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At least 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Optical detection of pulsed surface particle displacements

The detection of acoustic surface wave pulses by a wideband differential interferometric laser technique which is insensitive to low frequency noise is described. System bandwidth is determined by the relationship between acoustic wavelength and the variable spacing between two laser probe beams focused on the solid surface which supports the waves. Response to an ideal step function displacement is analyzed and experimental measurements of gated 2.5 MHz pulses generated on an aluminized pyrex plate are discussed.

Zewekh, P. S.↗
NASA NTRS1991

Correlation between some current parameters and optical radiation generated by 280 mm long laboratory sparks

Discussed here is the optical radiation generated by 280 mm long laboratory discharges and its relationship to the current flowing in the discharge channel. In an experiment, the optical radiation generated by the discharges was measured at wavelengths 777nm (bandwidth 10 nm) due to O I(1), and 500 nm (bandwidth 5 nm) due to N II(19), and the broadband optical radiation between the wavelengths 400 to 1100 nm. The shape of the current waveform, which had a rise time of 0.1 microsecond and a decay time of 5 microsecond, remained the same with increasing peak value. The experiment was conducted with peak current amplitudes in the range of 1-4 kA. In order to test the effect of current rise time on the optical radiation, researchers also conducted studies with a current waveform having a 5 microsecond rise time. It was observed that the peak amplitude of the optical radiation pulse at the wavelengths mentioned above is proportional to the peak amplitude of the current flowing through the discharge channel. The rise time of the optical radiation pulse at a given wavelength does not depend significantly on the peak amplitude of the current waveform. The rise time of the optical radiation pulse decreases with decreasing wavelength. A slight increase in the decay time of the optical pulse, at a given wavelength, is observed with increasing peak amplitude of the current waveform.The results show that the rise time of the optical radiation pulse increases with the increasing rise time of the current waveform. The relationship between peak current, peak optical power, and peak electrical power is studied. Also, the relationship between peak current, optical energy (400-1100 and 777 nm), and electrical energy is discussed.

Windmar, Dan↗
DOE OSTI2021

Dynamic strain evolution in an optically excited Pt thin film

The structural evolution of a Pt thin film following photo-thermal excitation by 1 ps optical laser pulses was studied with a time resolution of 100 ps over a total time period of 1 ms. Laser pulse fluences below 50 mJ/cm 2 were insufficient to relax the residual stress state of the as-prepared film even after 10 000 pulses. In this fluence regime, a rapid initial lattice expansion and a decrease in the lattice coherence length due to ultrafast photo-thermal heating were observed. The lattice expansion reached a maximum, and the coherence length reached a minimum, 100–200 ps after excitation before monotonically decaying back to their initial values in about 1 µs. Laser pulse fluences greater than 50 mJ/cm 2 produced irreversible stress relaxation within the first 10 optical pulses. In this regime, the lattice expansion was qualitatively similar to that in the low fluence regime, except that the initial structural state was not recovered. The evolution in the coherence length, however, was more complex. Following an initial decrease similar to that observed at low fluence, the coherence length then increased to a broad maximum greater than the initial value, before recovery.

36 MATERIALS SCIENCE↗
NASA NTRS1976

Optical pulsations in HZ Herculis. 5. Pulse-resolved spectrophotometry

Digital spectra of HZ Herculis were obtained with 10 A resolution in the 3,600 - 6,000 A region, synchronously dividing the 1.24-s optical pulsation period into eight 155-ms phase bins. The optical pulses are detected in the data, but their fractional amplitude is only 0.08 percent, a factor of 4 less than typically observed. The separate spectra of each one-eighth of the pulse are identical to within the statistics of the observation. If the X-ray to optical pulse reprocessing mechanism concentrates the optical pulsations into discrete spectral line features, data require the pulses to be distributed among more than four such lines.

