Seedless velocimetry of high-enthalpy hypersonic boundary layers by nitric oxide ionization induced flow tagging and imaging at 100 kHz.

This Letter describes the first, to the best of our knowledge, demonstration of a velocity measurement by nitric oxide ionization induced flow tagging and imaging (NiiFTI) of a high-enthalpy hypersonic flow utilizing naturally formed nitric oxide. The measurements were conducted in the hypervelocity expansion tunnel (HXT) at Texas A&M University in Mach 8.5 and Mach 10 flows near an ogive test article.

Non-paraxial effect in dual-pulse laser energy deposition.

In this paper we study defocusing and non-paraxial effects associated with nanosecond pulsed laser beam propagation through femtosecond laser generated plasmas. Simulations were performed using a non-reduced wave propagation equation to highlight the role of spatial beam shaping of the nanosecond pulse in improving the laser propagation characteristics. We show that optimizing the shape of the laser intensity profile at the focus using a beam with the ring-liked focus helps to reduce both defocusing and non-paraxial effects.

High-resolution spectroscopy of liquid water with dispersive atomic vapor prism cell.

This article presents an experimental demonstration of a spectroscopic method based on the dispersion of the scattering spectrum from laser-illuminated liquid water collected through a rubidium atomic vapor prism cell. Resonant absorption at 780 nm suppresses Mie/Rayleigh scattering and the steep gradients in refractive index near the 780 nm absorption lines separate Brillouin scattering from Raman scattering in liquid water. The opposing spatial displacements of the Stokes and Anti-Stokes shifted Brillouin peaks yield a measurement of their spectral shifts and thus the temperature or salinity of the water.

Two-dimensional high resolution electron properties of femtosecond laser-induced plasma filament in atmospheric pressure argon.

This work reports the measurement of two-dimensional electron properties over a nanosecond scale integration time across a femtosecond laser-induced plasma filament in atmospheric pressure argon. Radial electron properties across the [Formula: see text] [Formula: see text]m diameter filament are obtained at discrete axial locations at 2.5 mm steps by one-dimensional high-resolution laser Thomson scattering with a spatial resolution of 10 [Formula: see text]m.

Burst-mode velocimetry of hypersonic flow by nitric oxide ionization induced flow tagging and imaging.

This Letter describes, to the best of our knowledge, a new approach to flow tagging, nitric oxide (NO) Ionization Induced Flow Tagging and Imaging (NiiFTI), and presents the first experimental demonstration for single-shot velocimetry in a near Mach 6 hypersonic flow at 250 kHz. The mean velocity of 860 m/s was measured with a single-shot standard deviation of as low as 3.4 m/s and mean velocity uncertainty of 5.

Thermometry of liquid water through slow light imaging spectroscopy.

This work presents the first, to the best of our knowledge, experimental demonstration of slow light imaging spectroscopy for thermometry of liquid water. This novel technique for measuring temperature relies on detecting the spectral shift of Brillouin peaks in water using the temporal delay through a cell containing an atomic vapor. Stand-off sensing capabilities are achieved by time-domain measurements of Brillouin scattering tuned to be near a rubidium atomic resonance and passed through a cell filled with rubidium vapor.

1D time evolving electric field profile measurements with sub-ns resolution using the E-FISH method.

We present an approach for the measurement of time evolving electric field profiles in atmospheric pressure plasma discharges using electric field induced second harmonic generation (E-FISH). While the E-FISH effect has been known of for some time, recent advances in laser and detection technology have allowed the method to be utilized for spatial measurements of an arbitrarily applied electric field. A cylindrical lens is used to focus the femtosecond laser light to a line and an intensified charge coupled device is used for detection, allowing for one-dimensional (1D) spatial resolution on the order of ∼50  μm.

Kinetics Model of Femtosecond Laser Ionization in Nitrogen and Comparison to Experiment.

A zero-dimensional kinetics simulation of femtosecond laser ionization in nitrogen is proposed that includes fast gas heating effects, electron scattering (elastic and inelastic) rate coefficients from BOLSIG+ and photoionization based on filamentation theory. Key rate coefficients possessing significant uncertainty are tuned (within the range of variation found in literature) to reproduce the time-varying signal acquired by a bandpass-filtered photomultiplier tube with good agreement up to several hundred nanoseconds. Separate spectral measurements calibrate the relative strength of signal components.

Femtosecond laser tagging in R134a with trace quantities of air.

Femtosecond laser tagging is demonstrated for the first time in R134a (1,1,1,2-Tetrafluoroethane) gas, and in mixtures of R134a with small quantities of air. A systematic study of this tagging method is explored through the adjustment of gas pressure, mixture ratio and laser properties. It is found that the signal strength and lifetime are greatest at low pressures for excitation at both the 400 nm and 800 nm laser wavelengths.

Femtosecond laser tagging for velocimetry in argon and nitrogen gas mixtures.

Tagging is demonstrated in argon and nitrogen gases using a femtosecond laser with pulse energies of approximately 70 μJ through a nonresonant ionization process at 267 nm. The signal fluorescence lifetime in pure argon and nitrogen-argon mixtures are measured and found to be long enough to make mean velocity and turbulence measurements in a subsonic flow. In pure argon, the dominating processes involve atomic transitions between 700 and 900 nm.

FLEET velocimetry for combustion and flow diagnostics.

We report the use of femtosecond laser electronic excitation tagging (FLEET) for velocimetry at a 100-kHz imaging rate. Sequential, single-shot, quantitative velocity profiles of an underexpanded supersonic nitrogen jet were captured at a 100-kHz rate. The signal and lifetime characteristics of the FLEET emission were investigated in a methane flame above a Hencken burner at varying equivalence ratios, and room temperature gas mixtures involving air, methane, and nitrogen.

