Short distance optical interconnects, which has long promised to solve a number of problems assumed to exist in electronic systems, seems to be at the threshold of providing real value to future processing systems. In many cases, it has been the cost of optical interconnects which has prevented their widespread adoption. However, while data rates required in future computing systems are increasing, and fiber optic component costs have decreased, the distances over which optical interconnects become attractive have been shrinking. In addition the increasing demand for smaller footprints for the I/O at the card edge, plays into one of the strong advantages unique to optics. An effort called Optical Micro-Networks is underway, funded by DARPA, whose objective is to demonstrate cost effective system level benefits in parallel optics for intra- and inter-rack interconnects by leveraging recent advances in VCSEL arrays, high sped CMOS, and low cost glass fiber cabling, connectors and on-board fiber routing. The approach will integrate optical interconnect functionality directly into an ASIC package thus reducing size, cost, and power.
High quality optical fiber to OEIC pigtailing, using non-conventional technology, is required to create a real integrated optical
system for optical communications, computing, signal processing, control, and sensing. In this paper, Physical Optics
Corporation (POC) presents a novel singlemode fiber to singlemode GaAs channel waveguide pigtailing approach. This
pigtailing approach involves two key technologies. First, a fiber end-face lensing technology was used to improve modeprofile
matching between singlemode fiber and singlemode channel waveguide, so fiber to waveguide coupling efficiency
could be improved. Second, resistance layer assisted dual-carrier-soldering (RLADCS) technology was introduced to facilitate
fiber and waveguide chip alignment and fixing, so accurate, convenient, and reliable fiber to optoelectronic integrated cicuit
(OEIC) pigtailing could be achieved. By using radiation hardened fiber and special OEIC, this pigtailing and packaging
technology has potential applications in a space environment. This publication addresses all aspects of this pigtailing
approach, including theoretical analysis, design, fabrication, testing, and measurement results.
Optical interconnects have long promised significant advantages over their electrical counterparts. Specific advantages include increased bandwidths at long (ten meters or more) interconnection distances, immunity to EMI effects, negligible crosstalk, reduced size, and lower weight. Optical interconnects have been developed for, and are being used in, a range of ground based and aircraft applications, however they are only now beginning to gain acceptance in spaceborne systems. In addition to the maturity demanded from components destined for ground-based applications and the wider temperature excursions characteristic of airborne applications, spaceborne components must also be able to survive the radiation environments associated with their intended applications. The additional qualification required has resulted in delayed introduction of photonic interconnects. We describe the tradeoffs involved in implementing for the first time a spaceborne fiber optic data bus with a clock speed of 1.2 Gbps. The tradeoffs include emitter, detectors, fiber, connectors and packaging. We have selected a series of commercial grade optoelectronic devices which were then qualified for use in spaceborne environments and have developed a space qualifiable packaging scheme. We have designed and implemented the optoelectronic subsystem of the data bus and have simulated its operation.
We also describe recent advances in Vertical Cavity Surface Emitting Lasers (VCSELs) for spacebourne databuses. VCSELs also offer advantages in simplicity of packaging and electronic control. We summarize available initial radiation data on these devices and project their impact on spaceborne photonic interconnects.
