Low-cost robust optical interconnections interfacing solid state components, functional optical circuitry and optical fibers are increasingly needed to facilitate deployment of high capacity optical communication systems. Polymeric waveguide based optical circuits are being increasingly considered for these interconnect applications. Many applications are in development or early commercial stages. Some of these are highlighted in the presentation to provide a review of promising polymer based applications and technologies. In particular, Polyguide^TM polymeric integrated optic waveguide technology, currently being applied in the ARPA funded POLO and POINT programs is covered in more detail. These programs are focused on high speed optical data links and optical circuit board technology respectively.

Optics has the fundamental capability of dramatically improving computer performance via the reduction of capacitance for intrinsic high bandwidth communications and low power usage. Yet optical devices have not displaced silicon VLSI in any measure to date. The reason is clear. When placed into systems, the optical devices have not had significantly greater performance in equally complex information processing circuits and similarly low manufacturing cost. An approach demonstrated here uses the same system integration techniques that have been successful for silicon electronics, only applied to optics. Essential for creation of very large scale integrated optics (VLSIO), with over 50,000 high speed logic gates per square centimeter, is a new class of ultra high confinement (UHC) waveguides. These waveguides are created with high index difference (as high as 4.0 to 1.0) between guide and cladding. The waveguides have been demonstrated with infrared cross sections less than 5% of a square free space wavelength. These waveguides can be manufactured today only in the mid-infrared, but the concepts should scale to the near-infrared as lithography improves. Waveguide corners have been designed and demonstrated with a bend radius of less than one free space wavelength. Resonators have been designed which have over 100 times smaller volume than VCSELs, yet efficiently inter-connected laterally in high densities. A connector to the UHC waveguides has been developed and demonstrated using diffractive optical element arrays on the back side of the substrate. The coupler arrays can allow up to 10,000 Gaussian beam connections per square centimeter. This connectivity also has advantages for low cost three dimensional packaging for reduced cost and thermal dissipation. Experimental results on the above concepts and components are presented.

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 coupling of a vertical cavity surface emitting laser (VCSEL) array into an array of multimode polymer waveguides through a 45 degree endface mirror is investigated. A coupling model for the VCSEL and waveguide interface is developed and compared with our experimental results.

This paper reviews the use of circuit transfer processes for optoelectronic applications. In this process, a circuit comprising either Si or GaAs devices is removed from its original substrate and is transferred to a second substrate. For active matrix displays, CMOS circuits are transferred in this way to glass substrates, and active matrix liquid crystal displays with pixel format of 1280 by 1024 (with 1000 lines per inch) have been successfully formed. Photovoltaic circuits have also been transferred to glass and to other photovoltaic devices to further the formation of multi-bandgap tandem structures. LED arrays have been formed successfully by this technique as well. This work shows the potential for combining CMOS and III-V circuits to form integrated optical input and output devices, as well as optical power delivery to silicon circuits. We review progress in these areas and suggest new applications of the technology.

We present a technique for monolithic integration of vertical cavity lasers and detectors with refractive microlenses etched on the back side of the semiconductor substrate in a wafer-scale process. This integration provides collimated or focused laser beam sources for applications in free-space interconnections or for coupling to optical fibers, and it improves the collection efficiency of detectors.

A technique for the formation of self-aligning Sn-Ag solder balls is presented. By controlling the solder reflow conditions through the optimization of the solder adhesion to the surface, a very thin layer of solder can be used. This makes possible the use of conventional photolithography and metal evaporation for the definition of the solder volume, thereby assuring its precision. Both wettable and nonwettable materials and their effects on the balling- up process are presented. The solder ball formation process and optimization are discussed along with the uniformity and scalability of the results.

Two developments underway in the MAC multifiber connector technology and product line, the MiniMAC and the Metral MAC connectors, are explained as accommodating ongoing trends in optical (and electronic packaging): increased density, metric-based products, and increased use of standard rather than proprietary products.

Space-division optical switches will be indispensable for future fiber-optic communication systems such as fiber-optic subscriber-line concentrators, photonic inter-module connectors, and protection switches. This paper reports recent progress on both silica-based thermo-optic optical switches with drive electronics. Two types of free-space switch modules are also reviewed; analog free-space switching modules which use liquid crystal beam shifters and digital free-space switching modules which incorporate semiconductor photonic switch array devices.

