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At least 1,153 records · Page 64
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Quasinormal Coupled-Mode Analysis of Dynamic Gain in Exceptional-Point Lasers

One of the key features of lasers operating near exceptional points (EPs) is that the gain medium can support an oscillating population inversion above a pump threshold, leading to self-modulated laser dynamics. This unusual behavior opens up new possibilities for frequency comb generation and temporal modulation. However, the dynamic population inversion couples signals with different frequencies and is difficult to capture using conventional temporal coupled-mode theory (TCMT) based on stationary saturable gain. In this paper, we develop a perturbative coupled-mode analysis framework to capture the spatial-temporal dynamics of near-EP lasers. By decomposing discrete frequency generation into multiple excitations of resonant modes, our analysis establishes a minimal physical model that translates the local distribution of dynamic population-inversion into a resonant modal interpretation of laser gain. Furthermore, this work enables the exploration of unique properties in this self-time-modulated systems, such as time-varying scattering and nonreciprocal transmission.

Absorption↗
NASA NTRS1982

Chorus-related electrostatic bursts in the earth's outer magnetosphere

Studies of wideband plasma wave data from the ISEE 1 and ISEE 2 spacecraft are reported which reveal that whistler-mode chorus emissions in the earth's outer magnetosphere are often accompanied by high-frequency bursts of electrostatic noise. The chorus features that correlate to the electrostatic bursts have a hook-like frequency-time variation in the frequency range 200-400 Hz, and the associated electrostatic bursts have a frequency range of about 4-8 kHz. In some cases these electrostatic bursts have a harmonic frequency structure with a frequency spacing corresponding to the chorus frequency. This harmonic structure apparently results from the fact that the burst intensity is modulated at the chorus frequency.

Reinleitner, L. A.↗
NASA NTRS1992

Experimental measurement and theoretical modeling of microwave scattering and the structure of the sea surface influencing radar observations from space

The electromagnetic (EM) bias 'epsilon' is an error present in radar altimetry of the ocean due to the nonuniform reflection from wave troughs and crests. The EM bias is defined as the difference between the mean reflecting surface and the mean sea surface. A knowledge of the EM bias is necessary to permit error reduction in mean sea level measurements by satellite radar altimeters. Direct measurements of the EM bias were made from a Shell Offshore oil production platform in the Gulf of Mexico for a six month period during 1989 and 1990. Measurements of the EM bias were made at 5 and 14 Ghz. During the EM bias experiments by Melville et al., a wire wave gauge was used to obtain the modulation of the high frequency waves by the low frequency waves. It became apparent that the EM bias was primarily caused by the modulation of the short waves. This was reported by Arnold et al. The EM bias is explained using physical optics scattering and an empirical model for the short wave modulation. Measurements of the short wave modulation using a wire wave gauge demonstrated a linear dependence of the normalized bias on the short wave modulation strength, M. The theory accurately predicts this dependence by the relation epsilon = -alphaMH sub 1/3. The wind speed dependence of the normalized bias is explained by the dependence of the short wave modulation strength on the wind speed. While other effects such as long wave tilt and curvature will have an effect on the bias, the primary cause of the bias is shown to be due to the short wave modulation. This report will present a theory using physical optics scattering and an empirical model of the short wave modulation to estimate the EM bias. The estimated EM bias will be compared to measurements at C and Ku bands.

Arnold, David↗
DOE OSTI2022

Silicon nitride stress-optic microresonator modulator for optical control applications

