An evaluation of precision and systematic errors in vibrational CARS thermometry

Multiplex coherent anti-Stokes Raman scattering spectroscopy for trace chemical detection

Trace chemical detection is a particularly challenging problem of significant Army interest. Optical diagnostic techniques offer rapid, accurate, sensitive, and highly selective detection of hazardous materials in a variety of systems. Multiplex coherent anti-stokes Raman scattering (MCARS) spectroscopy generates a complete Raman spectrum from the material of interest using a combination of a supercontinuum pulse, which drives multiple molecular vibrations simultaneously, and a narrowband probe pulse. In this study, we demonstrated the ability of MCARS to detect trace amounts of both explosive materials and chemical warfare agent simulants with limits of detection below 0.2 ng and 0.1 nl, respectively. Integration times were on the order of 10 ms, using a compact USB spectrometer. Characteristics of supercontinuum generation were studied and compared to results in the literature. Finally, an algorithm that utilizes a combination of the maximum entropy method and advanced Fourier filtering to analytically remove the non-resonant background from the MCARS spectra without any a priori knowledge of the vibrational spectrum of the material of interest.

Development of a dual-pump coherent anti-Stokes Raman spectroscopy system for measurements in supersonic combustion

This work describes the development of a dual-pump coherent anti-Stokes Raman spectroscopy system for simultaneous measurements of the temperature and the absolute mole fraction of N2, O2, and H2 in supersonic combusting flows. Changes to the experimental setup and the data analysis to improve the quality of the measurements in this turbulent, high-temperature reacting flow are described. The accuracy and precision of the instrument have been determined using data collected in a Hencken burner flame. For temperatures above 800 K, errors in the absolute mole fraction are within 1.5%, 0.5%, and 1% of the total composition for N2, O2, and H2, respectively. Standard deviations based on 500 single shots are between 10 and 65 K for the temperature, between 0.5% and 1.7% of the total composition for O2, and between 1.5% and 3.4% for N2. The standard deviation of H2 is ~10% of the average measured mole fraction.

Development of multipoint vibrational coherent anti-Stokes Raman spectroscopy for flame applications

A novel technique for coherent anti-Stokes Raman spectroscopy (CARS) measurements in multiple points is presented. In a multipass cavity the pump and Stokes laser beams are multiply reflected and refocused into a measurement volume with an adjustable number of separated points along a line. This optical arrangement was used in a vibrational CARS setup with planar BOXCARS phase-matching configuration. The CARS spectra from spatially separated points were recorded at different heights on a CCD camera. Measurements of temperature profiles were carried out in the burned gas zone of a premixed one-dimensional flame to demonstrate the applicability of this method for temperature measurements in high-temperature regions. The ability to measure in flames with strong density gradients was demonstrated by simultaneous measurements of Q-branch spectra of N2 and CO in a Wolfhard-Parker burner flame. Interference phenomena found in multipoint spectra are discussed, and possible solutions are proposed. Merits and limitations of the technique are discussed.

Thermometry for turbulent flames by coherent anti-Stokes Raman spectroscopy with simultaneous referencing to the modeless excitation profile

An optimal system for temperature measurements by coherent anti-Stokes Raman spectroscopy (CARS) in turbulent flames and flows is presented. In addition to a single-mode pump laser and a modeless dye laser, an echelle spectrometer with a cross disperser is used. This system permits simultaneous measurement of the N2 CARS spectrum and the broadband dye laser profile. A procedure is developed to use software to transform this profile into the excitation profile by which the spectrum is referenced. Simultaneous shot-to-shot referencing is compared to sequential averaged referencing for data obtained in flat flames and in room air. At flame temperatures, the resultant 1.5% imprecision is limited by flame fluctuations, indicating that the system may have a single-shot imprecision below 1%. At room temperature, the 3.8% single-shot imprecision is of the same order as the best values reported for dual-broadband pure-rotational CARS. Using the unique shot-to-shot excitation profiles, simultaneous referencing eliminates systematic errors. At 2000 and 300 K, the 95% confidence intervals are estimated to be +/- 20 and +/- 10 K, respectively.

Pure rotational coherent anti-stokes Raman scattering: comparison of evaluation techniques for determining single-shot simultaneous temperature and relative n(2)-o(2) concentration

The accuracy and precision of time-resolved simultaneous temperature and O(2)-concentration measurements in binary N(2)-O(2) mixtures by single-pulse dual-broadband pure rotational coherent anti-Stokes Raman scattering (CARS) have been investigated. We present a detailed comparison of the applicability of six evaluation procedures to measurements of air in a temperature range 300-2050 K. Special emphasis is put on the dependence of the results on experimental restrictions and distortions. This comparison includes the least-sum-of-squared-differences fit (LSF) in the frequency space obtained by use of three different kinds of weighting with respect to signal intensity and in Fourier space by use of the complex or the cosine Fourier transformation, both of which permit a great reduction in the number of data points necessary for multidimensional evaluation. Additionally, a cross-correlation technique is tested that, to the best of our knowledge, was not previously applied to pure rotational CARS. We also present the results of measurements directed to the determination of low O(2)-concentration levels that were performed for various binary mixtures (1.0-15.6% O(2)) and for natural air within a temperature range of 300-773 K. A comparison is given for the three evaluation techniques that have proved most promising for the high-temperature investigations, i.e., the constant and the inverse weighted LSF in frequency space and the Fourier analysis technique.