To be competitive with electronic switching technologies, photonic switching systems must have stability that rivals that of electronic systems, requiring little or no intervention over time scales of years. This paper describes progress towards the goal of stable optical interconnects. We have built an extraordinarily stable free-space optical interconnect mounted in a standard electronics frame; the system operated successfully over a wide temperature range for three days and required no realignment after shipping. Robust optomechanical design played an important role in the successful operation of the system. The role of kinematic mounting principles, self-centering lens mounts, materials selection, and long-lever arm adjustments are described. Vigorous shaking of the system did not affect its bit-error rate -- measured to be less than 10E-12 on a single channel.

In order for optical interconnects to become a mature technology they must be amenable to electronic packaging technology. Two main obstacles to including free-space optical interconnects are alignment and heat dissipation issues. This paper presents work that studies the issues of alignment tolerancing over long distance (greater than 10 cm) board-level interconnects. In this work we demonstrate a computer-aided analysis procedure that permits one to determine the alignment tolerances needed to achieve some system level specification, such as yield or cost. The procedure that we employ relies upon developing a detailed design of the system to be studied in a standard optical design program such as CODE V. Using information from this model, we can determine the integrated power falling on the detector by performing Gaussian propagation and/or general Fresnel propagation (if significant vignetting occurs). With this computer-aided analysis technique, a sensitivity analysis of all the misalignments under study is made on a realistic system to find each misalignment's relative effects (with other misalignments being set to zero) on the power falling on the detector. This information is used to set initial tolerances for subsequent tolerancing analysis and design runs. An alignment tolerancing analysis using Monte Carlo techniques is applied to determine if the yield/cost (yield being defined as the percentage of systems that have acceptable system performance) is acceptable. By utilizing a technique called parametric sampling, a subsequent tolerancing design run can be applied to optimize this yield/cost with very little increase in computation. In this paper, we study a realistic design example and show that all tolerances can be achieved with current technology.

Silicon waferboard technology based on etched and deposited passive-alignment features has been applied to the fabrication of optoelectronic transmitter and receiver arrays for rf applications. Using silicon waferboards, we have aligned both 1 by 4 buried-heterostructure laser arrays and 1 by 4 PIN photodetector arrays to optical fiber ribbons. Besides serving as mechanical carriers and alignment guides, the silicon wafers can also be used as rf or microwave substrates. We introduce rf-optoelectronic receiver arrays based on such enhanced silicon waferboards.

Penetration into the fiber-in-the-loop (FITL) and fiber-to-the-curb (FTTC) markets requires a drastic cost reduction in optoelectronic packaging. To achieve this goal, passive alignment techniques were developed using micromachined silicon waferboard technology which showed great potential for the tight tolerances (plus or minus 0.5 micrometers) required to passively align optoelectronic devices to single mode fibers. In this technology, micromachined alignment pedestals and standoffs precisely locate the x, y, and z positions of optoelectronic devices, which have matching alignment notches, to the optical fiber confined by a v-groove. The tight tolerances are possible using precision photolithography and well controlled reactive ion etching (RIE). In this paper, we report our process development results of forming micromachined silicon alignment pedestals using RIE. We demonstrate RIE etch depth control for the 6 micrometer z-standoffs with a 0.13 micrometer standard deviation and etch profile control for the x and y alignment pedestals to within 0.25 micrometers for each edge, which were obtained across a 3-inch wafer and from run to run. Such high precision control on RIE etch profile and uniformity is extremely important in developing manufacturable processes.

Packaging of integrated optoelectronic devices (e.g., laser diode arrays and OEICs) is motivated by potential cost savings and the increased functionality of more highly integrated devices. To date, attempts to package integrated optoelectronic devices with arrays of single- mode fibers have tended to exhibit high optical losses. We have developed a single-mode array packaging process based on the use of an intermediate silica-on-silicon planar optical waveguides (POWs) assembly to which optical fibers are attached using V-grooves. By lensing the POWs, we have achieved coupling efficiencies of greater than 50%. The photolithographic registration of the POWs allows a large (greater than or equal to 8) array of POWs with attached fibers to be aligned to an array of optoelectronic devices in a single active alignment procedure. This single active alignment step is well-suited to automation, and our approach is thus well-suited to achieving low cost in a manufacturing environment. We also discuss our positioning and mounting techniques, which provide high-stability coupling in adverse temperature and vibration environments and are compatible with hermetic packaging.

