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Results for “FREQUENCY MODULATION”

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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At least 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Flicker: A review of temporal light modulation stimulus, responses, and measures

Flicker has been an important lighting system consideration for over a century. More precise terms are temporal light modulation (TLM) as the stimulus, and responses to TLM as the unwanted visual, cognitive, or physiological consequences. As lighting technology evolved, different forms of TLM emerged, and so did responses to them. Today, some LED systems – encompassing the LED, driver, and control – can result in TLM causing severe unwanted effects, while other LED systems produce no unwanted effects at all. LED systems can deliver a much wider range of luminous waveforms than conventional lighting systems, some exhibiting very high modulation depths. More than any light source before, they can elicit perceptions of the phantom array. Direct flicker effects at modulation frequencies less than about 80 Hz and the stroboscopic effect at frequencies greater than 80 Hz are fairly well understood, but the phantom array effect needs more exploration and characterisation. This review focuses on the technology and research history that led to current metrics for quantifying TLM and human responses to TLM. Notably, visually impaired individuals may exhibit alterations in their response to TLM, but such a discussion is beyond the intent of this review. Thus, the focus is on individuals with normal visual function.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2018

Fourier Spectroscopy as a Method of Investigation of Photoelectric Properties of Organic Systems

A new method of investigation of photoelectric properties of layered thin-film structures based on broadband Fourier spectroscopy exhibiting a harmonically modulated optical delay is proposed. In contrast to traditional approaches to study photoelectric properties, which are based on application of dispersive spectral devices, the proposed method allows not only simultaneously covering the ultraviolet, visible, and infrared spectral ranges, while demonstrating a wide dynamic range and high spectral resolution, but also easily varying low-frequency modulation of the action of light. The capabilities of the method are demonstrated using a polycrystalline organic heterostructure as an example. Its spectral sensitivity, speed, and specific detectivity are measured. A model and an equivalent electric circuit are proposed for explanation of the results of the measurements.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS2014

Novel Active Combustion Control Valve

This project presents an innovative solution for active combustion control. Relative to the state of the art, this concept provides frequency modulation (greater than 1,000 Hz) in combination with high-amplitude modulation (in excess of 30 percent flow) and can be adapted to a large range of fuel injector sizes. Existing valves often have low flow modulation strength. To achieve higher flow modulation requires excessively large valves or too much electrical power to be practical. This active combustion control valve (ACCV) has high-frequency and -amplitude modulation, consumes low electrical power, is closely coupled with the fuel injector for modulation strength, and is practical in size and weight. By mitigating combustion instabilities at higher frequencies than have been previously achieved (approximately 1,000 Hz), this new technology enables gas turbines to run at operating points that produce lower emissions and higher performance.

Caspermeyer, Matt↗
DOE OSTI2018

Generation regimes of an Nd : YAG laser with mode locking by a travelling-wave acousto-optic modulator and spherical mirror

The lasing regimes of a diode-pumped Nd : YAG laser with mode locking by a spherical mirror and a travelling-wave acousto-optic modulator (SMAOM method) are investigated. The lasing dynamics at mode spacing detuning from the doubled operating modulator frequency and with a change in the diffraction modulator efficiency is studied. The results of measuring the pulse duration and the frequency spectrum of output radiation are presented. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Passive harmonic mode-locking in an erbium-doped fibre laser

We report an all-fibre erbium laser operating in the regime of passive harmonic mode-locking based on the effect of nonlinear polarisation rotation. The generation of the 67th harmonic with a pulse repetition rate of 5.62 GHz and a duration of 1.8 ps is demonstrated. It is shown that as the harmonic number increases, both the frequency modulation parameters and the pulse duration change. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Spectral and temporal properties of optical signals with multiple sinusoidal phase modulations

Optical signals generated by multiple sinusoidal temporal phase modulations (multi-FMs) applied to a monochromatic field are studied from the viewpoint of their optical spectrum and temporal modulations arising from spectral impairments. Statistical analysis based on the central limit theorem shows that the signals' optical spectrum converges to a normal distribution as the number of modulations increases, allowing one to predict the frequency range containing a given fraction of the total energy with the associated cumulative density function. The conversion of frequency modulation to amplitude modulation is analyzed and simulated for arbitrary multi-FM signals. These developments are of theoretical and practical importance for high-energy laser systems, where optical pulses are phase modulated in the front end to smooth out the on-target beam profile and prevent potentially catastrophic damage to optical components.

