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.

In an effort to lower the cost of fiber optic couplers, integrated optic channel waveguide circuits are made of a UV-curable polymer using a molding technique, and then a novel fiber-to-channel connecting approach is employed in which UV light radiating from an optical fiber core cures the polymer in the channel, thus accomplishing a 'touchdown' of the core-extension waveguide onto the walls of the channel waveguide.

GaAs optoelectronic integrated circuits are having an increasing impact on the development of lightwave communication systems and very high sped digital and analog signal processors. However, cost-effective integration of complex electronic and optoelectronic circuits used in high speed networks or backplane interconnects still presents a significant technological challenge and substantial efforts are being devoted to the development of practical, low-loss integration processes. Photodetector/waveguide integration is a key aspect of the successful packaging of optoelectronic devices. In this work, metal-semiconductor- metal (MSM) photodetectors were integrated with polyimide ridge waveguides, and the processing parameters were varied for optimum performance. The absolute responsivities of the integrated MSM/polyimide waveguide structures were typically 0.5 A/W, and 3 dB bandwidths of 4-6 GHz were measured. Series of 1 X 2 and 1 X 4 photodetector arrays were interconnected, demonstrating a uniform division of the optical signal between the detectors. Results indicate that these arrays offer the potential to fabricate optoelectronic switches that can be used in a variety of high speed and broadband communication systems.

Thermal simulation was carried out by finite element analysis. Both 2D and 3D modeling of the technology demonstrators, usually based on quarter models facilitated by the VCSEL modules symmetries, were carried out. The models were parameterized so that the effects of material properties, dimension VCSEL power dissipation could be easily simulated. Based on these models, detailed performance figures were projected for the VCSEL modules. Similarly the effects of non-uniform power dissipation were simulated. Using the parameterized models, the simulations can be used to rapidly assess the effects of changes in the VCSEL modules design and structure and to select optimum configuration for a given application. The heat conduction from the VCSEL to a heat sink is modeled through different package elements. They models are also used to study the different in thermal behavior between a wire-bonded and a flip-chip VCSEL, which consisted of an 8 X 8 array. The dimension of the VCSEL was varied from 8 micrometers to 20 micrometers and the characteristics of (Delta) T vs. VCSEL's power dissipation are developed for all the different models. The analysis indicated that the VCSEL active junction temperature is higher than the heat sink temperature. Optimized package design can be a critical element in enhancing reliability. Trade-off issues resulting form conflicting performance requirements are also discussed. The FEM analysis was also used to simulate the VCSEL configuration in order to understand the thermo-mechanical response of the VCSEL and the flip-chip interconnect during the bond processing.

Intrinsically large mode semiconductor lasers and mode transformers monolithically integrated with semiconductor lasers, are two promising approaches for making alignment tolerant structures that can be used for passive alignment to single mode optical fibers. This technique, in conjunction with a recently developed silicon waferboard integration scheme, will significantly simplify the assembly process and the packaging of transmitter laser arrays. The passive alignment technique consists of octagonal electrodeposited copper bosses to physically register the laser chip with percussion etched inverted pyramidal receptacles and v-grooves in a silicon substrate.

MCC is currently applying a previously developed and patented optical submount approach for use in high- reliability/performance applications that include space, airframe, and outside telecommunication systems. The optical submount utilizes a novel alignment method integrated with low-cost, few-chip module packaging techniques originally developed for a laminate multichip module parallel link using vertical cavity surface emitting laser/optical electrical integrated circuit receiver arrays. This system is now being investigated for use in both single and parallel channel high-reliability/performance applications.

A reliability prediction model for optical interconnects has been developed and applied to two commercial optical interconnect systems. The reliability prediction model is based on the concept of a reliability figure of merit (RELFOM). The RELFOM methodology considers the entire system in a series structure and consists of the RELFOM of all of the individual components of the interconnect system, where each component of the RELFOM is further defined according to performance parameters, such as propagation delay, power dissipation and cross talk noise. The RELFOM model can be used effectively for technology comparisons and reliability prediction without utilizing extensive reliability testing.

The unrelenting demand for ever-higher data transfer rates between computing devices, coupled with the emerging ability to produce robust, monolithic arrays of optical sources and detectors has fueled the development of high-speed parallel optical data links, and created a need for connectorized, parallel, multifiber cable assemblies. An innovative approach to the cable assembly manufacturing process has been developed which incorporates the connector installation process into the cable fabrication process, thus enabling the production of connectorized cable assemblies in a continuous, automated manner. This cable assembly fabrication process, as well as critical details surrounding the process, will be discussed.

