of 100 Gb/s are envisioned as the solution to the dramatic increase in bandwidth and will be essential to support the future fixed and mobile broadband services. However, many challenging aspects have to be addressed in order to overcome the limitations imposed by the physical layer while meeting the economical requirements for mass deployment. In this thesis a comprehensive approach is taken in order to address the most compelling problems and investigate a series of solutions to the current capacity limitations of PONs. Advanced modulation formats are used to achieve bit-rate enhancement from 10 Gb/s to 25 Gb/s re-using the same optoelectronic devices in order to provide a 2.5x increase in transmission speed without resorting to a newer, more expensive generation of higher speed devices. The management of chromatic dispersion is also addressed in order to extend the reach of the networks beyond the standard 20 km using either electronic or optical based compensation strategies. Transmission of 25 Gb/s traffic over fibre lengths of 40 and 50 km is demonstrated confirming the suitability of the proposed technologies for extended reach networks which could greatly reduce the number of existing nodes and hence the capital and operational costs of PONs. Optical amplification strategies are also discussed as a means to improve the physical reach of the networks, both in terms of distance and number of customers. Raman amplifiers and semiconductor optical amplifiers are investigated in order to extend the reach of a PON upstream channel. The results demonstrate a reach of up to 50 km which is more than double the typical fibre length of 20 km adopted in deployed systems today. A number of customers, up to 512, was also demonstrated in a 20 km network, increased from the typical 32 or 64 users of most commercial networks

Opportunities for Optical Access Network Transceivers Beyond OOK

In response to the ever-growing challenge of using conventional direct-modulation/direct-detection transceivers for upgrading passive optical networks, there has recently been a surge in interest in alternative transceiver technologies. Candidate systems include (simplified) coherent receivers, and digital signal processing in combination with direct detection. Beyond these mainstream solutions, other, more esoteric, system designs have been proposed, including networks based on general purpose signal processing hardware, split carrier transmitters, and ultra-wide bandwidth coherent receivers. Herein, we review these techniques and detail an experimental investigation of an ultra-wide bandwidth coherent receiver. It is found that the use of a dual-local oscillator receiver in an ultra-dense wavelength division multiplexed passive optical network (UDWDM-PON) enables the simultaneous detection of multiple upstream channels while efficiently using the complete receiver optoelectronic bandwidth

Equalizer State Caching for Fast Data Recovery in Optically-Switched Data Center Networks

Optical switching offers the potential to significantly scale the capacity of data center networks (DCN) with a simultaneous reduction in switching time and power consumption. Previous research has shown that end-to-end switching time, which is the sum of the switch configuration time and the clock and data recovery (CDR) locking time, should be kept within a few nanoseconds for high network throughput. This challenge of low switching time has motivated research into fast optical switches, ultra-fast clock and amplitude recovery techniques. Concurrently, the data rate between server-to-server and server-to-switch interconnect is increasing drastically from the current 100 Gb/s (4×25 Gb/s) to 400 Gb/s and beyond, motivating the use of high order formats such as 50-GBaud four-level pulse-amplitude modulation (PAM-4) for signalling. Since PAM-4 is more sensitive to noise and distortion, digital equalizers are generally needed to compensate for impairments such as transceiver frequency rolloff, dispersion and optical filtering, adding additional time for equalizer adaptation and power consumption that are undesired for fast optical switching systems. Here we propose and investigate an equalizer state caching technique that reduces equalizer adaptation time and computation power consumption for fast optical switching systems, underpinning optically-switched DCNs using high baud rate and impairment-sensitive formats. Through a proof-of-concept experiment, we study the performance of the proposed equalizer state caching scheme in a three-node optical switching system using 56 GBaud PAM-4. Our experimental results show that the proposed scheme can tolerate up to 0.8-nm (100-GHz) instantaneous wavelength change with an adaptation delay of only 0.36 ns. Practical considerations such as clock phase misalignment, temperature-induced wavelength drift, and equalizer precision are also studied