Photonic Interconnects Research Articles

Breaking free of electrical constraints

Photonic interconnections provide a viable means to overcome the fundamental limitations imposed upon computer system topologies by electrical signaling. Direct board-to-board (or assembly-to-assembly) signaling, without need of a backplane, provides multiple points of connectivity between adjacent electronic complexes while obviating the need to route all board-level signals to and through a central connector choke point. Optical busses can permit full-speed signaling between multiple chips, memory units, or processing units without requiring an active control plane. One-to-many and all-to-all photonic broadcasts greatly simplify multi-processor and multi-core system design.

Scalable optical switches for computing applications [Invited

A scalable photonic interconnection network architecture is proposed whereby a Clos network is populated with broadcast-and-select stages. This enables the efficient exploitation of an emerging class of photonic integrated switch fabric. A low distortion space switch technology based on recently demonstrated quantum-dot semiconductor optical amplifier technology, which can be operated uncooled, is used as the base switch element. The viability of these switches in cascaded networks is reviewed, and predictions are made through detailed physical layer simulation to explore the potential for larger-scale network connectivity. Optical signal degradation is estimated as a function of data capacity and network size. Power efficiency and physical layer complexity are addressed for high end-to-end bandwidth, nanosecond-reconfigurable switch fabrics, to highlight the potential for scaling to several tens of connections. The proposed architecture is envisaged to facilitate high-capacity, low-latency switching suited to computing systems, backplanes, and data networks. Broadband operation through wavelength division multiplexing is studied to identify practical interconnection networks scalable to 100 Gbits/s per path and a power consumption of the order of 20 mW/(Gbits/s) for a 64×64 size interconnection network.

Patterned Growth of Vertically Aligned ZnO Nanowire Arrays on Inorganic Substrates at Low Temperature without Catalyst

We report an approach for growing aligned ZnO nanowire arrays with a high degree control over size, orientation, dimensionality, uniformity, and possibly shape. Our method combines e-beam lithography and a low temperature hydrothermal method to achieve patterned and aligned growth of ZnO NWs at <100degreesC on general inorganic substrates, such as Si and GaN, without using catalyst. This approach opens up the possibility of applying ZnO nanowires as sensor arrays, piezoelectric antenna arrays, two-dimensional photonic crystals, IC interconnects, and nanogenerators.

Optical 4x4 hitless slicon router for optical networks-on-chip (NoC)

We demonstrate here a spatially non-blocking optical 4x4 router with a footprint of 0.07 mm(2) for use in future integrated photonic interconnection networks. The device is dynamically switched using thermo-optically tuned silicon microring resonators with a wavelength shift to power ratio of 0.25nm/mW. The design can route four optical inputs to four outputs with individual bandwidths of up to 38.5 GHz. All tested configurations successfully routed a single-wavelength laser and provided a maximum extinction ratio larger than 20 dB.

Photonic Networks-on-Chip for Future Generations of Chip Multiprocessors

The design and performance of next-generation chip multiprocessors (CMPs) will be bound by the limited amount of power that can be dissipated on a single die. We present photonic networks-on-chip (NoC) as a solution to reduce the impact of intra-chip and off-chip communication on the overall power budget. A photonic interconnection network can deliver higher bandwidth and lower latencies with significantly lower power dissipation. We explain why on-chip photonic communication has recently become a feasible opportunity and explore the challenges that need to be addressed to realize its implementation. We introduce a novel hybrid micro-architecture for NoCs combining a broadband photonic circuit-switched network with an electronic overlay packet-switched control network. We address the critical design issues including: topology, routing algorithms, deadlock avoidance, and path-setup/tear-down procedures. We present experimental results obtained with POINTS, an event-driven simulator specifically developed to analyze the proposed idea, as well as a comparative power analysis of a photonic versus an electronic NoC. Overall, these results confirm the unique benefits for future generations of CMPs that can be achieved by bringing optics into the chip in the form of photonic NoCs.

Nanoelectronic and Nanophotonic Interconnect

A significant performance limitation in integrated circuits has become the metal interconnect, which is responsible for depressing the on-chip data bandwidth while consuming an increasing percentage of power. These problems will grow as wire diameters scale down and the resistance-capacitance product of the interconnect wires increases hyperbolically, which threatens to choke off the computational performance increases of chips that we have come to expect over time. We examine some of the quantitative implications of these trends by analyzing the International Technology Roadmap for Semiconductors. We compare the potential of replacing the global electronic interconnect of future chips with a photonic interconnect and see that there is in principle a four order of magnitude bandwidth-to-power ratio advantage for the latter. This indicates that it could be possible to dramatically improve chip performance without scaling transistors but rather utilize the capability of existing transistors much more efficiently. However, at this time it is not clear if these advantages can be realized. We discuss various issues related to the architecture and components necessary to implement on-chip photonic interconnect.

