Andrew D. Sappey

Optical imaging through turbid media with a degenerate four-wave mixing correlation time gate

A novel method for detection of ballistic light and rejection of unwanted diffusive light to image structures inside highly scattering media is demonstrated. Degenerate four wave mixing (DFWM) of a doubled YAG laser in Rhodamine 6G is used to provide an ultrafast correlation time gate to discriminate against light that has undergone multiple scattering and therefore lost memory of the structures inside the scattering medium. We present preliminary results that determine the nature of the DFWM grating, confirm the coherence time of the laser, prove the phase-conjugate nature of the signal beam, and determine the dependence of the signal (reflectivity) on dye concentration and laser intensity. Finally, we have obtained images of a test cross-hair pattern through highly turbid suspensions of whole milk in water that are opaque to the naked eye. These imaging experiments demonstrate the utility of DFWM for imagingthrough turbid media. Based on our results, the use of DFWM as an ultrafast time gate for the detectionof ballistic light in optical mammography appears to hold great promise for improving the current state of the art.

Planar laser‐induced fluorescence imaging of Cu atom and Cu2 in a condensing laser‐ablated copper plasma plume

Planar laser‐induced fluorescence (PLIF) is used to monitor Cu atoms and Cu2 produced by excimer laser ablation of a copper target (308 nm, ≳10 J/cm2, 1–3 GW/cm2) expanding rapidly into helium background gas at pressures ranging from 10 to 100 Torr. The Cu2 results from gas phase condensation of the copper atoms ablated from the target in the regions of highest Cu atom density as expected, but the maximum Cu2 laser‐induced fluorescence (LIF) signal occurs significantly after the maximum of the Cu signal. Rotationally resolved excitation scans of Cu2 utilizing the A–X (0,0) transition indicate that the Cu2 has reached equilibrium with the 300 K background gas. An extensive search for Cu3 via LIF failed, indicating that Cu3 is present only in very low ‘‘steady state’’ number density in the plume. This data is explained qualitatively by a simple kinetic model. In addition to the kinetic information, it is clear from the PLIF images that viscous eddy formation becomes more pronounced as the backing gas pressu...

Distribution of OH within silent discharge plasma reactors

Barrier discharges in humid air generate large quantities of OH and are well suited for the treatment of gas-based pollutants. In order to understand and optimize the formation and utilization of OH, we have imaged the spatial and temporal distribution of the OH radical with resolution of 40 /spl mu/m and 100 ns. Using a flowing mix of Ar/O/sub 2//H/sub 2/O (78.5/20/1.5), we have found that OH exists only within the 85-/spl mu/m radius channel formed by the transient microdischarge. At modest power densities of 0.1 W/cm/sup 2/, over l0/sup 6//cm/sup 2//s of these small filaments quickly fill the entire reactor volume.

Absolute copper atom density determination in laser-ablated copper plasmas using hook spectroscopy

Hook spectroscopy has been implemented to measure the absolute density of atomic species in a laser‐ablated plasma plume. We use the hook method to determine the density of copper atoms produced by ablation of a copper target in 25 Torr of helium at power densities of ∼1 GW/cm2. The measured copper densities when ablating into 25 Torr of helium backing gas range from 1.5×1015 cm−3 at a delay of 200 μs to 4.4×1013 cm−3 at 5 ms delay. This dramatic decrease in density is due to condensation of the metal vapor to form fine particulate.

Time-resolved PLIF imaging of Cu in a laser-ablated copper plasma plume

Planar laser-induced fluorescence (PLIF) has been used to determine the relative number density of ground state copper atoms in laser-ablated plasma plumes. An ablation laser power flux of /spl sim/1.5 GW/cm/sup 2/ is applied to a solid copper target in a background gas, producing a plasma plume suitable for studying homogeneous copper vapor condensation. Density is measured at postablation time delays ranging from 5 /spl mu/s to 10 ms with 1-100 torr of either argon or helium as the background gas. Planar laser-induced fluorescence images are used to spatially resolve the relative density within the plume, The decrease in density is due to the homogeneous condensation of copper vapor to form particulate.

Measurement of fuel corrosion products using planar laser-induced fluorescence

Characterizing the corrosion behavior of nuclear fuel material in a high‐temperature hydrogen environment is critical for ascertaining the operational performance of proposed nuclear thermal propulsion (NTP) concepts. In this paper, we describe an experimental study undertaken to develop and test non‐intrusive, laser‐based diagnostics for ultimately measuring the distribution of key gas‐phase corrosion products expected to evolve during the exposure of NTP fuel to hydrogen. A laser ablation technique is used to produce high temperature, vapor plumes from uranium‐free zirconium carbide (ZrC) and niobium carbide (NbC) forms for probing by various optical diagnostics including planar laser‐induced fluorescence (PLIF). We discuss the laser ablation technique, results of plume emission measurements, and we describe both the actual and proposed planar LIF schemes for imaging constituents of the ablated ZrC and NbC plumes. Envisioned testing of the laser technique in rf‐heated, high temperature gas streams is al...

Limitations in the application of hook spectroscopy for density measurement in high‐density‐gradient media

Hook spectroscopy has been successfully implemented to measure ground‐state Cu atom density in laser‐ablated copper vapor plumes in a background gas. In 1–25 Torr of background gas, the technique has been used to measure Cu atom densities up to 4×1015 cm−3. With higher background gas pressures, increasing distortion of the hook spectra is observed until they are rendered nearly uninterpretable at background gas pressures as low as 100 Torr. This effect is attributed to wavelength‐dependent beam deflection caused by large gradients in copper atom density present at high background gas pressures.

Varied line-space grating for flat spectral response of coupling to single-mode fiber

We use a planar linear grating with varied line-space grooves to introduce a tailored one-dimensional phase variation profile that results in an aberrated point-spread function at the focal plane. A design procedure for the period chirp map for such gratings is developed. As an example, we present theoretical and experimental results on a mechanically ruled, varied line-space echelle grating in single-mode fiber-coupled optical multiplexers in the wavelength region of 1545 nm. The varied line-space grating changes the multiplexers Gaussian spectral response function to a flat-top dependence with reduced sensitivity to source laser wavelength drift.

Hook spectroscopy as an atomic number density diagnostic applied to laser-ablated copper plasmas

Hook spectroscopy has been used to determine the absolute number density of ground state copper atoms in laser-ablated plasma plumes. An ablation laser power flux of {approximately} 1.5 GW/cm{sup 2} is applied to a solid copper target in a background gas, producing a plasma plume suitable for studying homogeneous copper vapor condensation. Density is measured at post-ablation time delays ranging from 10 {mu}s to 3 ms with 25 torr of argon as the background gas. Planar laser-induced fluorescence (PLEF) images containing relative density information are used in conjunction with the hook spectra to resolve spatially the absolute density within the plume. Copper atom densities thus measured ranged from 1.9 {times} 10{sup 15} cm{sup {minus}3} at a delay of 10 {mu}s to 2.7 {times} 10{sup 13} cm{sup {minus}3} at 3 ms delay in 25 torr of argon The decrease in density is due to the condensation of copper vapor to form fine particulate. As a combustion diagnostic, the hook method may prove extremely useful for the determination of metal impurity density in coal fired flames as well as a single -- shot OH density and temperature diagnostic.