O-Band Silicon Photonic Transmitters for Datacom and Computercom Interconnects

We are delighted to introduce a Special Issue on Optical Interconnects for the IET Optoelectronics magazine. This issue presents the current status as well as some of the outstanding progress that has been witnessed in the rapidly evolving area of optical interconnects. Today, data centres (DC) and high-performance computers are struggling with limited interconnect capacity and increased power consumption while trying to meet overwhelming bandwidth demands. As photonics technologies have been rapidly maturing, optical interconnects have come into the spotlight as the means to cope with the emerging bandwidth, energy, and capacity needs. As such, a wealth of photonic interconnect solutions across the DC hierarchy has unfolded, covering from electro-optic boards to transceiver development, on-chip and on-board switches and interconnects as well as novel noninvasive monitoring methods for light controlling for compensating thermal crosstalk, fabrication tolerances and unstable environmental conditions. At the same time, the increased penetration of optical interconnects in short-reach communications, such as board and chip levels, has triggered new challenges in photonics technologies calling for new innovations with respect to integration techniques and packaging methods. This Special Issue of IET includes integrated circuits, assembly techniques, systems-level architectures, and controlling methods related to short reach ‘chip-to-chip’, ‘in-the-box’, and ‘box-to-box’ communications. Specific areas covered by this special issue include: Silicon photonic transceiver technologies and flat DC topologies; High-speed electro-optic hosting boards and fast, energy-efficient electronics; DC switches and co-packaged optic (CPO) technologies; and Noninvasive controlling techniques for complex photonic integrated circuits. The four papers in this special section promote high-performance optical interconnect architectures and key technologies to tackle the rising bandwidth demands of DCs. They cover (a) co-packaged DC optics, (b) silicon technology transceiver modules and high-speed hosting boards for multisocket server board (MSB) interconnects and (c) automated controlling architecture for complex photonic systems; Items (b) and (c) draw from impressive achievements accomplished through the recently completed European H2020 Project ICT-STREAMS. This report written by C. Mickenberg tries to shed light on motivating factors behind co-packaged optics (CPO), the trade-offs involved and their potential impact on data centre network architectures. The article reviews the evolution of DC switch application-specific integrated circuit (ASICs) and DC optics, provides a future road map and finally discusses the potential impact of CPO on the network architecture. This topic is covered by two articles. The first, written by M. Morallis-Pegios, proposes using silicon photonics to advance interconnect throughput, socket connectivity and energy efficiency in MSB layouts by enabling a flat-topology wavelength division multiplexing (WDM) point-to-point optical MSB interconnect scheme. WDM silicon photonic transceivers coassembled with their electronic circuits achieving a 50-Gb/s line rate and 400-Gb/s aggregate data transmission are demonstrated. The article also proposes silicon photonic Arrayed Waveguide Grating Router (AWGRs) that can offer collision-less time-of-flight connectivity for up to 16 nodes. The second article covering that area is written by T. Lamprecht and reports on the development of electro-optic hosting boards for on-board transceiver subassemblies which is required to eventually form the Tb/s multi-socket onboard communication. The article also discusses the electrical interface as well as photonic board requirements, including the manufacturing technique and the assembly concept that was followed to deliver terabyte per second multichip optical communication. This article, written by F. Zanetto, introduces a novel dithering technique for real-time control of multiple cascaded photonic integrated devices, proposing a calibration-free strategy to support the development and real-life diffusion of increasingly complex photonic systems. The article elaborates on the implementation of a calibration-free automated control loop exploiting integral controllers to close a real-time feedback and stabilise the optimum working point of the employed photonic devices. The effectiveness of the proposed architectural approach is experimentally validated with the error-free routing and transmission of a 30-Gbit/s optical signal traversing a cascade of micro-ring resonator filters. All of the articles selected for this Special Issue present a wide variety of key topics and significant results addressing optical interconnects in high-performance data communication systems, including new progress on architectures, systems and packaging techniques as well as novel optimised controlling methods. As guest editors, we would like to thank all of the authors for their high-quality submissions that significantly contribute to the optical interconnect field. We would also like to thank the reviewers for their valuable and constructive technical comments that greatly enhance the Special Issue, because their feedback and comments ensure the high quality of the IET Optoelectronics Journal. We hope that this IET Special Issue will provide a useful reference of the state-of-the-art in optical interconnects to readers, and we anticipate such technological perspectives will stimulate new advancements and technological outcomes that will in turn enrich the growing field of optical interconnects. This work was supported by the E.C. through the H2020-ICT-STREAMS research project (688172).

