Quang-Viet Nguyen

Laser pulse-stretching with multiple optical ring cavities

We describe a simple and passive nanosecond-long laser-pulse stretcher using multiple optical ring cavities. We present a model of the pulse-stretching process for an arbitrary number of optical ring cavities. This new model explicitly includes the effects of cavity delay time, beam-splitter reflectivity, total number of optical cavities, and describes the effects of spatial profile sensitivity. Using the model, we optimize the design of a pulse stretcher for use in a spontaneous Raman-scattering excitation system that avoids laser-induced plasma spark problems. From the optimized design, we then experimentally demonstrate and verify the model with a three-cavity pulse-stretcher system that converts a 1000-mJ, 8.4-ns-long input laser pulse into an approximately 75-ns-long (FWHM) output laser pulse with a peak power reduction of 0.10x and an 83% efficiency.

Measurement and simulation of spontaneous Raman scattering in high-pressure fuel-rich H2–air flames

Rotational and vibrational spontaneous Raman scattering (SRS) spectra of H2, N2 and H2O have been measured in H2–air flames at pressures up to 30.4 bar as a first step towards establishing a comprehensive Raman spectral database for temperatures and species in high-pressure combustion. We have obtained an initial set of measurements that indicate the spectra are of sufficient quality in terms of spectral resolution, wavelength coverage and signal-to-noise ratio for use in the development of transferable standards for the cross-talk calibration matrix. The fully resolved Stokes and anti-Stokes shifted spectra were collected in the visible wavelength range (400–700 nm) using pulse-stretched 532 nm excitation and a spectrograph fitted with a non-intensified CCD detector and a high-speed shutter. Temperatures were determined via the intensity distribution of rotational H2 lines at stoichiometric and fuel-rich conditions. A discussion of the temperature measurement accuracy in terms of the number of laser shots, including a single-shot measurement, is presented. Theoretical Raman spectra of hydrogen were calculated using a semi-classical anharmonic-oscillator model with recent pressure broadening data and were compared with experimental data. The data and simulation showed good agreement at different equivalence ratios and pressures and indicate that high-J rotational lines of H2 may interfere with the N2 vibrational Q-branch lines, which could lead to errors in N2-Raman thermometry based on the line-fitting method. In addition, the relative intensities of the O- and S-branches to the Q-branch were determined theoretically and the result indicates that further studies of spectral interferences including contributions from O- and S-branches should be pursued. Finally, from a comparison of N2 Q-branch spectra in lean H2–air flames at nearly atmospheric (1.2 bar) and high pressure (30.4 bar), we found no significant line-narrowing or -broadening effects at a spectral resolution of 0.04 nm.

Single-shot rotational Raman thermometry for turbulent flames using a low-resolution bandwidth technique

An alternative optical thermometry technique that utilizes the low-resolution (order 101 cm−1) pure-rotational spontaneous Raman scattering of air is developed to aid single-shot multiscalar measurements in turbulent combustion studies. Temperature measurements are realized by correlating the measured envelope bandwidth of the pure-rotational manifold of the N2/O2 spectrum with a theoretical prediction of a species-weighted bandwidth. By coupling this thermometry technique with conventional vibrational Raman scattering for species determination, we demonstrate quantitative spatially resolved, single-shot measurements of the temperature and fuel/oxidizer concentrations in a high-pressure turbulent CH4–air flame. Our technique provides not only an effective means of validating other temperature measurement methods, but also serves as a secondary thermometry technique in cases where the anti-Stokes vibrational N2 Raman signals are too low for a conventional vibrational temperature analysis.

Subframe Burst Gating for Raman Spectroscopy in Combustion

We describe an architecture for spontaneous Raman scattering utilizing a frame-transfer CCD sensor operating in a subframe burst-gating mode to realize time-resolved combustion diagnostics. The technique permits all-electronic optical gating with microsecond shutter speeds (<5 micros) without compromising optical throughput or image fidelity. When used in conjunction with a pair of orthogonally polarized excitation lasers, the technique measures single-shot vibrational Raman scattering that is minimally contaminated by problematic optical background noise.

Measurements of Equivalence Ratio Fluctuations in a Lean Premixed Prevaporized (LPP) Combustor and Its Correlation to Combustion Instability

Experimental evidence correlating equivalence ratio fluctuations with combustion instabilities and NOX emissions in a jet-A fueled lean premixed prevaporized (LPP) combustor utilizing a non-proprietary ‘generic’ fuel injector is presented. Real-time laser absorption measurements of equivalence ratio, together with dynamic combustor pressure, flame luminosity and fuel pressure were obtained at inlet air conditions up to 16.7 atm and 817 K. From this data, an extensive database of real-time variables was obtained for the purposes of providing validation data for future studies of LPP combustion modeling. In addition, time and frequency space analysis of the data revealed measurable levels of acoustic coupling between all variables. Equivalence ratio and dynamic pressure cross-correlations were found to predict the level of combustion instability. Furthermore, NOX production was found to follow the root-mean-square (RMS) flame luminosity and RMS combustor dynamic pressure. However, the unmixedness of the fuel-air mixture was not found to predict NOX production in this combustor. The generic LPP injector, although not optimized for low-emissions or combustion stability, provides some of the essential features of real injectors for the purposes of studying the relationship between fluctuations in equivalence ratios and combustion instability. In particular, the fuel premixer advection time was found to have a significant and direct impact on the level of combustion instability. The results of this work support the time-lag concept for avoiding combustion instability when designing injector/premixers in LPP combustors.© 2002 ASME

