Experimental results are presented on LiNb0₃:Ti integrated optic device response to various nuclear irradiations. The results are compared to earlier studies performed on GaAlAs and LiNb0₃ and, the importance of various physical effects parameters such as photorefraction and photobleaching are presented for future consideration of studying device responses under varying temperature, optical power and irradiation levels. Data demonstrating the ability of LiNb0₃:Ti to withstand a gamma and X-ray accumulated dose of 204 Krad (Si) is reported and comparison of the data to a geminate ion recombination kinetic model is presented.

We discuss gallium arsenide integrated optical devices for high speed diagnostic systems. Specifically, we focus on the effect of radiation exposure on the performance characteristics of these devices.

We are developing and investigating homogeneous, photoconductive semiconductors as very fast radiation detectors. We irradiated GaAs, Cr-doped GaAs, and Fe-doped InP crystals with 14-MeV neutrons to produce lattice defects that act as fast-recombination centers for electrons and holes. Using short-pulse lasers and 17-MeV linear-accelerator electrons and bremsstrahlung x rays and r rays, we have measured the temporal response and sensitivity of these photoconductors as functions of fluence ranging from 10¹² to 10¹⁶ neutrons/cm². The carrier lifetime and mobility decrease monotonically as the neutron fluence increases, resulting in faster detector response at the expense of sensitivity. A resolving time of less than 30 ps (FWHM) was measured for the above photoconductors irradiated with ~10¹⁵ neutrons/cm².

We report the results of an experiment in which we used an ultrafast, photoconductive, radiation detector to drive a fast laser-diode transmitter. When we irradiated the neutron-damaged Cr-doped GaAs detector with 17-MeV electron beams, the temporal response of was measured to be less than 30 ps. The pulses from this detector modulated a fast GaA1As laser diode to transmit the laser output through 30- and 1100-m optical fibers. Preliminary results indicate that 50- and 80-ps time resolutions, respectively, are obtainable with these fibers. We are now working to integrate the photoconductive detector and the laser diode transmitter into a single chip.

The aim is to monitor the longitudinal distribution of electron bunches in LEP (the Large European Electron Positron collider) via the intense X-ray synchrotron radiation they emit. Very fast photoconductors inserted in an electronic circuit produce a time autocorrelation of the impinging X-ray pulses. The principle of the detector and its present status is given in this report.

Using x-ray absorption techniques, the Ti center in congruent LiNbO3 is shown to be concentration-dependent. The change in the Ti center takes place within the concentration range used for integrated optics and involves a displacement in the xy plane of the LiNbO₃ crystal lattice perpendicular to the polar axis. Reports in the literature indicate that as the ordinary optical index increases with increasing Ti concentration, an abrupt decrease in slope occurs at approximately the same concentration at which our data shows the Ti center itself changes. Simple theory based on a linear-combination-of-atomic-orbitals (LCAO) model suggests that the change in slope of the ordinary index vs Ti concentration can be accounted for by the observed xy displacement of the Ti.

Relationships of the waveguide properties of the reactively rf-sputtered tantalum oxide films with a variety of sputtering conditions have been studied. Sputtering gas pressure, gas composition, rf power, and substrate temperature have been varied for studying the effects on the refractive index, deposition rate, structure, and atomic composition of the deposited tantalum oxide films. For a certain total pressure and gas composition of the reactive atmosphere, the deposition rate and refractive index will reach a peak value; while, for lower or higher total pressure and/or composition, the deposition rate and refractive index will decrease simultaneously. Increasing the substrate temperature will lead to films with higher refractive index. Increasing the sputtering rf power will increase the deposition rate, but the refractive index of the deposited films will be decreased. Strong correlations between the atomic composition, refractive index, and the optical loss have been observed. The larger the atomic ratio of oxygen to tantalum, the higher the refractive index, and the lower the optical loss of the fabricated samples.

Rare earth ions are frequently incorporated into lasers by doping common glasses with the ions in the glass melt. This paper describes the potential of using diffusion of the rare earth ion from molten salt baths to incorporate it in the glass. The paper discusses the molten salts, the rare earths as a group, the diffusion phenomena, the glasses, and finally the interaction of all these to produce the process. General predictions of the waveguide profile and potential problems are presented.