We describe a novel vertical taper structure fabricated at the ends of polymer optical waveguide devices to improve the coupling between channel waveguides and single-m,ode fibers. The taper smoothly converts a highly elliptical waveguide mode into a bigger and more circular mode for low loss coupling and relaxed fiber alignment tolerances. A vertical taper 0.5-2 mm in length is made in the low index upper cladding to reduce its thickness from several micrometers to zero, followed by the coating of a second upper cladding with index higher than that of the previous upper cladding but slightly lower than that of waveguide core. In the taper, the channel waveguide mode gradually loses confinement by the upper cladding so that the mode size grows bigger a light propagates, whereas the confinement by the lower cladding and lateral confinement are hardly affected. The waveguide mode grows in the vertical direction away from the lossy ground electrode and substrate; therefore no compromise between mode size and propagation loss is involved. Two special but simple reactive ion etching techniques, shadow masked etching and tapered photoresist etching mask, are develop for making this vertical taper. Mode expansion and a 1.8 dB reduction in coupling los, which is not sensitive to waveguide width and polarization, is obtained in our preliminary experiment.

The performance of optical interconnects is directly related to the characteristics of the electronic interface between the optical interconnect components and the electronic processing elements. In this paper we examine this issue and determine limits on the power and bandwidth of optical interconnects with low threshold VCSEL sources. For a 0.8 micrometers CMOS process the effects of coupling parasitics and detector capacitance limit signal bandwidth below 500 MHz. The effects of device geometry, fanout, and line length are also studied and show that with existing optoelectronic devices optical interconnects are competitive with electrical connections at the board level, but not below this packaging level.

A procedure for designing and fabricating holograms which are used for generating free space optical interconnections is described. The design process uses the Gerchberg-Saxton algorithm and a random search method. Fabrication is performed by electron beam lithography and acetone etch of a polymethyl methacrylate film. The use of holograms fabricated by this method in an optoelectronic processing module is described. The interconnect patterns required to implement switching and data processing devices are discussed, including a crossbar switch, an adder, and a multiplier. Holograms which implement these interconnect patterns have been fabricated.

Advanced device technologies such as vertical cavity surface emitting lasers (VCSELs) and diffractive micro lenses can be combined with novel packaging techniques to allow low-power interconnection of parallel optical signals. These interconnections can be realized directly on circuit boards, in a multi-chip module format, or in packages that emulate electrical connectors. For applications such as stacking of multi-chip module (MCM) layers, the links may be realized in bi-directional form using integrated diffractive microlenses. In the stacked MCM design, consumed electrical power is minimized by use of a relatively high laser output from high efficiency VCSELs, and a receiver design that is optimized for low power, at the expense of dynamic range. WIthin certain constraints, the design may be extended to other forms such as board-level 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.

Epitaxial liftoff and grating of multiple thin integrated circuit-sized semiconductor films each containing over a hundred discrete devices confirms that multichip modules can be produced using this technology. the use of epitaxial liftoff membranes containing electronic and optoelectronic circuits and devices will provide (1) highly improved thermal management, (2) much higher density packaging, and (3) applications polylithically interconnecting electronic mixed-signal and optoelectronic devices. Bond strength data of thin epitaxial membranes grafted onto high thermal conductivity substrates quantifies for the first time to our knowledge the adhesion quality of van der Waals forces. In addition, the handling of very small discrete devices by means of integrated circuit-sized thin films permits the use of commercial off-the-shelf hardware in the development of an automated manufacturing assembly tool. This enabling technology will provide a manufacturing path to continue to commercialize new models of high performance optoelectronic modules with advanced features including higher data rates and increased function.

The increasing demand for clock speed is rapidly exhausting capabilities of interconnection techniques currently employed for high performance supercomputers. In order to address the bottleneck problem at the board level, we have taken a system's approach in developing optoelectronic interconnection layer for board-level high speed optical clock signal distribution. The reported approach employs polymer optical channel waveguides, waveguide clock signal distribution. The reported approach employs polymer based optical channel waveguides, waveguide splitters, and surface-normal waveguide couplers. This paper describes the system architecture, material choice, and fabrication process of board-level waveguides devices to achieve a synchronous global clock signal distribution.

We report the fabrication of tilted gratings on polyimide waveguides. The reported gratings provide an effective unidirectional surface-normal optical couple-in and couple- out for polymer-based optoelectronic interconnects. Such a planarized grating is particularly suitable for wafer-scale MCM optoelectronic interconnects due to its unique non- blocking fanout feature. both surface-normal input and output grating couplers have been demonstrated for the first time on polyimide waveguides on silicon substrate.

