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In this article, we present for the first time to our knowledge, a DWDM $16\times 16$ silicon photonic cyclic-frequency AWGR device. The AWGR features a channel spacing of 1.06 nm (189 GHz), a free spectral range of 17.80 nm (3.16 THz) and a 3-dB channel bandwidth of 0.65 nm (116 GHz). Its proper cyclic-frequency operation was experimentally verified for all $16\times 16$ channels with channel peak insertion loss values in the range of 3.90 dB to 8.37 dB, yielding a channel loss non-uniformity of 4.47 dB. Its compact footprint of 0.27 mm $\times0.71$ mm and its low crosstalk value of 21.65 dB highlight its potential for employment in future AWGR-based interconnect schemes in O-band.

Arrayed Waveguide Grating Router (AWGR)-based interconnections for Multi-Socket Server Boards (MSBs) have been identified as a promising solution to replace the electrical interconnects in glueless MSBs towards boosting processing performance. In this article, we present an 8-socket glueless optical flat-topology Wavelength Division Multiplexing (WDM)-based point-to-point (P2P) interconnect pursued within the H2020 ICT project ICT-STREAMS and we report on our latest achievements in the deployment of the constituent silicon (Si)-photonic transmitter and routing building blocks, exploiting experimentally obtained performance metrics for analyzing the 8-socket chip-to-chip (C2C) connectivity in terms of throughput and energy efficiency. We demonstrate an 8-channel WDM Si-photonic microring-based transmitter (Tx) capable of providing 400 (8 x 50) Gb/s non-return-to-zero (NRZ) Tx capacity and an 8 x 8 Coarse-WDM (CWDM) Si-AWGR with verified cyclic data routing capability in O-band. Following an overview of our recently demonstrated crosstalk (XT)-aware wavelength allocation scheme, that enables fully-loaded AWGR-based interconnects even for typical sub-optimal XT values of silicon integrated CWDM AWGRs, we validate the performance of a full-scale 8-socket interconnect architecture through physical layer simulations exploiting experimentally-verified simulation models for the underlying Si-photonic Tx and routing circuits. This analysis reveals a total aggregate capacity of 1.4 Tb/s for an 8-socket interconnect when operating with 25 Gb/s line-rates, which can scale to 2.8 Tb/s at an energy efficiency of just 5.02 pJ/bit by exploiting the experimentally verified building block performance at 50 Gb/s line. This highlights the perspectives for up to 69% energy savings compared to the standard QuickPath Interconnect (QPI) typically employed in electronic glueless MSB interconnects, while scaling the single-hop flat connectivity from 4- to 8-socket interconnection systems.

Following a decade of radical advances in the areas of integrated photonics and computing architectures, we discuss the use of optics in the current computing landscape attempting to re-define and refine their role based on the progress in both research fields. We present the current set of critical challenges faced by the computing industry and provide a thorough review of photonic Network-on-Chip (pNoC) architectures and experimental demonstrations, concluding to the main obstacles that still impede the materialization of these concepts. We propose the employment of optics in chip-to-chip (C2C) computing architectures rather than on-chip layouts towards reaping their benefits while avoiding technology limitations on the way to manycore set-ups. We identify multisocket boards as the most prominent application area and present recent advances in optically enabled multisocket boards, revealing successful 40Gb/s transceiver and routing capabilities via integrated photonics. These results indicate the strong potential to bring energy consumption down by more than 60% compared to current QuickPath Interconnect (QPI) protocol, while turning multisocket architectures into a single-hop low-latency setup for even more than 4 interconnected sockets, which form currently the electronic baseline. We go one step further and demonstrate how optically-enabled 8-socket boards can be combined via a 256x256 Hipo-aos Optical Packet Switch into a powerful 256-node disaggregated system with less than 335nsec latency, forming a highly promising solution for the latency-critical rack-scale memory disaggregation era. Finally, we discuss the perspective for disintegrated computing via optical technologies as a means to increase the number of synergized high-performance cores overcoming die area constraints, introducing also the concept of cache disintegration via the use of future off-die ultra-fast optical cache memory chiplets.

