Coupled Opto-Electronic Oscillators
These experiments demonstrate that COEOs are promising compact sources for generating low jitter optical pulses and low phase noise RF/millimeter wave signals.
opto-electronic oscillator (COEO) ring laser mode-↗
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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
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Semiconductor Laser Based Coupled Opto-Electronic Oscillators
These experiments demonstrate that COEOs are promising compact sources for generating low jitter optical pulses and low phase noise RF/millimeter wave signals.
Opto-Electronic Oscillator optical pulse RF photon↗
Mixed-mode oscillations in a three-timescale coupled Morris–Lecar system
Mixed-mode oscillations (MMOs) are complex oscillatory behaviors of multiple-timescale dynamical systems in which there is an alternation of large-amplitude and small-amplitude oscillations. It is well known that MMOs in two-timescale systems can arise either from a canard mechanism associated with folded node singularities or a delayed Andronov–Hopf bifurcation (DHB) of the fast subsystem. While MMOs in two-timescale systems have been extensively studied, less is known regarding MMOs emerging in three-timescale systems. In this work, we examine the mechanisms of MMOs in coupled Morris–Lecar neurons with three distinct timescales. We investigate two kinds of MMOs occurring in the presence of a singularity known as canard-delayed-Hopf (CDH) and in cases where CDH is absent. In both cases, we examine how features and mechanisms of MMOs vary with respect to variations in timescales. Our analysis reveals that MMOs supported by CDH demonstrate significantly stronger robustness than those in its absence. Moreover, we show that the mere presence of CDH does not guarantee the occurrence of MMOs. This work yields important insights into conditions under which the two separate mechanisms in two-timescale context, canard and DHB, can interact in a three-timescale setting and produce more robust MMOs, particularly against timescale variations.
97 MATHEMATICS AND COMPUTING↗
Analysis of a coupled roll spiral mode, pilot induced oscillation experienced with the M-2F2 lifting body
Analysis of coupled roll-spiral-mode, pilot induced oscillation occurring with M-2F2 lifting body
Kempel, R. W.↗
Fermi’s Golden Rule Rate Expression for Transitions Due to Nonadiabatic Derivative Couplings in the Adiabatic Basis
Starting from a general molecular Hamiltonian expressed in the basis of adiabatic electronic and nuclear position states, where a compact and complete expression for the nonadiabatic derivative coupling (NDC) Hamiltonian term is obtained, we provide a general analysis of the Fermi’s golden rule (FGR) rate expression for nonadiabatic transitions between adiabatic states. We then consider a quasi-adiabatic approximation that uses crude adiabatic states and NDC couplings, both evaluated at the minimum potential energy configuration of the initial adiabatic state, for the definition of the zeroth and first-order terms of the Hamiltonian. Although the application of this approximation is rather limited, it allows deriving a general FGR rate expression without further approximation while accounting for non-Condon contribution to the FGR rate arising from momentum operators of NDC terms and its coupling with vibronic displacements. For a generic and widely used model where all nuclear degrees of freedom and environmental effects are represented as linearly coupled harmonic oscillators, we derive a closed-form FGR rate expression that requires only Fourier transform. The resulting rate expression includes quadratic contributions of NDC terms and their couplings to Franck–Condon modes, which require evaluation of two additional bath spectral densities in addition to the conventional one that appears in a typical FGR rate theory based on the Condon approximation. Model calculations for the case where nuclear vibrations consist of both a sharp high-frequency mode and an Ohmic bath spectral density illustrate new features and implications of the rate expression. We then apply our theoretical expression to the nonradiative decay from the first excited singlet state of azulene, which illustrates the utility and implications of our theoretical results.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
On identification of structures with internal resonances
Identification of structures which exhibit modal interactions is considered, and the difficulties experienced due to these interactions are examined. Free oscillations of quadratically and cubically coupled pairs of oscillators are analytically studied to illustrate nonlinear interactions between structural modes involved in two-to-one and one-to-one frequency relationships. In light of this study, results obtained from application of the eigensystem realization algorithm toward identification of a beam-mass structure with a two-to-one frequency relationship and quadratic coupling are presented and discussed.
Balachandran, B.↗
Nonlinear behavior of acoustic waves in combustion chambers
The nonlinear growth and limiting amplitude of acoustic waves in a combustion chamber are considered. A formal framework is provided within which practical problems can be treated with a minimum of effort and expense. The general conservation equations were expanded in two small parameters, one characterizing the mean flow field and one measuring the amplitude of oscillations, and then combined to yield a nonlinear inhomogeneous wave equation. The unsteady pressure and velocity fields were expressed as syntheses of the normal modes of the chamber, but with unknown time-varying amplitudes. This procedure yielded a representation of a general unsteady field as a system of coupled nonlinear oscillators. The system of nonlinear equations was treated by the method of averaging to produce a set of coupled nonlinear first order differential equations for the amplitudes and phases of the modes. The analysis is applicable to any combustion chamber. The most interesting applications are probably to solid rockets, liquid rockets, or thrust augmentors on jet engines.
