James W. L. Lewis

Spatial and temporal profiles of pulsed laser-induced air plasma emissions

Abstract The laser-induced breakdown and energy deposition in air were studied using a full-width at half-maximum (FWHM) 6.5-ns pulse Nd : YAG laser. For the non-resonant breakdowns, two-dimensional spontaneous emission images were obtained with nanosecond resolution from breakdown to 10 times the FWHM of the pulse of the spatial and temporal variations of the energy deposition and plasma formation processes. The spatial emission features are compared with the predictions of the laser irradiance in the focal region that results from the focus lens aberrations. The statistical variation of the absorbed energy was determined over the energy range from below-to-approximately six times the breakdown threshold. Comparisons are made of the measured plasma wave speed with one-dimensional laser-supported radiation wave predictions.

Temperature measurements from CN spectra in a laser-induced plasma

Abstract The spontaneous-emission spectra of the CN violet system were observed following excimer-laser-induced breakdown of an atmospheric-pressure CO 2 /N 2 mixture. Using a triple monochromator and a gated linear-diode array, the spectra were acquired with spectral resolutions of 2 and 7 cm -1 within 1 μsec following the laser pulse. Comparison of the observed rotation-vibrational structure of the Δν=0 sequence and the synthetic spectra, which were calculated using direct diagonalization of the rotational and fine-structure Hamiltonians, yielded internal, molecular temperatures of approx. 8000 K. No evidence of internal nonequilibrium of the rotational and vibrational modes was observed.

Visualization of laser-induced breakdown and ignition

Laser-induced gas breakdown and ignition of atmospheric pressure NH3/O2 mixtures are investigated. The nanosecond-pulsed, 1064-nm Nd:YAG laser is used to create the cascade-type optical breakdown. The post-breakdown plasma and ignition are studied using spectroscopic techniques that include spontaneous emission and NH planar laser-induced fluorescence (PLIF). These time-resolved two-dimensional images provide not only radiative and gas dynamic information but also the space-time loci of the temperature and transient species concentrations. The results provide an understanding of the plasma kernel dynamics and the flame development that is essential to verify on-going simulation modeling of laser-ignition.

Spectroscopic temperature determination of aluminum monoxide in laser ablation with 266-nm radiation

We report time-resolved measurements of diatomic aluminum monoxide spectra in the study of laser ablation by the use of frequency-quadrupled 266 nm Nd:YAG laser radiation. Spectroscopic temperatures of 3432(35) K and 3329(13) K are obtained at a delay time of 20mu, respectively, by the use of the modified diatomic Boltzmann plot and by the use of the Nelder-Mead algorithm in the fitting of the recorded spectrum.

How keratoconus influences optical performance of the eye

Using a statistical description of keratoconus (KC) topography, schematic eye models of various KC conditions are constructed to study their optical influence on visual performance. The cone shape, protruding height and extent, and distance from the visual zone are independently investigated with the three-dimensional optical eye-modeling and ray-tracing techniques. The subsequent spherical equivalent (SE), cylinder, together with residual high-order ocular aberrations, are examined and related to each separated variable. The results show that myopic nature of SE is greatly dominated by the location of the cone. The cylinder is determined by the cone shape when the cone is inside the visual zone. It is dominated by the cone location when the cone is away from visual axis. The least myopic meridian always falls on the cone direction, and the high-order aberrations strongly relate to the cone dimension. This study investigates KC cone effect on optical quality and provides comprehension of clinical observations.

Spectroscopic temperature measurements in a decaying laser-induced plasma using the C2 Swan system

Abstract Spontaneous emission spectra of C2 Swan bands were recorded well after i.r. 1064 nm Nd: YAG laser-induced optical breakdown of carbon monoxide. Temperatures in excess of 6000 K were determined using fits to synthetic diatomic molecular spectra.

Electron number density and temperature measurement in a laser-induced hydrogen plasma

Abstract A Nd:YAG laser operated at 1064 nm and 7.5 nsec pulse duration is used to create optical breakdown in gaseous hydrogen. Time-resolved spectral measurements of the hydrogen Balmer series are reported and analyzed to characterized the electron number density and excitation temperature of the decaying plasma. The electron density is inferred from the Stark broadened H α linewidths; excitation temperatures are estimated using Boltzmann plots of the Balmer series. In the first few microseconds following laser breakdown, electron densities are found to be in the range of 10 19 -10 16 cc -1 , with corresponding excitation temperatures in the range of approx. 100,000–6600 K.

Fluid dynamics effects following laser-induced optical breakdown

The fluid flow phenomena resulting from laser-spark decay is described based on a numerical simulation. The transient model involves the solution of the two-dimensional axial-symmetric transport equations of mass, momentum, and energy in nitrogen gas. Initial temperature and pressure profiles are taken upon the termination of the laser pulse, and are based on the laserinduced plasma formation processes. The specific heat and transport property models include the effects of dissociation and ionization. Temperature, pressure, and velocity profiles are reported at selected times, and experimental ultra-high speed photographs are compared with the numerical analog.

Spherical aberration effects in lens–axicon doublets: theoretical study

Effects of spherical aberrations in converging and diverging lens-axicon doublets are investigated. Intensity profiles are obtained in the line and ring focal regions by numerically solving the Fresnel-Kirchhoff integral. Comparisons with aberration-free computations show that higher peak irradiances occur for the converging doublet when spherical aberrations are included. Results are presented for axicons with small apex angles in lens-axicon combinations illuminated by collimated Gaussian 1.064-mum laser beams.

TEMPERATURE MEASUREMENTS FROM FIRST-NEGATIVE N2+ SPECTRA PRODUCED BY LASER-INDUCED MULTIPHOTON IONIZATION AND OPTICAL BREAKDOWN OF NITROGEN

A 248-nm excimer laser was used to produce ionized nitrogen by the process of multiphoton excitation in gaseous nitrogen at room temperature. First-negative N(2)(+) emission spectra were analyzed to yield rotational temperatures of typically 600 to 1200 K. Rotational Raman scattering of H(2) in gaseous mixtures of N(2) and H(2) was used to determine if laser heating of the gas produced the observed increase in temperature, but the room temperature value of 295 K was inferred from the H(2) Raman data. Therefore the use of N(2)(+) spectra produced by multiphoton excitation at 248 nm does not appear to be acceptable for air-temperature diagnostics. N(2)(+) emission spectra were also recorded subsequent to optical breakdown in air induced by Nd:YAG 1064-nm radiation, and temperatures were determined to be greater than 5000 K in the decaying plasma.