Timothy J. Drabik

Comparison between electrical and free space optical interconnects for fine grain processor arrays based on interconnect density capabilities

Optically interconnected processor arrays are compared to conventional fully electronic processor arrays in terms of interconnect density capabilities. A complexity model is introduced that allows the calculation of the array area growth rate as an asymptotic function of the number of processing elements in the array Lower bounds on the area growth rate of electrically interconnected processor arrays are compared to upper bounds for free-space optically interconnected circuits that employ computer generated holograms. Results indicate that for connection networks such as the hypercube, perfect shuffle and crossbar networks, that have a high minimum bisection width (a measure of the global nature of an interconnect topology) and contain some degree of spatial invariance, optically interconnected circuit area growth rates are below lower bounds on VLSI circuit growth rates.

Ferroelectric-liquid-crystal/silicon-integrated-circuit spatial light modulator

We present the design and characterization of a spatial light modulator (SLM) comprising a ferroelectric-liquid-crystal light-modulating layer on top of a silicon integrated circuit. Our SLM consists of two electrically addressed arrays on the same integrated-circuit die. The first, a 1 x 128 linear array with a 20-microm center-to-center element spacing, used shift register addressing, while the second, a 64 x 64 square array with 60-microm pitch, used static random access memory addressing. The resulting SLM could be addressed at frame rates of up to 4.5 kHz and gave singleelement intensity contrast ratios of 12:1.

Optimized binary, phase-only, diffractive optical element with subwavelength features for 1.55 μm

We report the use of rigorous coupled-wave diffraction analysis in conjunction with the simulated annealing optimization method for performing optimal design of binary-level surface-relief diffractive elements with subwavelength, submicrometer features that exhibit quasi-linear-phase transmittance. An element designed for operation at 1.55 μm has been fabricated and is characterized.

Silicon VLSI/ferroelectric liquid crystal technology for micropower optoelectronic computing devices

Optoelectronic computing devices with high circuit complexity and a favorable speed-power product can be realized by fabricating a liquid crystal light modulating layer atop a conventional silicon die.

A new bonding technique for microwave devices

Over the past five years, a great deal of work has been done to perform semiconductor die attach with AuSn alloys. Successful die attach has recently been achieved using Au and Sn multilayers evaporated onto the die or the host substrate. However, bonding techniques with thin (below 5 /spl mu/m) AuSn layers for very thin semiconductor devices have not yet been reported. The increasing demand for more advanced optoelectronic integrated circuits has created the need to bond materials having different lattice constants (e.g., GaAs on Si). In this paper we report a new way for the bonding of epitaxial liftoff (ELO) devices onto host substrates. Three of the multilayer structures investigated in this work produce a AuSn alloy bond with approximately 84 wt.% gold, but can be bonded with a peak temperature below 280/spl deg/C. The bonded samples were investigated with several standard surface analysis techniques: Optical microscopy, scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDX). We conclude that much thinner bonding layers can be attained than thus far reported. The results of our research allow us to optimize the layer structure and bonding parameters.

Phase stability of ferroelectric liquid crystals upon repeated switching and static temperature characteristics

Surface-stabilized ferroelectric liquid crystals (FLCs) are promising materials for semiconductor integrated-circuit-based spatial light modulators. For coherent optical processing applications, phase stability upon repeated switching is critically important. The phase characteristics of an FLC device were measured at switching rates of up to 1 kHz and found to be very stable. The change in the total optical path length through the cell was found to be < 0.0025λ at a wavelength of 632.8 nm. The static optical characteristics were measured for a range of temperatures at and above room temperature in order to be able to identify any temperature-induced phase changes upon switching. The temperature of the FLC cell was externally varied, and changes in the birefringent optical path difference, the optical path length, and the optic axis tilt angle were measured. However, because of the observed phase stability of the FLC, the change of temperature caused by switching was determined to be < 0.046°C. It is clearly shown that FLCs can exhibit the stability needed for critical coherent and incoherent optical data-processing applications.

2D silicon/ferroelectric liquid crystal spatial light modulators

We have developed a spatial light modulator technology based on foundry silicon fabrication processes. This technology employs a thin, ferroelectric liquid crystal light-modulating layer at the substrates surface, producing electrically addressed display devices with resolutions up to 256/spl times/256 and frame rates up to 10 kHz. We have also fabricated optically addressed smart-pixel arrays for low-level image processing. Performance has advanced rapidly due to design innovations and effective use of modern process features. >

One-transistor DRAM ferroelectric-liquid-crystal spatial light modulator

We have made 128 X 128 and 256 X 256 spatial light modulators using active backplanes fabricated through a commodity silicon foundry and incorporating a thin ferroelectric liquid crystal light modulating layer at the backplanes surface by means of postprocessing of individual foundry die. These electrically addressed devices exhibit optical rise and fall times as short as 105 microsecond(s) , with contrast ratios in images as high as 100:1, and in zero-order diffracted light as high as 200:1. Total optical throughput to the zero-order diffracted beam exceeds 10% for the 256 X 256 devices and 17% for the 128 X 128 devices. Frame update times shorter than 100 microsecond(s) , corresponding to image information throughput of greater than 80 MBytes/s, were realized by employing pipelining techniques in conjunction with a wide digital input word.

Analog VLSI implementation of the Help If Needed Stereopsis Algorithm

This work introduces a novel clocked analog VLSI hardware system with an optical input that performs stereopsis. An algorithm called the Help If Needed Algorithm, developed previously, is readily mapped onto an analog VLSI platform. The system fits into the cellular neural network (CNN) paradigm. The circuit components that make up the cells of the CNN are designed with the constraint that they must function effectively and fit into the space available. In order to clarify the processing pathway, the system is described at the component and system levels. Each cell has an optical input, while the output is electrical. By utilizing an optical input, an analog VLSI silicon retina first stage can be connected to the stereopsis processor completely in parallel, creating a multi-stage artificial visual system. The physical system is composed of 2.0 /spl mu/m Tinychips fabricated through MOSIS. Experimental data are presented that verify that the system performs as desired and successfully implements the Help If Needed Stereopsis Algorithm. The novel stereopsis processor is ideally suited for autonomous robots, or any application that requires a low power visual processing system.

Coplanar refractive–diffractive doublets for optoelectronic integrated systems

A coplanar refractive-diffractive doublet array employing surface-relief diffractive phase elements embedded within poly(methyl methacrylate) microlenses is introduced as an optomechanical building block for optoelectronic integrated systems. The design method, fabrication technology, and results are described. Coplanarity of the quadratic- and linear-phase elements constituting the doublet can reduce optomechanical complexity in applications to unguided optical interconnects.