The ever-increasing energy consumption of Data Centers (DC), along with the significant waste of resources that is observed in traditional DCs, have forced DC operators to invest in solutions that will considerably improve energy efficiency. In this context, Rack- and board-scale resource disaggregation is under heavy research, as a groundbreaking innovation that could amortize the energy and cost impact caused by the vast diversity in resource demand of emerging DC workloads. However disaggregation, by breaking apart the critical CPU-to-memory path, introduces a challenging set of requirements in the underlying network infrastructure, that has to support low-latency and high-throughput communication for a high number of nodes. In this paper we present our recent work on optical interconnects towards enabling resource disaggregation both on Rack-level as well as on board-level. To this end, we have demonstrated the Hipoλaos architecture that can efficiently integrate Spanke-based switching with AWGR-based wavelength routing and optical feedforward buffering into highport switch layouts. The proof-of-concept Hipoλaos prototype, based on the 1024-port layout, provide latency performance of 456ns, while system level evaluations reveal sub-μs latency performance for a variety of synthetic traffic profiles. Moving towards high-capacity board-level interconnects, we present the latest achievements realized within the context of H2020-STREAMS project, where single-mode optical PCBs hosting Si-based routing modules and mid-board optics are exploited towards a massive any-to-any, buffer-less, collision-less and extremely low latency routing platform with 25.6Tb/s throughput. Finally, we combine the Hipolaos and STREAMS architectures in a dual-layer switching scheme and evaluate its performance via system-level simulations.

The rapid increase of bandwidth requirements across the entire hierarchy of Data Center (DC) networks, ranging from chip-to-chip, board-to-board up to rack-to-rack communications, puts strenuous requirements in the underlying network infrastructure that has to offer high-bandwidth and low-latency interconnection under a low-energy and low-cost envelope. Arrayed Waveguide Grating Router (AWGR)-based optical interconnections have emerged as a powerful architectural framework that can overcome the currently deployed electrical interconnect bottlenecks leveraging the wavelength division multiplexing (WDM) and the cyclic routing properties of AWGRs to offer one-hop, all-to-all communication when employed as N×N routers. However, the majority of previous silicon (Si)-based integrated AWGR demonstrations has either targeted C-band operation, despite the dominance of the O-band spectral region in the DC interconnection domain, or offered coarse-WDM (CWDM) functionality and, as such, were limited in terms of AWGR port count. In this article, we present for the first time to our knowledge, a Dense-WDM (DWDM) 16×16 Si-photonic cyclic-frequency AWGR device targeting O-band routing applications. The fabricated AWGR device features a channel spacing of 1.063 nm (189 GHz), a free spectral range of 17.8 nm (3.15 THz) and a 3-dB bandwidth of 0.655 nm (116 GHz). Its proper cyclic frequency operation was experimentally verified for all 16 channels with channel peak insertion loss values in the range of 3.9 dB to 8.37 dB, yielding a channel loss non-uniformity of 4.47 dB. Its compact footprint of 0.27×0.71 mm2 and low crosstalk of 21.65 dB highlight its potential for employment in future AWGR-based interconnection schemes.

Centralized Radio Access Network (C-RAN) comprises one of the main technologies for high capacity and low latency fronthauling in 5G networks. In this paper, we propose and evaluate an optical fronthaul 5G C-RAN architecture that targets to meet the bandwidth, latency and energy requirements of high traffic hot-spot areas. The proposed architecture employs Intermediate-Frequency-over-Fiber (IFoF) signal generation by Photonic Integrated Circuit (PIC) Wavelength Division Multiplexing (WDM) optical transmitters at the Base-Band Unit (BBU), while a novel design of Reconfigurable Optical Add-Drop Multiplexers (ROADMs) is used at the Remote Radio Head (RRH) site. An aggregate capacity up to 96 Gb/s has been reported by employing two WDM links with 4-band Orthogonal Frequency Division Multiplexing (OFDM) 64-QAM 0.5 Gbaud signals, showing error vector magnitude performance within the acceptable 8% limit. The physical layer evaluation of the proposed fronthaul is also extended with the evaluation of the network throughput and mean packet delay latency, using a Medium Transparent-Medium Access Control (MT-MAC) protocol, which employs gated service indicating latencies below 10ms.

