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Development of multi-mode, high-power, large-aperture two-dimensional VCSEL arrays, operating at a nominal wavelength of 940nm, with highly stable beam profile will be presented. They are designed and fabricated using Trilumina’s proprietary flip-chip-bondable back-side-emitting VCSEL array chip technology. We have demonstrated that a 150-element VCSEL chip array with the improved design shows divergence angle (FWHM) of less than 15°. Additionally, we have integrated this design into drive circuitry that allows us to achieve peak optical powers in excess of 400W.

scalable optical power outputs and the capability to separately address sub-array regions while maintaining fast turn-on and turn-off response times. Performance of these devices is critically dependent both on the design of the VCSEL devices and the design of the sub-mount, which provides both the electrical and thermal contacts for the array. Recent results for modelling and optimization of the VCSELs and their corresponding sub-mounts are discussed.

Substrate-emitting GaAs based oxide-confined 980-nm vertical-cavity surface-emitting lasers (VCSELs) with top-surface high-frequency ground-source-ground contact pads are designed, fabricated, and characterized. The devices are composed of standard top and bottom epitaxially-grown AlGaAs distributed Bragg reflectors (DBRs). The top (p)DBR is capped with p-contact Ti then Au thin-film metals for uniform current injection and laser emission is through the GaAs substrate. The devices are realized on a single epitaxial wafer with n-ohmic-contacts placed on a thick (n+)GaAs buffer layer beneath the bottom (n)DBR and alternatively with the n-ohmic-contacts placed on an (n)GaAs intra-cavity layer lying within the same bottom (n)DBR. Static device parameters including threshold current and rollover current, differential resistance, peak optical output power, and wall-plug efficiency are extracted for VCSELs with oxide-aperture diameters ranging from about 3 to 9-µm and at different temperatures. At room temperature threshold currents are achieved from the sub-mA range up to about 3.5-mA with maximum output powers exceeding 15-mW. Increasing the temperature up to 85 °C slightly increases the threshold current while the peak output power is about halved. The differential resistance at the thermal rollover current is comparable for standard and intra-cavity n-metal-contacts. Small-signal analysis is performed for different bias currents, temperatures, oxide-aperture diameters, and the two n-contact options. Under optimal bias conditions the 3-dB bandwidth exceeds 15 GHz. Direct current modulation-based on-off keying signal generation is investigated from 10 to 40-Gb/s. The influence of an anti-reflection-coated substrate, a thinned substrate, and the combination of both is investigated and discussed.

Flip-chip bonding enables a unique architecture for two-dimensional arrays of VCSELs. Such arrays feature scalable power outputs and the capability to separately address sub-array regions while maintaining fast turn-on and turn-off response times. These substrate-emitting VCSEL arrays can also make use of integrated micro-lenses for beam shaping and directional control. Advances in the performance of these laser arrays will be reviewed and emerging applications are discussed.

A unique architecture for two-dimensional arrays of VCSELs that allow for simultaneous high-power output and highbandwidth modulation has been developed for a variety of applications. The arrays use integrated micro-lenses for beam shaping and control, and to enable incoherent beam combining to make compact, high-brightness sources with low coherence noise. The fabrication and performance of the laser arrays are reviewed and sample applications are discussed.

Compact, radiation-hardened free-space optical data links are enabled by two-dimensional VCSEL arrays that can be modulated at high data rates while being scaled to produce high power levels. The combination of high modulation speed and scalability of power is enabled by the use of arrays which are flip-chip mounted onto sub-mounts that contain electrical strip line waveguides to provide an impedance match for the VCSELs. For laser wavelengths in the 910 nm to 1020 nm range, the lasers can be back-emitting through the GaAs substrate, which enables the use of etched microlenses to manipulate the beams from the individual elements. This approach of using VCSELs in arrays is inherently reliable and radiation-hard. The resulting free space optical data links are particularly advantageous for space-borne applications where size, weight, and power are important factors. Performance characterization of links constructed with these lasers demonstrates their suitability for short distance to medium distance data transfer at up to 10 Gb/s.

