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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Two-dimensional fluorescence spectroscopy of uranium isotopes in femtosecond laser ablation plumes

Here, we demonstrate measurement of uranium isotopes in femtosecond laser ablation plumes using two-dimensional fluorescence spectroscopy (2DFS). The high-resolution, tunable CW-laser spectroscopy technique clearly distinguishes atomic absorption from 235 U and 238 U in natural and highly enriched uranium metal samples. We present analysis of spectral resolution and analytical performance of 2DFS as a function of ambient pressure. Simultaneous measurement using time-resolved absorption spectroscopy provides information on temporal dynamics of the laser ablation plume and saturation behavior of fluorescence signals. The rapid, non-contact measurement is promising for in-field, standoff measurements of uranium enrichment for nuclear safety and security.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2015

Multidiagnostic analysis of ion dynamics in ultrafast laser ablation of metals over a large fluence range

The dynamics of ions in ultrafast laser ablation of metals is studied over fluences ranging from the ablation threshold up to ≈75 J/cm{sup 2} by means of three well-established diagnostic techniques. Langmuir probe, Faraday cup, and spectrally resolved intensified charge coupled device imaging simultaneously monitored the ions produced during ultrafast laser ablation of a pure copper target with 800 nm, ≈50 fs, Ti: Sapphire laser pulses. The fluence dependence of ion yield is analyzed, resulting in the observance of three different regimes. The specific ion yield shows a maximum at about 4–5 J/cm{sup 2}, followed by a gradual reduction and a transition to a high-fluence regime above ≈50 J/cm{sup 2}. The fluence dependence of the copper ions angular distribution is also analyzed, observing a gradual increase in forward-peaking of Cu ions for fluences up to ≈10 J/cm{sup 2}. A broader ion component is observed at larger angles for fluences larger than ≈10 J/cm{sup 2}. Finally, an experimental characterization of the ionic angular distribution for several metallic targets (Mg, Al, Cr, Fe, Cu, and W) is carried out at a relatively high fluence of ≈66 J/cm{sup 2}. Interestingly, the ion emission from the volatile metals shows a narrow, forward-peaked distribution, and a high peak ion yield compared to the refractory metals. Moreover, the width of ionic angular distributions presents a striking correlation with the peak ion yield.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Effect of laser radiation wavelength and reepithelization process on optical quality of eye cornea after laser correction of vision

The optical quality of the eye cornea surface after performing the laser vision correction essentially depends on the characteristic roughness scale (CRS) of the ablated surface, which is mainly determined by the absorption coefficient of the cornea at the laser wavelength. Thus, in the case of using an excimer ArF laser (λ = 193 nm) the absorption coefficient is equal to 39000 cm{sup -1}, the darkening by the dissociation products takes place, and the depth of the roughness relief can be as large as 0.23 mm. Under irradiation with the Er : YAG laser (λ = 2940 nm) the clearing is observed due to the rupture of hydrogen bonds in water, and the relief depth exceeds 1 μm. It is shown that the process of reepithelization that occurs after performing the laser vision correction leads to the improvement of the optical quality of the cornea surface. (interaction of laser radiation with matter)

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2017

Laser heating dynamics and glow spectra of carbon-, titanium- and erbium-containing optothermal fibre converters for laser medicine

Titanium- and erbium-containing optothermal fibre converters of laser radiation mounted at the distal end of quartz – quartz optical fibre are discussed for the first time. Technology of fabricating such converters is described. Carbon-containing converters are also considered. The laser heating dynamics of the converters and the glow spectra are studied by irradiating converters of each type by a 980 ± 10 nm semiconductor laser with an average power up to 4 W. It is shown that alongside with broadband thermal radiation accompanying the laser heating of all three types of converters in the temperature range 600 – 1100 °C, only in the spectrum of the erbium-containing converter the intense bands with the maxima at wavelengths 493, 523, 544, 660, and 798 nm, corresponding to the erbium radiative transitions {sup 4}F{sub 7/2} → {sup 4}I{sub 15/2}, {sup 2}H{sub 11/2} → {sup 4}I{sub 15/2}, {sup 4}S{sub 3/2} → {sup 4}I{sub 15/2}, {sup 4}F{sub 9/2} → {sup 4}I{sub 15/2} and {sup 4}I{sub 9/2} → {sup 4}I{sub 15/2}, respectively, are present. Such converters can be used in laser medicine for tissue surgery as well as in procedures combining laser, thermal, biostimulation or photodynamic action.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