We propose and experimentally demonstrate all optical format conversion from non-return-to-zero (NRZ) format to frequency-shift-keying (FSK) format based on free carrier dispersion effect in a silicon mode-split microring resonator. The injection of the high-power NRZ signal generates free carriers leading to the blue shift of the spectrum when a '1' comes. Therefore, there is a selective filtering for the two probes with certain separation located at different position of the split mode according to the information carried by the NRZ signal. Then the NRZ signal is converted to the FSK format. The microring resonator features ultra-compact size with a radius of 10 μm thus is suitable for integration with silicon-on-insulator (SOI) based optical and electronic devices. The split mode can provide large and variable frequency deviation for the FSK signal. 1 Gb/s NRZ signal is successfully converted to FSK format with a frequency deviation of 40 GHz, which can find application for interconnection between a metropolitan area networks (MAN) and a passive optical network (PON) system.

We study the problem of allocating optical bandwidth to sets of communication requests in all–optical networks that utilize Wavelength Division Multiplexing (WDM). WDM technology establishes communication between pairs of network nodes by establishing transmitter–receiver paths and assigning wavelengths to each path so that no two paths going through the same fiber link use the same wavelength. Optical bandwidth is the number of distinct wavelengths. Since state–of–the–art technology allows for a limited number of wavelengths, the engineering problem to be solved is to establish communication between pairs of nodes so that the total number of wavelengths used is minimized; this is known as the wavelength routing problem. In this paper we survey recent advances in bandwidth allocation in tree–shaped WDM all–optical networks. We present hardness results and lower bounds for the general problem and the special case of symmetric communication. We give the main ideas of deterministic greedy algorithms and study their limitations. We demonstrate how we can achieve optimal and nearly–optimal bandwidth utilization in networks with wavelength converters using simple algorithms. We also present recent results about the use of randomization for wavelength routing.

Optical transmitter is the carrier to realize optic communication. It is important to study the package design of high data rates optical transmitter and solve the engineering problems in the application of optical transmitter for optic communication. MRM-based wavelength division multiplexing (WDM) transmitter has become a hot topic because of the features of microring modulator (MRM). The development trend of MRM-based WDM transmitter is toward high density and high single-channel rate. In this paper, a 12-channel silicon photonics MRM-based WDM transmitter controlled by three CMOS drivers is designed. In addition, to increase the temperature stability of the MRM-based transmitter, a thermoelectric cooler (TEC) is introduced. Besides, high temperature co-fired ceramic (HTCC) is adopted in this study for hermetic encapsulation of TEC to ensure the high reliability of the TEC which could be impacted by the condensed water drop. After the carefully design and optimization, the WDM transmitter is fabricated. And four channels controlled by one of the drivers are measured. The results show that the 4-channel transmitter at 52 Gbps/ch PAM4 signals is obtained. And 12×52 Gbps PAM-4 signals transmission through a transmitter is potential.

The exponential growth of Internet data traffic pushes hard the telecommunication infrastructure for upgrading the transmission data rate. At the optical access level, a passive optical network (PON) is the most popular and lowest cost architecture which enables high speed access for enter prise and private users as well as various backhauling and fronthauling functionalities. Standardized PONs have undergone a number of upgrades in data rates. In order to guarantee a stable return of investment for operators, the co-existence of different generations of PONs on the same fiber infrastructure is ensured via a wavelength division multiplexing (WDM) approach. This leads to a spectrum shortage for future allocations if full co-existence is required. Today's fastest 40 Gb/s TWDM PONs [1] support 10 Gb/s lane rate non-return-to-zero (NRZ). Looking forward, the bandwidth demands continue to rise, thus increasing the per-wavelength bit rate is necessary for a future upgrade beyond 40 Gb/s. For such high data rates, the conventional NRZ format becomes demanding with respect to the required transceiver bandwidth and the acceptable fiber dispersion. As an alternative, advanced modulation formats and efficient signal processing can address the upgrade issue. Recently, research efforts have been focusing on 25 Gb/s or 40 Gb/s per wavelength next generation PONs using either NRZ with chirp management [2], or electrical/optical Duobinary [3–7], PAM-4 [3, 6–8], as well as more complex ODFM [9]. Considering PONs are cost-sensitive applications, intensity modulation and direct detection is highly desirable. Compared to DSP intensive OFDM, electrical/optical Duobinary and PAM-4 with low complex DSPs are potentially costeffective solutions. This paper aims to review and compare a number of implementation options for 40 Gb/s per wavelength high speed PONs using electrical/optical Duobinary and PAM-4. Optical link power budget performance, transceiver cost and power dissipation will be analyzed based on numerical approaches. Experimental demonstrations will also be presented.

