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At least 379 records · Page 21

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DOE OSTI2019

Modelling an electro-optical modulator based on a vertical p – n junction in a silicon-on-insulator structure

We report the results of numerical simulation of a Mach – Zehnder electro-optical modulator using beam splitters based on multimode interference in a silicon-on-insulator structure. The control is provided due to the depletion effect in the vertical p – n junction, which can be fabricated using the self-alignment technology. An optimal modulator design is proposed, which impedance is matched with an external 50-Ω load, for which, with a reverse bias of – 5 V and an active length of 1.7 mm, the optical frequency bandwidth of ∼ 50 GHz can be achieved. A special doping profile of the p – n junction of the modulator is presented, which provides an optical frequency bandwidth of 30 GHz with a reverse bias of –3 V and a modulator length of 2.5 mm. Such modulators can be used in integrated optics, optical communications and radio photonics devices. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

NASA NTRS1991

Baseband pulse shaping techniques for nonlinearly amplified pi/4-QPSK and QAM systems

A new generation of multi-stage pi/4-shifted QPSK and of superposed quadrature-amplitude-modulated (SQAM) modulators-coherent demodulators (modems) and of continuous phase modulated (CPM)-gaussian premodulation filtered minimum-shift-keying (MGMSK) systems is proposed and studied. These modems will lead to bandwidth and power efficient satellite communications systems designs. As an illustrative application, a baseband processing technique pi/4-controlled transition PSK (pi/4-CTPSK) is described. To develop a cost and power efficient design strategy, we assume that nonlinear, fully saturated high power amplifiers (HPA) are utilized in the satellite earth station transmitter and in the satellite transponder. Modem structures which could lead to application specific integrated circuit (ASIC) satellite on-board processing universal modem applications are also considered. Multistate GMSK (i.e., MGMSK) signal generation methods by means of two or more RF combined nonlinearly amplified SQAM modems and by one multistate (in-phase and quadrature-baseband premodulation filtered-superposed) SQAM architecture and one RF nonlinear amplifier are studied. During the SQAM modem development phase we investigate the potential system advantages of the pi/4-shifted logic. The bandwidth efficiency of the proposed multistate GMSK and baseband filtered PAM-FM modulator (a new class in the CPM family) will be significantly higher than that of conventional G-MSK systems. To optimize the practical P(sub e) = f((E sub b)/(N sub o)) performance we consider improved coherent demodulation MGMSK structures such as deviated-frequency locking coherent demodulators. For relative low bit rate SATCOM applications, e.g., bit rates less than 300 kb/s, phase noise tracking cancellation (for fixed site earth station) and phase noise cancellation as well as Doppler compensation (for satellite to mobile earth station) applications may be required. We study digital channel sounding methods which could cancel the phase noise-caused degradations of CPM and GMSK modems.

Feher, Kamilo↗

NASA NTRS2005

Implanted Silicon Resistor Layers for Efficient Terahertz Absorption

Broadband absorption structures are an essential component of large format bolometer arrays for imaging GHz and THz radiation. We have measured electrical and optical properties of implanted silicon resistor layers designed to be suitable for these absorbers. Implanted resistors offer a low-film-stress, buried absorber that is robust to longterm aging, temperature, and subsequent metals processing. Such an absorber layer is readily integrated with superconducting integrated circuits and standard micromachining as demonstrated by the SCUBA II array built by ROE/NIST (1). We present a complete characterization of these layers, demonstrating frequency regimes in which different recipes will be suitable for absorbers. Single layer thin film coatings have been demonstrated as effective absorbers at certain wavelengths including semimetal (2,3), thin metal (4), and patterned metal films (5,6). Astronomical instrument examples include the SHARC II instrument is imaging the submillimeter band using passivated Bi semimetal films and the HAWC instrument for SOFIA, which employs ultrathin metal films to span 1-3 THz. Patterned metal films on spiderweb bolometers have also been proposed for broadband detection. In each case, the absorber structure matches the impedance of free space for optimal absorption in the detector configuration (typically 157 Ohms per square for high absorption with a single or 377 Ohms per square in a resonant cavity or quarter wave backshort). Resonant structures with -20% bandwidth coupled to bolometers are also under development; stacks of such structures may take advantage of instruments imaging over a wide band. Each technique may enable effective absorbers in imagers. However, thin films tend to age, degrade or change during further processing, can be difficult to reproduce, and often exhibit an intrinsic granularity that creates complicated frequency dependence at THz frequencies. Thick metal films are more robust but the requirement for patterning can limit their absorption at THz frequencies and their heat capacity can be high. patterned absorber structures that offer low heat capacity, absence of aging, and uniform, predictable behavior at THz frequencies. We have correlated DC electrical and THz optical measurements of a series of implanted layers and studied their frequency dependence of optical absorption from .3 to 10 THz at cryogenic temperatures. We have modeled the optical response to determine the suitability of the implanted silicon resistor as a function of resistance in the range 10 Ohms/sq to 300 Ohms/sq.