High-resolution in-situ characterization of laser powder bed fusion via transmission X-ray microscopy at X-ray free electron lasers

In this work, we describe the instrumentation used to perform the first operando transmission X-ray microscopy (TXM) and simultaneous X-ray diffraction of laser melting simulating laser powder bed fusion on the XCS instrument at the Linac Coherent Light Source (LCLS) X-ray free-electron laser (XFEL). Our TXM with 40× magnification in the X-ray regime at 11 keV gave spatial resolutions down to 940 nm per line pair, with effective pixel sizes down to 206 nm, image integration times of <100 fs, and frame rates tunable between 2.1 and 119 ns for two probe frames (0.48 GHz to 8.4 MHz). Images were recorded on Zyla and Icarus (UXI) detectors to trade off between spatial resolution and time dynamics. A 1 kW CW IR laser was coupled into the interaction point to conduct pump–probe studies of laser melting and solidification dynamics. Our temporal and spatial resolution with attenuation-based contrast exceeds that currently possible with synchrotron-based high-speed radiography. This system was sensitive to feature velocities of 10–12000 m s −1 but we did not observe any motion in this range in the laser melting of Al6061 alloy. Shockwaves were not observed and hot cracking proceeded at velocities below the detection limits. Pore accumulation was observed between successive shots, indicating that bubble escape mechanisms were not active. With proper experimental design, the spatial resolution, contrast and field of view could be further improved or modified. The increased brightness and narrower bandwidth of the XFEL allowed for this imaging technique and it lays the groundwork for a wide range of operando techniques to study additive manufacturing.

47 OTHER INSTRUMENTATION↗
DOE OSTI2017

Characterization of laser produced plasma using laser induced breakdown spectroscopy

The plasma parameter studies of the Nd:YAG (neodymium-doped yttrium aluminum garnet, Nd:Y{sub 3}Al{sub 15}O{sub 12}) crystal by using the fundamental (1064 nm) and second (532 nm) harmonics of Nd:YAG laser are reported. The electron temperature (T{sub e}) and electron number density (N{sub e}) were determined using the Boltzmann plot method and the Stark-broadened line profile, respectively. An increase in the plasma parameters have been observed with an increase in the laser irradiance for both laser modes. The electron temperatures were calculated in the range of 0.53–0.66 eV for 1064 nm and 0.47–0.60 eV for 532 nm, and the electron number densities were determined in the range of 7.43 × 10{sup 15}–3.27 × 10{sup 16} cm{sup −3} for 1064 nm and 1.35 × 10{sup 16}–3.97 × 10{sup 16} cm{sup −3} for 532 nm in the studied irradiance range of 1.19–12.5 GW/cm{sup 2}. However, the spatial evolution of the plasma parameters investigated up to 2.75 mm away from the target surface at a fixed laser irradiance of 6.51 GW/cm2 showed a decreasing trend. In addition, the estimated values of the inverse bremsstrahlung (IB) absorption coefficients at both laser wavelengths showed that the IB process is dominant for the 1064-nm laser.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Guiding of laser pulses in plasma waveguides created by linearly-polarized femtosecond laser pulses

We experimentally demonstrate that plasma waveguides produced with ultra-short laser pulses (sub-picosecond) in gas jets are capable of guiding high intensity laser pulses. This scheme has the unique ability of guiding a high-intensity laser pulse in a plasma waveguide created by the same laser system in the very simple and stable experimental setup. A hot plasma column was created by a femtosecond class laser that expands into an on-axis parabolic low density profile suitable to act as a waveguide for high intensity laser beams. We have successfully guided ~10 15 W cm -2 laser pulses in a 8 mm long hydrogen plasma waveguide with a 35% guiding efficiency.