Wavelength division demultiplexer based on dispersive volume holograms and axial graded index (AGRIN) lenses is presented in this paper. The dispersive volume hologram was fabricated with DuPont photopolymer with two beam interference method. Close to 99 percent center wavelength diffraction efficiency was achieved and the dispersion property of a volume hologram was characterized with a mode-locked femto-second laser source. The experimental data were found to conform well with the theoretical derivations. Axial graded index (AGRIN) lenses have been sued as focusing elements to separate light with different incident angles after dispersive volume holograms. By using dispersive volume holograms and AGRIN lenses, a wavelength division demultiplexer with 1540 nm center wavelength and a 4 nm channel separation was demonstrated. By using two stacked holograms, light dispersion can be further enhanced up to five times. Therefore smaller devices and smaller channel separations can be realized.

AlliedSignal scientists have developed new polymeric materials for ultra-low-loss optical interconnection, particularly for the key wavelengths of 0.83, 1.3, and 1.55 microns. Developments of these materials has required a thorough understanding of fundamental principles of optical absorption due to both vibrational and electronic resonant absorptions. We have thus created materials with measure losses at 830 nm which are in the range of 0.02 dB/cm/ At longer wavelengths, the losses can be higher due to the vibrational absorption within the polymer. However through careful selection of chemical structure, polymeric materials with intrinsic loss below 0.08 dB/cm have been demonstrated at 1.55 micron wavelength. For wavelengths longer than 830 nm, single-mode and multimode waveguides with losses equal to the intrinsic loses have been fabricated.

Our research characterized the fundamental optical properties of poly(phenylsilsequioxane) (PPSQ) planar waveguides in an effort to determine the material's suitability as a guided wave optical interconnect in polymer. Material characterization included determining the refractive index, mode structure, and optical losses of 1-2 micrometers PPSQ films. Optical loss measurements indicate that PPSQ planar waveguides have a propagation loss of 0.17 dB/cm at 632.8 nm for first order TE modes in thin films between 1.72 micrometers and 1.32 micrometers . The silicon backbone polymer is experimentally shown to be thermally stable for temperatures up to 400 degree C, with no apparent change in index of refraction, volume, or optical loss. Experimental results indicate that the material is compatible with standard microelectronic fabrication and a strong candidate for use in optical waveguide applications.

Directional couplers with four sections poled in four perpendicular directions are proposed for the first time as a new electro-optic switch configuration in which complete conversion of both TE and TM light from one waveguide to the other can be achieved simultaneously by a low driving voltage adjustment. The perpendicularly poled sections of the switch make the device completely polarization- independent, and the inversely poled sections offer an extremely relaxed fabrication tolerance for the device. This configuration of each section poled in a different direction also makes it possible to drive the electro-optic coupler with a uniform electrode, which ensures high-speed operation of the device. Both the switching characteristics and the fabrication tolerance are simulated.

A thin colorless radiochromic diacetylene monomer sensor coated on a transparent polyester base undergoes solid-state polymerization via free radical mechanisms when irradiated with ultraviolet 266 nm wavelength, x- and gamma rays, and high-energy electron beam irradiation. The radiation-induced polymerization reaction leads to the formation of 1, 4-trans additions as polyconjugations along the ladder-like polymer chains.As a result, the colorless, transparent films responds to ultraviolet and to ionizing radiation by turning deep-blue, with an absorption spectrum that exhibits two distinct absorptions. Pulse radiolysis and flash photolysis techniques were used to measure the kinetics of the polymerization propagation reactions. The pulsed-electron- induced propagation of polymerization has an observed first- order rate constant of the order of 10³ s^-1, followed by much slower blue-shift of the primary absorption band. The activation energy of the polymerization was found to be approximately equals 50 kJ mol^-1. The fast kinetics of the UV-induced polymerization is faster by about one order of magnitude. The film can be utilized for optoelectronics, remote sensing, and radiation dosimetry.

New components, applications, and standards are driving parallel-optical interconnect development at HP Laboratories. Advances in the optoelectronics improve dynamic range and reduce power consumption. New electronics and packaging allow greater functional flexibility and compatibility with emerging standards.

It is one of the major bottlenecks to bridge various optoelectronic devices, such as laser diodes, optical waveguides and photodetectors in ultra low-loss optoelectronic interconnects, which are often fabricated using different technologies with different optical apertures. To solve this problem, 3D tapered optical polymeric waveguides are presented to provide the mode- matching among these optoelectronic components. Compression- molded polymeric waveguides presented herein is probably the only solution to bridge huge dynamic range of different optoelectronic device-depths varying from few microns to few hundreds microns. Both the design rule and fabrication technique are presented together with some experimental results. It is shown that such a 3D tapered waveguide can provide an effective optical coupling at a relaxed alignment tolerance.