Margon, B.↗
NASA NTRS2002

Laser Pulse-Stretching Using Multiple Optical Ring-Cavities

We describe a simple and passive nanosecond-long (ns-long) laser 'pulse-stretcher' using multiple optical ring-cavities. We present a model of the pulse-stretching process for an arbitrary number of optical ring-cavities. Using the model, we optimize the design of a pulse-stretcher for use in a spontaneous Raman scattering excitation system that avoids laser-induced plasma spark problems. From the optimized design, we then experimentally demonstrate and verify the model with a 3-cavity pulse-stretcher system that converts a 1000 mJ, 8.4 ns-long input laser pulse into an approximately 75 ns-long (FWHM) output laser pulse with a peak power reduction of 0.10X, and an 83% efficiency.

Kojima, Jun↗
NASA NTRS1982

Sequential-Impulse Generator Uses Fiber-Optics

Light pulse from a ruby or neodymium-glass laser enters miniature optics of repetitive-detonation apparatus. Traveling along a bundle of optical fibers, light strikes laser-sensitive microdetonator and charge explodes. Apparatus then advances next charge in train into position. Possible applications of sequential-impulse generator are in creating shock waves for aerodynamics research and in generating electrical power by magnetohydrodynamics.

Yang, L. C.↗
DOE OSTI2022

Effects and mitigation of pulsed power radiation on optical fiber velocimetry

We report optical fiber diagnostics are extensively used in pulsed power experiments, such as the Sandia Z machine. However, radiation produced in a pulsed power environment can significantly affect these measurements. Catastrophic fiber darkening may be mitigated with shielding, but no flexible material can stop all radiation produced by the machine and/or target. Radiation-induced refractive index modulations are particularly challenging for optical interferometry. Several approaches for radiation-tolerant photonic Doppler velocimetry are discussed here.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

A self-referenced in-situ arrival time monitor for X-ray free-electron lasers

We present a novel, highly versatile, and self-referenced arrival time monitor for measuring the femtosecond time delay between a hard X-ray pulse from a free-electron laser and an optical laser pulse, measured directly on the same sample used for pump-probe experiments. Two chirped and picosecond long optical supercontinuum pulses traverse the sample with a mutually fixed time delay of 970 fs, while a femtosecond X-ray pulse arrives at an instant in between both pulses. Behind the sample the supercontinuum pulses are temporally overlapped to yield near-perfect destructive interference in the absence of the X-ray pulse. Stimulation of the sample with an X-ray pulse delivers non-zero contributions at certain optical wavelengths, which serve as a measure of the relative arrival time of the X-ray pulse with an accuracy of better than 25 fs. We find an excellent agreement of our monitor with the existing timing diagnostics at the SACLA XFEL with a Pearson correlation value of 0.98. We demonstrate a high sensitivity to measure X-ray pulses with pulse energies as low as 30 μJ. Using a free-flowing liquid jet as interaction sample ensures the full replacement of the sample volume for each X-ray/optical event, thus enabling its utility even at MHz repetition rate XFEL sources.

47 OTHER INSTRUMENTATION↗
DOE OSTI2023

α-Quartz Phase Stabilization, Surface Texturing, and Tunable Optical Properties of Nanocrystalline GeO 2 Films Made by Pulsed-Laser Deposition: Implications for Optical and Optoelectronic Applications

Germanium oxide (GeO 2 ) has great potential in multifunctional devices and next-generation power electronics due to its high thermal conductivity and ambipolar doping capability. However, the complexity of synthesizing the desirable polymorph with a controlled phase, surface/interface quality, microstructure, and functional properties is the main barrier to GeO 2 utilization in advanced applications. Here, in this regard, we present a method to realize the hexagonal (h) or a-quartz type GeO 2 with nano-textured surface morphology on sapphire substrates using a hybrid synthesis strategy that comprises pulsed laser deposition (PLD) and post-deposition thermal annealing. We performed a comprehensive study to investigate the effect of annealing temperature, which was varied in a wide range (600-1100 °C), on the crystal structure, phase, surface morphology, chemical stoichiometry, defect states, and optical properties of PLD-grown GeO 2 films. As-deposited GeO 2 films at 500 °C were amorphous. Upon annealing, the GeO 2 films induced an amorphous-to-crystalline phase transformation; GeO 2 films annealed at higher annealing temperatures (≥900 °C) stabilized in the hexagonal phase and demonstrated excellent crystal quality and chemical stability. Thermally activated growth process showed increased average crystallite size, which was varied in the range of 20-130 (±2) nm, while the surface roughness followed a similar trend. The spectral transmittance and band gap also increased with increasing annealing temperature. The resulting h-GeO 2 films, particularly those obtained at annealing temperatures in the 900-1100 °C range, had a higher band gap of 6.2-6.3 eV and displayed excellent optical transmittance in the visible region. Moreover, the absence of extended valence band maxima and reduced optical defect density support the quality improvement upon annealing. When considering phase-pure bulk and nanostructured GeO 2 as a possible candidate for ultra wide band gap semiconductors in cutting-edge technological applications, the results of the current work can be beneficial to realize high structural and optical quality a-quartz structured GeO 2 films.