42 ENGINEERING↗
DOE OSTI2018

Betatron x-ray radiation from laser-plasma accelerators driven by femtosecond and picosecond laser systems

A comparative experimental study of betatron x-ray radiation from laser wakefield acceleration in the blowout and self-modulated regimes is presented. Our experiments use picosecond duration laser pulses up to 150 J (self-modulated regime) and 60 fs duration laser pulses up to 10 J (blowout regime), for plasmas with electronic densities on the order of 10 19 cm –3 . In the self-modulated regime, where betatron radiation has been very little studied compared to the blowout regime, electrons accelerated in the wake of the laser pulse are subject to both the longitudinal plasma and transverse laser electrical fields. As a result, their motion within the wake is relatively complex; consequently, the experimental and theoretical properties of the x-ray source based on self-modulation differ from the blowout regime of laser wakefield acceleration. In our experimental configuration, electrons accelerated up to about 250 MeV and betatron x-ray spectra with critical energies of about 10–20 keV and photon fluxes between 10 8 and 10 10 photons/eV Sr are reported. In conclusion, our experiments open the prospect of using betatron x-ray radiation for applications, and the source is competitive with current x-ray backlighting methods on multi-kilojoule laser systems.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Investigation of cadmium telluride grown by molecular-beam epitaxy using micro-Raman spectroscopy below and above the laser damage threshold

The effects of visible laser light on cadmium telluride (CdTe), grown by molecular beam epitaxy, are studied at low (48 μ W/ μ m 2 ) and high (480 μ W/ μ m 2 ) laser power densities using micro-Raman spectroscopy. The Raman spectrum of CdTe shows no notable change at low power density ~48 μ W/ μ m 2 for prolonged laser exposure. At higher power density ~480 μ W/ μ m 2 , the Raman spectrum is significantly changed and strong Te-related peaks appear in the spectrum, even for short laser exposure times suggesting that photo-induced Te enrichment happens at the CdTe surface at high laser power density. The temperature rise is estimated from observed shifts in the Te and CdTe optical phonon peaks and modeled using finite-element simulations. At laser power 480 μ W/ μ m 2 , the CdTe exhibits a rise of ~44 °C above room temperature while the observed change in Te temperature is significantly higher, ~179 °C. Furthermore, the approach illustrates steps needed to establish the laser damage threshold for CdTe.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Proof-of-concept Talbot–Lau x-ray interferometry with a high-intensity, high-repetition-rate, laser-driven K-alpha source

Talbot–Lau x-ray interferometry is a grating-based phase-contrast technique, which enables measurement of refractive index changes in matter with micrometric spatial resolution. The technique has been established using a variety of hard x-ray sources, including synchrotron, free-electron lasers, and x-ray tubes, and could be used in the optical range for low-density plasmas. The tremendous development of table-top high-power lasers makes the use of high-intensity, laser-driven K-alpha sources appealing for Talbot–Lau interferometer applications in both high-energy-density plasma experiments and biological imaging. To this end, we present the first, to the best of our knowledge, feasibility study of Talbot–Lau phase-contrast imaging using a high-repetition-rate laser of moderate energy (100 mJ at a repetition rate of 10 Hz) to irradiate a copper backlighter foil. The results from up to 900 laser pulses were integrated to form interferometric images. A constant fringe contrast of 20% is demonstrated over 100 accumulations, while the signal-to-noise ratio continued to increase with the number of shots. Phase retrieval is demonstrated without prior ex-situ phase stepping. Instead, correlation matrices are used to compensate for the displacement between reference acquisition and the probing of a PMMA target rod. The steps for improved measurements with more energetic laser systems are discussed. The final results are in good agreement with the theoretically predicted outcomes, demonstrating the applicability of this diagnostic to a range of laser facilities for use across several disciplines.

