Characterization of dielectric and electro-optic properties of PLZT 9/65/35 films on sapphire for electro-optic applications.

Lead lanthanum zirconate titanate (PLZT) thin films were deposited on r-plane sapphire at low temperatures by RF triode magnetron sputtering using lead compensated hot-pressed targets. To obtain fully perovskite phase in the films, two types of post-deposition processing were investigated: rapid thermal annealing (RTA) and furnace annealing (FA). Dielectric and electro-optic properties of PLZT films were found to be strongly dependent on annealing conditions. The peak dielectric constant of the films were 1200 and 2800 with Curie temperatures of 110 degrees C and 190 degrees C after RTA and FA processing, respectively. The dielectric losses in the films were fairly low; tan deltas were less than 0.02 after RTA and less than 0.04 after FA processing. The films showed good optical transmission characteristics after annealing and an anomalously large effective quadratic electro-optic effect was observed in one furnace annealed film.

Deposition and characterization of thin ferroelectric lead lanthanum zirconate titanate (PLZT) films on sapphire for spatial light modulators applications.

Ferroelectric lead lanthanum zirconate titanate (PLZT) films are deposited on R-plane sapphire using RF triode magnetron sputtering. Perovskite PLZT films with the desired composition (9/65/35) are obtained using compensated deposition techniques around 500 degrees C and postdeposition annealing at 650 degrees C. The deposited films exhibit good optical and electrooptical properties. The room temperature dielectric constant of the films was 1800 at 10 kHz. The refractive index of the films was in the range of 2.2-2.5. The films showed a quadratic electrooptic effect with R=0.6 x10(-16) m(2)/V(2). The development of PLZT on silicon-on-sapphire smart spatial light modulators using these films is also explored.

Partial response precoding for parallel-readout optical memories.

The data density that can be resolved in optical memories is limited by the inherent band-limited nature of optical systems. We show how a precoding technique used in serial communications, called partial response precoding, can be applied to parallel readout optical memories to precompensate for the spatial data broadening that occurs as result of this band limiting. We experimentally demonstrate a factor-of-15 improvement in average worst-case contrast ratio and an order-of-magnitude improvement in average contrast ratio. Over 50% more area was needed to achieve the same contrast ratio in a system without precoding.

Three-dimensional optical data storage in a fluorescent dye-doped photopolymer.

We propose a new, to our knowledge, monolithic multilayer optical storage medium in which data may be stored through the diffusional redistribution of fluorescent molecules within a polymer host. The active portion of the medium consists of a photopolymer doped with a fluorescent dye that is polymerized at the focal point of a high-numerical-aperture lens. We believe that as fluorescent molecules bond to the polymer matrix they become more highly concentrated in the polymerized regions, resulting in the modulated data pattern. Since data readout is based on detection of fluorescence rather than index modulation as in other photopolymer-based memories, the problems of media shrinkage and optical scatter are of less concern. An intensity threshold observed in the recording response of this material due to the presence of inhibitor molecules in the photopolymer allows for the three-dimensional confinement of recorded bits and therefore multilayer recording. The nonlinear recording characteristics of this material were investigated through a simple model of photopolymerization and diffusion and verified experimentally. Both single-layer and multilayer recordings were demonstrated.

Three-dimensional holographic stamping of multilayer bit-oriented nonlinear optical media.

The requirements and limitations on the use of a volume holographic element for the simultaneous optical stamping of multilayer data into a three-dimensional (3D) bit-oriented material that exhibits a suitable sensitivity threshold are investigated. The expected performance of such a holographic stamping element is examined through a model of the coherent noise effects that result from the interference of the many data layers with one another. We show that higher signal-to-noise values may be achieved through the use of semicoherent light during the readout of the hologram. The main limitations to this technique arise from the bandwidth requirements on the holographic element, the degree of nonlinearity required of the bit-oriented media, and the tolerance requirements for the optical exposure levels. As a demonstration of the concept, a two-layer stamping element is fabricated and used to simultaneously stamp two layers of data into a 3D dye-doped photopolymer storage medium.

Optimization and theoretical modeling of polymer microlens arrays fabricated with the hydrophobic effect.