Vertical cavity surface emitting lasers (VCSELs) have high potential for space applications, yet little is known of their sensitivity to radiation under vacuum conditions. The first observations of a commercially available proton implanted quantum well AlGaAs VCSEL operating at 850 nm in vacuo and irradiated by 4.5 MeV protons by a scanning ion microbeam is presented. Degradation of L-I-V responses at a proton dose of 1.19 MGy are discussed with particular attention drawn to heating arising from increased nonradiative carrier recombination and that resulting from the vacuum environment.

Damage resulting from irradiating oxide-confined vertical-cavity surface-emitting lasers became significant (threshold shift /spl ap/20%, peak power degradation /spl ap/20%) at fluence levels approaching 1/spl times/10/sup 13/ protons/cm/sup 2/. The threshold current shifted to higher values, and the peak light output power decreased. Forward-current annealing led to partial recovery of the performance of two of the three lasers for which annealing was attempted. Recent results on proton-implanted devices are summarized in a table.

As multichip modules (MCMs) grow in chip count and complexity, increasingly large numbers of input/output (I/O) channels will be required for connection to other MCMs or printed wiring boards. In applications such as digital signal processing, large increases in processing density (number of operations in a given volume) can be obtained in stacked MCM arrangements. The potential pin counts and required I/O densities in these stacked architectures will push beyond the limits of present interlevel coupling techniques. This problem is particularly acute if easy separation of layers is needed to meet MCM testing and yield requirements. Solutions to this problem include the use of laser-drilled, metal-filled electrical vias in the MCM substrate and also optoelectronic data channels that operate in large arrays. These arrays will emit and detect signals traveling perpendicular to the surface of the MCM. All of these approaches will require packaging and alignment that makes use of advanced MCM manufacturing techniques.

Numerical simulations of the transient response of gallium arsenide (GaAs) thyristors to bursts of ionizing radiation are reported. The device simulator BAMBI 2.0, with incorporated GaAs transport models, is used. BAMBI is a 2-D time-dependent transport code using accurate models, such as doping- and electric-field-dependent carrier mobilities and avalanche multiplication. Furthermore, the physical parameters can be very easily modified to isolate the effect of any particular parameter in the device response. This feature is used to study the effect of carrier lifetime on the switching behavior of the irradiated GaAs thyristors by comparing simulation results with the experimental data. The structures simulated are 1-D structures, but the full 2-D simulation is used. Good agreement exists between experimental and simulation results, including the effects of carrier lifetime on the radiation response and the phenomena of delayed switching. >

We have irradiated single- and multimode AlGaAs vertical-cavity surface-emitting laser (VCSEL) arrays operating at a nominal wavelength of 780 nm with 4.5-MeV protons and doses ranging from 10 to 30 Mrad in the active region. We observed a peak power reduction of about 2% per Mrad in the 14-/spl mu/m aperture, multimode VCSELs. Single-mode VCSELs having an aperture of 6 /spl mu/m exhibited a smaller peak power reduction of 0.4%-1% per Mrad. A slight shift in the current threshold was observed only for the multimode VCSELs at dose levels above 10 Mrad. First results indicate a reduced VCSEL peak laser power output that is dominated by a temperature shift caused by the radiation induced increase in resistive heating. In contrast, the power reduction in edge-emitting lasers is dominated by the enhanced radiation induced nonradiative recombination rate. The VCSEL irradiation was performed with a focused ion micro beam that was rastered over the device surface, ensuring a very uniform exposure of a single device in the array.

Historically, obtaining efficient coupling from single-mode waveguides in high performance GaAs modulator devices to single-mode fiber has been difficult. The reasons are; (1) the large modal mismatch between the elliptical waveguide output and the gaussian profile of the optical fiber; and (2) the large NA difference (0.9 for the waveguide in one direction) and 0.16 for fiber. Despite this difficulty, there exists a need for packaging devices with multiple fiber outputs, that have been gang-aligned, efficiently coupled, and hermetically sealed. (The latter item will be very important in automotive or aerospace applications.) Instead of trying to have fiber penetrate the package wall, the SNL approach to efficient coupling and hermeticity has been to allow light to penetrate the package wall. This has been accomplished by sending out the light normal to the waveguides and collecting it with a binary optic that focuses it on to a fiber outside the package. The optical design of this system requires that the beam be nearly collimated as it leaves the surface of the device. To accomplish this, a second-order grating was etched into a 200 /spl mu/m long section of an adiabatically expanded single-mode waveguide. >