Chervenak, J. A.↗

DOE OSTI2025

Optimizing Non-Terrestrial Hybrid RF/FSO Links With Reinforcement Learning: Navigating Through Clouds

In the pursuit of ubiquitous broadband connectivity, there has been a significant shift towards the vertical expansion of communication networks into space, particularly through the exploitation of low Earth orbit (LEO) satellite constellations, which are favored for their relatively low latency. However, this approach faces many challenges that need to be addressed, including atmospheric turbulence, high path loss, and dynamic cloud formations. High-altitude pseudo-satellites (HAPS) have emerged as promising relaying layers between LEO satellites and ground stations, enhancing coverage, latency, and direct terrestrial user connectivity. While radio frequency (RF) bands suffer from congestion and limited bandwidth, free space optical (FSO) communications offer higher data rates, but are susceptible to misalignment and weather-induced signal degradation. To address these challenges, a hybrid RF/FSO approach has been proposed to take advantage of both technologies by dynamic switching between RF and FSO based on propagation channel conditions. This paper introduces a reinforcement learning-based algorithm designed to optimize the trajectory of HAPS, maneuver around cloudy areas, and seamlessly switch between the RF and FSO communication modes to maximize the achievable capacity. The proposed approach aims to maximize system performance by intelligently adapting to environmental conditions and offering a promising solution for next-generation space communication networks.

actor-critic algorithm↗

DOE OSTI2021

Efficient photoconductive terahertz detection through photon trapping in plasmonic nanocavities

We present a photoconductive terahertz detector that employs a plasmonic nanocavity to offer high-sensitivity and broadband operation when used in a terahertz time-domain spectroscopy system even at very low optical pump power levels. By employing a plasmonic nanocavity, all of the photocarriers are generated within a 100 nm distance from the photoconductor contact electrodes, enabling a short transport time for almost all of the photogenerated carriers. As a result, the photoconductive detector maintains high quantum efficiency and ultrafast operation simultaneously, enabling high-sensitivity and broadband operation at very low optical pump power levels. We utilize a photoconductive detector based on a plasmonic nanocavity optimized for operation at a 770 nm optical wavelength in a terahertz time-domain spectroscopy system and demonstrate a 100 dB signal-to-noise ratio and a 0.1–6 THz noise-equivalent bandwidth at a record-low average optical pump power of 0.1 mW, compared to the state-of-the-art photoconductive terahertz detectors. The extremely low optical power budget of the demonstrated photoconductive detector makes this detector attractive for multi-pixel terahertz imaging systems.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2023

Smoothing Lexis diagrams using kernel functions: A contemporary approach

Lexis diagrams are rectangular arrays of event rates indexed by age and period. Analysis of Lexis diagrams is a cornerstone of cancer surveillance research. Typically, population-based descriptive studies analyze multiple Lexis diagrams defined by sex, tumor characteristics, race/ethnicity, geographic region, etc. Inevitably the amount of information per Lexis diminishes with increasing stratification. Several methods have been proposed to smooth observed Lexis diagrams up front to clarify salient patterns and improve summary estimates of averages, gradients, and trends. In this article, we develop a novel bivariate kernel-based smoother that incorporates two key innovations. First, for any given kernel, we calculate its singular values decomposition, and select an optimal truncation point—the number of leading singular vectors to retain—based on the bias-corrected Akaike information criterion. Second, we model-average over a panel of candidate kernels with diverse shapes and bandwidths. The truncated model averaging approach is fast, automatic, has excellent performance, and provides a variance-covariance matrix that takes model selection into account. We present an in-depth case study (invasive estrogen receptor-negative breast cancer incidence among non-Hispanic white women in the United States) and simulate operating characteristics for 20 representative cancers. The truncated model averaging approach consistently outperforms any fixed kernel. Our results support the routine use of the truncated model averaging approach in descriptive studies of cancer.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2025

Noise in Intense Electron Bunches (Final Technical Report)

