Publications

Abstract not provided.

Prior to this research, we have developed high-gain GaAs photoconductive semiconductor switches (PCSSs) to trigger 50-300 kV high-voltage switches (HVSs). We have demonstrated that PCSSs can trigger a variety of pulsed-power switches operating at 50300 kV by locating the trigger generator (TG) directly at the HVS. This was demonstrated for two types of dc-charged trigatrons and two types of field distortion midplane switches, including a ±100 kVDC switch produced by the High Current Electronics Institute used in the linear transformer driver. The lowest rms jitter obtained from triggering an HVS with a PCSS was 100 ps from a 300 kV pulse-charged trigatron. PCSSs are the key component in these independently timed fiber-optically controlled low jitter TGs for HVSs. TGs are critical subsystems for reliable and efficient pulsed-power facilities because they control the timing synchronization and amplitude variation of multiple pulse-forming lines that combine to produce the total system output. Future facility-scale pulsed-power systems are even more dependent on triggering, as they are composed of many more triggered HVSs, and they produce shaped pulses by independent timing of the HVSs. As pulsed-power systems become more complex, the complexity of the associated trigger systems also increases. One of the means to reduce this complexity is to allow the trigger system to be charged directly from the voltage appearing across the HVS. However, for slow or dc-charged pulsed-power systems, this can be particularly challenging as the dc hold-off of the PCSS dramatically declines. This paper presents results that are seeking to address HVS performance requirements over large operating ranges by triggering using a pulsed-charged PCSS-based TG. Switch operating conditions that are as low as 45% of the self-break were achieved. A dc-charged PCSS-based TG is also introduced and demonstrated over a 39-61 kV operating range. DC-charged PCSS allows the TG to be directly charged from slow or dc-charged pulsed-power systems. GaAs and neutron-irradiated GaAs (n-GaAs) PCSSs were used to investigate the dc-charged operation. © 2010 IEEE.

Advanced optically-activated solid-state electrical switch development at Sandia has demonstrated multi-kA/kV switching and the path for scalability to even higher current/power. Realization of this potential requires development of new optical sources/switches based on key Sandia photonic device technologies: vertical-cavity surface-emitting lasers (VCSELs) and photoconductive semiconductor switch (PCSS) devices. The key to increasing the switching capacity of PCSS devices to 5kV/5kA and higher is to distribute the current in multiple parallel line filaments triggered by an array of high-brightness line-shaped illuminators. Commercial mechanically-stacked edge-emitting lasers have been used to trigger multiple filaments, but they are difficult to scale and manufacture with the required uniformity. In VCSEL arrays, adjacent lasers utilize identical semiconductor material and are lithographically patterned to the required dimensions. We have demonstrated multiple-line filament triggering using VCSEL arrays to approximate line generation. These arrays of uncoupled circular-aperture VCSELs have fill factors ranging from 2% to 30%. Using these arrays, we have developed a better understanding of the illumination requirements for stable triggering of multiple-filament PCSS devices. Photoconductive semiconductor switch (PCSS) devices offer advantages of high voltage operation (multi-kV), optical isolation, triggering with laser pulses that cannot occur accidentally in nature, low cost, high speed, small size, and radiation hardness. PCSS devices are candidates for an assortment of potential applications that require multi-kA switching of current. The key to increasing the switching capacity of PCSS devices to 5kV/5kA and higher is to distribute the current in multiple parallel line filaments triggered by an array of high-brightness line-shaped illuminators. Commercial mechanically-stacked edge-emitting lasers have been demonstrated to trigger multiple filaments, but they are difficult to scale and manufacture with the required uniformity. As a promising alternative to multiple discrete edge-emitting lasers, a single wafer of vertical-cavity surface-emitting lasers (VCSELs) can be lithographically patterned to achieve the desired layout of parallel line-shaped emitters, in which adjacent lasers utilize identical semiconductor material and thereby achieve a degree of intrinsic optical uniformity. Under this LDRD project, we have fabricated arrays of uncoupled circular-aperture VCSELs to approximate a line-shaped illumination pattern, achieving optical fill factors ranging from 2% to 30%. We have applied these VCSEL arrays to demonstrate single and dual parallel line-filament triggering of PCSS devices. Moreover, we have developed a better understanding of the illumination requirements for stable triggering of multiple-filament PCSS devices using VCSEL arrays. We have found that reliable triggering of multiple filaments requires matching of the turn-on time of adjacent VCSEL line-shaped-arrays to within approximately 1 ns. Additionally, we discovered that reliable triggering of PCSS devices at low voltages requires more optical power than we obtained with our first generation of VCSEL arrays. A second generation of higher-power VCSEL arrays was designed and fabricated at the end of this LDRD project, and testing with PCSS devices is currently underway (as of September 2008).