High-performance polymer microlens arrays were fabricated by means of withdrawing substrates of patterned wettability from a monomer solution. The f-number (f(#)) of formed microlenses was controlled by adjustment of monomer viscosity and surface tension, substrate dipping angle and withdrawal speed, the array fill factor, and the number of dip coats used. An optimum withdrawal speed was identified at which f(#) was minimized and array uniformity was maximized. At this optimum, arrays of f/3.48 microlenses were fabricated with one dip coat with uniformity of better than Deltaf/f +/- 3.8%. Multiple dip coats allowed for production of f/1.38 lens arrays and uniformity of better than Deltaf/f +/-5.9%. Average f(#)s were reproducible to within 3.5%. A model was developed to describe the fluid-transfer process by which monomer solution assembles on the hydrophilic domains. The model agrees well with experimental trends.

Parallel fuzzy inference with an optoelectronic H-tree architecture.

Fuzzy inference is a method of reasoning with imprecise information. The mathematical operations of fuzzy inference can be stated in terms of generalized vector algebra, in which multiplication and summation are generalized to min and max operations. An optoelectronic H-tree architecture is ideally suited to perform these generalized vector operations in parallel and requires only a simple imaging optical interconnection. Appropriate data encodings and electronic circuitry permit large scale, pipelined systems.

Motionless-head parallel-readout optical-disk system: experimental results.

The motionless-head parallel-readout optical-disk system is designed to read out two-dimensional bit planes that are stored as one-dimensional Fourier-transform computer-generated holograms distributed radially on the disk active surface. Such a system, when built at full scale, could offer several potential advantages: high data-transfer rates of 1 Gbyte/s and higher, low access times of less than 15 ms, low retrieval times (the time required to read the entire memory) of less than 25 ms, and simple optical implementation. The experimental results of a scaled-down implementation of the system are presented.

Speed and energy analysis of digital interconnections: comparison of on-chip, off-chip, and free-space technologies.

We model and compare on-chip (up to wafer scale) and off-chip(multichip module) high-speed electrical interconnections withfree-space optical interconnections in terms of speed performance andenergy requirements for digital transmission in large-scalesystems. For all technologies the interconnections are firstmodeled and optimized for minimum delay as functions of theinterconnection length for both one-to-one and fan-outconnections. Then energy requirements are derived as functions ofthe interconnection length. Free-space optical interconnectionsthat use multiple-quantum-well modulators or vertical-cavitysurface-emitting lasers as transmitters are shown to offer aspeed-energy product advantage as high as 30 over that of the electrical interconnection technologies.

Biorthogonally accessed three-dimensional two-photon memory for relational database operations.

Memory bandwidth is a bottleneck for very large database machines. Parallel-access three-dimensional two-photon memories have the potential of achieving enormous throughput (>100 Gbit/s) and capacity (1 Tbit/cm(3)) [Appl. Opt. 29, 2058 (1990)] and, consequently, are well suited for this application. Our analysis shows that some operations can be completed more than 2 orders of magnitude faster with this type of memory than with a system based on serial-access storage. These particular memories have a further feature of being accessible in orthogonal directions. We show that this property, used in conjunction with a three-dimensional data-organization scheme designed for this approach, leads to improved performance by permitting the user a choice of accessing strategies for a given operation.

Characterization of a polymer microlens fabricated by use of the hydrophobic effect.

We report a means of fabricating hydrophilic domains in a hydrophobic background by lithographically patterning an adhesive hydrophobic layer. Polymer microlenses were fabricated on these substrates by use of a dip-coating technique. Various lens shapes (circular, elliptical, square) were fabricated on a variety of substrates (SiO(2), SiN, GaAs, InP, etc.), ranging in size from 2 to 500 microm in diameter, with fill factors of up to 90%. Plano-convex and double-convex lenses were fabricated, with f-numbers as low as 1.38 and 1.2, respectively. Optimum lens surfaces deviated from spherical by just +/-5 nm . The lenses are stable at room temperature and exhibit minimal degradation after 24 h at 105 degrees C. The transfer of these polymer lenses to an underlying substrate was also demonstrated.

Dual-scale topology optoelectronic processor.

The dual-scale topology optoelectronic processor (D-STOP) is a parallel optoelectronic architecture for matrix algebraic processing. The architecture can be used for matrix-vector multiplication and two types of vector outer product. The computations are performed electronically, which allows multiplication and summation concepts in linear algebra to be generalized to various nonlinear or symbolic operations. This generalization permits the application of D-STOP to many computational problems. The architecture uses a minimum number of optical transmitters, which thereby reduces fabrication requirements while maintaining area-efficient electronics. The necessary optical interconnections are space invariant, minimizing space-bandwidth requirements.

Parallel algorithms based on expander graphs for optical computing.

