Julian P. G. Bristow

Optical interconnections for massively parallel architectures

This paper presents a study of board-level interconnection requirements for highly parallel and massively parallel computing. Analytical models of the I/O bandwidth of popular interconnection networks have been developed and show that current electronic technologies are poor in supporting the necessary I/O density and bandwidth. Optical interconnects appear to offer greater potential in meeting these I/O requirements. Several possible optical implementations of interconnecting a network of electronic processors are compared. The use of polymer waveguides appears to offer the best solution compatible with existing multiboard system architectures.

Polymer Waveguide-Based Optical Backplane for Fine-Grained Computing

The interconnection requirements of fine-grained computing are examined and compared to the requirements of coarser grained, multiplexed systems. Specifications for the interconnection medium are developed and compared to the performance of available optical source and interconnection components. The use of polyimide waveguides for both applications is considered and the probable architecture of a multiboard fine-grained system is described.

Recent progress in short-distance optical interconnects

Short distance optical interconnects are under development for a range of applications including local area networks, optical backplanes, and optoelectronic accelerators or signal processors. In some applications, the aggregate bandwidth required cannot be provided with electrical interconnects, offering an obvious advantage for optics, while in others it is the density of available interconnects which motivates the use of optics. In most commercial applications, it is the cost of the interconnect solution which will affect its acceptance by system integrator. For optics to be applied in a broad range of applications, greater transparency must be provided to the system integrator. We describe both intercabinet and intracabinet interconnects in which the addition of optical interconnects has been designed to perturb the overall system as little as possible and yet still take advantage of optics.

Vertical cavity surface emitting lasers for spaceborne photonic interconnects

Vertical cavity surface emitting lasers (VCSELs) offer substantial advantages in performance and simplicity of packaging over the edge emitting lasers currently being applied to state-of-the-art photonic interconnects. We have demonstrated operation of VCSELs at cryogenic temperatures and at temperatures as high as 200 degrees Celsius, with a single device operating from minus 55 degrees Celsius to plus 125 degrees Celsius. The devices operate to 14 GHZ and can be operated in excess of 1 GHZ with bias-free operation. Initial radiation tests indicate an order of magnitude improvement in hardness with respect to neutron damage over an LED which is currently used in spaceborne photonic interconnect modules. We also describe the packaging of VCSELs in compact multichip modules. By using passive alignment techniques, optoelectronic devices can be packaged in established multichip module fabrication schemes without adding costly high precision assembly techniques.

Progress and status of guided-wave optical interconnection technology

Optical interconnects at the cabinet-to-cabinet, board-to-board, and multichip module-to- multichip module levels will enable future avionics systems requirements to be met by eliminating undesirable compromises associated with electrical interconnects. Fiber optics is the well established medium of choice for cabinet-to-cabinet applications, while planar polymeric interconnects are required at the backplane level. Significant advances have been made in demonstrating practical polymer interconnects compatible with existing board fabrication principles, however both waveguide loss and interfaces to optoelectronic components require further improvement before the technology is broadly applicable.

In vacuo responses of an AlGaAs vertical cavity surface emitting laser irradiated by 4.5 MeV protons

Vertical cavity surface emitting lasers (VCSELs) have high potential for space applications, yet little is known of their sensitivity to radiation under vacuum conditions. The first observations of a commercially available proton implanted quantum well AlGaAs VCSEL operating at 850 nm in vacuo and irradiated by 4.5 MeV protons by a scanning ion microbeam is presented. Degradation of L-I-V responses at a proton dose of 1.19 MGy are discussed with particular attention drawn to heating arising from increased nonradiative carrier recombination and that resulting from the vacuum environment.

Cost Effective Optoelectronic Packaging for Multichip Modules and Backplane Level Optical Interconnects

Optical backplanes are of increasing interest for commercial and military avionic processors, and for commercial supercomputers. Projected interconnection density limits of electrical interconnects are rapidly becoming a bottleneck, preventing optimal exploitation of electronic processor capability. A potential obstacle to the commercial development of optoelectronic interconnect components for backplane-based systems is the small market for such specialized technology. In order to ensure that a cost effective solution is available for backplane based systems, commonality with a higher volume application is required. We describe optical packaging techniques for board level waveguides and multichip modules which exploit materials, processes and equipment already in widespread use in the electronics industry, and which can also be applied to a wide range of optoelectronic modules for local area network and telecommunications applications. Rugged polyetherimide waveguides with losses of 0.24 dB/cm have been integrated with conventional circuit board materials, and optoelectronic die have been packaged in a multichip module process using equipment normally used for purely electronic packaging. Practical optical interfaces and connectors have been demonstrated for board-to-backplane and board-to-multichip module applications, and offer increased pincount over their electrical counterparts while retaining compatibility with existing electrical connector alignment and fabrication tolerances.

Physical interactions between charged particles and optoelectronic devices and the effects on fiber-based data links

Current developments in high performance satellite data links rely on fiber optic systems to take advantage of light weight, electromagnetic isolation, low power, and high bandwidth. Indications are that fiber data links operate with little degradation or interference in the earths trapped radiation belts. To quantify this, we report analyses of experimental investigations in which operating fiber bus components are subjected to proton bombardment at varying proton energy, proton flux, angle of incidence, data rate, and signal levels. Parameterization of bit error rate (BER) effects in terms of these variables offers insights into the physical mechanisms involved and suggests both circuit modification and device selection criteria to maximize link performance. We outline a method to predict BER in orbit and offer this as a basis for evaluating proposed hardening solutions. The method combines predicted trapped particle orbital environmental data, including spacecraft shielding effects, with the measured system response.

Hybrid integration of electrical and optical interconnects

In this paper, we describe a novel approach for fabrication of low-cost optoelectronic modules for optical interconnect applications. The concept includes: (1) placement of optical and electrical components on a common substrate using a chip-first MCM structure to improve thermal handling capabilities, (2) fabrication of both optical and electrical interconnects using planar processes compatible to standard IC processes in manufacturing to reduce nonrecurring engineering costs, and (3) application of adaptive interconnect for device-to-waveguide alignment to reduce recurring packaging costs. Preliminary results on waveguide fabrication and modeling of adaptive interconnect are discussed in this paper.

Optoelectronic backplane interconnect technology development - POINT

This paper describes the technical approach and progresses of the POINT program. This project is a collaborative effort among GE, Honeywell, AMP, AlliedSignal, Columbia University and University of California at San Diego, sponsored by DARPA/ETO to develop affordable optoelectronic packaging and interconnect technologies for board and backplane applications. In this paper, we report the development of a backplane interconnect structure using polymer waveguides to an interconnect length of 280 mm to demonstrate high density and high speed interconnect, and the related technical development efforts on: (a) a high density and high speed VCSEL array packaging technology that employs planar fabrication and batch processing for low-cost manufacturing, (b) passive alignment techniques for reducing recurrent cost in optoelectronic assembly, (c) low-cost optical polymers for board and backplane level interconnects, and (d) CAD tools for modeling multimode guided wave systems and assisting optoelectronic packaging mechanical design.