arXiv subjects

Keith Powell

Publications and source records attributed to Keith Powell.

16 recordsLinked to original sources

arXiv2026

Robo-AO-2: entering the era of automated science operations, hybrid wavefront sensing, and adaptive secondary integration

We present the first science results and new technical milestones from the Robo-AO-2 facility at the University of Hawaii 2.2-m telescope. Following successful commissioning, the system began science operations in 2025. We are starting a large-scale survey of young stars in the Scorpius-Centaurus association to detect sub-stellar companions, vetting the Habitable Worlds Observatory Target Stars and Systems list, and discovering stellar blends for TESS exoplanet host candidates. We report on the commissioning of the natural guide star wavefront sensor, supporting science and future hybrid laser-stellar sensing demonstrations. Finally, we detail the automation of the telescope facility, Robo88, via updated telescope control systems and absolute encoders, and the integration of Robo-AO-2 with the telescope's new adaptive secondary mirror.

astro-ph.IM↗

arXiv2026

Heterogeneous Transfer of Thin Film BaTiO$_3$ onto Silicon for Device Fabrication

Thin film BaTiO$_3$ has one of the highest known Pockels coefficients (>1200 pm/V), making it an attractive material for use in electro-optic devices. It is advantageous to integrate BaTiO$_3$ on silicon to enable complementary metal-oxide-semiconductor (CMOS) compatible processing. However, synthesis of high-quality BaTiO$_3$ directly on silicon remains a challenge. Here, we synthesize BaTiO$_3$ using hybrid metal-organic molecular beam epitaxy (hMBE) and demonstrate its transfer onto silicon using thermocompression bonding and chemical lift-off. Hybrid metal-organic MBE enables self-regulated synthesis of highly stoichiometric thin films at high growth rates (>100nm/hr). Our transfer method results in millimeter-scale areas of atomically flat, crack-free BaTiO$_3$ making it a potentially scalable method. Finally, we demonstrate the applicability of our process to device fabrication through characterization of lithographically-patterned and etch-transferred sub-micron features.

cond-mat.mtrl-sci↗

arXiv2025

Spectral-temporal processing using integrated recursive electro-optic circuit

Advances in integrated photonics have enabled unprecedented level of control of light, powering a wide range of photonic technologies from communications and computing to precision metrology and quantum information. However, the conventional on-chip optical signal processing approaches based on optical waveguides and cavities suffer from their large physical footprint and narrow operating bandwidth, respectively. To address this, we propose and experimentally demonstrate, using the thin film lithium niobate (TFLN) photonics, a modular, recursive optical signal processing framework. In our approach, fast electro-optic (EO) switch is used to either keep an optical packet inside the loop, with the processing element embedded within, or to release it from the loop and direct it towards the output waveguide. By configuring the switch on a timescale shorter than a time the photon spends traversing the loop, this architecture can achieve different optical pathlengths in a compact footprint without sacrificing optical bandwidth. As an example, we embed a phase-modulator (PM) inside the loop and demonstrate a frequency shift of optical packets up to 420 GHz, using only a 3 GHz sinusoidal microwave signal. By replacing the PM with chirped Bragg gratings (CBG), we demonstrate recursive delay line featuring large group delays of 28 ps/nm over a 30 nm optical bandwidth. Finally, by introducing asymmetric Mach-Zehnder interferometer (AMZI) inside the loop, we demonstrate a reconfigurable differentiation of the optical packet in time, up to an unprecedented fifth-order. Our results establish a powerful and scalable platform for multifunctional photonic processing, setting the stage for next-generation integrated systems with ultrahigh reconfigurability and spectral-temporal versatility.

