Quantitative three-dimensional optical tomographic imaging of supersonic flows.

Three-dimensional imaging of the density of nitrogen in a supersonic expansion from a nozzle by means of beam-deflection optical tomography is described. With a very simple apparatus, images could be obtained with high absolute accuracy, high spatial resolution, and wide dynamic range.

Congener-specific detection of dioxins using jet-REMPI.

Although 210 chemically different polychlorinated dibenzo-p-dioxin and dibenzofuran congeners can be produced during combustion, it is currently believed that fewer than 20 are toxic enough to warrant monitoring. SRI is developing a continuous emissions monitor to study the emission levels of these most toxic dioxins, leading eventually to an improved understanding of the formation of these molecules and to improved means of monitoring and control. We report here the first results of performing congener-specific detection for two dichloro dibenzo-p-dioxins present in low ppt concentrations in a mixture using the supersonic jet/resonantly enhanced multiphoton ionization time-of-flight mass spectrometer technique. In addition, we present preliminary data on the detection of chlorinated aromatic compounds using a two-color REMPI scheme with the same instrument.

Two-wavelength beam deflection technique for electron density measurements in laser-produced plasmas.

We describe the use of a two-wavelength beam deflection technique in the measurement of electron density and expansion velocity in a laser-produced plasma. Beam deflection measurements are made with a spatial resolution of 250 microm, temporal resolution of 25 ns, and a dynamic range of 1000. Several techniques for determining the spatial and temporal variation of the electron density from beam deflection measurements are described.

Wavelength dependence of the Raman cross section for liquid water.

We report measurements of the wavelength dependence of the 3400-cm(-1) Raman scattering cross section of liquid water for excitation wavelengths between 215 and 550 nm. Using previous measurements of the absolute Raman scattering cross section, we have determined an expression for the wavelength dependence of the absolute Raman cross section for water.

Ratio of oxygen and nitrogen Raman cross sections in the ultraviolet.

We report measurements of the ratio of oxygen and nitrogen Raman cross sections for excitation wavelengths between 220 and 290 nm. These measurements confirm strong enhancement of the oxygen Raman cross section in this region.

Two-photon excitation of neon at 133 nm.

High VUV powers, up to 2.5 kW at 133 nm, are obtained from two-photon-resonant, difference-frequency generation in hydrogen by using an ArF excimer laser and a frequency-doubled Nd:YAG-pumped dye laser. This radiation is used to perform two-photon excitation of neon at greater than 150000 cm(-1) , the shortest two-photon transition yet accessed to our knowledge.

Tunable differential interferometer for optical tomography.

We report the use of a tunable differential interferometer for optical tomography. This interferometer has several advantages over other methods for phase measurements in optical tomography, including good stability, variable sensitivity, and the elimination of fringe ambiguity. Quantitative images of the gas concentrations in subsonic jets of methane and oxygen issuing into air are presented, with absolute accuracies better than 3.5% and 4.5%, respectively.

Emission tomography of flame radicals.

We describe a simple experimental arrangement for emission tomographic measurements of radicals in flames. Experimental determinations of two-dimensional spatially resolved distributions of excited-state CH in atmospheric-pressure flames are presented. Calibration of the distributions to absolute number densities is performed with a Rayleigh scattering technique. Methods for simultaneous recording of two to six emission projections and reconstruction from very few (two or three) projections are experimentally investigated. The potential of emission tomography with high temporal resolution for monitoring, e.g., explosions or turbulent flames, is discussed.

Three-dimensional beam-deflection optical tomography of a supersonic jet.

We report 3-D imaging of density in a supersonic expansion using beam-deflection optical tomography. Quantitative high-resolution images with absolute accuracy of 3%, dynamic range of 500:1, and spatial resolution to within a factor of 1.7 of the diffraction limit were produced with a He-Ne laser and simple apparatus. Theory shows that the spatial frequency content of beam-deflection measurements is well suited for tomographic reconstruction. The theory for the diffraction-limited resolution for tomography is presented.

High-precision frequency-domain measurements of the optical properties of turbid media.

We report high-precision measurements of the absorption and the reduced scattering coefficients for turbid media. Using a frequency-domain measurement technique for a point-source infinite-medium geometry, we find that the standard deviations of multiple measurements of the absorption coefficient mu(a) and the reduced scattering coefficient mu(s)(?) are less than 0.6%, and three independently derived values for mu(a) and mu(s)(?) agree to better than 1%. Measurements of mu(a) agree with measurements of a nonscattering medium to within 1.2%. To obtain high precision requires attention to proper conditions for the spherical photon-density wave model, the detection linearity, and the ratio of the absorption rate to the source modulation rate. Frequency-domain amplitude and phase measurements deviate from fitted curves by 0.1% and 0.06 degrees rms, respectively.

Frequency-domain immersion technique for accurate optical property measurements of turbid media.