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Vertical-external-cavity surface-emitting lasers (VECSELs) combine high optical power and good beam quality in a device with surface-normal output. In this paper, we describe the design and operating characteristics of an electrically-pumped VECSEL that employs a wafer-scale fabrication process and operates at 850 nm. A curved micromirror output coupler is heterogeneously integrated with AlGaAs-based semiconductor material to form a compact and robust device. The structure relies on flip-chip bonding the processed epitaxial material to an aluminum nitride mount; this heatsink both dissipates thermal energy and permits high frequency modulation using coplanar traces that lead to the VECSEL mesa. Backside emission is employed, and laser operation at 850 nm is made possible by removing the entire GaAs substrate through selective wet etching. While substrate removal eliminates absorptive losses, it simultaneously compromises laser performance by increasing series resistance and degrading the spatial uniformity of current injection. Several aspects of the VECSEL design help to mitigate these issues, including the use of a novel current-spreading n-type distributed Bragg reflector (DBR). Additionally, VECSEL performance is improved through the use of a p-type DBR that is modified for low thermal resistance.

This paper describes the photonic component development, which exploits pioneering work and unique expertise at Sandia National Laboratories, ARDEC and the Army Research Laboratory by combining key optoelectronic technologies to design and demonstrate components for this fuzing application. The technologies under investigation for the optical fuze design covered in this paper are vertical cavity surface emitting lasers (VECSELs), integrated resonant cavity photodetectors (RCPD), and diffractive micro-optics. The culmination of this work will be low cost, robust, fully integrated, g-hardened components designed suitable for proximity fuzing applications. The use of advanced photonic components will enable replacement of costly assemblies that employ discrete lasers, photodetectors, and bulk optics. The integrated devices will be mass produced and impart huge savings for a variety of Army applications.

Abstract not provided.

Abstract not provided.

Abstract not provided.

This report summarizes an investigation of the use of high-gain Photo-Conductive Semiconductor Switch (PCSS) technology for a deployable impulse source. This includes a discussion of viability, packaging, and antennas. High gain GaAs PCSS-based designs offer potential advantages in terms of compactness, repetition rate, and cost.

A new class of semiconductor lasers that can potentially produce much more short pulse energy is presented. This new laser is not limited in volume or aspect ratio by the depth of a p-n junction and are created from current filaments in semi-insulating GaAs. A current filament semiconductor lasers (CFSL) that have produced 75 nJ of 890 nm radiation in 1.5 ns were tested. A filaments as long as 3.4 cm and several hundred microns in diameter in high gain GaAs photoconductive switches were observed. Their smallest dimension can be more than 100 times the carrier diffusion length in GaAs. The spectral narrowing, lasing thresholds, beam divergence, temporal narrowing and energies which imply lasing for several configurations of CFSL are reported.

The longevity of high-gain GaAs photoconductive semiconductor switches (PCSS) has been extended to over 100 million pulses. This was achieved by improving the ohmic contacts through the incorporation of a doped layer that is very effective in the suppression of filament formation, alleviating current crowding. Damage-free operation is now possible at much higher current levels than before. The inherent damage-free current capacity of the bulk GaAs depends on the thickness of the doped layers and is at least 100 A for a dopant diffusion depth of 4 μm. This current could be increased by employing multiple switches connected in parallel. The contact metal has a different damage mechanism, and the threshold for damage (∼40-80 A) is not further improved beyond a dopant diffusion depth of about 2 μm. In a diffusion-doped contact switch, the switching performance is not degraded at the onset of contact metal erosion, unlike a switch with conventional contacts. For fireset applications operating at 1-kV/l-kA levels and higher, doped contacts have not yet resulted in improved longevity. We employ multifllament operation and InPb solder/Au ribbon wirebonding to demonstrate >100-shot lifetime at 1-kV/l-kA. © 2000 IEEE.