Development of Hyperspectral remote sensing capability for the early detection and monitoring of Harmful Algal Blooms (HABs) in the Great Lakes

Hyperspectral imagers have significant capability for detecting and classifying waterborne constituents. One particularly appropriate application of such instruments in the Great Lakes is to detect and monitor the development of potentially Harmful Algal Blooms (HABs). Two generations of small hyperspectral imagers have been built and tested for aircraft based monitoring of harmful algal blooms. In this paper a discussion of the two instruments as well as field studies conducted using these instruments will be presented. During the second field study, in situ reflectance data was obtained from the Research Vessel Lake Guardian in conjunction with reflectance data obtained with the hyperspectral imager from overflights of the same locations. A comparison of these two data sets shows that the airborne hyperspectral imager closely matches measurements obtained from instruments on the lake surface and thus positively supports its utilization for detecting and monitoring HABs.

Observation of Turbulent Mixing in Lean-Direct-Injection Combustion at Elevated Pressure

We report the first quantitative single-shot multiscalar data obtained from a realistic air-fed lean-direct-injection burner operating on gaseous methane (CH 4 ) fuel at elevated pressure (5 atm) using single-shot spontaneous Raman spectroscopy. From a statistical analysis of the multiscalar data, we present spatially mapped probability density functions of the concentration of CH 4 and O 2 , and the instantaneous temperature. The measured three-scalar correlations and probability density functions provide insights into the nature and extent of the mixing process and its impact on the subsequent combustion process. The data will also be useful for comparison with the various turbulence-chemistry interaction models such as large-eddy simulation. The swirl-stabilized flame investigated in this paper was characterized as operating in a partially premixed combustion regime that was dominated by turbulent mixing provided by the lean-direct-injection configuration. Although a majority of the single-shot data indicated complete or near-complete reactions including stoichiometric combustion, a considerable number of the data points exhibited incomplete combustion characterized by a substantial amount of residual fuel at intermediate temperatures or were simply unreacted with little or no preheating of the mixture.

MISSE-X: An ISS external platform for space environmental studies in the Post-Shuttle era

Materials International Space Station Experiment-X (MISSE-X) is a proposed International Space Station (ISS) external platform for space environmental studies designed to advance the technology readiness of materials and devices critical for future space exploration. The MISSE-X platform will expand ISS utilization by providing experimenters with unprecedented low-cost space access and return on investment (ROI). As a follow-on to the highly successful MISSE series of ISS experiments, MISSE-X will provide advances over the original MISSE configurations including incorporation of plug-and-play experiments that will minimize return mass requirements in the post-Shuttle era, improved active sensing and monitoring of the ISS external environment for better characterization of environmental effects, and expansion of the MISSE-X user community through incorporation of new, customer-desired capabilities. MISSE-X will also foster interest in science, technology, engineering, and math (STEM) in primary and secondary schools through student collaboration and participation.1,2

Extremely Low Power Quantum Optical Communication Link for Miniature Planetary Sensor Stations

In this paper a very low power optical communications system is addressed that could be developed specifically for creating networks involving a planetary lander and fixed or mobile sensor stations that could be as small as 1 cm3. The communication system is a variant of photon-counting based communications. Instead of counting individual photons, the system only counts the arrival of time coincident sets of photons. Using sets of photons significantly decreases the bit error rate because they are highly identifiable in the presence of ambient light.An experiment demonstrating reliable communication over a distance of 70 m using less than a billionth of a watt of radiated power is presented. The experiment also compares this technique to traditional photon counting and successfully demonstrates that time coincident photon communications can achieve an equivalent bit error rate at a signal-to-noise ratio that is 5 to 7 dB lower than what is needed for classical photon counting communication. The components used in this system were chosen so that they could in the future be integrated into a cubic centimeter device.

Quantum Optical Communication for Micro Robotic Explorer s

One way to improve the acquisi tion of planetary information from robotic landers is to use many smaller robotic explorers that can cover more ground than a single conventional rover , given that the existing launch capabilities constrain the mass of planetary robotic landers to a relati vely fixed value . In addressing this vision, NASA has been challenged in the National Nanotechnology Initiative to research the development of miniature robots built from nano -sized components. These robots have very significant challenges, such as mobil ity and communication, given the small size and limited power generation capability. The research presented here has been focused on developing a communications system that has the potential for provid ing ultra -low power communications for robots such as t hese. In this paper an optical communications technique that is based on quantum entangled photons is presented. This technique both minimizes radiated power and is appropriate for the size scale of the robot. Simulations of this system show that the rad iated power may be reduced to extremely low levels for line -of -sight communication over distances of several kilometers, even in the presence of ambient light. The immunity to background noise makes this technique very promising in comparison to classical optical communications for ultra -low power applications. The results of t his research will generate requirements definitions for transceiver components built from nanotechnology. Results from experiments demonstrating aspects of this communications techn ique are presented.