Many of the potential applications for parallel photonic data links could benefit from a bi-directional Optoelectronic Multi-Chip Module (OEMCM), where the optical transmitter, receiver, and first-level interface electronics are combined into a single package. It would be desirable for such a module to exhibit low power consumption, have a simple electronic interface that can operate at a variety of speeds and possess a capability to use interchangeable optics for a variety of external connections. Here, we describe initial results for a parallel photonic link technology that exhibits those properties. This link uses high-efficiency, back-emitting, two-dimensional Vertical Cavity Surface-Emitting Laser (VCSEL) arrays operating at 980 nm. The lasers are matched, via integrated microlenses, to corresponding monolithically-integrated photoreceiver arrays that are constructed in a InGaAs/InP Heterojunction Bipolar Transistor (HBT) technology. In initial breadboard-level tests, the photonic data channels built with these devices have been demonstrated with direct (3.3 V) CMOS drive of the VCSELs and a corresponding CMOS interface at the photoreceiver outputs. These links have shown electrical power consumption as low as 42 mW per channel for a 50% average duty cycle while operating at 100 Mb/s.

An ultra-low-power, long-wavelength photoreceiver based on InGaAs-InP heterojunction bipolar transistors is reported. The photoreceivers were designed for massively parallel applications where low-power density is necessary for both electrical and thermal reasons. We demonstrate two-dimensional, four-by-four arrays of photoreceivers for free-space optical data links that interface directly with 3.3 V CMOS ASICs and dissipate less than 12 mW/channel; lower power is possible. Propagation delays of /spl sim/1 nsec were measured and large signal operation to 800 Mbits/sec is demonstrated. The array is on a 500 /spl mu/m pitch and can be easily scaled to much higher density. The photoreceivers can be utilized in both free-space and guided-wave applications.

The device designs used for this work are based on highly efficient vertical-cavity surface emitting lasers, connected to corresponding photoreceivers. In order to meet the constraints of packaging, the optical system used in the design is implemented by the integration of microlenses into the laser and photoreceiver substrates.

New applications of photonic interconnects will involve the insertion of parallel-channel links into Multi-Chip Modules (MCMs). Such applications will drive photonic link components into more compact forms that consume far less power than traditional telecommunication data links. MCM-based applications will also require simplified drive circuitry, lower cost, and higher reliability than has been demonstrated currently in photonic and optoelectronic technologies. The work described is a parallel link array, designed for vertical (Z-Axis) interconnection of the layers in a MCM-based signal processor stack, operating at a data rate of 100 Mb/s. This interconnect is based upon high-efficiency VCSELs, HBT photoreceivers, integrated micro-optics, and MCM-compatible packaging techniques.

Low-power photoreceivers based on InGaAs/InP heterojunction bipolar transistors (HBTs) and p-i-n diodes for highly-parallel optical data links have been designed, fabricated and characterized. The receivers are designed to operate from 980 nm to over 1.3 /spl mu/m and interface directly with 3.3 V CMOS. SPICE was utilized to investigate circuit topographies that minimize power dissipation while maintaining large signal operation required to interface directly with CMOS. Low-power dissipation of /spl sim/10 mW/channel has been achieved at bit rates up to 800 Mbits/sec. Performance characteristics of discrete HBTs and of low-power photoreceivers fabricated with p-i-n/HBT circuits are reported.

Long switching delays following optical triggering of GaAs thyristors are shown to result from near cancellation of carrier lifetime and current regeneration effects. Numerical simulations and analytical models account for the long delays, which are measured to be more than an order of magnitude larger than any lifetime or transit time in the device. Since the thyristor is in a low current state during the delay, this switching phenomena represents a form of dynamic information storage.