The goal of this project is to investigate density fluctuations in electron bunches. Noise and density fluctuations in relativistic electron bunches, accelerated in a linac, are of critical importance to various Coherent Electron Cooling (CEC) concepts as well as to free-electron lasers (FELs). For CEC, the beam noise results in additional diffusion that counteracts cooling; and if this noise is not controlled at sufficiently low level, the noise heating effects can overcome cooling. There have been several proposals in the past to suppress the noise in the beam in the frequency range of interest in order to optimize the cooling effects. In SASE FELs a microwave instability starts from the initial noise in the beam and eventually leads to the beam microbunching yielding coherent radiation, and the initial noise in the FEL bandwidth plays a useful role. In seeded FELs, in contrast, such noise interferes with the seed signal, so that reducing noise at the initial seed wavelength would lower the seed laser power requirement. Our major goals were (1) to measure the electron beam density noise level in a 0.5 – 10 um wavelength range, (2) to predict the beam noise level in order to compare with the measurements, and (3) as a stretch goal, to find mechanisms that affect the beam noise to control its level in a predictable manner.

43 PARTICLE ACCELERATORS↗

NASA NTRS1976

Analytical and experimental design and analysis of an optimal processor for image registration

The author has identified the following significant results. A quantitative measure of the registration processor accuracy in terms of the variance of the registration error was derived. With the appropriate assumptions, the variance was shown to be inversely proportional to the square of the effective bandwidth times the signal to noise ratio. The final expressions were presented to emphasize both the form and simplicity of their representation. In the situation where relative spatial distortions exist between images to be registered, expressions were derived for estimating the loss in output signal to noise ratio due to these spatial distortions. These results are in terms of a reduction factor.

Mcgillem, C. D.↗

DOE OSTI2019

NAMSA

In NAMSA, we introduce two main solutions to address the limitations of MSA-type algorithms for simulation of electron scattering/microscopy data: just-in-time compilation and work partitioning via MPI and GPU-to-GPU communication. First, the just-in-time (JIT) compilation strategy produces optimally tuned CUDA C++ kernels due to the data layout being perfectly known at runtime and compiled via calls from the Python interpreter. NAMSA automatically optimizes the 2-D FFT planning, and CUDA multi-streams scheduling/launching of probe simulations. With NAMSA, we can simulate electron scattering/microscopy on hundreds to thousands of GPUs, while removing the major limitations of memory and bandwidth from MSA.

Laanait, Nouamane↗

NASA NTRS1990

Design of localizer capture and track hold for a transport airplane - An H(infinity)/LTR approach

The authors describe the design of a lateral track-hold control law in a localizer capture and track autopilot using a procedure based on H(infinity)-optimization and loop transfer recovery. The recovery of the achievable performance using output feedback from a satisfactory track-hold design obtained in an H(infinity)-optimization is demonstrated. Simulation for a lateral track command is presented along with robustness properties of the output feedback design. Results indicate that the recovery of achievable performance must be carefully traded-off against control bandwidth limitation.

Chen, B. M.↗

NASA NTRS1992

High-performance acousto-optic materials - Hg2Cl2 and PbBr2

New results achieved with recently grown Hg2Cl2 and PbBr2 crystals are described. With an optimized crystal growth technique Hg2Cl2 crystals were grown that show a significantly reduced acoustic attenuation compared to prior crystals, from 13.4 to 8 dB/microsec-GHz-squared. These crystals allow the development of Hg2Cl2 Bragg cells with time-bandwidth product figures in the 5100 to 6900 range, frequency operation as high as that for TeO2, and resolution about 25 percent higher than TeO2 for similar crystal lengths. PbBr2 crystals were also grown that exhibit a large figure of merit (M2 = 550) with an attenuation coefficient of 12 dB/microsec-GHz-squared. This material may be the choice for infrared devices where large diffraction efficiencies are needed.

Gottlieb, Milton↗

DOE OSTI2017

New Type of 2D Perovskites with Alternating Cations in the Interlayer Space, (C(NH 2 ) 3 )(CH 3 NH 3 ) n Pb n I 3 n +1 : Structure, Properties, and Photovoltaic Performance