A ne optical interconnect architecture for a 3D switch array using multiple LiNbO₃-based electro-optically modulated gratings in conjunction with substrate guided waves is reported. First the operating mechanism of the system is studied in detail, and the momentum mismatch in the operating process of the system is also demonstrated. Then, a new method for calculating coupling efficiency is derived by introducing a compensation for the mismatch. With the new calculation method and by introducing a substrate guided wave with 45 degree bouncing angle and 100-V applied voltage, an optimized coupling efficiency is obtained even though it is under the case of momentum-mismatch. Finally, the experimental result of coupling efficiency of the electro-optically modulated grating is provided, and agreements between the experimental results and the theoretical prediction are obtained.

In this paper, concerning with the practical applications of substrate guided wave optical interconnects, one-to-many surface-normal fanout devices with equalized fanout energy distribution are addressed and then demonstrated. DuPont photopolymer film HRF-600X001-20 is used in our experiment. The optimum recording beam intensity is investigated for obtaining large dynamic region of diffraction efficiency versus exposure dosage. Based on the experimental diffraction efficiency curve, 1-to-5 and 1-to-9 surface- normal fanout devices are fabricated operating at a wavelength of 850 nm. Output energy fluctuations of +/- 4 percent and +/- 10 percent are obtained for the two devices.

The expanding information revolution has been made possible by the development of optical communication technology. To meet the escalating demand for information transmitted and processed at high data rates and the need to circumvent the growing electronic circuit bottlenecks, mass deployment of not only optical fiber networks but manufacturable optical interconnect circuits, components and connectors for interfacing fibers and electronics that meet economic and performance constraints are absolutely necessary. Polymeric waveguide optical interconnection are considered increasingly important to meet these market needs. DuPont's polyguide polymeric integrated optic channel waveguide system is thought by many to have considerable potential for a broad range of passive optical interconnect applications. In this paper the recent advances, status, and unique attributes of the technology are reviewed. Product and technology developments currently in progress including parallel optical ink organization and polymer optical interconnect technology developments funded by DARPA are used as examples to describe polyguide breadth and potential for manufacture and deployment of optical interconnection products for single and multimode telecom and datacom waveguide applications.

An ATM-based buffered HyperPlane smart pixel array (SPA) utilizing the Hybrid CMOS-SEED technology has been designed, fabricated, and tested. Multiple quantum well p-i-n photodiodes (SEEDs) are used as the optoelectronic interface in the SPA. The SPA consists of a 4 X 9 array of smart pixels comprised of 4 parallel ATM node channels. The chip fabricated is an experimental version extensible to a 256 X 256 array for implementing an ATM-based HyperPlane switching architecture on a free space optical backplane. Experimental performance of the SPA is presented.

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.

We are assembling a crosspoint switch system to demonstrate the free-space interconnects based on the smart-pixel-array (SPA) technology. The hybrid SPA including VCSEL arrays, microlens array, hologram array, and CMOS detector array will be described. The whole system is packaged using a custom designed optomechanics component. The design, fabrication and performance of the system are discussed.

This paper presents the construction of the smart pixel arrays which perform AND and XOR functions with three-input and one-output optical signals for the application of an optical database filter. The device is based on oxide confined VCSELs bump bonded to GaAs MESFET pixels. The MSM photodetectors are monolithically integrated with MESFETs.

Recent advances in the design of high-speed optical switches and transceivers for a reconfigurable, spatially-multiplexed optical interconnection network are described. Monolithic switches based on the integration of vertical-cavity surface-emitting lasers with heterojunction bipolar transistors and photodetectors have achieved switching operation at a data rate of close to 1 Gb/s. Optical transmission experiments through fibers have been carried out using these switches at a data rate between 650 Mb/s and 1 Gb/s. For future improvements in performance, the photonic and electronic elements should be separately integrated and independently optimized. To facilitate photonic integration, VCSELs and resonance-enhanced photodetectors have been integrated on the same substrate.

We explore the potential of using optical waveguides and switches to build high performance parallel processing systems by introducing a new class of parallel computing architectures. This class of architectures are based on a special kind of reconfigurable buses called segmented buses. A segmented bus is a bus that can be dynamically partitioned into segments, called sub-buses, under program control. Such a bus can be used as a basic building block for constructing powerful parallel architectures. We show that parallel architectures based on segmented buses are versatile by embedding parallel communication patterns supported by a wide variety of networks such as linear array, ring, complete binary tree, X-tree, mesh-of-trees, multidimensional mesh, torus, multigrid and pyramid into segmented bus based architectures, and show that all these networks can be simulated with small slowdown factors.