The authors have recently demonstrated two prototypes where photonics and microelectromechanical system (MEMs) technologies have been integrated to show proof-of-principle functionality for weapon surety functions. These activities are part of a program which is exploring the miniaturization of electromechanical components for making weapon systems safer. Such miniaturization can lead to a low-cost, small, high-performance ``systems-on-a-chip``, and have many applications ranging from advanced military systems to large-volume commercial markets like automobiles, rf or land-based communications networks and equipment, or commercial electronics. One of the key challenges in realization of the microsystem is integration of several technologies including digital electronics; analog and rf electronics, optoelectronics (light emitting and detecting devices and circuits), sensors and actuators, and advanced packaging technologies. In this work the authors describe efforts in integrating MEMs and photonic functions and the fabrication constraints on both system components. Here, they discuss two examples of integration of MEMs and a photonic device. In the first instance, a MEMs locking device pin is driven by a voltage generated by photovoltaic cells connected in series, which are driven by a laser. In the second case, a VCSEL emitting at 1.06 {micro}m is packaged together with a metallized MEMs shutter. By appropriate alignment to the opening in the shutter, the VCSEL is turned on and off by the movement of the Si chopper wheel.

Vertical-cavity surface-emitting lasers (VCSELs) are uniquely suited to miniaturized free-space optical systems in which surface-mounting and hybrid assembly techniques can be used to combine different technologies together. Two examples are described of such microsystems that are being developed for sensing applications. The first example is a optical position sensing system for rotating parts. Progress on fabricating similar systems by flip-chip bonding techniques is then discussed. The second example is a chemical sensing/analysis system which uses a miniature fluorescence detection module that is based on surface-mounted VCSELs and diffractive optical elements. The detection module is integrated with a capillary electrochromatography separation system and uses substrate-mode light propagation to focus the VCSEL beam on the capillary channel.

High quality optical fiber to OEIC pigtailing, using non-conventional technology, is required to create a real integrated optical system for optical communications, computing, signal processing, control, and sensing. In this paper, Physical Optics Corporation (POC) presents a novel singlemode fiber to singlemode GaAs channel waveguide pigtailing approach. This pigtailing approach involves two key technologies. First, a fiber end-face lensing technology was used to improve modeprofile matching between singlemode fiber and singlemode channel waveguide, so fiber to waveguide coupling efficiency could be improved. Second, resistance layer assisted dual-carrier-soldering (RLADCS) technology was introduced to facilitate fiber and waveguide chip alignment and fixing, so accurate, convenient, and reliable fiber to optoelectronic integrated cicuit (OEIC) pigtailing could be achieved. By using radiation hardened fiber and special OEIC, this pigtailing and packaging technology has potential applications in a space environment. This publication addresses all aspects of this pigtailing approach, including theoretical analysis, design, fabrication, testing, and measurement results.

Present and future space-based applications such as sensors, low-weight and low-power data links for satellites, communication between electromagnetically-shielded modules, and short-distance cross-links within satellite constellations may benefit from the inclusion of small, low-power, and high-efficiency lasers such as the recently-developed Vertical Cavity Surface-Emitting Laser (VCSEL). Many factors influence the application of these devices to space. Temperature response, operational lifetime and reliability, and power consumption are all important considerations for space applications. In addition, the space radiation environments must be considered. In this work, the effects of ionizing radiation on VCSELs are studied with an emphasis on proton damage, and with comparisons to related neutron and gamma-induced phenomena. The influence of proton irradiation is studied in-depth for selected VCSEL structures by the use of an ion microbeam. The experiments indicate that VCSELs exhibit much less threshold current shift for a given radiation dose, compared to the more traditional edge-emitting semiconductor lasers, but that self-heating is a more important consideration for VCSELs. The high current densities associated with VCSELs also lead to a strong influence from forward-bias annealing. These effects are common to various VCSEL types (780 nm and 850 nm) and their magnitude at a given dose is strongly dependent onmore » device size. This indicates that, while VCSELs appear to be very insensitive to ionizing radiation when compared with alternative technologies, there are a number of factors that must be taken into account when optimizing for the space environment.« less

Advanced device technologies such as Vertical Cavity Surface-Emitting Lasers (VCSELs) and diffractive micro lenses can be obtained with novel packaging techniques to allow low-power interconnection of parallel optical signals. These interconnections can be realized directly on circuit boards, in a multi-chip module format, or in packages that emulate electrical connectors. For applications such as stacking of Multi-Chip Module (MCM) layers, the links may be realized in bi-directional form using integrated diffractive microlenses. In the stacked MCM design, consumed electrical power is minimized by use of a relatively high laser output from high efficiency VCSELs, and a receiver design that is optimized for low power, at the expense of dynamic range. Within certain constraints, the design may be extended to other forms such as board-level interconnects.

