Applications of Kalman filtering to real-time trace gas concentration measurements

A Kalman filtering technique is applied to the simultaneous detection of NH₃ and CO₂ with a diode-laser-based sensor operating at 1.53 μm. This technique is developed for improving the sensitivity and precision of trace gas concentration levels based on direct overtone laser absorption spectroscopy in the presence of various sensor noise sources. Filter performance is demonstrated to be adaptive to real-time noise and data statistics. Additionally, filter operation is successfully performed with dynamic ranges differing by three orders of magnitude. Details of Kalman filter theory applied to the acquired spectroscopic data are discussed. The effectiveness of this technique is evaluated by performing NH₃ and CO₂ concentration measurements and utilizing it to monitor varying ammonia and carbon dioxide levels in a bioreactor for water reprocessing, located at the NASA–Johnson Space Center. Results indicate a sensitivity enhancement of six times, in terms of improved minimum detectable absorption by the gas sensor. Received: 13 July 2001 / Revised version: 11 October 2001 / Published online: 29 November 2001     

Quantitative NO-LIF imaging in high-pressure flames

Planar laser-induced fluorescence (PLIF) images of NO concentration are reported in premixed laminar flames from 1–60 bar exciting the A-X(0,0) band. The influence of O₂ interference and gas composition, the variation with local temperature, and the effect of laser and signal attenuation by UV light absorption are investigated. Despite choosing a NO excitation and detection scheme with minimum O₂-LIF contribution, this interference produces errors of up to 25% in a slightly lean 60 bar flame. The overall dependence of the inferred NO number density with temperature in the relevant (1200–2500 K) range is low (<±15%) because different effects cancel. The attenuation of laser and signal light by combustion products CO₂ and H₂O is frequently neglected, yet such absorption yields errors of up to 40% in our experiment despite the small scale (8 mm flame diameter). Understanding the dynamic range for each of these corrections provides guidance to minimize errors in single shot imaging experiments at high pressure. Received: 13 May 2002 / Published online: 8 August 2002     

Attenuation corrections for in-cylinder NO LIF measurements in a heavy-duty Diesel engine

Quantification of the nitric oxide (NO) concentration inside the cylinder of a Diesel engine by means of laser-induced fluorescence (LIF) measurements requires, amongst others, knowledge of the attenuation of the ultraviolet radiation involved. We present a number of laser diagnostic techniques to assess this attenuation, enabling a correction for laser intensity and detection efficiency of the raw NO LIF data. Methods discussed include overall laser beam transmission, bidirectional laser scattering (bidirectional LIF), spectrally resolved fluorescence imaging, and Raman scattering by N₂. A combination of techniques is necessary to obtain the complete attenuation of laser beam and NO fluorescence. The overall laser beam transmission measurements and bidirectional LIF measurements (the latter yielding spatially resolved transmission) provide evidence of a non-uniform attenuation distribution, with predominant attenuation within or near the piston bowl. Fluorescence imaging of multiple vibrational bands through a spectrograph is shown to be a powerful method for obtaining spatially resolved data on the transmission losses of fluorescence. Special attention is paid to the role of CO₂ and O₂ as UV light absorbers, and the consequences to different excitation-detection schemes for NO. PACS 82.33.Vx; 42.62.Fi; 33.20.t     

UV planar laser induced fluorescence imaging of hot carbon dioxide in a high-pressure flame

UV planar laser-induced fluorescence (PLIF) images of hot carbon dioxide (CO₂) are obtained in a laminar flame (CH₄/air) at high pressure (20 bar) with excitation wavelengths at 239.34 nm and 242.14 nm. Excitation wavelengths are chosen to minimize the contribution of nitric oxide and molecular oxygen LIF signals. Spectrally resolved single point measurements are used for correction of the remaining oxygen LIF interference. The continuum LIF signal from electronically excited CO₂ is detected in a broad (280–400 nm) emission region. The UV PLIF of hot CO₂ has the potential for application to a wide variety of diagnostic needs in high-pressure flames, combustors, and engines. PACS 42.62.Fi; 42.30.Va; 07.25+k; 39.30+w     