We present the fabrication of polyimide-based H-tree waveguides for a multi-GBit/sec optical clock signal distribution in a Si CMOS process compatible environment. Such a clock distribution system is to replace the existing electronic counterpart associated with high-performance computers. A waveguide propagation loss of 0.21 dB/cm at 850 nm was experimentally confirmed for the l-to-48 waveguide fanout device, l-to-2 splitting loss and bending loss were measured to be 0.25 dB and higher. The planarization requirement of the optical interconnection layer among many electrical interconnection layers makes the employment of tilted grating a choice of desire. Theoretical calculation predicts the 1-to-l free-space to waveguide coupling with an efficiency as high as 95%. Currently, a coupling efficiency of 35% was experimentally confirmed due to the limited index difference between guiding and cladding layers. Further experiments aimed at structuring a larger guiding/cladding layer index differences are under investigation. To effectively couple an optical signal into the waveguide through the tided grating coupler, the accuracy of the wavelength employed is pivotal. This makes the usage of the vertical cavity surface-emitting lasers (VCSELs) and VCSEL arrays the best choice when compared with edge-emitting lasers. Modulation bandwidth as high as 6 GBit/sec was demonstrated at 850 nm. Such a wavelength is compatible with Si-based photodetectors. Temperature dependence of the threshold current up to 155 °C was measured which will determine the power dissipation issue of the optoelectronic packaging. Finally, the first fully monolithic Si-MOSFET integrated receiver was made as the optical clock signal detector. To further enhance the bandwidth of such a detector, a resonant cavity structure with Si/Si02 as the bottom mirror was employed. The measured demodulation bandwidth is over 10 GHz. A fully integrated guided-wave optical clock signal distribution system having planarized grating couplers, H-tree Si- CMOS process compatible waveguides, VCSELs and Si-based photo-receivers will be demonstrated in the near future.
In this paper, we focus on how vertical-cavity surface- emitting lasers (VCSELs) and arrays have led to many feasible advanced technological applications. Their intrinsic characteristics, performance, and producibility offer substantial advantages over alternative sources. Demonstrated performance of `commercial-grade' VCSELs include low operating powers (< 2 V, mAs), high speeds (3 dB BWs > 15 GHz), and high temperature operating ranges (10 K to 400 K and -55 degree(s)C to 125 degree(s)C, and T > 200 degree(s)C). Moreover, their robustness is manifest by high reliability in excess of 107 hours mean time between failures at room temperature and tenfold improvement over existing rad-hard LEDs. Hence, even these `commercial-grade' VCSELs offer potential within cryogenic and avionics/military or space environments. We have also demonstrated submilliamp ITH, stable, single-mode VCSELs utilized within bias-free 1-Gbit/s data links. These low- power VCSELs may also serve in applications from printers to low-cost atomic clocks. The greatest near-term VCSEL applications are upgrades to low-cost LEDs and high-grade copper wire in data links and sensors. Exploiting their surface-emitting geometry, VCSELs are also compatible with established multichip module packaging. Hence VCSELs and VCSEL arrays are ideal components for interconnect-intensive processing applications between and within computing systems.
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.
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.
The development of multimode passive polymer optical waveguide components for board and backplane interconnect applications, such as in the DARPA-sponsored, polymer optical interconnect technology (POINT) program, require several optics design issues to be addressed including efficiency and modal noise. For example, the mating of arrays of sources, detectors, and fibers requires appropriate fanout structures to match the component pitch. Here we consider designs for such structures employing multimode polymer waveguides, including both abrupt and smooth bending elements. We investigate these structures using a new multimode BPM simulation CAD tool, and consider the bend losses as a function of geometry, angle, and source condition. The results are compared with experimental observations on devices fabricated for use in the POINT demonstration module. The simulation closely matches the experiment, demonstrating the utility of such efforts in practical component development.
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.
We have demonstrated a complete optical data between a pair of optical transmitter (Tx) and receiver (Rx) multichip modules (MCMs) using polymer optical waveguide array as interconnect media. The Tx and Rx MCMs each has three data channels and are fabricated using conventional MCM-C technology. The components used in our MCMs include Honeywell's vertical cavity surface emitting lasers (VCSELs) at wavelength of 850 nm, and other commercial off-the-shelf Si devices such as Si detectors, laser drivers, and amplifiers. The waveguide interconnect is over 4-inch long, and includes both flexible ribbons and board integrated polymer waveguides with passively aligned connectors. The waveguides are finished with 45-degree mirrors at each end facets for interfacing with the VCSELs and the p-i-n detectors in the Tx and Rx modules, respectively. The loss of the overall optical link is less than 10 dB. We have measured a bit-error-rate of less than 10-12 at a data rate of 1 Gbps per channel.