The integration of optical components with polysilicon surface micromechanical actuation mechanisms show significant promise for signal switching, fiber alignment, and optical sensing applications. Monolithically integrating the manufacturing process for waveguide structures with the processing of polysilicon actuators allows actuated waveguides to take advantage of the economy of silicon manufacturing. The optical and stress properties of the oxides and nitrides considered for the waveguide design along with design, fabrication, and testing details for the polysilicon actuators are presented.

Future advances in the application of photonic interconnects will involve the insertion of parallel-channel links into Multi-Chip Modules (MCMs) and board-level parallel connections. Such applications will drive photonic link components into more compact forms that consume far less power than traditional telecommunication data links. These will make use of new device-level technologies such as vertical cavity surfaceemitting lasers and special low-power parallel photoreceiver circuits. Depending on the application, these device technologies will often be monolithically integrated to reduce the amount of board or module real estate required by the photonics. Highly parallel MCM and board-level applications will also require simplified drive circuitry, lower cost, and higher reliability than has been demonstrated in photonic and optoelectronic technologies. An example is found in two-dimensional point-to-point array interconnects for MCM stacking. These interconnects are based on high-efficiency Vertical Cavity Surface Emitting Lasers (VCSELs), Heterojunction Bipolar Transistor (HBT) photoreceivers, integrated micro-optics, and MCM-compatible packaging techniques. Individual channels have been demonstrated at 100 Mb/s, operating with a direct 3.3V CMOS electronic interface while using 45 mW of electrical power. These results demonstrate how optoelectronic device technologies can be optimized for low-power parallel link applications.

An optical interconnection system is being developed to provide vertical, digital data channels for stacked multichip modules. A key component of the system is an array of individually addressable vertical-cavity surface-emitting lasers with diffractive lenses integrated into the substrate to control beam divergence and direction. The lenses were fabricated by direct-write e-beam lithography and reactive ion beam etching into the GaAs substrate. Preliminary device performance data and the design and fabrication issues are discussed.

As Multi-Chip Modules (MCMs) grow in chip count and complexity, increasingly large numbers of Input/Output (I/O) channels will be required for connection to other MCMs or printed wiring boards. In applications such as digital signal processing, large increases in processing density (number of operations in a given volume) can be obtained in stacked MCM arrangements. The potential pin counts and required I/O densities in these stacked architectures will require technology beyond the limits of present day capabilities. This problem is particularly acute if easy separation of layers is needed to meet MCM testing and yield requirements. One solution to this problem is the use of optoelectronic data channels that operate in large arrays [1]. These arrays will emit and detect signals traveling normal to the surface of the MCM stack as in Fig. 1. In order to maximize compatibility with present MCM packaging techniques, the chips that form the arrays should include both the active devices (sources such as vertical cavity surface emitting lasers (VCSELs) and detectors as shown in the demonstration cross-section in Fig. 2) and any collimation and/or focusing optics that are needed to compensate for misalignments and accommodate small detector areas.

A high-speed distributed electrode phase modulator has been designed and fabricated. The processing of this device presented many challenges: (1) smooth rib waveguides etched to an accuracy of +/- 200angstroms; (2) polyimide planarization of 5micrometers step heights, with vias patterned as small as 1.6 micrometers ; (3) contact metalization with resistivities as low as 1 X 10-6(Omega) -cm2; (4) coplanar n and p gold contacts 2.5 micrometers thick, with a 0.5 micrometers gap between contacts; (5) ion- implantation to achieve both electrical and optical isolation. A brief description of how each of these processes have been accomplished will be presented. The epitaxial growth structure of this device will be discussed, including SEM cross-sections of the completed device.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A high-performance high-speed optical phase modulator for photonic integrated circuit (PIC) use is described. Integration of these optical phase modulators into a real system (COMPASS) is also discussed. The optical phase modulators are based on depletion-edge translation and have experimentally provided optical phase shifts in excess of 60/spl degV/spl middot/mm with approximately 4 dB/cm loss while simultaneously demonstrating bandwidths in excess of 10 GHz. >