Three-photon femtosecond pumped backwards lasing in argon.

We demonstrate backwards lasing in atomic argon directly excited via a three-photon pumping in air mixtures with argon mole fractions down to 10%. We achieve well collimated, narrowband coherent emission at 1327nm by using both broadband femtosecond excitation and narrow linewidth picosecond excitation in the vicinity of 261nm. This approach shows promise for standoff trace detection in the atmosphere.

New diagnostic methods for laser plasma- and microwave-enhanced combustion.

The study of pulsed laser- and microwave-induced plasma interactions with atmospheric and higher pressure combusting gases requires rapid diagnostic methods that are capable of determining the mechanisms by which these interactions are taking place. New rapid diagnostics are presented here extending the capabilities of Rayleigh and Thomson scattering and resonance-enhanced multi-photon ionization (REMPI) detection and introducing femtosecond laser-induced velocity and temperature profile imaging. Spectrally filtered Rayleigh scattering provides a method for the planar imaging of temperature fields for constant pressure interactions and line imaging of velocity, temperature and density profiles.

Femtosecond laser electronic excitation tagging for quantitative velocity imaging in air.

Time-accurate velocity measurements in unseeded air are made by tagging nitrogen with a femtosecond-duration laser pulse and monitoring the displacement of the molecules with a time-delayed, fast-gated camera. Centimeter-long lines are written through the focal region of a ∼1 mJ, 810 nm laser and are produced by nonlinear excitation and dissociation of nitrogen. Negligible heating is associated with this interaction.

Standoff spectroscopy via remote generation of a backward-propagating laser beam.

In an earlier publication we demonstrated that by using pairs of pulses of different colors (e.g., red and blue) it is possible to excite a dilute ensemble of molecules such that lasing and/or gain-swept superradiance is realized in a direction toward the observer.

Detecting localized trace species in air using radar resonance-enhanced multi-photon ionization.

A microwave-scattering-based resonance-enhanced multi-photon ionization technique is used to detect molecular species such as NO, CO, Xe, and Ar in pure form, and for standoff detection of trace species in atmospheric pressure air. In this paper,the spectra, dynamics, and the detection limits of trace species in air are studied. We demonstrate 10 m scale standoff detection of NO, and show that the system has a linear response down to the parts in 10(9) NO levels in ambient air.

High-gain backward lasing in air.

The compelling need for standoff detection of hazardous gases and vapor indicators of explosives has motivated the development of a remotely pumped, high-gain air laser that produces lasing in the backward direction and can sample the air as the beam returns. We demonstrate that high gain can be achieved in the near-infrared region by pumping with a focused ultraviolet laser. The pumping mechanism is simultaneous resonant two-photon dissociation of molecular oxygen and resonant two-photon pumping of the atomic oxygen fragments.

Temperature measurements by coherent Rayleigh scattering.

We demonstrate, for the first time to our knowledge, the utility of coherent Rayleigh scattering (CRS) for temperature measurements in low-density gases and weakly ionized plasmas by measuring the translational temperature of neutral argon in a glow discharge. By analysis of the near-Gaussian spectral profile of the CRS signal, we determine temperatures with an uncertainty of View Article and Find Full Text PDF

Guiding radar signals by arrays of laser-induced filaments: finite-difference analysis.

Circular arrays of plasma filaments induced by femtosecond laser pulses in atmospheric air are shown to support guided modes of electromagnetic radiation in the centimeter and millimeter wavelength range. With the refractive index of laser-induced filaments being lower than the refractive index of nonionized air, arrays of such filaments can serve as a structured waveguide cladding, providing an index guiding of radar signals in a nonionized gas region. In spite of attenuation of radar radiation induced by plasma absorption, filament-array waveguides are shown to enhance radar signal transmission relative to freely propagating radar beams.

Coherent microwave rayleigh scattering from resonance-enhanced multiphoton ionization in argon.

Multiphoton ionization and electron recombination processes are studied in argon using coherent microwave Rayleigh scattering from a localized, resonance-enhanced multiphoton ionization produced plasma. A time dependent one-dimensional plasma dynamic model is developed to predict the time evolution of the microwave scattering from the plasma. Experimental results of the argon ionization spectrum and electron recombination rates are in good agreement with the model predictions.

One GHz linewidth, 33 line per mm, wide angle imaging filter at the potassium resonant line.

This paper presents a narrow linewidth, high resolution, and high quantum efficiency imaging transmission filter based on optical trapping of resonance radiation in potassium vapor. The filter can be used to image radiation over a bandwidth narrow enough to fall within a Fraunhofer dark zone in the solar spectrum, and it can be applied to the imaging of flames, plumes or discharges containing potassium. It may also be applicable to the imaging of Raman scattering from a tunable laser.

Coherent Rayleigh-Brillouin scattering.

Coherent Rayleigh-Brillouin scattering in gases has been studied experimentally for the first time in the kinetic regime and shown to give line shapes that differ significantly from the spontaneous Rayleigh-Brillouin scattering. A kinetic model was developed to obtain an analytic solution of the line shape for monatomic gases, and good agreement with the experimental data was achieved.

Gas temperature measurements in weakly ionized glow discharges with filtered Rayleigh scattering.

We report the first gas temperature measurements in plasmas to our knowledge obtained by filtered Rayleigh scattering (FRS). A narrow-linewidth Ti:sapphire laser is used as the illumination source, and a mercury filter provides strong suppression of elastic background. We perform measurements in weakly ionized glow discharges in pure argon and in an argon-plus-1%-nitrogen mixture.