Culick, F. E. C.↗
Solar activity and the coupling of g-mode oscillations
The solar g-mode oscillations depend strongly on the difference between actual and adiabatic temperature gradients in the solar interior. Solar activity anchored at the base of the convection zone disturbs this difference and couples modes with similar eigenfrequencies. This coupling is computed, and the possible observational effects at the photosphere are discussed. These include a scatter in the frequencies deduced for weakly coupled modes and the apparent temporal behavior of strongly coupled modes.
Wentzel, D. G.↗
The rate of occurrence of dayside Pc 3,4 pulsations - The L-value dependence of the IMF cone angle effect
When the angle of the IMF to the earth sun line is 15 deg or less, the occurrence rate of dayside Pc 3,4 pulsations in 7-8 times the average at L values of 2.4-2.8, and 2.2-3.5 times the average at L of 4-4.3. These waves disappear when the IMF is nearly at right angles to the sun-earth line. Such observations are consistent with a source originating in the waves upstream of the subsolar bow shock, which are transported by convection to the magnetopause. There, they couple to oscillations of magnetospheric field lines. Because the magnetospheric plasma's index of refraction decreases with radial distance except at the plasmapause, inwardly propagating waves should be refracted away from the radial direction. To reach low L values, the waves should therefore couple near the stagnation point and propagate nearly radially inwards. The streamline geometry and its connection to the foreshock region is illustrated for various IMF orientations, using a simple approximation to the magnetosheath flow field.
Russell, C. T.↗
Viscous/inviscid interaction procedure for high-amplitude oscillating airfoils
A coupling procedure between an inviscid potential theoretical panel method and an unsteady interactive boundary layer calculation procedure has been developed for calculating unsteady airloads on oscillating airfoils in the high-incidence regime. A coordinate transformation into a time-dependent surface coordinate has been incorporated into the boundary layer code to avoid a quasi-steady treatment of the viscous flow equations in the leading edge region. The upper and lower sides of the airfoil are calculated with the complete set of unsteady equations. The interactive region of the boundary layer calculation can be straightforwardly extended over the complete upper surface of the airfoil while accounting for interaction areas in the region of strong adverse pressure gradients with incipient separation. Comparisons of sample calculations with experimental data show reasonably good agreement even for unsteady drag coefficients.
Geissler, W.↗
Injection locking and coupling dynamics in superconducting nanowire-based cryogenic oscillators
Oscillators designed to function at cryogenic temperatures play a critical role in superconducting electronics and quantum computing by providing stable, low-noise signals with minimal energy loss. Here, in this work, we present a comprehensive numerical study of injection locking and mutual coupling dynamics in superconducting nanowire (ScNW)-based cryogenic oscillators. Using the design space of a standalone ScNW-based oscillator, we investigate two critical mechanisms that govern frequency synchronization and signal coordination in cryogenic computing architectures: (1) injection locking induced by an external AC signal with a frequency near the oscillator's natural frequency, and (2) the mutual coupling dynamics between two ScNW oscillators under varying coupling strengths. We identify key design parameters—such as shunt resistance, nanowire inductance, and coupling strength—that govern the locking range. Additionally, we examine how the amplitude of the injected signal affects the amplitude of the locked oscillation, offering valuable insights for power-aware oscillator synchronization. Furthermore, we analyze mutual synchronization between coupled ScNW oscillators using capacitive and resistive coupling elements. Our results reveal that the phase difference between oscillators can be controlled by tuning the coupling strength, enabling programmable phase-encoded information processing. These findings could enable building ScNW-based oscillatory neural networks, synchronized cryogenic logic blocks, and on-chip cryogenic resonator arrays.
Artificial neural networks↗
SympGNNs: Symplectic Graph Neural Networks for identifying high-dimensional Hamiltonian systems and node classification
Existing neural network models to learn Hamiltonian systems, such as SympNets, although accurate in low-dimensions, struggle to learn the correct dynamics for high-dimensional many-body systems. Herein, we introduce Symplectic Graph Neural Networks (SympGNNs) that can effectively handle system identification in high-dimensional Hamiltonian systems, as well as node classification. SympGNNs combine symplectic maps with permutation equivariance, a property of graph neural networks. Specifically, we propose two variants of SympGNNs: (i) G-SympGNN and (ii) LA-SympGNN, arising from different parameterizations of the kinetic and potential energy. We demonstrate the capabilities of SympGNN on two physical examples: a 40-particle coupled Harmonic oscillator, and a 2000-particle molecular dynamics simulation in a two-dimensional Lennard-Jones potential. Furthermore, we demonstrate the performance of SympGNN in the node classification task, achieving accuracy comparable to the state-of-the-art. Finally, we also empirically show that SympGNN can overcome the oversmoothing and heterophily problems, two key challenges in the field of graph neural networks.