Photonics activities at Sandia National Laboratories are founded on an extensive materials research program. In 1988, the Compound Semiconductor Research Laboratory (CSRL) was established at Sandia to bring together device and materials research and development, in support of Sandia`s role in weapons technologies. Recently, industrial competitiveness has been added as a major mission for the national laboratories. As a result, present photonics programs are not only directed towards internal applications-driven projects, but are increasingly tied to the Department Of Energy`s (DOE`s) Technology Transfer Initiatives (TTIs), Cooperative Research and Development Agreements (CRADAs), and participation in partnerships and consortia. This evolution yields a full range of photonics programs, ranging from materials synthesis and device fabrication to packaging, test, and subsystem development. This paper presents an overview of Sandia`s photonics-program directions, using three applications as examples.

Dielectric optical waveguides exhibit properties that are well suited to sensor applications. They have low refractive index and are transparent to a wide range of wavelengths. They can react with the surrounding environment in a variety of controllable ways. In certain sensor applications, it is advantageous to integrate the dielectric waveguide on a semiconductor substrate with active devices. In this work, we demonstrate a tamper sensor based on dielectric waveguides that connect epitaxial GaAs-GaAlAs sources and detectors. The tamper sensing function is realized by attaching particles of absorbing material with high refractive index to the surface of the waveguides. These absorbers are then attached to a lid or cover, as in an integrated circuit package or multi-chip module. The absorbers attenuate the light in the waveguides as a function of absorber interaction. The absorbers are placed randomly on the waveguides, to form a unique attenuation pattern that is registered by the relative signal levels on the photodetectors. When the lid is moved, the pattern of absorbers changes, altering the photodetector signals. This dielectric waveguide arrangement is applicable to a variety of sensor functions, and can make use of resonant coupling properties between low refractive index dielectric waveguides and high-index absorbing materials.

Arrangements of discrete photonic waveguide devices are currently being integrated to form circuits. These Photonic Integrated Circuits (PICs) require test methods that are adaptable as circuit size and complexity grows. Here, we describe tests used to characterize a GaAs/GaAlAs waveguide-based PIC being developed for phase-shift control in phased-array antenna applications. The various elements of the PIC include digital and analog waveguide modulators, light-guide splitters and combiners, turning mirrors, and input-output polarization-maintaining single-mode optical fibers. These basic elements are combined to perform higher-order functions such as optical frequency translation (single side-band suppressed carrier modulation), and phase shifting on multiple optical taps. Tests used to characterize the elements include optical loss, guided-wave modal characteristic, antireflection coating effectiveness, electro-optical (phase) modulation efficiency, electrical frequency response, and optical frequency translation. These tests are integrated with the fiber attachment and RF packaging sequence to construct working device prototypes. They are currently being applied to discrete components of the circuit, and will be adapted as the various parts are integrated.

Semiconductor optical waveguide circuits are now moving from the laboratory into application-driven components.

Progress will be presented regarding circuit elements to be eventually incorporated into a Photonic Integrated Circuit (PIC). Data from elements such as phase modulators and couplers will be reviewed. They have been developed employing ridge waveguides in MBE grown GaAs/AlGaAs material structures. A general approach to PICs will be discussed as well as their incorporation into optical frequency translation (OFT) modulation circuits for coherent optical systems. Detailed theoretical discussion of one, two, and four-arm phase-modulator-based OFT circuits is included.

Optically-triggered thyristor switches often operate in adverse environments, such as high temperature and high dose-rate transient radiation, which can result in lowered operating voltage and premature triggering. These effects can be reduced by connecting or monolithically integrating a reverse-biased compensating photodiode or phototransistor into the gate of the optically-triggered thyristor. We have demonstrated the effectiveness of this hardening concept in silicon thyristors packaged with photodiodes, and in gallium arsenide optically-triggered thyristors monolithically integrated with compensating phototransistors.