Currently, the Hamamatsu and Sandia Corporations use lithium niobate (LiNbO₃) in their crystal streak cameras. However, these systems have low resolution due to the low electrooptic coefficient of LiNbO₃, 30 x 10^-12, which requires excessive voltage drives for the system. For a practical system, new tungsten bronze crystals such as barium niobate [Sr_1-xBa_xNb₂O₆, or SBN:1-x(%) for short] must be used to achieve higher resolution and lower voltage drive requirements. Electrooptic coefficients of 400 x 10^-12 m/V for SBN:60 and 1400 x 10-12 m/V for SBN:75 give improvements in resolution of 13 and 47, respectively. In this paper, we discuss the highlights of a light deflector developed at Lawrence Livermore National Laboratory (LLNL) for a streak camera that uses tungsten bronze crystals grown at Rockwell International Science Center. We address the design and fabrication of these crystals and include experimental results from our deflector experiment, the purpose of which is to determine the feasibility and expected performance of the crystal streak camera using tungsten bronze crystals.

Photocurrent and dark current precise time-dependent measurements have been conducted on semi-insulating GaAs as functions of pulsed voltage at room and low temperature, employing pulsed and continuous illumination. These experiments have been performed also as functions of fast voltage interruptions and slow voltage interruptions and slow voltage relaxations, and their respective position in time (epoch) as well as the time of imposition of a field pulse. Further work was conducted on the response of photocurrent to follow rapidly oscillating cw voltage superimposed onto a field pulse. The objective of these studies is to assist in characterizing trapping levels in semi-insulating GaAs, including EL2, and to determine time-constants for current decay. From these experiments we deduced a trap saturation time of 300 ns and a trap liberation time of 8.5 - 10.0 ns.

The Einstein relation for the diffusivity-mobility ratio of the carriers in semi-conductors (hereinafter referred to as DMR) is a very important one, since the simplest method of analyzing semiconductor devices taking into account the degeneracy of the bands is to use the DMR to express the performance of them at the device terminals and the switching speed in terms of carrier concentration. In recent years, the connection of the DM with the velocity auto-correlation function, its relation with the screening of the carriers in semiconductors and the vario modifications of the DMR in degenerate semiconductors having different types of band structures under varying physical conditions have extensively been investigated in the literature.

Hybrid systems which merge silicon (Si) and gallium arsenide (GaAs) devices will produce a technology which combines high performance with ease of fabrication. Such heteroepitaxial optoelectronic circuits can help alleviate the interchip communication bottleneck created by present day VLSI technology and provide substantial immunity to electromagnetic interference. This paper describes the selective area growth of GaAs on Si for optical interconnects. The design issues and conflicts inherent in the fabrication of optical interconnects are discussed. Data relating to the monolithic integration of GaAs light emitting diodes (LED's) and Si field effect transistor (FET) drivers is presented.

The staircase impact electroluminescent devices (SIED) and the staircase photon amplifier converter (SPAC) are propos6d. The devices are a multilayered heterojunction structures in which the acceleration and collision excitation processes are spatially separated and permits independent optimization of each function in different materials. In collision excitation region, ballistic electrons excite the rare earth ions by direct impact. By changing conduction band step AEc we can tune up the energy of excited electrons to the resonance condition of a particular excited state of RE³⁺ ions. The emission from the rare earth ions with transition wavelength shorter than semiconductor's band edge emission can be generated. The emission will contain narrow lines with gradual change in wavelength with temperature. The advantages of the new devices and excitation mechanism are discussed in detail. Both proposed devices STED and SPAC can be built using the possible combinations containing Si,III-V and II-VT compounds doped with rare earth. Laser action should also be obtainable by properly choosing device geometry. The laser oscilation at the rare earth transition assure the same wavelength from devices to devices with weak temperature dependence.

Ordered structures of nonrandom diluted-alloy semiconductors are examined. These ordered-alloys may be created by selectively depositing atomic monolayers along specific crystal directions. We show that ordered structures of dilute-magnetic semiconductors have enhanced magnetic properties. Ordered structures also lack the disorder-induced potential fluctuations associated with random alloying, and in addition might provide totally new band structures, via zone-folding.