We demonstrate a digital Optical Physical-layer Network Coding (OPNC) for mm-wave fiber-wireless signals modulated with up to 2.5 Gb/s On/OFF Keyed (OOK) data. The proposed OPNC concept uses an all-optical XOR gate comprising a Semiconductor Optical Amplifier-Mach Zehnder Interferometer (SOA-MZI) with SOAs being driven at low moderate electrical currents in order to perform the all-optical encoding between the OOK envelopes of the data, ignoring the high-frequency Sub-Carrier (SC) signal. In this scheme, network coding is performed on-the-fly at the central office and the resultant packet is broadcasted at the client nodes, where the decoding takes place. We demonstrate experimental results of OPNC using OOK data signals modulated on a 10 GHz SC with the aid of a second all-optical XOR gate for the decoding process at the client's site, reporting error-free performance for both synchronous/asynchronous data packets. The scenario of all optical encoding for 60 GHz SC frequencies followed by electrical decoding at the end-users is evaluated via PHY-layer simulations. Going a step further and considering the network level, we present a performance improvement on the network throughput by using the proposed network coding.

We demonstrate the first digital optical physical-layer network coding (OPNC) scheme for 2.5-Gb/s ON/OFF keyed (OOK) subcarrier modulated (SCM) signals suitable for the application in fiber-wireless networks. The proposed OPNC scheme employs all optical XOR logic gates that rely on cross-phase modulation-based Semiconductor Optical Amplifier (SOA) Mach–Zehnder interferometers to perform encoding and decoding operations. The proposed encoding and decoding scheme is targeting deployment at the central office and the clients site at the remote antenna units, respectively. Proof of concept verification is experimentally demonstrated for 2.5-Gb/s OOK-SCM data using a 10-GHz subcarrier. Both synchronous and asynchronous operations are evaluated, achieving error free operation.

An all optical multi-wavelength tag comparator unit followed by a semiconductor optical amplifier-read access gate (SOA-RAG) is presented for all-optical cache memory architectures. Proof-of-principle operation of the Tag Comparison (TC) circuit as a dual-bit tag-comparison stage is presented by exploiting cross-phase modulation-based XOR logic gates based on SOA-Mach–Zehnder interferometers (SOA-MZIs), in order to decide upon a cache hit or cache miss operation, while the two SOA-MZI outputs control the ON/OFF cross-gain-modulation of the SOA-RAG unit. Experimental error free operation is demonstrated for a $8\,\times \,10$ -Gb/s wavelength division multiplexing (WDM) formatted optical word signal and a $4\times 10$ -Gb/s WDM tag.

We report an 8 × 8 silicon photonic integrated Arrayed Waveguide Grating Router (AWGR) targeted for WDM routing applications in O-band. The AWGR was designed for cyclic-frequency operation with a channel spacing of 10 nm. The fabricated AWGR exhibits a compact footprint of 700 × 270 μm2. Static device characterization revealed 3.545 dB maximum channel loss non-uniformity with 2.5 dB best-case channel insertion losses and 11 dB channel crosstalk, in good agreement with the simulated results. Successful data routing operation is demonstrated with 25 Gb/s signals for all 8 × 8 AWGR port combinations with a maximum power penalty of 2.45 dB.

Analog optical fronthaul for 5G network architectures is currently being promoted as a bandwidth- and energy-efficient technology that can sustain the data-rate, latency and energy requirements of the emerging 5G era. This paper deals with a new optical fronthaul architecture that can effectively synergize optical transceiver, optical add/drop multiplexer and optical beamforming integrated photonics towards a DSP-assisted analog fronthaul for seamless and medium-transparent 5G small-cell networks. Its main application targets include Urban and Hot-Spot Area networks, promoting the deployment of mmWave massive MIMO Remote Radio Heads (RRHs) that can offer wireless data-rates ranging from 25 to 100 Gb/s and beyond depending on the fronthaul technology employed (e.g. spatial or wavelength division multiplexing transport). Small-cell access and resource allocation is ensured via a Medium-Transparent (MT-) MAC protocol that enables the transparent communication between the central office and the wireless end-users or the access cells via V-band massive MIMO RRHs.

We demonstrate the first all-optical digital physical-layer network coding (AOPNC) for Differential Phase Shift Keyed (DPSK) Sub-Carrier Modulated (SCM) signals in mm-wave Radio-over-Fiber (RoF) networks. AOPNC is performed at the Central Office (CO) by employing a Delay Interferometer (DI) for phase-to-amplitude conversion, two Semiconductor Optical Amplifier-Mach Zehnder Interferometers (SOA-MZIs) performing the XOR and XNOR operation between the OOK converted signals and a third SOA-MZI for the phase regeneration of the encoded phase-formatted XOR signal. Encoding and decoding are evaluated via simulations for 2.5 Gb/s NRZ-DPSK data modulated on 60 GHz SC for both synchronous and asynchronous operation.