Both multichip modules (MCMs) and optical interconnects are expected to play a pivotal role in the development of high performance electronic systems. Only by packaging optoelectronic components within the multichip modules can the advantages of both technologies be realized. We have demonstrated the incorporation of optoelectronic components into two different MCM technologies. In the first, we have used existing equipment and proven polymer materials to implement optoelectronic interfaces in a high density 'chip-first' technology. In the second, we have demonstrated optoelectronic packaging in 'chip-and-wire' packaging schemes. In both cases, the optical characteristics are compatible with commercial vertical cavity surface emitting lasers and optoelectronic receivers, allowing the implementation of practical MCM-to-MCM interconnects.
The Polymer Optical Interconnect Technology (POINT) represents a major collaborative effort among GE, Honeywell, AMP, AlliedSignal, Columbia University and the University of California at San Diego (UCSD), sponsored by ARPA, in developing affordable optoelectronic module packaging and interconnect technologies for board- and backplane- level optical interconnect applications for a wide range of military and commercial applications. The POINT program takes a novel development approach by fully leveraging the existing electronic design, processing, fabrication and module packaging technologies to optoelectronic module packaging. The POINT program further incorporates several state-of-the-art optoelectronic technologies that include high-speed VCSEL for multichannel array data TM transmission; flexible optical polymers such as Polyguide or coupling of device-to-fiber using a passively alignment process; a low-loss polymer for backplane interconnect to provide a high I/O density; low-cost diffractive optical elements (DOE) for board-to-backplane interconnect; and use of molded MT array ferrule to reduce overall system size, weight, and cost. In addition to further reducing design and fabrication cycle times, computer simulation tools for optical waveguide and mechanical modeling will be advanced under the POINT program.
Several interconnect Junctions within cabinets comprising high performance computers are pushing the limits of electrical interconnect technology. A widespread problem of limited interconnect density at the board-to-backplane interface has been identified, while advances in clock speed and cabinet functionality are expected to stress electrical interconnects first for high fanout applications, then for high speed point-to-point parallel interconnects. Key components are described, and from these components the design of a generic optical backplane technology is described. While components are now available to implement a practical solution, the use of proven electrical techniques is considered an important step in hastening the introduction of the technology.
As large factories, chemical plants and oil refineries become more and more automated, it becomes necessary to relay information from a large number of sensors to central control sites. In addition, it has also become important to transmit large amoutns of information in the form of data, voice, video, etc., to workers around the plant or factory. A unique technique based on optical microwave cellular system is described which is highly suitable for such an application.
We present the characteristics of a prototype all-fiber sensor network that is useful in structure-health management and distributed sensor data acquisition. In this network, each remote sensor node is powered over a fiber by a laser in the base station. The sensor data are sent back to a base station through a different fiber. Issues concerning power consumption per node, data rate, fault-tolerance, packaging, cost, and network expandability will be discussed.
Acceptance of optical interconnects into spaceborne and ground-based military systems will be limited by the risk and maturity of the technology. Multimode fibers are currently the interconnect medium of choice for military application due to the availability of qualified parts. Multimode optical interconnects also offer lower assembly cost than singlemode interconnects, making them attractive for use in commercial computers. In both applications, modal noise may limit the attainable bit error rate in a digital system. We report on an investigation of the validity of simple expressions for determining modal noise in multimode systems, and consider in particular waveguides supporting fewer modes than typical multimode fibers and waveguides with incomplete modal excitation. We conclude that the simple expressions are valid for losses greater than approximately 0.5 dB per interface, but that the actual signal-to-noise ratio is significantly poorer than that predicted by theory for large displacements of waveguides in which a small fraction of the total modal volume is excited. We describe two simulation techniques for determining the statistics of the transmission associated with a waveguide bend, and find that the simple formula widely used for straight waveguide intersections do not describe curved waveguide interconnects accurately. We also describe a practical demonstration of two optical interconnect systems in which multimode interconnect media incorporating several imperfect interfaces and excited with coherent sources can achieve useful bit error rates in digital systems. In the first interconnect, a fiber optica data bus for satellite use employs six multimode connectors and multimode fiber to transmit data at 1.2 Gbps over distances from 1 meter to 100 meters. In the second interconnect, passive multimode polymide waveguides compatible with intracabinet optical interconnects are used to implement a multichip module (MCM)-to-MCM interconnect in which optoelectronic die are incorporated in a 'chip first' multichip module technology.