Abstract not provided.

The electrical properties of semi-insulating (SI) Gallium Arsenide (GaAs) have been investigated for some time, particularly for its application as a substrate in microelectronics. Of late this material has found a variety of applications other than as an isolation region between devices, or the substrate of an active device. High resistivity SI GaAs is increasingly being used in charged particle detectors and photoconductive semiconductor switches (PCSS). PCSS made from these materials operating in both the linear and non-linear modes have applications such as firing sets, as drivers for lasers, and in high impedance, low current Q-switches or Pockels cells. In the non-linear mode, it has also been used in a system to generate Ultra-Wideband (UWB) High Power Microwaves (HPM). The choice of GaAs over silicon offers the advantage that its material properties allow for fast, repetitive switching action. Furthermore photoconductive switches have advantages over conventional switches such as improved jitter, better impedance matching, compact size, and in some cases, lower laser energy requirement for switching action. The rise time of the PCSS is an important parameter that affects the maximum energy transferred to the load and it depends, in addition to other parameters, on the bias or the average field across the switch. High field operation has been an important goal in PCSS research. Due to surface flashover or premature material breakdown at higher voltages, most PCSS, especially those used in high power operation, need to operate well below the inherent breakdown voltage of the material. The lifetime or the total number of switching operations before breakdown, is another important switch parameter that needs to be considered for operation at high bias conditions. A lifetime of {approximately} 10{sup 4} shots has been reported for PCSS's used in UWB-HPM generation [5], while it has exceeded 10{sup 8} shots for electro-optic drivers. Much effort is currently being channeled in the study related to improvements of these two parameters high bias operation and lifetime improvement for switches used in pulsed power applications. The contact material and profiles are another important area of study. Although these problems are being pursued through the incorporation of different contact materials and introducing doping near contacts, it is important that the switch properties and the conduction mechanism in these switches be well understood such that the basic nature of the problems can be properly addressed. In this paper the authors report on these two basic issues related to the device operation, i.e., mechanisms for increasing the hold-off characteristics through neutron irradiation, and the analysis of transport processes at varying field conditions in trap dominated SI GaAs in order to identify the breakdown mechanism during device operation. It is expected that this study would result in a better understanding of photoconductive switches, specifically those used in high power operation.

This paper reports on a recent comparison made between the Air Force Research Laboratory (AFRL) gallium arsenide, optically-triggered switch test configuration and the Sandia National Laboratories (SNL) gallium arsenide, optically-triggered switch test configuration. The purpose of these measurements was to compare the temporal switch jitter times. It is found that the optical trigger laser characteristics are dominant in determining the PCSS jitter.

The longevity of high gain GaAs photoconductive semiconductor switches (PCSS) has been extended to over 100 million pulses at 23A, and over 100 pulses at 1kA. This is achieved by improving the ohmic contacts by doping the semi-insulating GaAs underneath the metal, and by achieving a more uniform distribution of contact wear across the entire switch by distributing the trigger light to form multiple filaments. This paper will compare various approaches to doping the contacts, including ion implantation, thermal diffusion, and epitaxial growth. The device characterization also includes examination of the filament behavior using open-shutter, infra-red imaging during high gain switching. These techniques provide information on the filament carrier densities as well as the influence that the different contact structures and trigger light distributions have on the distribution of the current in the devices. This information is guiding the continuing refinement of contact structures and geometries for further improvements in switch longevity.

The authors have developed diode lasers for short pulse duration and high peak pulse power in the 0.01--100.0 m pulsewidth regime. A primary goal of the program was producing up to 10 W while maintaining good far-field beam quality and ease of manufacturability for low cost. High peak power, 17 W, picosecond pulses have been achieved by gain switching of flared geometry waveguide lasers and amplifiers. Such high powers area world record for this type of diode laser. The light emission pattern from diode lasers is of critical importance for sensing systems such as range finding and chemical detection. They have developed a new integrated optical beam transformer producing rib-waveguide diode lasers with a symmetric, low divergence, output beam and increased upper power limits for irreversible facet damage.

A new class of semiconductor laser is presented that does not require p-n junctions. Spectral narrowing, lasing thresholds, beam divergence, temporal narrowing, and energies are shown for these lasers based on current filaments in bulk GaAs.