In this study, we present the new homologous series (C(NH 2 ) 3 )(CH 3 NH 3 ) n Pb n I3 n+1 (n = 1, 2, 3) of layered 2D perovskites. Structural characterization by single-crystal X-ray diffraction reveals that these compounds adopt an unprecedented structure type, which is stabilized by the alternating ordering of the guanidinium and methylammonium cations in the interlayer space (ACI). Compared to the more common Ruddlesden–Popper (RP) 2D perovskites, the ACI perovskites have a different stacking motif and adopt a higher crystal symmetry. The higher symmetry of the ACI perovskites is expressed in their physical properties, which show a characteristic decrease of the bandgap with respect to their RP perovskite counterparts with the same perovskite layer thickness (n). The compounds show a monotonic decrease in the optical gap as n increases: E g = 2.27 eV for n = 1 to E g = 1.99 eV for n = 2 and E g = 1.73 eV for n = 3, which show slightly narrower gaps compared to the corresponding RP perovskites. First-principles theoretical electronic structure calculations confirm the experimental optical gap trends suggesting that the ACI perovskites are direct bandgap semiconductors with wide valence and conduction bandwidths. To assess the potential of the ACI perovskites toward solar cell applications, we studied the (C(NH 2 ) 3 )(CH 3 NH 3 ) 3 Pb 3 I 10 (n = 3) compound. Compact thin films from the (C(NH 2 ) 3 )(CH 3 NH 3 ) 3 Pb 3 I 10 compound with excellent surface coverage can be obtained from the antisolvent dripping method. Planar photovoltaic devices from optimized ACI perovskite films yield a power-conversion-efficiency of 7.26% with a high open-circuit voltage of ~1 V and a striking fill factor of ~80%.

14 SOLAR ENERGY↗

DOE OSTI2016

Dynamic Multi-Sensor Multi-Mission Optimal Planning Tool

Remote sensing systems have firmly established a role in providing immense value to commercial industry, scientific exploration, and national security. Continued maturation of sensing technology has reduced the cost of deploying highly-capable sensors while at the same time increased reliance on the information these sensors can provide. The demand for time on these sensors is unlikely to diminish. Coordination of next-generation sensor systems, larger constellations of satellites, unmanned aerial vehicles, ground telescopes, etc. is prohibitively complex for existing heuristics-based scheduling techniques. The project was a two-year collaboration spanning multiple Sandia centers and included a partnership with Texas A&M University. We have developed algorithms and software for collection scheduling, remote sensor field-of-view pointing models, and bandwidth-constrained prioritization of sensor data. Our approach followed best practices from the operations research and computational geometry communities. These models provide several advantages over state of the art techniques. In particular, our approach is more flexible compared to heuristics that tightly couple models and solution techniques. First, our mixed-integer linear models afford a rigorous analysis so that sensor planners can quantitatively describe a schedule relative to the best possible. Optimal or near-optimal schedules can be produced with commercial solvers in operational run-times. These models can be modified and extended to incorporate different scheduling and resource constraints and objective function definitions. Further, we have extended these models to proactively schedule sensors under weather and ad hoc collection uncertainty. This approach stands in contrast to existing deterministic schedulers which assume a single future weather or ad hoc collection scenario. The field-of-view pointing algorithm produces a mosaic with the fewest number of images required to fully cover a region of interest. The bandwidth-constrained algorithms find the highest priority information that can be transmitted. All of these are based on mixed-integer linear programs so that, in the future, collection scheduling, field-of-view, and bandwidth prioritization can be combined into a single problem. Experiments conducted using the developed models, commercial solvers, and benchmark datasets have demonstrated that proactively scheduling against uncertainty regularly and significantly outperforms deterministic schedulers.

47 OTHER INSTRUMENTATION↗

DOE OSTI2016

Fast multipurpose Monte Carlo simulation for proton therapy using multi- and many-core CPU architectures

Purpose: Accuracy in proton therapy treatment planning can be improved using Monte Carlo (MC) simulations. However the long computation time of such methods hinders their use in clinical routine. This work aims to develop a fast multipurpose Monte Carlo simulation tool for proton therapy using massively parallel central processing unit (CPU) architectures. Methods: A new Monte Carlo, called MCsquare (many-core Monte Carlo), has been designed and optimized for the last generation of Intel Xeon processors and Intel Xeon Phi coprocessors. These massively parallel architectures offer the flexibility and the computational power suitable to MC methods. The class-II condensed history algorithm of MCsquare provides a fast and yet accurate method of simulating heavy charged particles such as protons, deuterons, and alphas inside voxelized geometries. Hard ionizations, with energy losses above a user-specified threshold, are simulated individually while soft events are regrouped in a multiple scattering theory. Elastic and inelastic nuclear interactions are sampled from ICRU 63 differential cross sections, thereby allowing for the computation of prompt gamma emission profiles. MCsquare has been benchmarked with the GATE/GEANT4 Monte Carlo application for homogeneous and heterogeneous geometries. Results: Comparisons with GATE/GEANT4 for various geometries show deviations within 2%–1 mm. In spite of the limited memory bandwidth of the coprocessor simulation time is below 25 s for 10{sup 7} primary 200 MeV protons in average soft tissues using all Xeon Phi and CPU resources embedded in a single desktop unit. Conclusions: MCsquare exploits the flexibility of CPU architectures to provide a multipurpose MC simulation tool. Optimized code enables the use of accurate MC calculation within a reasonable computation time, adequate for clinical practice. MCsquare also simulates prompt gamma emission and can thus be used also for in vivo range verification.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2018