High-performance server boards rely on multi-socket architectures for increasing the processing power density on the board level and for flattening the data center networks beyond leaf-spine architectures. Scaling, however, the number of processors per board and retaining at the same time low-latency and high-throughput metrics puts current electronic technologies into challenge. In this article, we report on our recent work carried out in the H2020 projects ICT-STREAMS and dREDBox that promotes the use of Silicon Photonic transceiver and routing modules in a powerful board-level, chip-to-chip interconnect paradigm. The proposed on-board platform leverages WDM parallel transmission with a powerful wavelength routing approach that is capable of interconnecting multiple processors with up to 25.6 Tbps on-board throughput, providing direct and collision-less any-to-any communication between multiple compute and memory sockets at low-energy 50 Gbps OOK line-rates. We demonstrate recent advances on the Si-based WDM transceiver, cyclic AWGR router and polymer-based electro-optical circuit board key-enabling technologies, discussing also potential applications in disaggregated rack-scale architectures. We also demonstrate our recent research on optical RAM technologies and optical cache memory architectures that can take advantage of the on-board interconnect technology for yielding true disintegrated computing resolving both power and memory bandwidth bottlenecks of current computational settings.

Future broadband access networks in the 5G framework will need to be bilateral, exploiting both optical and wireless technologies. This paper deals with new approaches and synergies on radio-over-fiber (RoF) technologies and how those can be leveraged to seamlessly converge wireless technology for agility and mobility with passive optical networks (PON)-based backhauling. The proposed convergence paradigm is based upon a holistic network architecture mixing mm-wave wireless access with photonic integration, dynamic capacity allocation and network coding schemes to enable high bandwidth and low-latency fixed and 60GHz wireless personal area communications for gigabit rate per user, proposing and deploying on top a Medium-Transparent MAC (MT-MAC) protocol as a low-latency bandwidth allocation mechanism. We have evaluated alternative network topologies between the central office (CO) and the access point module (APM) for data rates up to 2.5 Gb/s and SC frequencies up to 60 GHz. Optical network coding is demonstrated for SCM-based signaling to enhance bandwidth utilization and facilitate optical-wireless convergence in 5G applications, reporting medium-transparent network coding directly at the physical layer between end-users communicating over a RoF infrastructure. Towards equipping the physical layer with the appropriate agility to support MT-MAC protocols, a monolithic InP-based Remote Antenna Unit optoelectronic PIC interface is shown that ensures control over the optical resource allocation assisting at the same time broadband wireless service. Finally, the MT-MAC protocol is analysed and simulation and analytical theoretical results are presented that are found to be in good agreement confirming latency values lower than 1msec for small- to mid-load conditions.

As data centers constantly expand, electronic switches are facing the challenge of enhanced scalability and the request for increased pin-count and bandwidth. Photonic technology and wavelength division multiplexing have always been a strong alternative for efficient routing and their potential was already proven in the telecoms. CWDM transceivers have emerged in the board-to-board level interconnection, revealing the potential for wavelength-routing to be applied in the datacom and an AWGR-based approach has recently been proposed towards building an optical multi-socket interconnection to offer any-to-any connectivity with high aggregated throughput and reduced power consumption. Echelle gratings have long been recognized as the multiplexing block exhibiting smallest footprint and robustness in a wide number of applications compared to other alternatives such as the Arrayed Waveguide Grating. Such filtering devices can also perform in a similar way to cyclical AWGR and serve as mid-board routing platforms in multi-socket environments. In this communication, we present such a 3x3 Echelle grating integrated on thick SOI platform with aluminum-coated facets that is shown to perform successful wavelength-routing functionality at 10 Gb/s. The device exhibits a footprint of 60x270 μm2, while the static characterization showed a 3 dB on–chip loss for the best channel. The 3 dB-bandwidth of the channels was 4.5 nm and the free spectral range was 90 nm. The echelle was evaluated in a 2x2 wavelength routing topology, exhibiting a power penalty of below 0.4 dB at 10-9 BER for the C-band. Further experimental evaluations of the platform involve commercially available CWDM datacenter transceivers, towards emulating an optically-interconnected multi-socket environment traffic scenario.