A free space chip-to-chip optical interconnection system using hybrid architecture is presented here. The Gaussian beam propagation model and the ray transform matrices are utilized to design the optical interconnect link. The optical performance of the hybrid optical receiver is computed using a scalar diffraction theory and the results compared with ray transform matrices. Also, the diffraction loss and the optical crosstalk are computed as a function of microlens diameter for various microlens pitch.

Packard-Hughes Interconnect has developed a detachable connector for plugging to military and commercial aerospace fiber-optic modules. The connector comprises floating spring loaded fiber-optic termini with 1-mm ceramic ferrules, all contained within a 0.136-in thick, low-profile connector plug body. The connector is mated to the package via a patented retention clip mechanism which secures the plug body to metal posts attached to the package sidewall. Optical alignment between the connector plug terminus and the package is accomplished by an alignment sleeve and mating 1-mm ceramic ferrule mounted in the package nosetube. Connector demating is performed by actuating a release button mechanism integral to the connector plug body. The fiber-optic termini in the connector plug body are easily maintained without replacing the entire connector. This makes the repair/replacement process for a broken fiber pigtail or damaged terminus endface a low cost, fast, and simple operation. The insertion loss for a simplex connector mated to a Boeing FDDI Transmitter receptacle package using 100/140 micrometers graded index optical fiber is less than 0.5 dB at 1.3 micrometers wavelength.

This paper discusses free-space optical interconnects from four perspectives: need, system architectures, major enabling technology, and component and system packaging. The need for such interconnection in telecommunication/datacommunication switching networks and fine- grained parallel computers is described. Since most of the envisioned systems fall into the two architectural classes of optical backplanes and 3-D systems, the paper describes these classes in detail. And the reader is introduced to the major enabling technology of smart pixel arrays. Finally, the paper describes attempts to implement free-space optical interconnect systems and some of the interesting thermal management issues that these implementations raise.

Multiple quantum well reflection modulators are used in photonic switching systems. A change in device temperature will vary the band gap of the modulator material and therefore the location of the exciton absorption maxima. At some change in temperature the shift in maxima location will be too great and the device will become inoperative. External temperature controls can hold a specific point on the chip to a fraction of a degree, however, spacial non-uniformities in heat transfer from the chip to the environment can result in a temperature variation across the chip. The design of the chip, the materials and methods used to attach the chip to a mount, and the design of the mount can all affect the spacial temperature variation. We have used finite element analysis (FEA) to analyze the affects many of these design factors have on the temperature variation across the chip. By careful package design, the calculated temperature spread across a chip can be significantly reduced. We have also used the temperature dependence of the exciton absorption maxima to map the temperature of an existing chip. The chip and mount were then modeled by FEA. The experimentally measured temperatures and those calculated by FEA were found to be in good agreement.

We discuss our measurements on thermal impedance and thermal crosstalk of etched-pillar vertical-cavity lasers and laser arrays. The average thermal conductivity of AlAs-GaAs Bragg reflectors is estimated to be 0.28 W/(cmK) and 0.35W/(cmK) for the transverse and lateral direction, respectively. Lasers with a Au-plated heat spreading layer exhibit a 50% lower thermal impedance compared to standard etched-pillar devices resulting in a significant increase of maximum output power. For an unmounted laser of 64 micrometer diameter we obtain an improvement in output power from 20 mW to 42 mW. The experimental results are compared with a simple analytical model showing the importance of heat sinking for maximizing the output power of vertical-cavity lasers.

An integrated tool has been developed to predict how vertical cavity surface emitting laser (VCSEL) light output power is affected by packaging components. The set that has been developed includes modeling tools for devices and packages. The integrated tool can be useful for choosing an optimum specific package design for a VCSEL structure and to predict its characteristics in different packages under a variety of operating conditions. In this paper the integrated tool has been applied to study how VCSEL light output power is affected by different assembly technologies and packaging materials.