Atomically clean and flat surfaces of poly- and single-crstalline metallic Ni and half-metallic NiMnSb and PtMnSb are studied by electron capture spectroscopy (ECS) and by magneto-optical Kerr effect (MOKE). Long-ranged ferromagnetic order is found at all surfaces.The measured ECS-data for the electron spin polarization are consistent with predictions from recent spin-polarized band structure calculations. MOKE data ob-tained at surfaces of the Heusler alloys PtMnSb and NiMnSb contrast strikingly to data measured at Ni surfaces. At surfaces of single-crystalline PtMnSb, the Kerr effect (Kerr rotation and ellipticity) is approximately 20 times stronger compared to single-crystalline Ni(100) surfaces. This dramatic effect is attributed to the peculiar band structure of PtMnSb.

Quantum well systems lattice matched to GaAs and InP have emerged as important materials for use in long wavelength optoelectronic devices. Intrinsic problems associated with the growth of these quantum wells by molecular beam epitaxy are discussed and the luminescence properties of state-of-art quantum wells, using novel growth techniques are described. Finally, the properties of detectors, modulators and integrated devices made with these heterojunction materials are described and discussed.

The application of the InGaAlAs/InP material system to low loss waveguides and monolithically integrated waveguide photodiodes for opto-electronic integrated circuits is described. Low loss waveguiding was obtained on ridge waveguides at 1.3 and 1.55 μm and then applied on the monolithic integration of a ridge waveguide with an InGaAs pin photodiode for detection at 1.55 μm wavelength. The structure was entirely grown by molecular beam epitaxy on a InP substrate. Absorption of the guided light is provided by leaky coupling from the InGaAlAs guiding layer into the higher index InGaAs absorbing layer. External quantum efficiencies as high as 20% have been obtained. This is the first demonstration of a monolithic integrated waveguide device in the InGaAlAs/InP material system.

Charge-coupled device (CCD) imagers have been fabricated on gallium arsenide (GaAs) with very closely spaced (<100nm) Schottky-barrier metal electrodes. The short interelectrode spacing was achieved by using anodic oxidation in an ethylene glycol based electrolyte. All the active device regions of the CCD imagers were formed by silicon implantation into semi-insulating GaAs substrates followed by rapid thermal activation. The photodetectors were Schottky barrier diodes formed with thin aluminum metal anodes over silicon-implanted active regions in the semi-insulating substrate. The detectors formed a linear array along one side of the CCD channel and a three-phase transfer electrode structure was used. The imagers have been tested with front side illumination at clocking speeds up to 25 MHz.

The heterostructure phototransistor (HPT) has applications as an optical detector/amplifier in receivers for fibre optic communications systems and analysis has shown that its potential performance may be equal to, or even superior to, that of the PIN/FET combination. It is well known that the provision of an external bias, either optically or electrically via a third (base) terminal, can enhance the performance of the HPT in terms of gain, speed of response and signal to noise ratio. This is principally due to an increase in emitter efficiency and a reduction in the RC time constant of the emitter base junction. In this paper we show that crowding of the bias current and the signal current, due to base resistance, can be a serious problem and will limit the advantage given by the external bias in 3-terminal devices but, on the other hand, will improve the performance of 2-terminal devices. To illustrate this point, the current-crowding has been modelled for 2-terminal and 3-terminal phototransistors with cylindrical geometry (this shape being optimal for low leakage operation). The model is used to relate the gain-bandwidth product of an HPT to the electrical bias current and to the optical signal level.

Generation of short optical pulses at gigabit rate from laser diodes has been demonstrated by several techniques. These includes gain switching, mode locking, optoelectronic feedback, and mutual phase-locked loop. In this paper we present an alternative technique to obtain self-sustained operation of semiconductor laser diodes generating picosecond optical pulses. The short pulse train is obtained forcing the laser into the relaxation oscillation. As the laser is turned on fast enough, it emits short optical pulses of decaying amplitudes. A replica of oscillation will also be present on the laser current and the electrical circuit. By implementing a wideband electronic gain media in the laser circuit, the current transient can be amplified and returned to the laser. The amplified feedbacked signal is phased so that it enhances the laser gain which, after a few cycle of "growing transient", produces steady-state repetitive optical pulses. The system is then, effectively, an optoelectronic oscillator reaching steady state operation after an initial imposed transients. Experimental results on the performance of Gb/s picosecond optical pulse generator for high speed data communication system will be presented. Circuit arrangement of a compact self-sustained pulse generator suitable for an integrated optoelectronic circuit will be discussed.