Memory cubes (MCs), following the general concept of Micron's Hybrid Memory Cube (HMC), represent a promising memory architecture for high scalability and energy efficiency due to their partitioned 3D-stacked DRAM, high-speed serial links, abstract packet-switched interface, and on-die switching fabric connecting the host processor with the MC's different partitions ('vaults'). While previous studies have shown that implementing processor-to-MC links with silicon-photonic (SiP) integrated optical links offers higher energy efficiency and bandwidth density, they keep the electrical switching fabric inside the MC die and perform signal conversion prior to routing packets across the switch. We believe that the technological limitations of electrical interconnects in terms of energy consumption, the large size of MC dies, the high radix of the on-die switch, and the bandwidth demands will all ultimately turn the on-MC switching fabric into a critical issue in terms of energy and latency. Using an integrated optical switching fabric alleviates all of these issues and allows the host processor to directly communicate with the MC vaults by exploiting wavelength routing, thereby eliminating the need for electrical switch traversal. In particular, we propose to use Arrayed Waveguide Grating Routers (AWGRs) which offer a compact SiP switching fabric with a connectivity pattern that is ideal as the on-MC switch. Our simulation results show that exploiting AWGRs and direct processor-to-vault communication reduces both MC access energy and latency by up to 40% (on average) on PARSEC/SPLASH-2 workloads.

DOI: 10.1049/ic:20070242 ISBN: 978-3-8007-3042-1 Location: Berlin, Germany Conference date: 16-20 Sept. 2007 Format: PDF For the first time, we have demonstrated error-free operation of slow-light via SBS in optical fiber for 10-Gb/s NRZ and PSBT formats. The PSBT signal sensivity is 5.2 dB better than the NRZ case for a 35-ps delay. (2 pages) Inspec keywords: delays; optical modulation; stimulated Brillouin scattering; optical fibre communication Subjects: Modulation and coding methods; Stimulated Raman scattering and spectra; CARS; stimulated Brillouin and stimulated Rayleigh scattering and spectra; Optical communication; Optical communication devices, equipment and systems; Nonlinear optics and devices

Optically preamplified receiver performance according to the vestigial sideband (VSB) filtering has been numerically investigated for 40-Gb/s optical signals modulated with nonreturn-to-zero, duobinary nonreturn-to-zero (NRZ), return-to-zero (RZ), carrier-suppressed RZ, and duobinary carrier-suppressed RZ formats. The VSB filtering enables the spectral widths of NRZ, duobinary NRZ, and RZ signals to be reduced without severe power penalties at the receiver. On the other hand, carrier-suppressed RZ and duobinary carrier-suppressed RZ signals have no large advantages over VSB filtering because of the characteristics of their signals. Our results suggest that RZ signals are the most suitable modulation format for VSB filtering, without considering the filter loss, because of the tolerance of the intersymbol interference and a large spectral width. However, duobinary NRZ signals are the most suitable modulation format for VSB filtering, considering the filter loss, because of their narrow spectral width.

Summary form only given. The large scale deployment of all-optical wavelength-division multiplexing (WDM) networks will require signal regeneration due to signal impairments occurring in transmission spans and optical switching nodes/cross-connects. A 2R regenerative interface for non-return to zero (NRZ) signals based on semiconductor optical amplifiers (SOAs) has been proposed by Chiaroni et al. (1997), with clear demonstration of regenerative properties in the amplitude domain. In this paper, a 3R regenerative interface based on the same core structure is described. The core of these interfaces is composed of the association of a wavelength converter (WC) under cross-gain modulation (XGM-WC) and a wavelength converter under cross-phase modulation (XPM-WC), both operating in polarity-inverting mode. Basically, the role of the first WC is to convert power fluctuations into extinction ratio variations and to provide the second WC with a constant input power. The second converter is in turn mainly used to regenerate the extinction ratio thanks to its nonlinear transfer function. This interface in addition limits the accumulation of amplified spontaneous emission (ASE) noise.