Bouffetier, V. (ORCID:0000000160791260)↗
DOE OSTI2016

Two-dimensional fluorescence spectroscopy of laser-produced plasmas

We use a two-dimensional laser-induced fluorescence spectroscopy technique to measure the coupled absorption and emission properties of atomic species in plasmas produced via laser ablation of solid aluminum targets at atmospheric pressure. Emission spectra from the Al I 394.4 nm and Al I 396.15 nm transitions are measured while a frequency-doubled, continuous-wave, Ti:Sapphire laser is tuned across the Al I 396.15 nm transition. The resulting two-dimensional spectra show the energy coupling between the two transitions via increased emission intensity for both transitions during resonant absorption of the continuous-wave laser at one transition. Time-delayed and gated detection of the emission spectrum is used to isolate the resonantly-excited fluorescence emission from the thermally-excited emission from the plasma. In addition, the tunable continuous-wave laser measures the absorption spectrum of the Al transition with ultra-high resolution after the plasma has cooled, resulting in narrower spectral linewidths than observed in emission spectra. Our results highlight that fluorescence spectroscopy employing continuous-wave laser re-excitation after pulsed laser ablation combines benefits of both traditional emission and absorption spectroscopic methods.

Harilal, Sivanandan S.↗
NASA NTRS1975

Improved semi-conductor laser device, operating, at room temperature, with an array of three lasers in the spatially coherent, free running mode

The peak pulse power was increased by operating an array of three homostructure Ga As lasers in the laser device. A spatial filter in the laser device selects the spatially coherent, free running, mode. The optical peak power is 5 watts, which is three times the peak power of a single laser in the array. The far-field distribution of the three laser array is a single Gaussian beam of spatial coherence without sidelobes or grating lobes. The length of the optical pulses of spatial coherence was increased to 200 ns by improved heat transfer from the p-n junctions of the lasers to the metal housing of the pulse transformer, and by doubling the core area and increasing the turns of the primary windings of the pulse transformer. The mechanical stability of the laser device was improved and the transition from mechanical alignment to electro-mechanical alignment control, was facilitated.

Rutz, E. M.↗
NASA NTRS1975

Single laser beam of spatial coherence from an array of GaAs lasers - Free-running mode

Spatially coherent radiation from a monolithic array of three GaAs lasers in a free-running mode is reported. The lasers, with their mirror faces antireflection coated, are operated in an external optical cavity built of spherical lenses and plane mirrors. The spatially coherent-beam formation makes use of the Fourier-transformation property of the internal lenses. Transverse mode control is accomplished by a spatial filter. The optical cavity is similar to that used for the phase-controlled mode of spatially coherent-beam formation; only the spatial filters are different. In the far field (when restored by an external lens), the intensities of the lasers in the array are concentrated in a single laser beam of spatial coherence, without any grating lobes. The far-field distribution of the laser array in the free-running mode differs significantly from the interference pattern of the phase-controlled mode. The modulation characteristics of the optical waveforms of the two modes are also quite different because modulation is related to the interaction of the spatial filter with the longitudinal modes of the laser array within the optical cavity. The modulation of the optical waveform of the free-running mode is nonperiodic, confirming that the fluctuations of the optical fields of the lasers are random.

Philipp-Rutz, E. M.↗
NASA NTRS1977

The NASA high-power carbon dioxide laser - A versatile tool for laser applications

The NASA Lewis Research Center has designed and fabricated a closed-cycle, continuous wave (CW), carbon dioxide (CO2) high-power laser to support research for the identification and evaluation of possible high-power laser applications. The device is designed to generate up to 70 kW of laser power in annular-shape beams from 1 to 9 cm in diameter. Electric discharge, either self-sustained or electron-beam-sustained, is used for excitation. This laser facility can be used in two ways. First, it provides a versatile tool on which research can be performed to advance the state-of-the-art technology of high-power CO2 lasers in such areas as electric excitation, laser chemistry, and quality of output beams, all of which are important whether the laser application is government or industry oriented. Second, the facility provides a well-defined, continuous wave beam for various application experiments, such as propulsion, power conversion, and materials processing.