A phase-sensitive technique to measure small changes in laser frequency: application to measure the shift and broadening of a saturated absorption line of OsO4

A phase-sensitive technique was used to measure the sensitivity to operating conditions of the output frequency of a highly stable carbon dioxide laser, which operated on the 10-μmm R(0) transition (28.8 THz) and was locked to the saturated absorption of the Q(15) transition of OsO₄. Periodic changes in the operating conditions were imposed on the laser and the resulting changes in the output frequency were measured through phase-sensitive demodulation of the heterodyne beat with a second CO₂ laser of much lower stability. Shifts as small as 50Hz could be detected by this technique. The results are in good agreement with previous measurements which employed an infrared frequency chain.     

Quasi-simultaneous detection of CH2O and CH by cavity ring-down absorption and laser-induced fluorescence in a methane/air low-pressure flame

The combination of two-dimensional, planar laser-induced fluorescence (PLIF) and cavity ring-down (CRD) absorption spectroscopy is applied to map quantitatively the spatial distributions of CH₂O and CH in a methane/air flame at 25 Torr. Both species are detected in the same spectral region using the overlapping CH₂O A ¹ A ₂ -X ¹ A ₁ 4₁ ⁰ and CH B-X(1,0 )bands. The combination of diagnostic techniques exploits the spatial resolution of LIF and the quantitative CRD absorption measure of column density. The spatially resolved PLIF provides the distribution of absorbers and line-of-sight CRD absorption the absolute number density needed for quantitative concentration images. The peak CH₂O concentration is (3.5±1.4 )×10¹⁴ cm^-3, or 1450±550 ppm at 1000 K. The lack of precise absorption cross-section data produces these large error limits. Although a flame model predicts lower amounts, these large uncertainties limit this measurement’susefulness as a test of the flame chemistry. Received: 24 April 2001 / Revised version: 10 July 2001 / Published online: 10 October 2001     

Experimental evaluation of strategies for quantitative laser-induced-fluorescence imaging of nitric oxide in high-pressure flames (1-60 bar)

Nitric oxide laser-induced-fluorescence (NO-LIF) 2-D imaging measurements using a new multi-spectral detection strategy are reported for high-pressure flames (1-60 bar). This work builds on previous research that identified interference LIF from O₂ and CO₂ in high-pressure flames and optimized the choice of excitation strategies as a function of application conditions. In this study, design rules are presented to optimize the LIF detection wavelengths for quantitative 2-D NO-LIF measurements over a wide range of pressures (1-60 bar) and temperatures. Simultaneous detection of LIF in multiple wavelength regions enables correction of the NO signal for interference from O₂ and CO₂ and allows simultaneous imaging of all three species. New experiments of wavelength-resolved 1-D LIF in slightly lean (? = 0.9) and slightly rich (? = 1.1) methane/air flames are used to evaluate the design rules and estimate the NO detection limits for a wide range of flame conditions. The quantitative 2-D measurements of NO in the burnt gas are compared with model calculations (using GRI-Mech 3.0) versus pressure for slightly lean and slightly rich flames. The discussions and demonstrations reported in this study provide a practical guideline for application of instantaneous 1-D or 2-D NO-LIF imaging strategies in high-pressure combustion systems.     

Self- and foreign-broadening and shift coefficients for C2H2 lines at 1.54 μm

In this article we present experimental data for line parameters measurements of several transition lines of overtone bands of C₂H₂ at 1540 nm. The measurements were done with a spectrometer based on a semiconductor diode laser and a direct absorption spectroscopy scheme. Broadening and shift coefficients have been measured for the pure gas or in mixtures with N₂, O₂ and CO₂ as perturbing gas. Received 6 November 2000 and Received in final form 13 March 2001     

Methods for controlling of the laser-induced absorption in a BTO crystal by using of cw-laser radiation

The iron-atom concentration distribution as well as the gas-phase temperature was measured via laser-induced fluorescence (LIF) during iron-oxide nanoparticle synthesis in a low-pressure hydrogen/oxygen/argon flame reactor using ironpentacarbonyl (Fe(CO)₅) as precursor. Temperature measurements based on multi-line NO-LIF imaging are used to correct for temperature-dependent ground-state populations. The concentration measurement is calibrated based on line-of-sight absorption measurements. The influence of the precursor on the flame is observed at precursor concentrations larger than 70 ppm as the flame front moves closer to the burner surface with increasing Fe(CO)₅ concentration.     