The fabrication and performance of a vertical cavity surface emitting laser array integrated with microlenses are reported. The lasers emit at 967 nm through the substrate with integral polyimide microlenses. This design reduces beam divergence and provides an integrated device that emits a nearly collimated beam for applications such as optical interconnects.

Semiconductor-clad optical waveguides possess unique properties which arise from the periodic coupling effect between the lossless modes of the dielectric waveguide and the lossy modes which are supported by the thin semiconductor layer. This paper presents a detailed analysis of this mode structure showing mode conversion as a function of cladding thickness. The analysis predicts that several types of mode may exist for certain guide thicknesses, depending on the magnitude of the absorption in the semiconductor. The combined effects of waveguide coupling and loss are determined and illustrated in devices using silicon claddings.

Gallium arsenide optically triggered thyristors that exhibit tolerance to high X-ray dose rates have been fabricated. These two-terminal epitaxial devices feature breakover voltages of 18 V to 38 V with no radiation. They trigger at less than 2 V with only tenths of a milliwatt of laser light, but they do not trigger at 2*10/sup 9/ rad(Si)/s with a bias level as much as 40% to 60% of the zero-radiation breakover voltage. When these devices are bombarded with neutrons, the reduced carrier lifetimes result in a decreased sensitivity to triggering by light and ionizing radiation. The thyristors show some decrease in optical and X-ray sensitivity upon neutron exposure in excess of 10/sup 13/ n/cm/sup 2/. At 10/sup 14/ n/cm/sup 2/, there are major reductions in photon sensitivity, and some instability is observed. >

Optical systems which make use of incoherent illumination provide potential benefits in noise immunity over systems using coherent illumination. Because incoherent systems are linear with respect to intensity, however, complex number representations and operations are not readily obtainable. This paper proposes using the tristimulus (three-color) system as found in color television to establish a vector space, which then permits the complex number operations of polar-form multiplication and rectangular-form addition to be achieved with incoherent illumination. When these operations are combined in a hybrid (optical-electronic) processor using color television equipment, discrete approximations of the general 1-D and 2-D linear complex superposition integrals may be obtained.

Unique propagation characteristics arise from the coupling of light between the lossless modes of a dielectric waveguide and the lossy modes supported by an absorptive cladding layer such as a semiconductor. These effects occur because the propagating lightwave couples into a variety of modes that exist in the semiconductor-clad waveguide. Properties such as cladding thickness, refractive index, and absorption level greatly affect modes of the clad waveguide. Such mode effects are due to periodic coupling between the dielectric layer and the semiconductor cladding. The resulting characteristics suggest use in various device applications. Here, we characterize the periodic coupling effect and how it creates unique modes in the clad waveguide structure. We also describe how the refractive index and absorption of the cladding work together to influence the properties of those modes.

New integrated optical modulator designs are presented that are based on a GaAs-clad dielectric waveguide technology. These devices exhibit unique features that arise due to the periodic coupling between the propagating lossless guided modes of a dielectric optical waveguide and the lossy modes supported by a semiconductor cladding layer. An incident light beam that propagates with a photon energy above the band gap of the GaAs introduces carriers that shift the absorption of the GaAs cladding. The carriers induce shifts in the periodic interaction between the modes of the dielectric portion of the waveguide and those of the GaAs cladding. Depending on the cladding thickness and material absorptions that are used, the induced shifts can modulate either the amplitude or phase of a guided lightwave that propagates at a photon energy near the band gap of the GaAs. The coupling properties of the clad-waveguide structure and the effects of shifts in the GaAs cladding permittivities are evaluated analytically. Proposed device structures will allow the use of a dielectric waveguide and a thin isolated GaAs layer with a GaAs substrate so the modulator will be fully integrable with other guided-wave devices.

A GaAs thyristor with 35 V breakover has been fabricated that exhibits large discrimination between light and flash X-ray power levels for triggering. Approximately 108 times more X-ray than light power is needed to switch the device over a bias voltage range of 25 V. This discrimination property represents a form of radiation hardening for a lightactivated switch.