We propose a pipelined asynchronous time division multiplexing optical bus. Such a bus can use one of the two hardwared priority schemes, the linear priority scheme and the round-robin priority scheme. Our simulation results show that the performances of our proposed buses are significantly better than the performances of known pipelined synchronous time division multiplexing optical buses. We also propose a class of processor arrays connected by pipelined asynchronous time division multiplexing optical buses. We claim that our proposed processor array not only have better performance, but also have better scalabilities than the existing processor arrays connected by pipelined synchronous time division multiplexing optical buses.

Low latency, high bandwidth interconnecting networks that directly link arbitrary pairs of processing elements without contention are very desirable for parallel computers. The simultaneous optical multiprocessor exchange bus (SOME-Bus) based on a fiber optic interconnect is such a network. The SOME-Bus provides a dedicated channel for each processor for data output and thus eliminates global arbitration. Each processor can receive data simultaneously from all other processors in the system using an array of receivers. The architecture allow for simultaneous multicast and broadcast messages using several processors with zero setup time and no global scheduling. In this paper, we discuss the design of a possible opto-electronic implementation of the SOME-Bus along with an optical power budget analysis. Slant Bragg fiber grains arranged to couple light out of a fiber ribbon cable into an array of amorphous silicon detectors vertically integrated on a silicon are presented as a low cost novel means of interconnecting 10 to 120 processors.

Arrays of 8 X 8 of GaAs/AlGaAs vertical cavity surface emitting lasers (VCSELs), which operate at approximately 850 nm, are being fabricated for integration with low power, optoelectronic integrated circuits. These high performance optoelectronic computing modules are being developed for high speed switching and data processing applications. The VCSEL array is a gain-guided, top-surface emitting device. It is fabricated from a GaAs/AlGaAs p-i-n distributed Bragg reflector structure, which is grown by metal organic chemical vapor deposition. The VCSEL fabrication process involves two stages of hydrogen ion implantation and three stages of metallization. A set of five photolithography masks has ben developed for VCSEL fabrication.

The design of a 1.2 Gbit/sec CMOS laser driver with a novel temperature controlled feedback loop is presented. It was designed to be used for digital fiber applications, e.g. fiber-to-the-curb telecommunications or for datacom. The laser-driver operates at a maximum speed of 1.2 Gbit/sec between the temperature ranges of -40 C to 85 C using low-cost CMOS technology. Simulation results at these speeds, and experimental results at a lower speed of 200Mbit/sec, are presented.

Optical interconnects provide wide bandwidth, lowloss, and high fanout as compared to those for traditional electrical interconnects. In the past years many high performance optoelectronic circuits have been demonstrated. However, most of them require complicated process and exotic devices. To make optical interconnects in real system and commercial use, circuits utilizing manufacturable, robust, and low-cost technology have to be realized. Ion implanted GaAs MESFETs provide great promise due to their simplicity in manufacturing and their high speed performance. The optical characteristics of GaAs materials also make this technology favorable in realizing low-cost, high-performance OEICs.

We report on the fabrication of a prototype multichannel optical transmitter based on vertical-cavity surface- emitting lasers (VCSELs). The package consists of an array of four VCSELs, mounted directly on a RF circuit board, and UV-curable polymer microlenses and waveguides which couple the laser output to a multimode fiber ribbon. Light from the lasers is captured by refractive polymer microlenses positioned on a glass substrate above the VCSEL array. The lenses focuses the light signals onto angled reflective end facets in the polymer waveguides. These waveguides are situated on a separate glass substrate which is bonded to the lens substrate. The light is then coupled form the waveguides to a multimode fiber ribbon; the average measured coupling efficiency was 47.5 percent +/- 3 percent. Experimental measurements reveal an analog bandwidth of 2.65 GHz per channel with better than 30 dB isolation between adjacent channels for frequencies up to 2 GHz without active heatsinking. In addition, it has been experimentally verified that this isolation is limited by the parasitics inherent in the VCSEL array rather than the parasitics of the device driver circuitry.

The film shrinkage effect of photopolymeric phase media failed to provide the desired volume holograms for point-to- point optical interconnects. In this paper, we report a new compensation method to physically correct the shrinkage effect resulted from the holographic recording and the post baking. DuPont photopolymer HRF-600X001 is studied. The correction of the Bragg diffraction angle shift of 85', which is induced by a 5.25 percent film shrinkage, is successfully demonstrated with the surface-normal configuration. A shrinkage-corrected volume hologram with 80 percent diffraction efficiency is experimentally confirmed. The methodology reported herein is applicable to other phase media when the associated film shrinkage data are experimentally determined.