CH4/air/SO2 premixed flame spectroscopy with a 7.5-μm diode laser

As sulfur dioxide (SO₂) is often involved in combustion processes, we present here SO₂-concentration measurements in the post-flame region of a CH₄/air/SO₂ premixed flame. SO₂ concentrations were deduced from high-resolution absorption spectra recorded with a mid-infrared tunable diode-laser (TDL) source operating at liquid nitrogen temperature. Single-mode, continuous frequency tuning around 1384.5 cm^-1 (or 7.5 μm) is achieved by a fine TDL temperature ramp. These experiments lead us to develop in situ combustion-pollutant measurements with compact apparatus. We show that this non-intrusive method is efficient for detection and allows the retrieval of SO₂ concentration and temperature. Received: 19 February 2001 / Revised version: 18 April 2001 / Published online: 7 June 2001     

Fluorescence quantum yield of carbon dioxide for quantitative UV laser-induced fluorescence in high-pressure flames

The fluorescence quantum yield for ultraviolet laser-induced fluorescence of CO₂ is determined for selected excitation wavelengths in the range 215–250 nm. Wavelength-resolved laser-induced fluorescence (LIF) spectra of CO₂, NO, and O₂ are measured in the burned gases of a laminar CH₄/air flame (φ=0.9 and 1.1) at 20 bar with additional NO seeded into the flow. The fluorescence spectra are fit to determine the relative contribution of the three species to infer an estimate of fluorescence quantum yield for CO₂ that ranges from 2–8×10^?6 depending on temperature and excitation wavelength with an estimated uncertainty of ±0.5×10^?6. The CO₂ fluorescence signal increases linearly with gas pressure for flames with constant CO₂ mole fraction for the 10 to 60 bar range, indicating that collisional quenching is not an important contributor to the CO₂ fluorescence quantum yield. Spectral simulation calculations are used to choose two wavelengths for excitation of CO₂, 239.34 and 242.14 nm, which minimize interference from LIF of NO and O₂. Quantitative LIF images of CO₂ are demonstrated using these two excitation wavelengths and the measured fluorescence quantum yield.     

Comparative first-principles analysis of the absorption spectra of ZnAl2S4 and ZnGa2O4 crystals doped with Cr3+

Systematic first-principles analysis of the energy level schemes and ground state absorption spectra of trivalent chromium in ZnAl₂S₄ and ZnGa₂O₄ crystals has been performed in the present paper. The recently developed first-principles approach to the analysis of the absorption spectra of impurity ions in crystals based on the discrete variational multi-electron (DV-ME) method [K. Ogasawara et al., Phys. Rev. B 64, 115413 (2001)] was used in the calculations. The method is based on the numerical solution of the Dirac equation; no phenomenological parameters are used in the calculations. As a result, complete energy level schemes of the Cr³⁺ ion and its absorption spectra in both crystals were calculated, assigned and compared with experimental data. By performing analysis of the molecular orbital (MO) population, it was shown that the covalency of the chemical bonds between the Cr³⁺ and S^2- ions is more significant than that one between the Cr³⁺ and O^2- ions.     