Modulation-based control and locking of lasers, filters and other photonic components is a ubiquitous function across many applications that span the visible to infrared (IR), including atomic, molecular and optical (AMO), quantum sciences, fiber communications, metrology, and microwave photonics. Today, modulators used to realize these control functions consist of high-power bulk-optic components for tuning, sideband modulation, and phase and frequency shifting, while providing low optical insertion loss and operation from DC to 10s of MHz. In order to reduce the size, weight and cost of these applications and improve their scalability and reliability, modulation control functions need to be implemented in a low loss, wafer-scale CMOS-compatible photonic integration platform. The silicon nitride integration platform has been successful at realizing extremely low waveguide losses across the visible to infrared and components including high performance lasers, filters, resonators, stabilization cavities, and optical frequency combs. Yet, progress towards implementing low loss, low power modulators in the silicon nitride platform, while maintaining wafer-scale process compatibility has been limited. Here we report a significant advance in integration of a piezo-electric (PZT, lead zirconate titanate) actuated micro-ring modulation in a fully-planar, wafer-scale silicon nitride platform, that maintains low optical loss (0.03 dB/cm in a 625 µm resonator) at 1550 nm, with an order of magnitude increase in bandwidth (DC - 15 MHz 3-dB and DC - 25 MHz 6-dB) and order of magnitude lower power consumption of 20 nW improvement over prior PZT modulators. The modulator provides a >14 dB extinction ratio (ER) and 7.1 million quality-factor (Q) over the entire 4 GHz tuning range, a tuning efficiency of 162 MHz/V, and delivers the linearity required for control applications with 65.1 dB·Hz 2/3 and 73.8 dB·Hz 2/3 third-order intermodulation distortion (IMD3) spurious free dynamic range (SFDR) at 1 MHz and 10 MHz respectively. We demonstrate two control applications, laser stabilization in a Pound-Drever Hall (PDH) lock loop, reducing laser frequency noise by 40 dB, and as a laser carrier tracking filter. This PZT modulator design can be extended to the visible in the ultra-low loss silicon nitride platform with minor waveguide design changes. This integration of PZT modulation in the ultra-low loss silicon nitride waveguide platform enables modulator control functions in a wide range of visible to IR applications such as atomic and molecular transition locking for cooling, trapping and probing, controllable optical frequency combs, low-power external cavity tunable lasers, quantum computers, sensors and communications, atomic clocks, and tunable ultra-low linewidth lasers and ultra-low phase noise microwave synthesizers.

Wang, Jiawei (ORCID:0000000257965220)↗
NASA NTRS1983

Noise generation by a finite span swept airfoil

An analysis is developed for the noise generated by a convected gust interacting with a single swept airfoil. The airfoil spans a channel with infinite parallel walls which contains a uniform subsonic mean flow. High frequency gusts, for which the noise generation is concentrated at the airfoil leading edge, are considered. The solution utilizes the Wiener-Hopf technique applied to the equations in nonorthogonal coordinates. Closed form expressions for the farfield acoustic pressure are obtained. The convected gust is assumed to be a plane wave, i.e., the lowest order spanwise mode. Because of the swept leading edge, this convected gust generates acoustic modes of all orders. At high frequencies and low Mach numbers, moderate sweep angles produce substantial noise reductions. The residual acoustic energy is carried mainly by the higher modes. The airfoil sweep also modulates the basic high frequency farfield directivity, producing lobed patterns. The angular modulation of the directivity patterns is most pronounced for the lower order modes.

Kerschen, E. J.↗
DOE OSTI2022

Dispersive optical systems for scalable Raman driving of hyperfine qubits

Hyperfine atomic states are among the most promising candidates for qubit encoding in quantum information processing. In atomic systems, hyperfine transitions are typically driven through a two-photon Raman process by a laser field which is amplitude modulated at the hyperfine qubit frequency. Here we introduce a method for generating amplitude modulation by phase modulating a laser and reflecting it from a highly dispersive optical element known as a chirped Bragg grating. This approach is passively stable, offers high efficiency, and is compatible with high-power laser sources, enabling large Rabi frequencies and improved quantum coherence. We benchmark this approach by globally driving an array of approximately 300 neutral 87 Rb atomic qubits trapped in optical tweezers and obtain Rabi frequencies of 2 MHz with photon-scattering error rates of less than 2×10 -4 per π pulse. This robust approach can be directly integrated with local addressing optics in both neutral atom and trapped ion systems to facilitate high-fidelity single-qubit operations for quantum information processing.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2020

Identity of the JET M-mode and the ASDEX Upgrade I-phase phenomena

An H-mode plasma state free of edge-localized mode (ELM), close to the L-H transition with clear density and temperature pedestal has been observed both at the Joint European Torus (JET) and at the ASDEX Upgrade (AUG) tokamaks usually identified by a low frequency (LFO, 1–2 kHz), m = 1, n = 0 oscillation of the magnetics and the modulation of pedestal profiles. The regime at JET is referred to as M-mode while at AUG as intermediate phase or I-phase. This contribution aims at a comparative analysis of these phenomena in terms of the density and temperature pedestal properties, the magnetic oscillations and symmetries. Lithium beam emission spectroscopy (Li-BES) and reflectometer measurements at JET and AUG show that the M-mode and the I-phase modulates the plasma edge density. A high frequency oscillation of the magnetics and the density at the pedestal is also associated with both the M-mode and the I-phase, and its power is modulated with the LFO frequency. The power modulation happens simultaneously in every Mirnov coil signal where it can be detected. The bursts of the magnetic signals and the density at the pedestal region are followed by the flattening of the density profile, and by a radially outward propagating density pulse in the scrape-off layer (SOL). The analysis of the helium line ratio spectroscopy (He-BES) signals at AUG revealed that the electron temperature is modulated in phase with the density, thus the I-phase modulates the pressure profile gradient. Overall, this analysis gave opportunity to compare Li-BES and He-BES density profiles at different locations suggesting a toroidal and poloidal symmetry of the density modulation. The presented results indicate that the regimes, the AUG I-phase and the JET M-mode, exhibit similar characteristics, which leads to the conclusion that the regimes are likely the same.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