The Kramers-Kronig relations have been used to determine the change in index of refraction for crystalline Silicon (Si) from absorption data. The Franz-Keldysh effect and the presence of free carriers both have an effect on the index of refraction of the pn junction diode. The results have been used to predict the sensitivity of a technique for measuring on-chip voltages in VLSI devices. A difference in sensitivity for PMOS and NMOS on a single substrate is calculated, in contrast to the simple theoretical result.

The preparation of hydrogenated amorphous silicon. (a-Si:H) thin films by RF sputtering has been investi-gated. Subsequent characterization of the properties of these films indicates that this manner of deposition is suitable for use in the fabrication of thin-film waveguide photodetectors.

Two approaches to fabrication of optical nonlinear waveguides consisting of thin glass films doped with CdS_xSe_{1_x} and CdS microcrystallites, deposited onto standard glass substrates are described. The films were fabricated by RF planar magnetron sputtering in Ar atmosphere of two types of targets: a monocomponent one made of CdS_xSe_{1_x} microcrystallites-doped glass and a composite one consisting of an undoped glass plate and CdS powder. The Raman spectroscopy measurements revealed the presence of semiconductor microcrystallites only in the films fabricated with the composite target. Sharp optical cut-off near 500 nm is visible on the transmission spectra of these films. The films can be used as planar optical waveguides. The refractive index of these films at λ= 632.8 nm varies from 1.55 to 1.70 depending on RF power supplied to the sputtering system. Propagation losses for TE_o mode at the same wavelength are about 10 dB/cm.

Effective mass theory and zero-field absorption data are used to calculate electric-field-induced changes in optical refraction and absorption (the Franz-Keldysh effect) for five direct gap III-V semiconductors: InP, GaAs, GaSb, InAs, and InSb. Results are given for fields of 10⁴ V/cm to the estimated breakdown strength and for photon energies of 40 and 80 meV below the band gap for each semiconductor.

Photorefractive GaAs is a potential optical processing material with the capability of integration with optoelectronic and electronic devices. Recent results on photorefractive properties of liquid encapsulated Czochralski(LEC)-grown, undoped, semi-insulating GaAs crystals are presented.

In recent years, there has been considerable interest in studying the carrier statistics in nonlinear optical materials made of small gap semiconductors since the various electronic properties and the derivation of different important physical parameters of optoelectronic devices under varying situations are based on the relation between the Quasi-Fermi energy and the carrier concentration in such devices.

Important properties of heterojunction and quantum well structures and how these structures have been used for optical waveguides and guided wave intensity and phase modulation (switching) are reviewed in this paper. Further investigations necessary to obtain large bandwidth, low insertion loss and efficient modulation are discussed.

The analysis and design of a GaA1As Mach-Zehnder electro-optic modulator operating at 0.82μm is presented. Optical modulations is achieved through coplanar 50Ωtraveling wave microwave electrodes with a resulting 3 db bandwidth-length product of 11.95 GHz-cm.

Multiple layer GaA1As optical waveguide heterostructures have been grown by molecular beam epitaxy. These samples were designed to operate at a wavelength of .84 microns with negligible coupling of guided light to the absorbing GaAs substrate. The aluminum concentration was 13% for the guiding layer and was 16% for the cladding layers. The process for growing waveguide layers was calibrated primarily by high energy electron diffraction with the optical quality confirmed by photoluminescence measurements. Channel waveguide structures having widths of 5 microns were etched in a low pressure, magnetically confined, multipolar plasma reactor. The resulting waveguide structures were characterized by Raman spectroscopy, ellipsometry, Auger electron spectroscopy and optical waveguide loss measurements.