Future broadband access networks in the 5G framework will need to be bilateral, exploiting both optical and wireless technologies. This paper deals with new approaches and synergies on radio-over-fiber (RoF) technologies and how those can be leveraged to seamlessly converge wireless technology for agility and mobility with passive optical networks for stability, extended coverage and ultra-high speeds. The proposed convergence paradigm is based upon a holistic network architecture mixing mm-wave wireless access with photonic integration, dynamic capacity allocation and network coding schemes to enable high bandwidth fixed and 60GHz wireless personal area communications for gigabit rate per user. We have evaluated the link between the central office (CO) and the access point module (APM) of the proposed architecture by using both SCM OOK-SSB and BPSK modulation formats obtaining error free transmission for 40 km and 15 km respectively for power penalties below 2 dB while the concept was investigated for data rates up to 2.5 Gb/s and SC frequencies up to 60 GHz. In parallel a new optical network coding scheme is reported and demonstrated for SCM-based signalling to enhance bandwidth utilization and facilitate optical-wireless convergence in 5G applications.

We demonstrate an optical tag comparator for optical cache memory peripheral circuitry that decides upon cache “hit” or “miss” controlling the access of a 4-bit WDM-encoded optical word at 10Gb/s line-rate into a cache line.

We realized an all-optical digital physical-layer network coding in Fiber-Wireless networks by employing two SOA-MZI XOR gates. Encoding and decoding operations are demonstrated for 1Gb/s data on 10GHz subcarrier and extended to 60GHz with simulations.

We propose and experimentally demonstrate the first all optical digital network coding between OOK-NRZ Subcarrier Modulated (SCM) signals for Radio over Fiber (RoF) networks using a Semiconductor Optical Amplifier-Mach Zehnder Interferometer (SOA-MZI) as an XOR encoder. Successful encoding and decoding is demonstrated for 1 Gb/s data on a 10 GHz sub-carrier for synchronous and asynchronous NRZ data.

We present a heterogeneous passive WDM RoF system that is capable of simultaneously delivering broadband wireless 500 Mb/s services at 5 GHz RF carriers and 2.5 Gb/s LAN traffic at 60 GHz RF carriers over a standard single mode fiber. Our system employs phase modulation formats and optical frequency multiplication in order to increase the transmission distance and reduce the chromatic dispersion, allowing also for the use of cost-effective low-frequency RF components at the CO premises. Detailed performance evaluation results are derived by means of simulations that confirm the successful transmission of both RF carriers with less than 2 dB power penalty over a long distance optical path. The proposed system is fully compatible with the requirements of Medium-Transparent MAC protocols proposed for seamlessly and dynamically allocating resources in 60 GHz RoF LANs.

We propose a crosstalk (XT)-aware wavelength (λ)-detuning scheme to overcome the non-optimal in-band XT limiting all-to-all optical interconnects based on sub-optimal arrayed waveguide grating routers (AWGR). Experimental validation is demonstrated employing an 8 × 8 O-band cyclic Si-AWGR with 10 nm channel spacing and a 5.5 nm channel bandwidth, where the in-band XT limits the simultaneous use of the same λ in only 2 out of the 8 possible AWGR I/O routing scenarios, yielding a 10 -7 error-floor when >3 channels at the same λ are launched. We present experimentally an 8 × 25 Gb/s λ-switched interconnect exploiting slightly detuned λ s around the central λ for realizing all 8 possible I/O routing scenarios, enabling an error-free all-to-all interconnect even for sub-optimal AWGRs. Error-free simultaneous transmission via the AWGR of all 8 channels using 4λs and a λ utilization factor 2 is shown, with a max. power penalty of 4.64 dB at 10 -9 error-rate.

We present an 8×8 silicon photonics AWGR with 10 nm channel spacing for O-band cyclic-routing operation. Successful transmission at 25 Gb/s is demonstrated for all 8×8 AWGR channel combinations with a maximum power penalty of 0.82 dB.

We present the error-free demodulation of a 10Gb/s DPSK-signal based on a novel Delayed Interferometer with ultra-small footprint. The device is fabricated on thick um-scale SOI platform and exhibits similar dimension with nanophotonic based DIs.

We present an O-band energy-efficient broadcast-friendly optical architecture relying on a silicon photonics broadband MZM and an 8×8 AWGR. Experimental validation is demonstrated at 10Gb/s followed by an energy-saving analysis for the proposed broadcast scheme.

In the present communication we review recent advances in o-band silicon photonics transceivers and wavelength routers and demonstrate their potential application in board-level and rack level interconnection for edge and disaggregated data centers.