System and method for ultra-high-resolution ranging using RFID

A radio-frequency method for range finding includes modulating a reference signal having an intermediate frequency to a downlink signal having a carrier frequency using a clock signal. The downlink signal is transmitted to a tag using a transceiver. An uplink signal backscattered from the tag is received and demodulated using the clock signal. The uplink signal has a frequency that is a harmonic of the carrier frequency. A distance between the tag and the transceiver is calculated based on a phase of the demodulated uplink signal. A system for range finding includes a transceiver and a processor. The transceiver modulates a reference signal to a downlink signal and transmits the downlink signal. The transceiver receives and demodulates an uplink signal. The processor is configured to receive the demodulated uplink signal and calculate a distance between the tag and the transceiver using a phase of the demodulated uplink signal.

Hui, Xiaonan↗
DOE OSTI2024

System and method for ultra-high-resolution ranging using RFID

A radio-frequency method for range finding includes modulating a reference signal having an intermediate frequency to a downlink signal having a carrier frequency using a clock signal. The downlink signal is transmitted to a tag using a transceiver. An uplink signal backscattered front the tag is received and demodulated using the clock signal. The uplink signal has a frequency that is a harmonic of the carrier frequency. A distance between the tag and the transceiver is calculated based on a phase of the demodulated uplink signal. A system for range finding includes a transceiver and a processor. The transceiver modulates a reference signal to downlink signal and transmits the downlink signal. The transceiver receives and demodulates an uplink signal. The processor is configured to receive the demodulated uplink signal and calculate a distance between the tag and the transceiver using a phase of the demodulated uplink signal.

Hui, Xiaonan↗
NASA NTRS1985

The northern Utah satellite (NUSAT) communications link

During the planning Stages of the NUSAT satellite, an obvious issue to be discussed was the method of communications to be used. The frequencies would have to be high enough to pass through the atmosphere relatively unattenuated but low enough that antennas and transmission lines would not be so critical in length and properties that unexperienced students would have difficulty with handling them. In conversations with the Federal Communications Commission (FCC) in Washington D.C., the frequencies of 450.000 MHz and 137.900 MHz were decided upon and applied for licensing. The 450.000 MHt up-link is used on a "non-interference" basis to control the satellite while the 137.900 MHz frequency down-links data from the satellite. A reasonable amount of time was spent discussing the mods of transmission to be used in communication with the satellite. Representatives of the amateur radio satellite organization, AMSAT, were contacted for ides. This organization seems to favor AM types of emissions such as CW to control their OSCAR series of satellites and also the current Phase 111 unit. NUSAT personnel felt, however, that there would be merit in the improved signal to noise ratio usually obtained in an FM mode. Doppler shift of the transmitted information on the NUSAT also had to be considered. The final decision WBS to use Audio Frequency Shift Keying (AFSK) modulated on an FM carrier. In this mode the audio tones used would not shift frequency with Doppler - only the carrier would shift in frequency. If necessary, the receivers could be fitted with a form of AFC control or simply have enough bandwidth to handle the Doppler shift with an acceptable increase in noise.

Lee Barrett↗
NASA NTRS2018

Solar Irradiance Variability, Influenced by r Modes

A spectrum of the four-decade solar irradiance record has a prominent cluster of power for periodicities near 1 yr. Correlating irradiance with a bandpass filter showed that periodicity values were not constant, but varied sinusoidally with each cycle lasting 14 ± 1 yr. The large modulation amplitude makes solar frequencies ≥1 yr(exp −1) hard to detect at the solar surface. After removing the modulation, a Lomb–Scargle spectrum exposed two true periodicities: 1.006 and 0.920 yr. They are interpreted as the synodic rotation periods of r modes of lowest angular degree (ℓ = 1). The first propagates in the stable interior and the second in the convective envelope perturbed by its several flow fields. The rotational beat period of the two modes is about 10.9 yr. This is close to the average length of a solar cycle and possibly controls this average. The 1.006 yr periodicity dominates most of the filtered irradiance record but an abrupt change to about 0.8 yr occurs in mid-2010. Also found was evidence for higher-degree r modes (ℓ = 2 to 8) and a curious sawtooth modulation with a recurrence period of 2.6 yr.