It is well-known that the gate capacitance of MOS structures of n-channel inversion layers on small-gap semiconductors is very important parameter since the MOS capacitance can be very easily controlled by varying the gate voltage and also since it explores various other fundamental features of semconductor surfaces in MOS structures. However, the gate capacitance of MOS structures of n-channel inversion layers on ternary semiconductors in the presence of crossed electric and magnetic fields has yet to be investigated since the cross-field configuration is of fundamental importance for classical and quantum transport phenomena.

A multimode channel waveguide that branches into several single-mode channels can route each of the modes of the multimode waveguide into a different single-mode channel. Such branches are useful for performing a variety of functions in guided wave optoelectronic devices. The two basic functions on which such devices rely are routing and its inverse, selective mode excitation. (For example, switching is time-dependent routing.) We describe the fundamentals of routing and selective mode excitation, and then analyze the performance of proposed two-and three-channel optical switches using parameters appropriate for GaAs. Because of the larger induced changes of refractive index that are possible in this material, we find that such switches can be substantially shorter than in other materials such as LiNbO₃.

Proposals in the literature for the development of devices based on ultrafast, nonresonant nonlinearities have concentrated on demonstrating the existence and measuring the size and speed of the physical phenomenon. However, even when an interaction between light pulses can be described in terms of a logic operation, a viable device will not result unless it can be made robust in the system sense. It is necessary to show, for example, that requirements for fanin and fanout, extinction, geometrical and power tolerances, and immunity from thermal effects and linear interference can be satisfied. These and other fundamental device design issues are discussed here for general travelling-wave structures which will be required in systems designed to exploit the speed of nonresonant optical nonlinearities.

The Photothermal Deflection Technique is applied to the determination of thermal nonlinearity in samples of Corning-glasses doped with CdS_xSe_l-x (color filter glasses).

The coupling characteristics and nonlinear all-optically controlled switching/modulation behavior is studied analytically and numerically for a GaAs/A1GaAs multiple quantum well (MQW) structure with self-defocusing nonlinearities. An exact dispersion equation is derived in terms of Jacobian elliptic functions for this MQW waveguiding structure. The optical-intensity dependent coupling length is treated as a combination of symmetrical-like and antisymmetrical-like modes of propagation. A single analytical formula is derived for the critical power of this structure. The new exact approach provides an insight into the very complicated nature of the structure.

Two designs for fibre based optical logic and switching elements are presented. The problem of switching entire ultrashort pulses is addressed and the solution of employing dispersion to generate communication within the pulse is presented. This work shows how solitons can be exploited in the devices to give excellent switching characteristics as well as ensuring that there are no detrimental dispersive effects.

We present a review of optical bistability in active and passive optoelectronic devices, with particular reference to switching speeds and powers. The large resonant nonlinearities available near the bandgaps of semiconductors may be utilized to develop bistable devices operating at low powers and high speeds. Passive devices are driven by external optical sources and include nonlinear etalons, nonlinear interfaces, self-focusing configurations and electro-optic modulators with electrical feedback. Active devices include injection lasers and superluminescent amplifiers. Potential advantages of active devices include speed, gain, sensitivity and the ability to drive other devices, but complexity and high power dissipation may limit their use in applications requiring high density.

The coupling of two codirectional modes in waveguides exhibiting, intensity-dependent refractive index may be spatially unstable: small changes in the input light field yeld large variations at the output of the guiding structure. Based on this effect, a number of devices can be designed whose common feature is that an intense beam can be switched by means of a much weaker controlling light field. Fast responding nonlinear glass fibers require high switching power and large dimensions. Moreover, for certain incidence conditions fiber imperfections may cause chaotic switching at the output of a few beat lengths long fiber. Sub-watt switching powers are possible with resonant nonlinearities. In this case, however, large absorptive losses and saturation of the nonlinear index changes may inhibit the instability (and nonlinear switching). We show that using a directional coupler with a two-level saturable nonlinearity imposes rather strict allowances on the material characteristics, as well as considerable wavelength selectivity.