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.
Recent advances in optical devices, polymeric materials, and in the electronic MCM packaging and interconnect technologies could bring the cost of optical interconnect to a level affordable for module, board, and backplane level interconnect applications. Specifically, we discuss how the development in (1) vertical-cavity surface-emitting-laser devices, (2) multichip module packaging technologies, (3) optical polymers, and (4) adaptive interconnect can be applied to benefit optoelectronic packaging and interconnect. We show how these advancements will allow widely used planar processes and the already developed packaging technology in electronics to be applicable to optoelectronic packaging to reduce both recurring and nonrecurring engineering costs for this new technology insertion into computers and advanced electronic systems.
The concept of an all-fiber, IC-based sensor network is presented. Fundamental issues including topology tradeoffs, power budget, and power distribution subsystem are analyzed. Some potential applications of such sensor networks are discussed.
This paper presents a fiber optic packaging technique using qualifiable materials and processes that has been developed for space applications. The technique is consistent with a Multi Chip Module implementation. This packaging technique allows multiple fiber optic interfaces to be combined with high density electronics within a single easily integrable package and can be used wherever fiber optics are incorporated into spaceborne systems, including the Fiber Optic Data Bus, the Space Station FDDI interface, any upgrades of the Space Station Local Bus from 1553 to 1773, and various Dual Rate 1773 applications.
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.
The design of the optical portion of a fiber optic data bus for space applications is described. A maximum NRZ data rate of 1.9 Gbps is predicted using low risk components which have either been demonstrated in space or for which sufficient qualification data are available. Substitution of lasers and fibers for higher bandwidth components are expected to extend the data rates to 4 Gbps but require additional qualification effort.
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 earth's 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.
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.
Message congestion in heavily loaded mesh networks can limit system throughput. While electronic express routers increase mesh system throughput by alleviating congestion and reducing average message delivery time, optical express routers offer the possibility of a further order of magnitude performance increase by exploiting the latency advantages of decoupling data rate from optical router reconfiguration time. We show how a self-routing optical mesh can be interfaced to an existing electrical mesh, and describe the use of optimized default switch settings to minimize latency. We show that circuit switching exploits the benefits of optics in express routers to a greater extent than does packet switching, and illustrate likely architectures in which waveguide switches, detectors, and amplifiers are integrated to form self-routing nodes. The self-routing mesh algorithm and data format required to guarantee that the correct path is established are described. Our calculations show that for meshes of 512 nodes or more, optical express routers allow an order of magnitude increase in throughput compared to electronic express routers, and possibly two orders of magnitude compared to non-express meshes.
Analytical modeling and practical experience reveal that interconnection networks for large-scale parallel architectures are severely limited by the I/O bandwidth of electrical interconnects. Optical interconnects offer far greater potential in meeting these bandwidth demands. The development of an operational 1024x1024 polyimide waveguide perfect shuffle network and high-density modulator arrays demonstrate how optics can meet this challenge. Further optical switching networks would be possible with the development of single-mode 2x2 waveguide switches. We envisage as feasible the insertion of active optical interconnection networks in future large-scale parallel architectures using integrated arrays of waveguide modulators photodetectors switches and interconnects.
A new design of GaAs/AlGaAs waveguide amplitude modulator intended for array applications at 830nm wavelength is reported. The device is based on polarization rotation and has bandwidths in excess of 1 GHz and an extinction ratio of 1 7dB. Mach-Zehnder modulators with 23dB have also been fabricated and arrays with 20 microns device separation fabricated. The arrays have crosstalk less than -20dB and feature electrical interconnection compatible with standard packaging techniques.
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