physics.optics↗

arXiv2025

A sub-volt near-IR lithium tantalate electro-optic modulator

We demonstrate a low-loss integrated electro-optic Mach-Zehnder modulator in thin-film lithium tantalate at 737 nm, featuring a low half-wave voltage-length product of 0.65 V$\cdot$cm, an extinction ratio of 30 dB, low optical loss of 5.3 dB, and a detector-limited bandwidth of 20 GHz. A small $<2$ dB DC bias drift relative to quadrature bias is measured over 16 minutes using 4.3 dBm of on-chip power in ambient conditions, which outperforms the 8 dB measured using a counterpart thin-film lithium niobate modulator. Finally, an optical loss coefficient of 0.5 dB/cm for a thin-film lithium tantalate waveguide is estimated at 638 nm using a fabricated ring resonator.

physics.optics↗

arXiv2025

Robust Poling and Frequency Conversion on Thin-Film Periodically Poled Lithium Tantalate

We explore a robust fabrication process for periodically-poled thin-film lithium tantalate (PP-TFLT) by systematically varying fabrication parameters and confirming the quality of inverted domains with second-harmonic microscopy (SHM). We find a periodic poling recipe that can be applied to both acoustic-grade and optical-grade film, electrode material, and presence of an oxide interlayer. By using a single high-voltage electrical pulse with peak voltage time of 10 ms or less and a ramp-down time of 90 s, rectangular poling domains are established and stabilized in the PP-TFLT. We employ our robust periodic poling process in a controllable pole-after-etch approach to produce PP-TFLT ridge waveguides with normalized second harmonic generation (SHG) conversion efficiencies of 208 %W-1cm-2 from 1550 nm to 775 nm in line with the theoretical value of 244 %W-1cm-2. This work establishes a high-performance poling process and demonstrates telecommunications band SHG for thin-film lithium tantalate, expanding the capabilities of the platform for frequency mixing applications in quantum photonics, sensing, and spectroscopy.

physics.optics↗

arXiv2024

Integrated electro-optic digital-to-analog link for efficient computing and arbitrary waveform generation

The rapid growth in artificial intelligence and modern communication systems demands innovative solutions for increased computational power and advanced signaling capabilities. Integrated photonics, leveraging the analog nature of electromagnetic waves at the chip scale, offers a promising complement to approaches based on digital electronics. To fully unlock their potential as analog processors, establishing a common technological base between conventional digital electronic systems and analog photonics is imperative to building next-generation computing and communications hardware. However, the absence of an efficient interface has critically challenged comprehensive demonstrations of analog advantage thus far, with the scalability, speed, and energy consumption as primary bottlenecks. Here, we address this challenge and demonstrate a general electro-optic digital-to-analog link (EO-DiAL) enabled by foundry-based lithium niobate nanophotonics. Using purely digital inputs, we achieve on-demand generation of (i) optical and (ii) electronic waveforms at information rates up to 186 Gbit/s. The former addresses the digital-to-analog electro-optic conversion challenge in photonic computing, showcasing high-fidelity MNIST encoding while consuming 0.058 pJ/bit. The latter enables a pulse-shaping-free microwave arbitrary waveform generation method with ultrabroadband tunable delay and gain. Our results pave the way for efficient and compact digital-to-analog conversion paradigms enabled by integrated photonics and underscore the transformative impact analog photonic hardware may have on various applications, such as computing, optical interconnects, and high-speed ranging.

physics.optics↗

arXiv2024

Integrated lithium niobate photonic computing circuit based on efficient and high-speed electro-optic conversion

Here we show a photonic computing accelerator utilizing a system-level thin-film lithium niobate circuit which overcomes this limitation. Leveraging the strong electro-optic (Pockels) effect and the scalability of this platform, we demonstrate photonic computation at speeds up to 1.36 TOPS while consuming 0.057 pJ/OP. Our system features more than 100 thin-film lithium niobate high-performance components working synergistically, surpassing state-of-the-art systems on this platform. We further demonstrate binary-classification, handwritten-digit classification, and image classification with remarkable accuracy, showcasing our system's capability of executing real algorithms. Finally, we investigate the opportunities offered by combining our system with a hybrid-integrated distributed feedback laser source and a heterogeneous-integrated modified uni-traveling carrier photodiode. Our results illustrate the promise of thin-film lithium niobate as a computational platform, addressing current bottlenecks in both electronic and photonic computation. Its unique properties of high-performance electro-optic weight encoding and conversion, wafer-scale scalability, and compatibility with integrated lasers and detectors, position thin-film lithium niobate photonics as a valuable complement to silicon photonics, with extensions to applications in ultrafast and power-efficient signal processing and ranging.