High quality optical waveguides have been produced by incorporating nonlinear organic molecules into polymeric substrate materials. Doping levels in excess of 30% have been achieved and waveguide propagation losses below ldBcm^-1 have been measured. Selective doping has been used to produce simple monomode channel waveguide structures and a variety of nonlinear waveguiding phenomena have been demonstrated including intensity-dependent prism and grating coupling and quadratic electro-optic modulation.

Propagation of laser light through a saturable absorber is theoretically studied. Computed steady state solutions of the Maxwell equations describing the unidimensional propagation of a plane monochromatic wave without introducing the slowly-varying envelope approximation are presented showing how saturation effects can influence the absorption of the field. At a certain range of refractive index and extintion coefficients, computed solutions display a very susprising behaviour, and a self-reflected wave appears inside the absorber. This can be useful for a new kind of biestable device, similar to a standard bistable cavity but with the back mirror self-induced by the light.

Through use of an acousto-optic modulator in a feedbackicircuit having an instability and a short carrier round trip time, an optical oscillator has been constructed. The frequency and character of this oscillation is very sensitively dependent on the position of the detector for the 1st order diffraction, as well as on amplifier balance, and on the total cavity round trip time. Under certain conditions the termination and reinstatement of the oscillation (with its modulation) are initiated by the slightest perturbations of the diffracted laser beam. By continuous monitoring of the signal which energizes the acousto-optic modulator and the oscillatory signal of the first order diffraction, the use of the X/Y mode on a digital oscilloscope provides the instantaneous output vs input data fc2r this instability. These data agree in detail with the complex theoretical predictions of previous work.

We present a theoretical model for analyzing resonant tunneling processes in a multi-layer superlattice structure. The model is then applied to the analysis of asymmetric superlattices formed by varying the well widths in a natural series progression, i.e., Fibonacci and Odd series variations. These structures are analyzed at zero and applied bias. Plots of the transmissivity vs. energy, current density vs. applied voltage, and the wavefunction amplitude vs. real space position are presented. Finally, we show that a new superlattice concept, the variably spaced superlattice energy filter, leads to resonant alignment under bias with subsequent control of carrier injection energy.

The measuring method described is based on the GALAI-CIS-1 particle sizing system. The instrument scans the fiber by a focused laser beam and the transmitted light is analyzed to evaluate the required information. It is demonstrated that core centering can be determined within one micron accuracy. The simple construction and operation of the system makes it applicable on a fiber production line.

In this paper we describe a new fiber lensing technique in which an index-matched microlens is fabricated on the tip of a single-mode fiber. The lens is formed by etching the fiber until only a "whisker" of pure core material remains. This whisker then is cut and arc melted back to form a lens of proper radius for optimal laser-to-single-mode fiber coupling. High average coupling efficiencies of greater than 45% (best value 61%) have been measured and will be reported. The ability to etch many fibers at the same time with reproducible results indicates that the technique is compatible with batch processing. We will explain the lens formation technique in detail and describe the conditions necessary to obtain optical coupling efficiencies. Finally, the coupling sensitivity to lateral misalignment between laser and fiber will be presented.

A new interconnect method for multiple fibers is reported. The design concept proposes novel silicon components to align and position multifiber subassemblies, which have V-grooves to capture either fiber ribbon or monocoated fibers. The silicon components are fabricated by using preferential crystallographic etching techniques. This approach differs from earlier efforts, as preferential etching of silicon is used not only for the fiber placement, but for array alignment as well. Preliminary results are presented to demonstrate the concept.

An all-electrical circuit monitors the laser terminal voltage and adjusts the laser current and temperature to bias the laser above threshold and to maintain a constant wavelength output.

An optical coupling technique utilizing a high index lens to provide high coupling efficiencies (>75%) into single-mode fiber is described. This laser package design provides improved lateral tolerances (>1.5μm) over alternative designs. The improved tolerance provides for easier fiber alignment and assembly as well as excellent temperature stability. Data will be presented on coupling efficiency, lateral tolerance, temp-erature stability and laser performance characteristics.

Miniaturized optical transmitter and receiver modules find widespread application in high-speed local area networks and FDDI applications. Both transmitter and receiver modules implement active fiber optic components and associated bipolar circuits in a compact 16-pin dual-in-line package. A typical transmitter/receiver pair utilizing an LED source and a p-i-n photodetector for data rates up to 200 Mb/s is the focus of this paper.