Performance evaluation of the augmented data vortex switch fabric: An all-optical packet switched interconnection network

Modern high-performance computing systems require networks with high capacity, extremely high throughput and low latency in order to pass messages between thousands of processors and memory elements. Optical Interconnection Networks (OIN) offer a potentially viable solution to this requirement. An all-optical packet switched interconnection network called a Data Vortex (DV) switch has already been proposed by Yang et al. for the purpose of large scale photonic interconnections. For any interconnection network, fault tolerance and reliability are crucial issues, evaluation of which lacked attention for the case of the DV switch. In our earlier work we therefore presented the results for fault tolerance and reliability analysis of the primary DV switch. We also proposed a new Augmented Data Vortex (ADV) switch fabric, to improve the fault tolerance of the primary DV switch. The performance as regards fault tolerance of the ADV switch was computed and detailed results were obtained. In this paper, performance of ADV is investigated with reference to parameters such as latency and injection ratio (throughput) by means of numerical simulations. A uniform random traffic model has been used for the performance evaluation. The results obtained are compared with the results reported for the DV switch. The results show that the ADV switch with enhanced fault tolerance also improves the performance regarding latency. For same switch sizes (i.e. the same number of angles A , and height H ) the injection ratios (throughput) for the DV and the ADV switches are comparable. Hence it can serve as a suitable candidate for high performance computing.

Photonic crystal-based optical interconnect

This article presents theoretical optical data pathway model. The goal is to evaluate the physics behind photonic crystal/photon relationships and to develop a theoretical model for photonic crystal implementation into very large scale integration (VLSI) system design. One of the most popular applications of photonic crystals is the possibility of creating a new type of optical waveguide that surpasses the capabilities of present day optical communication equipment, i.e., fiber optics. Thus, further research used to determine the bandwidth capabilities of the data highway and clock synchronization between the signal generator and the receiving IC chip and the experiments based on laser diode switching are practiced to increase the speed

Broadcast MIMO over multimode optical interconnects by modal beamforming

We introduce broadcast MIMO communication systems over multimode optical fibers or waveguides. Based on BeamForming (BF) at the transmitter, decoupled virtual subchannels are provided to multiple uncoordinated conventional direct detection receivers. This optical technique, extending Zero-Forcing BF wireless MIMO techniques to quadratic detection, is applicable to photonic interconnects, e.g. short-reach point-to-(multi)point transmission over MMF, up to rates of 100 Gb/s for distances up to 100 m.

Hierarchical clustering of the data vortex optical interconnection network

The data vortex photonic interconnection network is studied for application to clustering and hierarchical layering of nodes. Performance is examined for varying cluster counts and under loads of varying network locality. In today's technology, similar performance is attained at high network communication locality loads (&gt;2/3), and a 19% latency reduction is obtained at the highest locality loads (&gt;95%) for current optical switching technology. For projected future technology, the clustered system is shown to yield up to a 55% reduction in latency for applications with 2/3 or better locality.

Recent progress in photonic interconnection technology

Recent progress in photonic interconnection technology

Photonic Combinatorial Network for Contention Management in 160 Gb/s-Interconnection Networks Based on All-Optical 2$\,\times\,$2 Switching Elements

A modular photonic interconnection network based on a combination of basic 2times2 all-optical nodes including a photonic combinatorial network for the packet contention management is presented. The proposed architecture is synchronous, can handle optical time division multiplexed (OTDM) packets up to 160 Gb/s, exhibits self-routing capability, and very low switching latency. In such a scenario, OTDM has to be preferred to wavelength division multiplexing (WDM) because in the former case, the instantaneous packet power carries the information related to only one bit, making the signal processing based on instantaneous nonlinear interactions between packets and control signals more efficient. Moreover, OTDM can be used in interconnection networks without caring about the propagation impairments because of the very short length (<100 m) of the links in these networks. For such short-range networks, the packet synchronization can be solved at the network boundary in the electronic domain without the need of complex optical synchronizers. In this paper, we focus on a photonic combinatorial network able to detect the contentions, and to optically drive the contention resolution block and the switching control block. The implementation of the photonic combinatorial network is based on semiconductor devices, which makes the solution very promising in terms of compactness, stability, and power consumption. This implementation represents the first example of complex photonic combinatorial network for ultrafast digital processing. The network performance has been investigated for bit streams at 10 Gb/s in terms of bit error rate (BER) and contrast ratio. Moreover, the suitability of the 2times2 photonic node architecture exploiting the earlier mentioned combinatorial network has been verified at a bit rate up to 160 Gb/s. In this way, the potential of photonic digital processing for the next generation broad band and flexible interconnection networks has been demonstrated.