physics.optics↗

arXiv2024

Commissioning results from the Robo-AO-2 facility for rapid visible and near-infrared AO imaging

We installed the next-generation automated laser adaptive optics system, Robo-AO-2, on the University of Hawaii 2.2-m telescope on Maunakea in 2023. We engineered Robo-AO-2 to deliver robotic, diffraction-limited observations at visible and near-infrared wavelengths in unprecedented numbers. This new instrument takes advantage of upgraded components, manufacturing techniques and control; and includes a parallel reconfigurable natural guide star wavefront sensor with which to explore hybrid wavefront sensing techniques. We present the results of commissioning in 2023 and 2024.

astro-ph.IM↗

arXiv2024

High-power and narrow-linewidth laser on thin-film lithium niobate enabled by photonic wire bonding

Thin-film lithium niobate (TFLN) has emerged as a promising platform for the realization of high performance chip-scale optical systems, spanning a range of applications from optical communications to microwave photonics. Such applications rely on the integration of multiple components onto a single platform. However, while many of these components have already been demonstrated on the TFLN platform, to date, a major bottleneck of the platform is the existence of a tunable, high-power, and narrow-linewidth on-chip laser. Here, we address this problem using photonic wire bonding to integrate optical amplifiers with a thin-film lithium niobate feedback circuit, and demonstrate an extended cavity diode laser yielding high on-chip power of 78 mW, side mode suppression larger than 60 dB and wide wavelength tunability over 43 nm. The laser frequency stability over short timescales shows an ultra-narrow intrinsic linewidth of 550 Hz. Long-term recordings indicate a high passive stability of the photonic wire bonded laser with 58 hours of mode-hop-free operation, with a trend in the frequency drift of only 4.4 MHz/h. This work verifies photonic wire bonding as a viable integration solution for high performance on-chip lasers, opening the path to system level upscaling and Watt-level output powers.

physics.optics↗

arXiv2024

Stable electro-optic modulators using thin-film lithium tantalate

We demonstrate electro-optic modulators realized in low-loss thin-film lithium tantalate with superior DC-stability (<1 dB power fluctuation from quadrature with 12.1 dBm input) compared to equivalent thin-film lithium niobate modulators (5 dB fluctuation) over 46 hours.

physics.optics↗

arXiv2022

Handover Experiments with UAVs: Software Radio Tools and Experimental Research Platform

Mobility management is the key feature of cellular networks. When integrating unmanned aerial vehicles (UAVs) into cellular networks, their cell association needs to be carefully managed for coexistence with other cellular users. UAVs move in three dimensions and may traverse several cells on their flight path, and so may be subject to several handovers. In order to enable research on mobility management with UAV users, this paper describes the design, implementation, and testing methodology for handover experiments with aerial users. We leverage software-defined radios (SDRs) and implement a series of tools for preparing the experiment in the laboratory and for taking it outdoors for field testing. We use solely commercial off-the-shelf hardware, open-source software, and an experimental license to enable reproducible and scalable experiments. Our initial outdoor results with two SDR base stations connected to an open-source software core network, implementing the 4G long-term evolution protocol, and one low altitude UAV user equipment demonstrate the handover process.

eess.SP↗

arXiv2021

Open-Source Software Radio Platform for Research on Cellular Networked UAVs -- It Works!