A 1.3μm connectorized laser package has been proposed which uses a low-cost Compact Disk package coupled to single-mode fiber using two graded-index lenses. The threaded laser module consists of the Compact Disk package mounted directly behind a graded-index lens. This lens serves to collimate the laser radiation. The laser module can then be connected to single-mode fiber by means of a single-mode connector with a second graded-index lens permanently attached to the connector end. The second lens serves to focus the laser radiation onto the fiber core. Alignment sensitivity at the lens-lens interface is reduced, resulting in a highly tolerant connection. The decrease in lateral alignment sensitivity is achieved by trading it off for some unneeded angular tolerance, and some loss of coupling efficiency. In this paper, we will present results of calculations showing how to determine given coupling efficiency/sensitivity trade-offs by making appropriate lens selection. Graded-index lenses are identified and selected on the basis of pitch, which is simply proportional to the length of the lens. In addition, we note that the effective angular alignment at the lens-lens interface can be quite sensitive, depending on the fiber connector used. We propose a means of avoiding this difficulty, using a modified biconic connector design.

Conditions are determined for which free space optical interconnects can transmit information at a higher data rate and consume less power than the equivalent electrical interconnections. Effects of circuit dimension scaling and improved optical link efficiency are discussed. The packing densities of optical and electrical interconnects are also compared.

The area requirements for routing and distributing guided-wave optical signals using gradual waveguide curves and branches will severely restrict the utilization of optical interconnects in future multichip IC packages. This paper addresses the use of total internal reflection in abrupt guided-wave optical components for realizing high-density optical interconnect circuits that eliminate this area constraint. We have measured excess insertion losses of 0.2-0.5 dB at 830 nm for 90° waveguide corner bends, branches, and crossovers for both E^y and E^x polarizations in 5-micron- and 10-micron-wide multimode channels. These loss figures are expected to provide sufficient optical power margins for all point-to-point interconnects in a targeted high-speed multichip VLSI/GaAs package for signal processing applications.

Progress on optically addressing a CMOS RAM for a feasibility demonstration of free space optical interconnection is reported in this paper. The optical RAM chip has been fabricated and functional testing is in progress. Initial results seem promising. New design and SPICE simulation of optical gate cell (OGC) circuits have been carried out to correct the slow fall time of the 'weak pull-down' OGC, which has been characterized experimentally and reported previously. Methods of reducing the response times of the photodiodes and the associated circuits are discussed. Even with the current photodiode, it appears that an OGC can be designed with a performance that is compatible with a CMOS circuit such as the RAM.

Optical intrachip communication offers the potential advantages of high speed and lack of electrical interference. We report on progress made on an interconnect design using GaAs LEDs, polymer waveguides, and photodiodes in a silicon substrate. The polymer waveguides are fabricated in polyimide or polystyrene materials, and are patterned by reactive ion etching with a tri-level resist system. The photodiodes are of two designs, including one in which the depletion layer lies directly below the waveguide. The LEDs are fabricated from GaAs deposited by MOCVD on a Ge lattice-matching layer placed between the GaAs and the silicon substrate. Test results are presented for the individual components.

To even attempt to match the bandwidth capability of fiber optic transmission, electronics must be designed with the fastest chips and the next generation packaging- the multichip module. While individual chip packages are a convenience for testing and in handling with clumsy fingers, they are detrimental to system performance. They demand long path lengths, frequently using materials of high dielectric constant, and often create impedance mismatches. The next generation multichip substrates developed and manufactured by Raychem Corporation, cut path lengths typically 4 :1 and allow interconnect densities of more than 500 in/sq.in of board. They are constructed from multilayer thin film metal lines less than 25μ wide and a polymer insulation. The precision thin film technique provides chip attach footprints that can match the chip I/O, resulting in low bonding parasitics. The electrical properties are very well described by traditional models for planar interconnects, and agree with the actual measurements up to 10 GHz, the limit of reliable data. These generic substrates will help bridge the "impedance mismatch" between the transmission bandwidth of fibers and the information exchange bandwidth of digital electronics.