Unsupervised classification-based hyperspectral data processing: lossy compression

The existing lossy compression algorithms play an important role in reducing the cost of storage equipment and bandwidth for hyperspectral (HS) application. However, none of the lossy compression algorithms considers the real-time classification of HS data. In this paper, we present a new lossy compression method for HS data that aims to optimally compress in both spatial and spectral domains and simultaneously maximize classification performance. For this target, Harsanyi–Farrand–Chang (HFC) and k-means++ algorithms are applied to achieve a spectral library and an index matrix for HS image. Spectral angle mapping and Euclidean distance are used to update the spectral library and the index matrix. The experiment results indicate that the proposed method has a good classification performance. The results also reveal that the proposed method works well in real-time classification and compression of HS data with a large volume and achieves a high compression ratio. It is noteworthy to mention that the superiority of our method in compression becomes more apparent as the volume of HS data grows. Consequently, the proposed method has a strong advantage in HS applications that require both compression and classification.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2024

Flexible silicon photonic architecture for accelerating distributed deep learning

The increasing size and complexity of deep learning (DL) models have led to the wide adoption of distributed training methods in datacenters (DCs) and high-performance computing (HPC) systems. However, communication among distributed computing units (CUs) has emerged as a major bottleneck in the training process. In this study, we propose Flex-SiPAC, a flexible silicon photonic accelerated compute cluster designed to accelerate multi-tenant distributed DL training workloads. Flex-SiPAC takes a co-design approach that combines a silicon photonic hardware platform with a tailored collective algorithm, optimized to leverage the unique physical properties of the architecture. The hardware platform integrates a novel wavelength-reconfigurable transceiver design and a micro-resonator-based wavelength-reconfigurable switch, enabling the system to achieve flexible bandwidth steering in the wavelength domain. The collective algorithm is designed to support reconfigurable topologies, enabling efficient all-reduce communications that are commonly used in DL training. The feasibility of the Flex-SiPAC architecture is demonstrated through two testbed experiments. First, an optical testbed experiment demonstrates the flexible routing of wavelengths by shuffling an array of input wavelengths using a custom-designed spatial-wavelength selective switch. Second, a four-GPU testbed running two DL workloads shows a 23% improvement in job completion time compared to a similarly sized leaf-spine topology. We further evaluate Flex-SiPAC using large-scale simulations, which show that Flex-SiPAC is able to reduce the communication time by 26% to 29% compared to state-of-the-art compute clusters under representative collective operations.

Wu, Zhenguo (ORCID:0000000322847985)↗

NASA NTRS1973

Study and design of laser communications system for space shuttle

The design, development and operation are described of the laser communications system developed for potential space shuttle application. A brief study was conducted to identify the need, if any, for narrow bandwidth space-to-space communication on the shuttle vehicles. None have been specifically identified that could not be accommodated with existing equipments. The key technical features developed in this hardware are the conically scanned tracker for optimized track while communicating with a single detector, and the utilization of a common optical carrier frequency for both transmission and detection. This latter feature permits a multiple access capability so that several transceivers can communicate with one another. The conically scanned tracker technique allows the received signal energy to be efficiently divided between the tracking and communications functions within a common detector.

Source record↗

NASA NTRS1979

Concepts for 18/30 GHz satellite communication system study. Executive summary

An examination of a multiplicity of interconnected parameters ranging from specific technology details to total system economic costs for satellite communication systems at the 18/30 GHz transmission bands are presented. It was determined that K sub A band systems can incur a small communications outage during very heavy rainfall periods and that reducing the outage to zero would lead to prohibitive system costs. On the other hand, the economics of scale, ie, one spacecraft accommodating 2.5 GHz of bandwidth coupled with multiple beam frequency reuse, leads to very low costs for those users who can tolerate the 5 to 50 hours per year of downtime. A multiple frequency band satellite network can provide the ultimate optimized match to the consumer performance/economics demands.

Baker, M.↗