Lancashire, R. B.↗
NASA NTRS1986

Observation of atmospheric ozone by dial with Raman lasers pumped by a KrF laser

Since the XeCl excimer laser (308 nm) was first used in Differential Absorption Lidar (DIAL) for stratospheric ozone detection, the XeCl ozone lidar became a useful tool for the monitoring of the stratospheric ozone concentration. Shorter wavelength lasers are needed for the observation of ozone in the troposphere where the ozone concentration is about one order of magnitude smaller than in the stratosphere. In 1983, tropospheric ozone was observed with the combination of the second Stokes line (290.4 nm) of stimulated Raman scattering from methane pumped by a KrF laser and the XeCl laser line. The measurement of the ozone distribution from ground to 30 km was reported, using three Stokes lines of Raman lasers pumped by a KrF laser. At wavelengths shorter than 295 nm, the background solar radiation is effectively suppressed by atmospheric ozone. Such a solar-blind effect can be expected when two wavelengths 277 and 290.4 nm are used for DIAL ozone detection. A preliminary measurement of the day time ozone distribution in the troposphere is presented using these wavelengths generated by a KrF laser with a Raman shifter. Analysis using the lidar equation predicts the maximum detectable range is 7 km.

Maeda, M.↗
NASA NTRS1991

Environmental testing of a Q-switched Nd:YLF laser and a Nd:YAG ring laser

A Q-switched Nd:YLF laser (model 110-02) and a Nd:YAG ring laser (model 120-04) from Lightwave Electronics were subjected to thermal and vibration tests similar to what can be expected during launch and flight on a spacecraft. Even though these lasers were not designed for space flight, environmental tests were performed to identify major design weaknesses. Laser performance (output power, energy, pulsewidth, lasing threshold etc.) were measured prior to and after thermal vibration tests. Average output power of the Q-switched laser degraded 15-20 percent after thermal tests and an additional 20-25 percent after vibration tests. Post diagnostic tests revealed that degradation of the Q-switched laser was due to misalignment of pump focusing optics and the laser cavity.

Robinson, D. L.↗
NASA NTRS1991

Narrowband alexandrite laser injection seeded with frequency dithered diode laser

Narrowband radiation is produced from a pulsed alexandrite laser when injection seeded with the output of a low power, tunable, continuous wave single mode diode laser. Injection seeded power oscillators are easier to frequency stabilize than etalon narrowed lasers, are more efficient and less prone to optical damage. AlGaAs diode lasers are available with wavelengths from 760 to 770 nm in the oxygen A band that can be used for differential absorption lidar remote sensing of atmospheric pressure and temperature. Diodes with room temperature output at 740 nm may be cooled sufficiently to emit in the water vapor absorption band at 720-730 nm for humidity remote sensing. The diode laser linewidth of 200 MHz is sufficient to seed 2 or 3 longitudinal modes of the multi-transverse mode alexandrite laser, giving the pulsed laser a bandwidth of 0.007 to 0.014/cm.

Schwemmer, Geary↗
NASA NTRS1992

Optimization of rod diameter in solid state lasers side pumped with multiple laser diode arrays

Results of a study to determine the optimum laser rod diameter for maximum output energy in a solid state neodymium laser transversely pumped with multiple laser diode arrays are reported here. Experiments were performed with 1.0 mm, 1.5 mm and 2.0 mm rod radii of both neodymium doped Y3Al5O12 (Nd:YAG) and La2Be2O5 (Nd:BeL) pumped with laser diode arrays having a maximum combined energy of 10.5 mJ. Equations were derived which predict the optimum rod radius and corresponding output mirror reflectivity for a given laser material and total pump energy. Predictions of the equations agreed well with the experiments for each of the laser materials which possessed significantly different laser properties from one another.

Sims, Newton, Jr.↗