Cellular network-connected unmanned aerial vehicles (UAVs) experience different radio propagation conditions than radio nodes on the ground. Therefore, it has become critical to investigate the performance of aerial radios, both theoretically and through field trials. In this paper, we consider low-altitude aerial nodes that are served by an experimental cellular network. We provide a detailed description of the hardware and software components needed for establishing a broadband wireless testbed for UAV communications research using software radios. Results show that a testbed for innovation in UAV communications and networking is feasible with commercial off-the-shelf hardware, open-source software, and low-power signaling.

eess.SP↗

arXiv2021

Network and Physical Layer Attacks and countermeasures to AI-Enabled 6G O-RAN

Artificial intelligence (AI) will play an increasing role in cellular network deployment, configuration and management. This paper examines the security implications of AI-driven 6G radio access networks (RANs). While the expected timeline for 6G standardization is still several years out, pre-standardization efforts related to 6G security are already ongoing and will benefit from fundamental and experimental research. The Open RAN (O-RAN) describes an industry-driven open architecture and interfaces for building next generation RANs with AI control. Considering this architecture, we identify the critical threats to data driven network and physical layer elements, the corresponding countermeasures, and the research directions.

cs.CR↗

arXiv2020

UAV-Assisted Attack Prevention, Detection, and Recovery of 5G Networks

Unmanned aerial vehicles (UAVs) are emerging as enablers for supporting many applications and services, such as precision agriculture, search and rescue, temporary network deployment or coverage extension, and security. UAVs are being considered for integration into emerging 5G networks as aerial users or network support nodes. We propose to leverage UAVs in 5G to assist in the prevention, detection, and recovery of attacks on 5G networks. Specifically, we consider jamming, spoofing, eavesdropping and the corresponding mitigation mechanisms that are enabled by the versatility of UAVs. We introduce the hot zone, safe zone and UAV-based secondary authorization entity, among others, to increase the resilience and confidentiality of 5G radio access networks and services. We present simulation results and discuss open issues and research directions, including the need for experimental evaluation and a research platform for prototyping and testing the proposed technologies.

eess.SP↗

arXiv2020

Software Radios for Unmanned Aerial Systems

As new use cases are emerging for unmanned aerial systems (UAS), advanced wireless communications technologies and systems need to be implemented and widely tested. This requires a flexible platform for development, deployment, testing and demonstration of wireless systems with ground and aerial nodes, enabling effective 3D mobile communications and networking. In this paper, we provide a comparative overview of software-defined radios (SDRs), with a specific focus on SDR hardware and software that can be used for aerial wireless experimentation and research. We discuss SDR hardware requirements, features of available SDR hardware that can be suitable for small UAS, and power measurements carried out with a subset of these SDR hardware. We also present SDR software requirements, available open-source SDR software, and calibration/benchmarking of SDR software. As a case study, we present AERPAW: Aerial Experimentation and Research Platform for Advanced Wireless, and discuss various different experiments that can be supported in that platform using SDRs, for verification/testing of future wireless innovations, protocols, and technologies.

cs.NI↗

arXiv2008

On-sky wide field adaptive optics correction using multiple laser guide stars at the MMT

We describe results from the first astronomical adaptive optics system to use multiple laser guide stars, located at the 6.5-m MMT telescope in Arizona. Its initial operational mode, ground-layer adaptive optics (GLAO), provides uniform stellar wavefront correction within the 2 arc minute diameter laser beacon constellation, reducing the stellar image widths by as much as 53%, from 0.70 to 0.33 arc seconds at lambda = 2.14 microns. GLAO is achieved by applying a correction to the telescope's adaptive secondary mirror that is an average of wavefront measurements from five laser beacons supplemented with image motion from a faint stellar source. Optimization of the adaptive optics system in subsequent commissioning runs will further improve correction performance where it is predicted to deliver 0.1 to 0.2 arc second resolution in the near-infrared during a majority of seeing conditions.

astro-ph↗