Dorrer, C.↗
DOE OSTI2016

Stratocumulus Precipitation and Entrainment Experiment (SPEE) Field Campaign Report

The scientific focus of this project was to examine precipitation and entrainment processes in marine stratocumulus clouds. The entrainment studies focused on characterizing cloud turbulence at cloud top using Doppler cloud radar observations. The precipitation studies focused on characterizing the precipitation and the macroscopic properties (cloud thickness, and liquid water path) of the clouds. This project will contribute to the U.S. Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Climate Research Facility’s overall objective of providing the remote-sensing observations needed to improve the representation of key cloud processes in climate models. It will be of direct relevance to the components of ARM dealing with entrainment and precipitation processes in stratiform clouds. Further, the radar observing techniques that will be used in this study were developed using ARM Southern Great Plains (SGP) facility observations under Atmospheric System Research (ASR) support. The observing systems operating automatously from a site located just north of the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) aircraft hangar in Marina, California during the period of 1 May to 4 November 2015 included: 1. Microwave radiometer: ARM Microwave Radiometer, 3-Channel (MWR3C) with channels centered at 23.834, 30, and 89 GHz; supported by Dr. Maria Cadeddu. 2. Cloud Radar: CIRPAS 95 GHz Frequency Modulated Continuous Wave (FMCW) Cloud Radar (Centroid Frequency Chirp Rate [CFCR]); operations overseen by Drs. Ghate and Albrecht. 3. Ceilometer: Vaisala CK-14; operations overseen by Drs. Ghate and Albrecht.

54 ENVIRONMENTAL SCIENCES↗
NASA NTRS1980

Measuring coal deposits by radar

Front-surface, local-oscillator radar directly compares frequency of signals reflected from front and back surfaces of coal deposits. Thickness is measured directly as frequency difference. Transmitter is frequency modulated, so thickness is computed directly from frequency difference. Because front and back reflections are detected in combination rather than separately, masking of comparatively weak back signal is less problem. Also system is not sensitive to extraneous reflections from targets between transmitting antenna and coal surface.

Barr, T. A.↗
DOE OSTI2015

Injection locking of a two-mode electron oscillator with close frequencies

Theory of injection locking is developed for a two-mode electron maser with close frequencies, when the driving signal affects both modes. There exist two regimes of phase locking in which either first or second mode dominates. Hard transitions between the two regimes are observed with variation of the driving frequency. The results of numerical simulations are presented for the case of driving by a signal with linear frequency chirp, as well as by a signal with sinusoidal frequency modulation. The effect of bifurcation delay is observed with the increase of chirp rate.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Numerical investigation into the injection-locking phenomena of gain switched lasers for optical frequency comb generation

We present detailed numerical simulations of the laser dynamics that describe optical frequency comb formation by injection-locking a gain-switched laser. The typical rate equations for semiconductor lasers including stochastic carrier recombination and spontaneous emission suffice to show the injection-locking behavior of gain switched lasers, and we show how the optical frequency comb evolves starting from the free-running state, right through the final injection-locked state. Unlike the locking of continuous wave lasers, we show that the locking range for gain switched lasers is considerably greater because injection locking can be achieved by injecting at frequencies close to one of the comb lines. The quality of the comb lines is formally assessed by calculating the frequency modulation (FM)-noise spectral density and we show that under injection-locking conditions the FM-noise spectral density of the comb lines tend to that of the maser laser.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Propagation of a Self-Similar Pulse in a Medium with One-Photon Transient Absorption

The shape of an optical self-similar pulse that propagates in a medium of finite length with noninstantaneous nonlinear absorption is found analytically. The pulse has an asymmetric shape and, in addition, frequency modulation, which confirms that it belongs to a new type of self-similar propagation modes of laser radiation—chirped pulses with a self-similar shape. The existence of pulses of this kind is also confirmed by results of computer simulation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Three axis vector atomic magnetometer utilizing polarimetric technique

The three axis vector magnetic field measurement based on the interaction of a single elliptically polarized light beam with an atomic system is described. The magnetic field direction dependent atomic responses are extracted by the polarimetric detection in combination with laser frequency modulation and magnetic field modulation techniques. The magnetometer geometry offers additional critical requirements like compact size and large dynamic range for space application. Further, the three axis magnetic field is measured using only the reflected signal (one polarization component) from the polarimeter and thus can be easily expanded to make spatial array of detectors and/or high sensitivity field gradient measurement as required for biomedical application.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2017

Rotational Spectra in 29 Vibrationally Excited States of Interstellar Aminoacetonitrile

We report a detailed spectroscopic investigation of the interstellar aminoacetonitrile, a possible precursor molecule of glycine. Using a combination of Stark and frequency-modulation microwave and millimeter wave spectroscopies, we observed and analyzed the room-temperature rotational spectra of 29 excited states with energies up to 1000 cm{sup −1}. We also observed the {sup 13}C isotopologues in the ground vibrational state in natural abundance (1.1%). The extensive data set of more than 2000 new rotational transitions will support further identifications of aminoacetonitrile in the interstellar medium.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Optimisation of parameters of Raman laser pulse compression in a plasma for its implementation using the PEARL laser facility (IAP RAS)