High-precision 2-D SM fiber connectors fabricated through deep proton writing

High-precision two-dimensional (2-D) fiber alignment modules would offer great benefits for high-density photonic interconnects at the multichip-module level, where parallel light signals have to be transferred between integrated dense 2-D emitter and detector arrays, or for massive parallel sensing applications. In telecom, the availability of highly accurate low-cost field installable 2-D fiber couplers would boost the further integration of fiber optics in future fiber-to-the-home networks. We present deep proton writing as a prototyping technology for the mastering of small-form-factor 2-D fiber connector components. The alignment components, which we present here, consist of 4 times 8 arrays of circular conically shaped holes for single-mode fibers and feature average insertion losses of 0.062 dB and a maximum loss of 0.15 dB, when used in a fiber butt-coupling configuration

On cascades of resonators for high-bandwidth integrated optical interconnection networks

The power penalty characteristics of high-speed optical signals transmitted through a variety of filters based on multiple microring resonator devices are analyzed by numerical simulation. The technique used here has been verified with single-ring experimental measurements. Butterworth and Chebyshev filters are investigated, as are serial cascades of resonant devices. Although the power penalty is generally not prohibitive, it is a parameter which cannot be ignored for the design of complex high-bandwidth photonic interconnect systems that utilize microring resonators as filters and switches.

Control of a free-space adaptive optical interconnect using a liquid-crystal spatial light modulator for beam steering

This paper is a revision of a paper presented at the SPIE Conference on Micro-Optics, VCSELs, and Photonic Interconnects, Apr. 2004, Strasbourg, France. The paper appears (unrefereed) in SPIE Proceedings Vol. 5453. An adaptive beam position control system is reported, for use in alignment correction for free-space parallel optical interconnects at the board-to-board level. A liquid-crystal spatial light modulator (SLM) displaying binary phase gratings is used as a diffractive optical element to steer the beam. The design and implementation of a feedback controller for the SLM are presented, and shown in experimental results to completely correct for step disturbances within 1 to 2 sample periods. Extension of the system to tolerate random vibrations is considered, in terms of both the required SLM technology and the design of the control algorithms. The discussion highlights some of the trade-offs in choosing components when using this technology to correct for different levels of vibration.

VLSI Photonic Ring Multicomputer Interconnect: Architecture and Signal Processing Performance

This paper presents the architectural design of a multicomputer interconnection network based on the use of optical technology. The performance of the system is evaluated on a set of signal processing applications. The interconnect uses Vertical Cavity Surface Emitting Lasers (VCSELs) and flexible fiber image guides to implement a physical ring topology that is logically configured as a multiring. Processors in the multicomputer are nodes on the ring and extremely high communication bandwidth is possible. Using the Laser Channel Allocation (LCA) algorithm and the Deficit Round Robin (DRR) media access protocol, the bandwidth available in the optical interconnect can be reconfigured to make efficient use of the interconnect resources. A discrete-event simulation model of the interconnect is used to examine performance issues such as throughput, latency, fairness, and the impact of reconfigurability.

Highly polarized photoluminescence and photodetection from single indium phosphide nanowires.

We have characterized the fundamental photoluminescence (PL) properties of individual, isolated indium phosphide (InP) nanowires to define their potential for optoelectronics. Polarization-sensitive measurements reveal a striking anisotropy in the PL intensity recorded parallel and perpendicular to the long axis of a nanowire. The order-of-magnitude polarization anisotropy was quantitatively explained in terms of the large dielectric contrast between these free-standing nanowires and surrounding environment, as opposed to quantum confinement effects. This intrinsic anisotropy was used to create polarization-sensitive nanoscale photodetectors that may prove useful in integrated photonic circuits, optical switches and interconnects, near-field imaging, and high-resolution detectors.

WDM packet routing for high-capacity data networks

We present experimental and numerical studies of a novel packet-switch architecture, the data vortex, designed for large-scale photonic interconnections. The selfrouting multihop packet switch efficiently scales to large port counts (>10 k) while maintaining low latencies, a narrow latency distribution, and high throughput. To facilitate optical implementation, the data-vortex architecture employs a novel hierarchical topology, traffic control, and synchronous timing that act to reduce the necessary routing logic operations and buffering. As a result of this architecture, all routing decisions for the data packets are based on a single logic operation at each node. The routing is further simplified by the employment of wavelength division multiplexing (WDM)-encoded header bits, which enable packet-header processing by simple wavelength filtering. The packet payload remains in the optical domain as it propagates through the data-vortex switch fabric, exploiting the transparency and high bandwidths achievable in fiber optic transmission. In this paper, we discuss numerical simulations of the data-vortex performance and report results from an experimental investigation of multihop WDM packet routing in a recirculating test bed.

Recent developments and applications in electroabsorption semiconductor modulators

Abstract State of the art developments in electroabsorption modulators that utilize quantum well-semiconductors are reviewed. Special emphasis is given to recent progress made in materials for modulators. On technological grounds, optimized Multiple Quantum Well modulators in the GaAs/AlGaAs system have been driven by applications in photonic switching and optical interconnects. Surprisingly, these same structures exhibit a wealth of new behavior that ranges from Bloch oscillations to excitons in Coupled Well, Wannier–Stark and Shallow Well superlattices. Several types of excitonic phases have been identified, optically and found to transform as the system dimensionality changes from 2D to 3D. In addition, new material systems have shown that quantum well excitonic absorption quality can be transferred to technologically important wavelengths at 1.06 or 1.55 μm.

Introduction to the issue on smart photonic components, interconnects, and processing

Introduction to the issue on smart photonic components, interconnects, and processing