We consider the task of interconnecting processors to realize efficient parallel algorithms. We propose interconnecting processors using certain graphs called expander graphs, which can provide fast communication from any group of processors to the rest of the network. We show that these interconnections would result in a number of efficient parallel algorithms for sorting, routing, associative memory, and fault-tolerance networks. As the interconnections based on expander graphs are global and irregular, we reason that optical interconnections are preferred to electronic and propose implementation of these interconnections using the programmable optoelectronic multiprocessor architecture.

Comparison between optical and electrical interconnects based on power and speed considerations.

Conditions are determined for which optical interconnects can transmit information at a higher data rate and consume lc3s power than the equivalent electrical interconnections. The analysis is performed for free-space optical intrachip communication links. Effects of scaling circuit dimensions, presence of signal fan-out, and the use of light modulators as optical signal transmitters are also discussed.

Grain-size considerations for optoelectronic multistage interconnection networks.

This paper investigates, at the system level, the performance-cost trade-off between optical and electronic interconnects in an optoelectronic interconnection network. The specific system considered is a packet-switched, free-space optoelectronic shuffle-exchange multistage interconnection network (MIN). System bandwidth is used as the performance measure, while system area, system power, and system volume constitute the cost measures. A detailed design and analysis of a two-dimensional (2-D) optoelectronic shuffle-exchange routing network with variable grain size K is presented. The architecture permits the conventional 2 x 2 switches or grains to be generalized to larger K x K grain sizes by replacing optical interconnects with electronic wires without affecting the functionality of the system. Thus the system consists of log(k) N optoelectronic stages interconnected with free-space K-shuffles. When K = N, the MIN consists of a single electronic stage with optical input-output. The system design use an effi ient 2-D VLSI layout and a single diffractive optical element between stages to provide the 2-D K-shuffle interconnection. Results indicate that there is an optimum range of grain sizes that provides the best performance per cost. For the specific VLSI/GaAs multiple quantum well technology and system architecture considered, grain sizes larger than 256 x 256 result in a reduced performance, while grain sizes smaller than 16 x 16 have a high cost. For a network with 4096 channels, the useful range of grain sizes corresponds to approximately 250-400 electronic transistors per optical input-output channel. The effect of varying certain technology parameters such as the number of hologram phase levels, the modulator driving voltage, the minimum detectable power, and VLSI minimum feature size on the optimum grain-size system is studied. For instance, results show that using four phase levels for the interconnection hologram is a good compromise for the cost functions mentioned above. As VLSI minimum feature sizes decrease, the optimum grain size increases, whereas, if optical interconnect performance in terms of the detector power or modulator driving voltage requirements improves, the optimum grain size may be reduced. Finally, several architectural modifications to the system, such as K x K contention-free switches and sorting networks, are investigated and optimized for grain size. Results indicate that system bandwidth can be increased, but at the price of reduced performance/cost. The optoelectronic MIN architectures considered thus provide a broad range of performance/cost alternatives and offer a superior performance over purely electronic MIN's.

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.

Photonic content-addressable memory system that uses a parallel-readout optical disk.

We describe a high-performance associative-memory system that can be implemented by means of an optical disk modified for parallel readout and a custom-designed silicon integrated circuit with parallel optical input. The system can achieve associative recall on 128 × 128 bit images and also on variable-size subimages. The system's behavior and performance are evaluated on the basis of experimental results on a motionless-head parallel-readout optical-disk system, logic simulations of the very-large-scale integrated chip, and a software emulation of the overall system.

Small-signal-equivalent circuits for a semiconductor laser.

Passive electrical circuits whose voltage and current equations are exactly equivalent to the small-signal rate equations of a semiconductor laser are derived to model an electrically modulated laser (verified to be the same as that given in the literature), an optically modulated laser (i.e., a laser used as an optical amplifier), and a multimode laser. These circuits offer a fast and efficient simulation tool with little computational complexity in which the small-signal assumption (i.e., small modulation range) is neither violated nor insufficient for the simulation.

Photorefractive Beam splitter for Free-Space Optical Interconnections.

A photorefractive beam splitter (PRBS) is introduced as an alternative to a polarizing beam splitter (PBS) for coupling optical power into reflective modulators in a free-space optical interconnection system. The PRBS uses a single diffraction grating recorded in a photorefractive material to redirect the incident laser light into the first diffraction order and onto the modulators. Reflected interconnection light not matching the Bragg angle criteria transmits uncoupled through the beam splitter. Experimental results show that the PRBS provides better, more uniform transmission for off-axis beams than the currently used PBS.