GeO2↗
NASA NTRS1976

Pulse height response of an optical particle counter to monodisperse aerosols

The pulse height response of a right angle scattering optical particle counter has been investigated using monodisperse aerosols of polystyrene latex spheres, di-octyl phthalate and methylene blue. The results confirm previous measurements for the variation of mean pulse height as a function of particle diameter and show good agreement with the relative response predicted by Mie scattering theory. Measured cumulative pulse height distributions were found to fit reasonably well to a log normal distribution with a minimum geometric standard deviation of about 1.4 for particle diameters greater than about 2 micrometers. The geometric standard deviation was found to increase significantly with decreasing particle diameter.

Wilmoth, R. G.↗
DOE OSTI2025

Cryogenically enhanced quasi-optical resonator for megawatt pulsed millimeter-wave sources

We report the design, cryogenic optimization, and performance modeling of a compact quasi-optical ring resonator intended to compress microwaves pulses at 170 and 250 GHz to the megawatt level. By combining ultra-low-loss CVD diamond and gold-doped silicon wafers with high-RRR copper mirrors, the calculated unloaded quality factor exceeds 4.3 × 10 5 at 20 K and yields simulated gains up to $\mathscr{G}$ = 4.1 × 10 3 . Coupling the resonator with a laser-driven semiconductor switch described by an extended Vogel model shows that 1 MW, nanosecond pulses can be generated from only 445 W of microwave drive power while dissipating 272 W into the cryostat. A practical cooling architecture using two Gifford–McMahon stages (20 and 80 K) is proposed, demonstrating that high-repetition-rate (10–20 kHz) operation is feasible with commercially available cryocoolers. The results outline a clear path toward cost-effective, table-top sources for extreme-ultraviolet lithography, dynamic nuclear polarization, and fusion systems.

Panisset, Constant [Ecole Polytechnique Federale L↗
NASA NTRS2013

Some New Lidar Equations for Laser Pulses Scattered Back from Optically Thick Media Such as Clouds, Dense Aerosol Plumes, Sea Ice, Snow, and Turbid Coastal Waters

I survey the theoretical foundations of the slowly-but-surely emerging field of multiple scattering lidar, which has already found applications in atmospheric and cryospheric optics that I also discuss. In multiple scattering lidar, returned pulses are stretched far beyond recognition, and there is no longer a one-to-one connection between range and return-trip timing. Moreover, one can exploit the radial profile of the diffuse radiance field excited by the laser source that, by its very nature, is highly concentrated in space and collimated in direction. One needs, however, a new class of lidar equations to explore this new phenomenology. A very useful set is derived from radiative diffusion theory, which is found at the opposite asymptotic limit of radiative transfer theory than the conventional (single-scattering) limit used to derive the standard lidar equation. In particular, one can use it to show that, even if the simple time-of-flight-to-range connection is irretrievably lost, multiply-scattered lidar light can be used to restore a unique profiling capability with coarser resolution but much deeper penetration into a wide variety of optical thick media in nature. Several new applications are proposed, including a laser bathymetry technique that should work for highly turbid coastal waters.

turbid water↗
DOE OSTI2022

Spatiotemporal Structure in Intense THz Pulsed Beams

Optically generated terahertz radiation, with gigavolt per meter (GV/m) electric fields accessible in tabletop experiments, provides a promising source of accelerating gradients for future particle accelerator applications. Manipulation and characterization of radiation is essential for efficiently producing high fields and effectively delivering them to an accelerating structure or interaction region. The talk will cover a method of generating and characterizing high quality and structured terahertz pulsed laser beams for compact particle acceleration.

