There is a growing demand to increase the throughput of high speed processors and computers. To meet this demand, denser and higher speed integrated circuits and new computing architectures are being developed. IElectrical interconnects and switching have been identified as bottlenecks to the throughput of computing systems. Two trends brought on by the need for faster computing systems have pushed the requirements on various levels of interconnects to the edge of what is possible with conventional interconnects. The first trend is the development of higher speed and denser switching devices in silicon and gallium arsenide. Switching speeds of logic devices are now exceeding 1 Gb/s, and high density integration has resulted in the need for interconnect technologies to handle hundreds of output pins. The second trend is the development of new architectures for increasing the parallelism and, hence, the throughput of a computing system.

We compare optical interconnect techniques with conventional electrical interconnects as applied to high speed electronic computing systems. Theoretical and semiempirical data are used to identify the advantages of optical interconnects at various levels. Besides the inherent advantages of optics such as reduced electromagnetic and radio frequency interference and crosstalk, optical interconnects are capable of providing larger fanouts at higher bandwidths, with possibly lower system power and complexity. In addition, optics has an advantage over conventional interconnects via the density potential of free-space interconnects. We also present issues related to the implementation of optical interconnects within conventional hardware configurations and describe a packaging technique to overcome several of these issues.

The use of optics as an alternative method for achieving very high speed (10 Gb/s > bit rate > 500 Mb/s) electrical interconnects is the subject of this paper. Optical interconnect media considered include plastic channel waveguides, glass waveguides, fibers, and free-space interconnects. Typical interconnection distances considered are inches or less. The problems of cou-pling and interconnecting and their overall effect on system power budgets are also discussed. As a means of quantifying the results, link budgets for a 565 Mb/s, a 2.3 Gb/s, and a 4.6 Gb/s interconnect scenario are made. Multipoint as well as single-point-to-single-point situations are considered.

Timing constraints for state-of-the-art very large scale integrated circuits (VLSI) in silicon are rapidly approaching communication limits available with layered two-dimensional metal and polysilicon wiring approaches. For such communication-limited systems, reliable clock distribution is a key concern. The range of finite differences in signal delays over clock wires of various lengths for large chips creates a timing skew that is significant when compared to the switching time of transistors in the circuit. The high bandwidth and three-dimensionality of imaging optical systems suggest that optical clock distribution systems have the potential to overcome the timing barriers presented by planar wiring. Clock signals can be holographically mapped to detector sites within small functional cells on a chip surface. Within each functional cell, the clock is distributed with negligible delays via surface wires, reducing skew effects to the variation in reaction times of the photodetectors on the chip. This paper includes the presentation of an optical clock distribution system assuming holographic mapping of beams from an off-chip optical source. Computer simulations of the electronic response of optical clock detection circuits in standard 4 µm CMOS technology have been performed.

This paper introduces new applications and design trade-offs anticipated for free-space optical interconnections of VLSI chips. New implementations of VLSI functions are described that use the capability of making optical inputs at any point on a chip and take advantage of greater flexibility in on-chip signal routing. These include n-port addressable memories, CPU clock phase distribution, hardware multipliers, and dynamic memory refresh, as well as enhanced testability. Fault tolerance and production yields may be improved by reprogramming the optical imaging system to circumvent defective elements. These attributes, as well as those related to performance alone, will affect the design methodology of future VLSI ICs. This paper focuses on identifying the design issues, their possible solutions, and their impact on VLSI design tech-niques and, finally, presents some preliminary measurements on various sys-tem components.

Advances in computational speed and system complexity are pro-ceeding at a rapid pace. As systems become more complex and demands for computation speed increase, parallel processing becomes the solution to meet-ing the requirements for future systems. A significant bottleneck in these and other systems is interconnect and communication problems between chips and subsystems. In addition, communication systems designers need rugged and reliable high speed transmitters and receivers. This paper discusses the design, development, and performance of GaAs optoelectronic integrated circuits for application in optical interconnection control and signal processing. We review the requirements and approaches for realization of practical optical intercon-nects and report on work in progress at Rockwell. Performance and design criteria of optical transmitters and receivers as well as data obtained on 1:8 demultiplexed receivers are given.

Concepts for optical interconnects between electronic circuits and systems based on multichip integration, a new form of hybrid integration, are presented. In this integration scheme, chips of different substrate types, such as Si and GaAs, are embedded in a potting material and connected with photolithographically defined metallizations to form a multichip. Within a multichip, the interconnection scheme would retain the high packing density and the minimal number of packaging operations achievable with monolithic integration and still permit the individual optimization of material types and device designs to perform electronic and optical functions. Also, the elimination of large bonding pads on the component chips within a multichip will alleviate the pinout problem of complex integrated circuits at the chip level by allowing the pitch between input/output lines to be decreased. Multiplex-ing many slow lines with fast GaAs multiplexers can alleviate the same problem at the multichip level. Off-chip connections between multichips can be made either electrically or optically. Optical interconnections would have low crosstalk. Preliminary results with an epoxy potting material show that continuous lines can be photolithographically patterned from the chip to the epoxy. Tests with a discrete breadboard demonstrate that fast edge transitions can be retained with a diode laser driven directly by a commercial GaAs digital circuit at rates of 1.4 GHz.