Spectroscopic detection of biological NO with a quantum cascade laser

Two configurations of a continuous wave quantum cascade distributed feedback laser-based gas sensor for the detection of NO at a parts per billion (ppb) concentration level, typical of biomedical applications, have been investigated. The laser was operated at liquid nitrogen temperature near λ=5.2 μm. In the first configuration, a 100 m optical path length multi-pass cell was employed to enhance the NO absorption. In the second configuration, a technique based on cavity-enhanced spectroscopy (CES) was utilized, with an effective path length of 670 m. Both sensors enabled simultaneous analysis of NO and CO₂ concentrations in exhaled air. The minimum detectable NO concentration was found to be 3 ppb with a multi-pass cell and 16 ppb when using CES. The two techniques are compared, and potential future developments are discussed. Received: 1 November 2000 / Revised version: 19 January 2001 / Published online: 20 April 2001     

Laser frequency measurements and spectroscopic assignments of optically pumped 13CH3OH

A three-laser heterodyne system was used to measure the frequencies of twelve optically pumped laser emissions from ¹³CH₃OH in the far-infrared (FIR) region. These emissions, ranging from 54 to 142 μm, are reported with fractional uncertainties up to ±2×10^-7 along with their polarization relative to the CO₂ pump. Using the 9P32 and 10R14 CO₂ lines, complete spectroscopic assignments for two laser systems were confirmed. Received: 31 May 2001 / Published online: 19 September 2001     

First principles analysis of the MgAl2O4:Ni absorption spectrum

Analysis of the energy-level scheme and absorption spectrum of the Ni²⁺ ion in MgAl₂O₄ was performed. The recently developed first-principles approach to the analysis of the absorption spectra of impurity ions in crystals based on the discrete variational multi-electron (DV-ME) method [K. Ogasawara, et al., Phys. Rev. B 64 (2001) 115413) was used in the calculations. The method is based on the numerical solution of the Dirac equation; no phenomenological parameters are used in the calculations. As a result, complete energy-level scheme of Ni²⁺ and its absorption spectra were calculated, assigned and compared with experimental data on the ground and excited state absorption spectra. Numerical contributions of all possible electron configurations into the calculated energy states were determined. By performing analysis of the molecular orbitals population, numerical contributions of the oxygen 2p- and 2s-orbitals into the 3d molecular orbitals were determined.     

Absolute absorber quantification in turbid media at small source–detector separations

The aim of our studies was to develop a method to determine the absorption coefficient of a turbid medium in a reflection geometry with small source–detector separations. Therefore, the time-integrated microscopic Beer–Lambert law (MBL) was modified in order to obtain the absolute absorption coefficient from mean time of flight and dc-intensity measurements. The new technique was evaluated using turbid phantoms having varying scattering (μ_s ^′ between 0.2 and 2.2 mm^-1) and absorbing properties (μ_a between 0.04 and 0.14 mm^-1), comparable to many biological tissues at various source–detector separations between 3 and 11 mm. The measurements were performed at a wavelength of 1064 nm. We found that this new method was able to determine the absolute absorption coefficient of the selected phantoms with a standard error of less than 0.005 mm^-1 over the range of optical properties investigated. Received: 23 November 2001 / Revised version: 28 February 2002 / Published online: 14 May 2002     

Investigations of the interactions of CO2, O3 and UV light with silver surfaces by in situ IRRAS/QCM and ex situ TOF-SIMS

The characterization of the reactions occurring during the atmospheric corrosion of silver surfaces at the metal-atmosphere interface is of great importance as only the exact knowledge of the degradation mechanisms enables the development of suitable corrosion inhibiting steps. For in situ surface analysis three complementary experimental techniques have been combined in order to study the surface chemistry of silver samples exposed to humidified air, CO₂, ozone (O₃) and ultraviolet (UV) light. In situ infrared reflection absorption spectroscopy (IRRAS) combined with a quartz crystal microbalance (QCM) as well as ex situ time of flight secondary ion mass spectrometry (TOF-SIMS) provide an overall picture of the processes occurring at the metal-atmosphere interface. Investigations were carried out by exposing polycrystalline silver samples to synthetic air containing 50-90% relative humidity (RH) and 250 ppm CO₂. Additionally, the samples were exposed to UV light or O₃ for certain time periods. The formation of the main corrosion products on the sample surfaces, depending on the relative humidity, O₃ content and UV light exposure could be observed in a time resolved way. These investigations reveal the different reaction mechanisms and reaction rates occurring on a silver surface.     