Hine, George↗
NASA NTRS1985

Direct-detection optical communication with color coded pulse position modulation signaling

The performance characteristics of a direct-detection optical communication system which is based on a laser transmitter which produces single light pulses at selected nonoverlapping optical center frequencies are discussed. The signal format, called color coded pulse position modulation (CCPPM), uses more of the total available response bandwidth characteristics of the photodetector than does ordinary PPM signaling. The advantages of CCPPM signaling are obtained at the expense of an increased optical bandwidth of the transmitted signal and a more complicated transmitter and receiver structure. When the signal format is used in conjunction with block length Reed-Solomon codes, high data rates and reliable high-speed optical communications under conditions of optimal energy efficiency are obtained.

Davidson, F.↗
NASA NTRS1997

Validity Using Pump-Probe Pulses to Determine the Optical Response of Niobate Crystals

A variety of niobate crystals have found their places in nonlinear optical applications as well as in laser devices. In recent years much attention has been paid to study the ultrafast optical response in a variety of photorefractive crystals such as KTa(1-x)Nb(x)O3 and KNbO3 crystals, glasses, semiconductors and polymers for applications in optical switching, information processing, optical computing, and all-optical device systems. Third-order optical nonlinearity is the most important property for realization of all-optical switching. Therefore experiments have been performed on the third order susceptibility using a variety of techniques such as the third-order harmonic generation, EFISH and degenerate four-wave mixing(DFWM). The latter has been conducted with a variety of pump wavelengths and with nanosecond, picosecond and femtosecond pulses. Niobate crystals, such as potassium niobate KNbO3, potassium tantalate niobate KTN family (KTa(1-x)Nb(x)O3), strontium barium niobate SBN (Sr(x)Ba(1-x)Nb2O6) and potassium-sodium niobate SBN (KNSBN) are attractive due to their photorefractive properties for application in optical storage and processing. The pulsed probe experiments performed on theses materials have suggested two types of time responses. These responses have been associated with an coherent response due to Chi(sup 3), and a long lived component due to excited state population. Recent study of DFWM on KNbO3 and KTN family reveals that the long lived component of those crystals depends on the crystal orientation. A slowly decaying signal is observable when the grating vector K(sub g) is not perpendicular to the C-axis of those photorefractive crystals', otherwise the optical response signal would be only a narrow coherent peak with FWHM equal to the cross-correlation width of the write beam pulses. Based on this understanding, we study the photodynamical process of a variety of niobate crystals using DFWM in a Kg perpindicular to C geometry with a ps-YAG: Nd laser operating at 532nm. However, the discrepancies in numerical estimations of Chi(sup 3) in these materials and other nonlinear optical media have resulted in a number of discussions. In order to better understand the photodynamical process of niobate crystals after an short pulse excitation we analyze the factors governing the coherent signal and present the DFWM spectra of niobate crystals.

Liu, Huimin↗
NASA NTRS1996

Pulse Shepherding in Nonlinear Fiber Optics

In a wavelength division multiplexed fiber system, where pulses on different wavelength beams may co-propagate in a single mode fiber, the cross-phase-modulation (CPM) effects caused by the nonlinearity of the optical fiber are unavoidable. In other words, pulses on different wavelength beams can interact with and affect each other through the intensity dependence of the refractive index of the fiber. Although CPM will not cause energy to be exchanged among the beams, the pulse shapes and locations on these beams can be altered significantly. This phenomenon makes possible the manipulation and control of pulses co-propagating on different wavelength beams through the introduction of a shepherd pulse at a separate wavelength. How this can be accomplished is demonstrated in this paper.

Yeh, C.↗