Temperature dependence of laser-induced fluorescence of nitric oxide in laminar premixed atmospheric-pressure flames

The temperature dependence of laser-induced NO A (2)?(+)-X (2)? fluorescence in the hot gases of natural gas-air flames, seeded with known quantities of NO, has been determined experimentally by means of a difference method. The flame temperature at three fixed equivalence ratios was changed when the mixture velocity was varied through a water-cooled, flat-flame burner and was measured by coherent anti-Stokes Raman spectroscopy. When the possible reburning of part of the seeded NO is allowed for, the results in the range 1700-2150 K are best described by the temperature dependence obtained from a model in which quenching corrections are neglected, as in the case of a saturated two-level system, when millijoule pulse energies are used. Measurements of the fluorescence intensity at constant seed concentration as a function of equivalence ratio between 0.75 and 1.3 also indicate that quenching corrections are unnecessary under these excitation conditions. Using the measured intensities of the seeded flame as a calibration factor, we determined the absolute NO concentrations as functions of the equivalence ratio at 1 cm above the burner. The results indicate that, with the calibration method presented here, a relative accuracy of 5% should be obtainable.

Experimental comparison of single-shot broadband vibrational and dual-broadband pure rotational coherent anti-Stokes Raman scattering in hot air

Broadband vibrational and dual-broadband pure rotational coherent anti-Stokes Raman scattering (CARS) have been compared in a high-temperature oven, in which the accuracy and single-shot precision of gas temperature and relative O(2)- and N(2)-concentration measurements in hot air were probed over a temperature range that is typical for many combustion processes. To ensure a realistic comparison, we used nearly the same experimental setup for both CARS techniques. Besides temperature information, dual-broadband pure rotational CARS offers the possibility of achieving simultaneous single-shot concentration measurements. The comparison shows that this technique also has significant advantages in temperature evaluation over a large temperature range in comparison with vibrational CARS.

Analysis of laser-induced-fluorescence carbon monoxide measurements in turbulent nonpremixed flames

The influence of fluctuating concentrations and temperature on the laser-induced-fluorescence (LIF) measurement of CO in turbulent flames is described, under conditions in which the fluorescence and the temperature are measured independently. The analysis shows that correlations between CO concentration and temperature can bias the averaged mole fraction extracted from LIF measurements. The magnitude of the bias can exceed the order of the average CO mole fraction. Further, LIF measurements of CO concentrations in a turbulent, nonpremixed, natural gas flame are described. The averaged CO mole fractions are derived from the fluorescence measurements by the use of flame temperatures independently measured by coherent anti-Stokes Raman spectroscopy. Analysis of the fluctuations in measured temperature and fluorescence indicates that temperature and CO concentrations in flame regions with intensive mixing are indeed correlated. In the flame regions where burnout of CO has ceased, the LIF measurements of the CO mole fraction correspond to the probe measurements in exhaust.

Simultaneous coherent anti-Stokes Raman scattering and two-dimensional laser Rayleigh thermometry in a contained technical swirl combustor

The simultaneous application of vibrational coherent anti-Stokes Raman scattering (CARS) and the two-dimensional (2D) UV laser Rayleigh technique is reported for the investigation of a highly turbulent swirl frame inside a contained technical combustor. The CARS technique has been used to determine accurate temperature values at one point within the 2D Rayleigh-probed combustion field. These values were necessary to normalize the Rayleigh data to overcome influences of absorption effects along the detection path of the Rayleigh-scattered light through the exhaust gas volume and by the sealing window of the combustion chamber. At several different downstream positions, 500 simultaneous measurements with the point and with the 2D technique were performed to cover the whole combustion field. These data can be used for both the evaluation of 2D temperature structures in single frames and for the calculation of temperature probability density functions from the Rayleigh data at one single camera pixel over 500 frames. With this information, characterization of a highly turbulent flame is possible.

Simultaneous temperature and sensitive two-species concentration measurements by single-shot CARS

Simultaneous spatially and temporally resolved measurements of N(2) and O(2) mole fractions and of temperature are performed using coherent anti-Stokes Raman scattering (CARS). The CARS setup is used with the crossed-beam arrangement (BOXCARS) and nonresonant-background suppression. The technique employs two Stokes lasers, broadband and narrowband, in combination with a frequency-doubled Nd:YAG laser. Temperature and N(2) mole fractions are obtained by single-shot multiplex CARS spectra of N(2) using the broadband laser; O(2) mole fractions are deduced from a particular rovibrational Q-line of O(2) using the narrowband dye laser. The single-shot detectivity limit is better than 0.4% for oxygen at 2200 K and atmospheric pressure, i.e., 10(16) molecules x cm(-3). The capability of the technique for measuring 2-D probability density functions is demonstrated in the simple cases of an isothermal jet and a laminar premixed flame of air and ethylene. The experimental work reveals grave difficulties in using CARS for precise measurements of mole fractions: appreciable signals can be created very far from the geometrical focus; beam disruption by turbulence and the Stark effect cause large mole fraction measurement errors. These problems are discussed. Referencing the mole fraction of the second species by nitrogen mole fraction is demonstrated to be a solution for the turbulence effect in premixed flames.