Wolff, Charles L.↗
NASA NTRS1974

Signal simulator model A-111

A signal simulator designed to function as a signal source for Doppler tracking high data rate receivers is described. The simulator produces modulated signals whose carrier frequency can be varied between 200 and 900 MHz at rates greater than 20 MHz/sec. The modulation is phase shift keying with data rate up to 300 megabits per second.

Flattau, T.↗
NASA NTRS1985

Two-frequency laser-induced fluorescence technique for rapid velocity-field measurements in gas flows

A technique is presented for measurements of two-dimensional velocity fields in gas flows. The single-mode frequency of an argon-ion laser is fixed in the wing of an absorption line of iodine molecules that are seeded at low level in the flow of interest. The emitted fluorescence is detected with an image-intensified 100 x 100 photodiode-array camera. two pairs of counterpropagating laser sheets sequentially probe the flow to determine two velocity components. The frequency in one pair is shifted with respect to the other by an acousto-optic modulator. This two-frequency scheme eliminates the need to determine the slope of the line externally and offers the potential for combined pressure and velocity-field measurements.

Hiller, B.↗
NASA NTRS1987

Pseudomorphic InGaAs/AlGaAs modulation-doped FET's with reduced low-frequency noise and thermally stable performance

A high-performance MODFET structure grown by MBE with the incorporation of a single quantum well In(0.15)Ga(0.85)As layer for the transport of two-dimensional electron gas has been critically examined for its thermal stability at 80 K and low-frequency noise form 0.01 to 10 to the 8th Hz. Experimental results indicate that the behavior of this device in both these respects is much superior when compared with the same behavior of conventional MODFETs. A maximum low-field carrier mobility of 29,000 sq cm/s at 80 K and an average carrier saturation velocity of 2 x 10 to the 7th cm/s at 300 K in a 1-micron gate device clearly indicate that the quality of the pseudomorphic quantum well (InGaAs) layer is either comparable or better than that of the usual GaAs buffer layer. The deep level spectra, obtained through photo-FET measurements, and the low-frequency noise spectra at different temperatures obtained for the new pseudomorphic and conventional MODFET's have clearly indicated that contributions from various deep levels present in the new structure are significantly reduced.

Liu, Shih-Ming↗
NASA NTRS2018

In-Line Electromagnetic Actuator for Fuel Modulation

An in-line electromagnetic actuator with normally-open configuration has been developed for the purpose of exploring its application as a fuel modulator for the active control of combustor thermo-acoustic instabilities. The actuator is based on the spring-coil-plunger mechanism with the plunger designed to be suspended by crossed cantilever beam springs. Operational specification was set for a 1000 psi maximum inlet fuel pressure and modulation for a broad frequency bandwidth that encompasses frequencies that are typically associated with combustor thermo-acoustic instabilities. Various test results demonstrated fuel modulation up to 1200 Hz for coil excitation voltage between 2.3 and 4.6 V and current between 16 and 48 mA, respectively. The initial goal of this work was to demonstrate fuel modulation based on the concept of a normally-open fuel modulator with tethered plunger displacement.

Actuator↗
NASA NTRS2018

In-Line Electromagnetic Actuator for Fuel Modulation

An in-line electromagnetic actuator with normally-open configuration has been developed for the purpose of exploring its application as a fuel modulator for the active control of combustor thermo-acoustic instabilities. The actuator is based on the spring-coil-plunger mechanism with the plunger designed to be suspended by crossed cantilever beam springs. Operational specification was set for a 1000 psi maximum inlet fuel pressure and modulation for a broad frequency bandwidth that encompasses frequencies that are typically associated with combustor thermo-acoustic instabilities. Various test results demonstrated fuel modulation up to 1200 Hz for coil excitation voltage between 2.3 and 4.6 V and current between 16 and 48 mA, respectively. The initial goal of this work was to demonstrate fuel modulation based on the concept of a normally-open fuel modulator with tethered plunger displacement.

Okojie, Robert S.↗