We demonstrate transmission of 36 10-Gbit/s WDM signals over 4134 km of standard single-mode fiber using all-Raman amplification and NRZ format. Wide-band fiber delay loop depolarizers are used in the Raman pump unit to reduce the depolarization dependent Raman gain. Keywords: Optical fiber transmission, Raman amplifier, non-return-to-zero, wavelength-division multiplexing, dispersion compensation, forward error correction INTRODUCTION In recent years ultra-long haul large capacity dense wavelength-division multiplexing (DWDM) optical transmission systems have attracted considerable attention due to their potential to greatly reduce the cost per bit × distance. One of the main limitations for such ultra-long haul transmission is the degradation of the link optical signal-to-noise ratio (OSNR) because of the amplified spontaneous emission (ASE) noise accumulation. The channel power cannot be increased unlimitedly due to the fiber nonlinearity impairment. All distributed Raman amplification (DRA) has recently demonstrated its potential in DWDM transmission [1-3]. In addition to its low noise, another attractive characteristics of Raman amplifier is the freedom in designing the amplification band through properly choosing the wavelength of the pump. Several record experiments have been demonstrated based on all-Raman amplification [1-4]. In this paper, we report a transmission experiment of 10Gb/s DWDM signals over 4000 km of standard single-mode fiber (SSMF) with 106 km spans using all-Raman amplification and non-return-to-zero (NRZ) format. In our Raman pump units, rather than polarization multiplexing, the WDM pumps are depolarized by fiber delay loops depolarizer. This Raman pumping scheme eliminates the polarization multiplexing and reduces the number of pump diodes by a half.

Third generation fiber-to-the-x (FTTx) will introduce wavelength division multiplexing (WDM) technology to increase data rates up to 40 Gbit/s, as well as coexist with current communication systems. WDM passive optical network (WDM-PON) based fiber-to-the-home (FTTH) system architecture together with the Next-generation passive optical network (NG-PON) can provide solution related to nowadays problems with transmission capacity. It is expected that NG-PON systems will become more cost-efficient at high per user data rates, providing different transmission speeds for end users, based on pay-as-you-grow approach. In this paper, our introduced system model is used to obtain WDM-PON system effectiveness for different data rates of up to 32 Gbit/s per downstream transmission channel. We created combined simulative-experimental WDM-PON transmission system model in environment of RSoft OptSim simulation software to compare the performance of non-return-to-zero (NRZ) on-off-keying (OOK) modulated PON transmission system versus transmission bitrate and distance. Results showed that proposed transmission system is capable to provide data transmission of up to 32 Gbit/s per channel with BER $ , if additional forward error correction (FEC) is used. Additionally, to increase the reach of investigated system, fiber Bragg grating-based dispersion compensation module (FBG-DCM) is implemented, therefore enabling full pre-compensation of chromatic dispersion.

Crosstalk power penalties were calculated for a bidirectional fiber-optic wavelength division multiplexed (WDM) system (622 Mb/s at 1300 nm in one direction and 155 Mb/s at 1550 nm in the other) using the system simulation program Simulation of Fiber-Optic Communication Systems (SIFOCS). Nonreturn-to-zero (NRZ) and return-to-zero (RZ) modulation formats were investigated with the result that, in most cases, the power penalty was shown to depend only weakly on individual system properties and is close to the values found in the literature, but there are exceptions where it may be greater than usual (e.g. at 622 Mb/s RZ). These can be detected and investigated with the methods reported. The crosstalk power penalty becomes excessive only for signal-to-crosstalk power ratios well below 10 dB and is negligible for values greater than 15 dB. The calculation methods used can be easily adapted to other systems, e.g. to systems with more closely spaced wavelengths. >

The issue of what is the best duty cycle in an intensity-modulated direct-detection optical system keeps resurfacing. Here, this issue is addressed from a transmission point of view for a wavelength-division-multiplexed (WDM) system. It is shown that nonreturn-to-zero (NRZ) is more robust to optical fiber transmission impairments for both already deployed systems and new systems in which the dispersion map is a design parameter.

A dynamic, wideband model for simulation of spatial-temporal distribution of photons and carriers in a bulk InP-InGaAsP reflective semiconductor optical amplifier (RSOA) is presented. Saturation and noise characteristics are successfully predicted and in good accordance with theoretical predictions found in literature. Based on RSOA model and a model for access network link, numerical optimization of multiplexing (MUX) and demultiplexing (DMUX) 3-dB filter bandwidths when transmitting non-return-to-zero (NRZ) format is preformed. The access network simulated scenario is based on high density (< 25 GHz channel spacing) arrayed waveguide grating (AWG) and remodulation of 10 Gbit/s NRZ signal by using RSOA.

Simultaneous all-optical multi-channel RZ and CSRZ to NRZ format conversion