We demonstrate that absorption coefficient micro(a) and reduced scattering coefficient micro(s)(?) of a small turbid object can be measured to high accuracy with a frequency-domain immersion technique. For this technique the sample is immersed in a calibrated scattering medium and the optical properties are obtained from a differential measurement. Compared with conventional approaches, the immersion technique improves accuracy, minimizes variations owing to probe coupling and motion, reduces the effects of boundary conditions, and offers simple and rapid measurement once the immersion medium is calibrated. Accuracy tests of immersion-based measurements of micro(a) and micro(s)(?) agree with reference values to within 3.6% and 2.6%, respectively. These tests are limited by the accuracy of the reference samples rather than by the accuracy of the immersion medium or the precision of the immersion approach. We demonstrate the in vivo capabilities of the technique through time-resolved measurements of micro(a) and micro(s)(?) for a human hand during cuff occlusion on the upper arm.

Quantitative optical tomographic imaging of a supersonic jet.

Two-dimensional quantitative images of the density of chlorine in a supersonic expansion are produced with an absolute accuracy of 5% in 150-microm pixels by using optical absorption tomography. An incoherent arc-lamp source provides ample ultraviolet radiation for high-resolution optical absorption tomographic measurements. A measurement of the chlorine absorption coefficient is reported.

Beam-deflection optical tomography.

We report two-dimensional density measurements in a supersonic expansion using beam-deflection optical tomography. Quantitative, high-resolution images with absolute accuracy to 3.5% and spatial resolution to 50 microm were taken with a helium-neon laser and simple apparatus.

Beam-deflection optical tomography of a flame.

We report three-dimensional index-of-refraction measurements in a methane-air slot diffusion flame and a methane-air slot subsonic jet using beam-deflection optical tomography. The horizontal and vertical spatial resolutions are 0.26 and 0.635 mm, respectively. The peak temperature is calculated from the index-of-refraction map for the flame and compared with thermocouple temperature measurements.

High-power Lyman-alpha source generated with an ArF excimer laser.

We report high-power vacuum-ultraviolet (vuv) generation at the Lyman- alpha wavelength of 121.6nm , using a simple experimental system. vuv radiation is produced through two-photon-resonant difference-frequency mixing with a tunable ArF excimer laser and a Nd:YAG-pumped dye laser. Using phase-matched mixtures of Kr and Ar at a total pressure of 650 mbar, we produced 7-microJ energies at Lyman- alpha in approximately 5ns (1.3kW) , as measured directly with a pyroelectric energy probe. Measurements indicate that higher powers are possible with system optimization. A tuning range of 0.1nm was achieved for a fixed gas mole fraction at a total pressure of 650 mbar. Qualitative agreement is found between measured tuning profiles and theoretical predictions.

Compensated transillumination.

We demonstrate imaging through human tissue in vivo , using a new optical technique, compensated transillumination. Immersion in a scattering medium with absorption and scattering coefficients matched to the tissue is used for drastic improvement of image contrast. The immersion medium is composed of polymer microspheres and methylene blue dye. The optical properties of the medium are matched to those of the tissue by use of a frequency-domain measurement technique. Images of a human hand taken with this technique show the internal structure, including the outlines of bones. The mechanism for the contrast is likely the absence of blood between the bones.

Transient effects on stimulated Brillouin scattering.

We present a detailed comparison of theory and experiment for transient stimulated Brillouin scattering for a pump pulse with Gaussian temporal profile. A new approach for measuring Brillouin linewidths is demon-strated, and an unexplained asymmetry is observed.

Electrostrictive and thermal stimulated Rayleigh spectroscopy in liquids.

We have observed the spectrum of electrostrictive stimulated Rayleigh scattering in a liquid and created a transition to stimulated thermal Rayleigh scattering with the addition of an absorbing liquid. With the proper amount of absorption, the electrostrictive and thermal contributions to the scattering exactly cancel, resulting in no stimulated Rayleigh scattering. An absorption coefficient of 0.000 12 cm(-1) is sufficient to cancel the electrostrictive Rayleigh scattering in Freon 113.

Laser linewidth narrowing induced by amplifier gain saturation.

We demonstrate that temporal broadening of transform-limited nanosecond pulses during gain saturation in a Nd:YAG amplifier can produce substantial linewidth narrowing. Measurements of linewidths with two-color Doppler-free saturated spectroscopy in I(2) show reductions in linewidth by more than a factor of 2.5 from 58 to 20 MHz when a double-pass amplifier is used. The observed temporal broadening is described well by a simple gain saturation theory.

Frequency-domain technique for optical property measurements in moderately scattering media.

We demonstrate a reflection geometry frequency-domain technique that is suitable for measurement of the optical properties of a medium with moderate scatter and absorption (mu(s) and mu(a) ~ 1 m(-1)). Variations in absorption and scatter produce opposing tends in the amplitude signal and common trends in the phase signal, allowing unique determination of absorption and scattering coefficients for a given phase function. An analytical model based on single-scattering events gives good agreement with experimental measurements.