Conventional interconnect is rapidly becoming a critical issue in the realization of VLSI systems. Large silicon systems are especially limited by the bandwidth of available interconnect and the number of pinouts presently available. Optical I/O technology promises to provide relief from the pinout problem and to enhance performance significantly. Power requirements, interconnect flux, and noise immunity of metallized interconnects and electro-optic links are compared. Several alternatives for optical clock distribution and optical fiber interfacing are discussed. An architecture for an ultralarge electro-optic switch using optical interconnect is presented. The low noise characteristics of optical interconnect allow the use of novel synchronization circuitry that is both fast and compact.

A nonblocked zero-frequency filtering technique is used to improve the in-line holographic recording of small objects at extended distances from the hologram. Since the filter is coated on the relay lens, the irradiance distributions in three planes (i.e., in projected images, diffraction patterns, and reconstructed images) are calculated. Influences of filter diameter, filter transmittance, and exposure level of holograms are considered in discussing fringe visibility, image forms, and image contrast.

A 100 X100 pixel charge-coupled-device (CCD) area detector combined with an image intensifier, all-sky optics, and a filter wheel has been developed to provide quantitative monochromatic images of the aurora. The design of the instrument provides for a variable exposure rate to accommodate the large dynamic range of auroral intensities, from a minimum of 20 ms up to several minutes. Standard auroral emission features are monitored by four selectable filters, normally at 6300, 5577, 3914, and 4278 A. Depending on the filter utilized, sensitivity can be from 180 to 250 R ADU-1 s-1. The instrument has been used successfully in support of balloon, rocket, and satellite campaigns.

A design is presented for an imaging spectropolarimeter/photometer based on the Landsat multispectral scanner (MSS) for the wavelength range 0.5 to 1 .1 µm. The proposed scanner has essentially the same wavelength sensitivity, field of view, and scanning parameters of the original MSS except for the duplication of sensing and data systems for the determination of the amount of plane polarization. A unique nonpolarizing (i.e., polarization-com-pensated) scanning system is described, as well as a nonpolarizing beam splitter.

Literature in the field of optomechanical design is reviewed, including selected papers and texts that should provide the optical engineer with information needed in various phases of design. These references are accompanied by short annotations. Beyond citation of general texts in optics, the areas covered include materials, tolerancing and specification, mounting, mechanical and thermal analysis, positioning, stabilization, baffling, assembly and alignment, and scanning.

Laser damage to optical surfaces, particularly coated surfaces, is typically initiated by material defects that couple strongly to the laser radiation. Knowledge of damage-related defect characteristics is therefore essential to optical material development efforts and to quality control. Such characteristics include individual defect failure levels, defect densities, and content of optically absorbing impurities. Defect failure distributions containing defect density information can be inferred from properly designed laser damage measurements. The data also permit distinctions among different damage mechanisms, including non-defectrelated mechanisms involving the substrate. Inclusions containing volatile, optically absorbing impurities can be detected by mass analysis of laser-desorbed vapors. These methods are described, and examples of results are presented.

Alkali halide crystals can be used to provide a quick and simple method for obtaining a three-dimensional ultraviolet laser beam profile. The crystals have a large dynamic range, and the same crystal may be used an indefinite number of times.

Using arrays of optical bistable devices addressed and read out by arrays of holograms, we can make an optical symbolic computer that can learn languages, play games, invent stories, etc.

One of the most difficult tasks I have encountered as Editor of Optical Engineering has been to write an editorial for each issue. A few of these editorials were not particularly difficult to compose, but the remainder required a considerable amount of thought and time and underwent many revisions before I felt they were acceptable. However, I have not always been totally pleased with the end result nor confident that it would convey the message that I intended to convey.

Amorphous Silicon Solar Cells by K. Takahashi and M. Konagai is one of the first books dealing exclusively with the subject of amorphous silicon solar cells. The book was first published in Japanese in 1983 and was translated and published in English in 1986. Part 1, covering 94 pages, is a general introduction to solar energy, including the basic concepts, the prospects for cost reduction of the various competing photovoltaic technologies, and a discussion of several types of solar power systems, while Part 2 deals exclusively with the technical issues surrounding the application of amorphous silicon to solar cells. Throughout, reference is made to the impact of photovoltaics on the Japanese economy, both as a business activity and as a domestic supply of electrical energy. As the authors point out, photovoltaics is a national priority for the Japanese, with increasing business as well as government support. Although this was also once the case in the U.S., as memories of the recent energy crises fade, the bulk of this activity is shifting toward the Far East.