We consider Raman compression of laser pulses in a plasma under the conditions of an experiment planned at the Institute of Applied Physics of the Russian Academy of Sciences on the PEARL laser facility. The analysis is based on the equations describing, among other things, the effect of plasma dispersion and relativistic nonlinearity, as well as the dynamics of the field near the plasma wave breaking threshold. It is shown that the main limiting factors are excessive frequency modulation of the pump pulse and a too low plasma density in which the plasma wave breaking can occur. To reduce the negative influence of these effects, we suggest using an intense and short (on the order of the plasma period) seed laser pulse. Numerical simulation shows the possibility of a hundredfold increase in the intensity of the compressed pulse in comparison with the intensity of the pump pulse at a length of uniform plasma of 2 cm. (paper)

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Influence of Laser Nitriding on the Surface and Corrosion Properties of Ti-20Nb-13Zr Alloy in Artificial Saliva for Dental Applications

Ti alloys have been used for dentistry due to their lower density and improved compatibility. However, the resistance to corrosion and surface hardness of Ti alloys need to be improved to avoid the formation of debris, which affects the long-term biocompatibility. In the present work, the surface characteristics and corrosion protection in artificial saliva produced by laser nitriding of a new biomedical Ti-20Nb-13Zr at.% alloy have been investigated for dental applications. The laser nitrided and untreated alloy surfaces were examined using XRD, FE-SEM, x-ray photon spectroscopy, Vickers microhardness measurements, atomic force microscopy, scratch measurements, Raman spectroscopy, and surface energy measurements. The susceptibility of the laser nitrided TNZ alloy to corrosion in an artificial saliva medium was investigated by electrochemical corrosion tests, which were performed by utilizing open circuit potential monitoring, electrochemical frequency modulation, and electrochemical impedance spectroscopic methods. The findings revealed that a TiN layer formed on the alloy surface after nitriding. The surface hardness of the treated alloy was 2.4 times that of the as-received samples. Moreover, the results showed improved friction coefficient and hydrophilic surface characteristics. The corrosion analysis revealed that the compact and dense TiN layer produced during laser nitriding improved the corrosion resistance of the Ti-20Nb-13Zr sample in artificial saliva medium.

36 MATERIALS SCIENCE↗
NASA NTRS2010

Frequency Stabilization of DFB Laser Diodes at 1572 nm for Spaceborne Lidar Measurements of CO2

We report a fiber-based, pulsed laser seeder system that rapidly switches among 6 wavelengths across atmospheric carbon dioxide (CO2) absorption line near 1572.3 nm for measurements of global CO2 mixing ratios to 1-ppmv precision. One master DFB laser diode has been frequency-locked to the CO2 line center using a frequency modulation technique, suppressing its peak-to-peak frequency drifts to 0.3 MHz at 0.8 sec averaging time over 72 hours. Four online DFB laser diodes have been offset-locked to the master laser using phase locked loops, with virtually the same sub-MHz absolute accuracy. The 6 lasers were externally modulated and then combined to produce the measurement pulse train.

Numata, Kenji↗
DOE OSTI2014

Molecular-scale investigations of structures and surface charge distribution of surfactant aggregates by three-dimensional force mapping

Surface charges on nanoscale structures in liquids, such as biomolecules and nano-micelles, play an essentially important role in their structural stability as well as their chemical activities. These structures interact with each other through electric double layers (EDLs) formed by the counter ions in electrolyte solution. Although static-mode atomic force microscopy (AFM) including colloidal-probe AFM is a powerful technique for surface charge density measurements and EDL analysis on a submicron scale in liquids, precise surface charge density analysis with single-nanometer resolution has not been made because of its limitation of the resolution and the detection sensitivity. Here we demonstrate molecular-scale surface charge measurements of self-assembled micellar structures, molecular hemicylinders of sodium dodecyl sulfate (SDS), by three-dimensional (3D) force mapping based on frequency modulation AFM. The SDS hemicylindrical structures with a diameter of 4.8 nm on a graphite surface were clearly imaged. We have succeeded in visualizing 3D EDL forces on the SDS hemicylinder surfaces and obtaining the molecular-scale charge density for the first time. The results showed that the surface charge on the trench regions between the hemicylinders was much smaller than that on the hemicylinder tops. The method can be applied to a wide variety of local charge distribution studies, such as spatial charge variation on a single protein molecule.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