Nitric oxide detection inside the cylinder of an SI engine by direct UV absorption spectroscopy

The present work is a first step for the implementation and the development of a rapid and quantitative measurement technique using the UV broadband-lamp absorption spectroscopy. This technique is applied to detect nitric oxide molecules inside the combustion chamber of a methane-fueled spark-ignition engine. The ability of this technique to detect NO in very harsh thermodynamic conditions is proved under certain engine operating conditions. In spite of the broadband absorption, mainly due to the hot CO₂ and H₂O, clear NO absorption γ-bands are observed when the engine is skip-fired or when NO is added at the intake port. It was however, not possible to detect any nitric oxide absorption during the combustion time when each cycle was ignited and no nitric oxide was added. Finally, using the skip-ignition mode, the acquired NO absorption spectra allowed to conclude that evacuation of the in-cylinder residual gases can be complete at the end of the sixth cycle.     

Measurements of high-pressure CO2 absorption near 2.0 μm and implications on tunable diode laser sensor design

A tunable diode laser (TDL) is used to measure the absorption spectra of the R46 through R54 transitions of the 20012←00001 band of CO₂ near 2.0 μm (5000 cm⁻¹) at room temperature and pressures to 10 atm (densities to 9.2 amagat). Spectra are recorded using direct absorption spectroscopy and wavelength modulation spectroscopy with second-harmonic detection (WMS-2f) in a mixture containing 11% CO₂ in air. The direct absorption spectra are influenced by non-Lorentzian effects including finite-duration collisions which perturb far-wing absorption, and an empirical χ-function correction to the Voigt line shape is shown to greatly reduce error in the spectral model. WMS-2f spectra are shown to be at least a factor of four less-influenced by non-Lorentzian effects in this region, making this approach more resistant to errors in the far-wing line shape model and allowing a comparison between the spectral parameters of HITRAN and a new database which includes pressure-induced shift coefficients. The implications of these measurements on practical, high-pressure CO₂ sensor design are discussed.     

Sensitive detection of temperature behind reflected shock waves using wavelength modulation spectroscopy of CO2 near 2.7?μm

Tunable diode-laser absorption of CO₂ near 2.7 μm incorporating wavelength modulation spectroscopy with second-harmonic detection (WMS-2f) is used to provide a new sensor for sensitive and accurate measurement of the temperature behind reflected shock waves in a shock-tube. The temperature is inferred from the ratio of 2f signals for two selected absorption transitions, at 3633.08 and 3645.56 cm⁻¹, belonging to the ν ₁+ν ₃ combination vibrational band of CO₂ near 2.7 μm. The modulation depths of 0.078 and 0.063 cm⁻¹ are optimized for the target conditions of the shock-heated gases (P∼1–2 atm, T∼800–1600 K). The sensor is designed to achieve a high sensitivity to the temperature and a low sensitivity to cold boundary-layer effects and any changes in gas pressure or composition. The fixed-wavelength WMS-2f sensor is tested for temperature and CO₂ concentration measurements in a heated static cell (600–1200 K) and in non-reactive shock-tube experiments (900–1700 K) using CO₂–Ar mixtures. The relatively large CO₂ absorption strength near 2.7 μm and the use of a WMS-2f strategy minimizes noise and enables measurements with lower concentration, higher accuracy, better sensitivity and improved signal-to-noise ratio (SNR) relative to earlier work, using transitions in the 1.5 and 2.0 μm CO₂ combination bands. The standard deviation of the measured temperature histories behind reflected shock waves is less than 0.5%. The temperature sensor is also demonstrated in reactive shock-tube experiments of n-heptane oxidation. Seeding of relatively inert CO₂ in the initial fuel-oxidizer mixture is utilized to enable measurements of the pre-ignition temperature profiles. To our knowledge, this work represents the first application of wavelength modulation spectroscopy to this new class of diode lasers near 2.7 μm.