Deep learning↗
Limit cycle oscillations in the zonal-flow-catalyzed interactions of ion-temperature-gradient turbulence
Limit-cycle oscillations are studied for ion temperature gradient turbulence, which, in the absence of large diamagnetic (mean) shear flows, saturates through energy transfer from unstable modes to large-scale stable modes via zonal-flow intermediary modes. Oscillations of zonal flow and turbulence levels are strongly constrained by the reactive, largely non-dissipative character of the zonal flows. Since existing predator–prey models for observed oscillations in experiments do not include energy transfer through zonal flows to stable modes, low-order fluid models with this physics are constructed and investigated. A simple three-wave truncation produces low-amplitude zonal flows that slowly oscillate around a zero mean, with turbulence oscillations between coupled wavenumbers that exceed linear frequencies by orders of magnitude. This inconsistency with experimental observations is caused by the weak non-linear drive of zonal flows in three-wave systems and the lack of multiple-wavenumber turbulent interactions. A more comprehensive model that preserves multiple wavenumber interactions within the context of conservative zonal-flow-mediated energy transfer to stable modes accurately reflects observed dynamics when the phase between stable and unstable modes is occasionally randomized.
Li, P. -Y. (ORCID:0000000295254171)↗
Twists and turns: stacking and structure-dependent optical response in MoS 2 nanoscrolls
Transition metal dichalcogenide (TMD) nanoscrolls (NSs), specifically MoS 2 NSs, present unique structural and optical properties, exhibiting prominent photoluminescence (PL) signals despite their multilayer nature. Here, this study investigates the structural and spectroscopic characteristics of MoS 2 NSs, correlating them to show the effects of reduced interlayer interactions on excitons of MoS 2 NSs. The reduction in interlayer interaction arises from two main factors: (1) symmetry-broken mixed stacking due to misalignment between layers and (2) a highly inhomogeneous strain profile generated by the Archimedean spiral geometry with positive eccentricity. Transmission electron microscopy, field emission scanning electron microscopy, atomic force microscopy, Raman spectroscopy, and second harmonic generation measurements confirm these findings. Low-temperature PL spectroscopy explores the impact of reduced interlayer interactions on exciton properties such as exciton–phonon coupling and oscillator strength. This study provides crucial insights into the structure, stacking and unique optical properties of TMD NSs, advancing the understanding of interlayer interactions and their impacts in complex quasi-one-dimensional nanostructures.
25 ENERGY STORAGE↗
Mechanical cosmology: Simulating scalar fluctuations in expanding universes using synthetic mechanical lattices
Inspired by recent advances in observational astrophysics and continued explorations in the field of analog gravity, we discuss the prospect of simulating models of cosmology within the context of synthetic mechanical lattice experiments. We focus on the physics of expanding universe scenarios described by the Friedmann-Lemaître-Robertson-Walker (FLRW) metric. Specifically, quantizing scalar fluctuations in a background FLRW spacetime leads to a quadratic bosonic Hamiltonian with temporally varying pair production terms. Here we present a mapping that provides a one-to-one correspondence between these classes of cosmology models and feedback-coupled mechanical oscillators. As proof of principle, we then perform experiments on a synthetic mechanical lattice composed of such oscillators. We simulate two different FLRW expansion scenarios with universes dominated by vacuum energy and matter and discuss our experimental results. Published by the American Physical Society 2025
Rhyno, Brendan (ORCID:0000000235266998)↗
Atlas-Centaur-Surveyor longitudinal dynamics test
Dynamics tests of engine-structure coupled longitudinal oscillations of Atlas Centaur Surveyor
LONGITUDINAL STABILITY↗
Linear physical chains with Sturm-Liouville characteristic polynomials
Linear physical chains with Sturm-Liouville characteristic polynomials used for natural frequency determination of coupled harmonic oscillators or electrical analog
Cook, F. L.↗
Investigation of characteristics of feed system instabilities
The relationship between the structural and feed system natural frequencies in structure-propulsion system coupled longitudinal oscillations (pogo) is investigated. The feed system frequencies are usually very dependent upon the compressibility (compliance) of cavitation bubbles that exist to some extent in all operating turbopumps. This document includes: a complete review of cavitation mechanisms; development of a turbopump cavitation compliance model; an accumulation and analysis of all available cavitation compliance test data; and a correlation of empirical-analytical results. The analytical model is based on the analysis of flow relative to a set of cascaded blades, having any described shape, and assumes phase changes occur under conditions of isentropic equilibrium. Analytical cavitation compliance predictions for the J-2 LOX, F-1 LOX, H-1 LOX and LR87 oxidizer turbopump inducers do not compare favorably with test data. The model predicts much less cavitation than is derived from the test data. This implies that mechanisms other than blade cavitation contribute significantly to the total amount of turbopump cavitation.
Vaage, R. D.↗